5 Earth’s Tectonic Forces

5.1 Geologic Structures

Stress and Strain

Stress is the force exerted per unit area, and strain is a material’s response to that force. Strain is deformation caused by stress. Strain in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. There are three types of stress: tensional, compressional, and shear. Tensional stress involves pulling something apart in opposite directions, stretching and thinning the material. Compressional stress involves things coming together and pushing on each other, thickening the material. Shear stress involves transverse movement of material moving past each other, like a scissor.


When rocks are stressed, the resulting strain can be elastic, ductile, or brittle, called deformation. Elastic deformation is strain that is reversible after the stress is released. For example, when you compress a spring, it elastically returns to its original shape after you release it. Ductile deformation occurs when enough stress is applied to a material that the changes in its shape are permanent, and the material is no longer able to revert to its original shape. For example, if you stretch a spring too far, it can be permanently bent out of shape. Note that concepts related to ductile deformation apply at the visible (macro) scale, and deformation is more complex at a microscopic scale. Research of plastic deformation, which touches on the atomic scale, is generally beyond the scope of introductory texts. The yield point is the amount of strain at which elastic deformation is surpassed, and permanent deformation is measurable. Brittle deformation is when the material undergoes another critical point of no return. When sufficient stress to pass that point occurs, it fails and fractures.

Important factors that influence how a rock will undergo elastic, ductile, or brittle deformation is the intensity of the applied stress, time, temperature, confining pressure, pore pressure, strain rate, and rock strength. Pore pressure is the pressure exerted by fluids inside of the open spaces (pores) inside of a rock or sediment. Strain rate is how quickly material is deformed. Rock strength is a measure of how readily a rock will respond to stress. Shale has low strength and granite has high strength.

Removing heat, such as decreasing temperature, makes the material more rigid. Likewise, heating materials make them more ductile. Heating glass makes it capable of bending and stretching. Regarding strain response, it is easier to bend a piece of wood slowly without breaking it.

Sedimentary rocks are essential for deciphering the geologic history of a region because they follow specific rules. First, sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top. Second, sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some volcanic rocks, such as ash falls. Finally, sedimentary rock layers that are not horizontal are deformed in some manner. Often looking like they are tiling into the earth.

Scientists can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest-on-bottom position. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses.

Stress and Mountain Building

It is the sheer power and strength of two or more converging continental plates smash upwards that create mountain ranges. Stresses from geologic uplift cause folds, reverse faults, and thrust faults, which allow the crust to rise upwards. Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges. When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults. The result is alternating mountains and valleys, known as a basin-and-range.


Geologic folds are layers of rock that are curved or bent by ductile deformation. Terms involved with folds include axis, which is the line along which the bending occurred, and limbs, which are the dipping beds that make up the sides of the folds. Compressional forces most commonly form folds at depth, where hotter temperatures and higher confining pressures allow ductile deformation to occur.

Folds are described by the orientation of their axes, axial planes, and limbs. They are made up of two or more sets of dipping beds, generally dipping in opposite directions, that come together along a line, called the axis. Each set of dipping beds is known as a fold limb. The plane that splits the fold into two halves is known as the axial plane.

Symmetrical folds have mirrored limbs across their axial planes. The limbs of a symmetrical fold are inclined at the same, but opposite, angle indicating equal compression on both sides of the fold. Asymmetrical folds have dipping, non-vertical axial planes, where limbs dip into the ground at different angles. Recumbent folds are very tight folds with limbs compressed near the axial planes, and are generally horizontal, and overturned folds are where the angles on both limbs dip in the same direction. The fold axis is where the axial plane intersects the strata involved in the fold. A horizontal fold has a horizontal fold axis. When the axis of the fold plunges into the ground, the fold is called a plunging fold.

Drawing of a geologic syncline and anticline.


Anticlines are arch-like (“A”-shaped) folds, with downward curving limbs that have beds that dip away from the central axis of the fold. They are convex-upward in shape. In anticlines, the oldest rock strata are in the center of the fold, along the axis, and the younger beds are on the outside. An antiform has the same shape as an anticline, but in antiforms, the relative ages of the beds in the fold cannot be determined. Oil geologists have interest in anticlines because they can form oil traps, where oil migrates up along the limbs of the fold and accumulates in the high point along the axis of the fold.


Synclines are trough-like (“U” shaped), upward curving folds that have beds that dip in towards the central axis of the fold. They are concave-upward in shape. In synclines, the older rock is on the outside of the fold, and the youngest rock is on the inside of the fold along the axis. A synform has the shape of a syncline but, like an antiform, does not distinguish between the ages of the units.

2x1 Anticline & Syncline


Oblique aerial photograph of a long line of multicolored rock beds dipping into the ground. The beds are fractured and erode in a way that makes the parts sticking out look like triangles.

Monoclines are step-like folds, in which flat rocks are upwarped or downwarped, then continue flat. They are relatively common on the Colorado Plateau where they form “reefs,” which are ridges that act as topographic barriers and should not be confused with ocean reefs. Capitol Reef is an example of a monocline in Utah. Monoclines can be caused by bending of shallower sedimentary strata as faults grow below them. These faults are commonly called “blind faults” because they end before reaching the surface and can be either normal or reverse faults.


A dome is a symmetrical to semi-symmetrical upwarping of rock beds. Domes have a shape like an inverted bowl, similar to domes on buildings, like the Capitol Building. Domes in Utah include the San Rafael Swell, Harrisburg Junction Dome, and the Henry Mountains. Some domes are formed from compressional forces, while other domes are formed from underlying igneous intrusions, by salt diapirs, or even impacts, like upheaval dome in Canyonlands National Park.


A basin is the inverse of a dome. The basin is when rock forms a bowl-shaped depression. The Uinta Basin is an example of a basin in Utah. Technically, geologists refer to rocks folded into a bowl-shape as structural basins. Sometimes structural basins can also be sedimentary basins in which large quantities of sediment accumulate over time. Sedimentary basins can form as a result of folding, but are much more commonly produced in mountain building, between mountain blocks or via faulting. Regardless of, the cause, as the basin sinks, called subsidence, it can accumulate even more sediment as the weight of the sediment causes more subsidence in a positive-feedback loop. There are active sedimentary basins all over the world. An example of a rapidly subsiding basin in Utah is the Oquirrh Basin of Pennsylvanian-Permian age in which over 30,000 feet of fossiliferous sandstones, shales, and limestones accumulated. These strata can be seen in the Wasatch Mountains along the east side of Utah Valley, especially on Mt. Timpanogos and in Provo Canyon.

5.2 Faults

Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. There are three major fault types: normal, reverse, and strike-slip. Normal and reverse faults display vertical, also known as dip-slip, motion. Dip-slip motion consists of relative up and down movement along a dipping fault between two blocks, the hanging wall, and the footwall. In a dip-slip system, the footwall is below the fault plane, and the hanging-wall is above the fault plane. An excellent way to remember this is to imagine a mine tunnel through a fault; the hanging wall would be where a miner would hang a lantern, and the footwall would be at the miner’s feet. Faults are more prevalent near and related to plate boundaries, but can occur in plate interiors as well. Faults can show evidence of movement along the fault plane. Slickensides are polished, often grooved surfaces along the fault plane created by friction during the movement. A joint or fracture is a plane of breakage in a rock that does not show movement or offset. Joints can result from many processes, such as cooling, depressurizing, or folding. Joint systems may be regional affecting many square miles.

Moab Fault splays in Moenkopi Formation rocks of early Triassic age, in a road cut on Route 191 in Moab Canyon, south of Arches National Park visitor centre, Utah, USA

Normal faults move by a vertical motion where the hanging-wall moves downward relative to the footwall along the dip of the fault. Tensional forces create normal faults in the crust. Normal faults and tensional forces are commonly caused at divergent plate boundaries and where the crust is being stretched by tensional stresses. Utah examples of normal faults are the Wasatch Fault, the Hurricane Fault, and other faults bounding valleys in the Basin and Range.

Grabens, horsts, and half-grabens are all blocks of crust or rock that are bounded by normal faults. Grabens drop down relative to adjacent blocks and create valleys. Horsts go up relative to adjacent down-dropped blocks and become areas of high topography. Where together, horsts and grabens create a symmetrical pattern of valleys surrounded by normal faults on both sides and mountains. Half-grabens are a one-sided version of a horst and graben, where blocks are tilted by a normal fault on one side, creating an asymmetrical valley-mountain arrangement. The mountain-valleys of the Basin and Range Province of Western Utah and Nevada consist of a series of full and half-grabens from the Salt Lake Valley to the Sierra Nevada Mountains. When the dip of a normal fault decreases with depth (i.e., the fault becomes more horizontal as it goes deeper), the fault is a listric fault. Extreme versions of listric faulting occur when large amounts of extension occur along very low-angle normal faults, known as detachment faults. The normal faults of the Basin and Range appear to become detachment faults at depth.

Diagram of a normal fault.

Reverse faults, caused by compressional forces, are when the hanging wall moves up relative to the footwall. A thrust fault is a reverse fault where the fault plane has a low dip angle (generally less than 45 degrees). Thrust faults bring older rocks on top of younger rocks and can cause repetition of rock units in the stratigraphic record. Convergent plate boundaries with subduction zones create a particular type of “reverse” fault called a megathrust fault. Megathrust faults cause the most significant magnitude earthquakes and commonly cause tsunamis.

Diagram of a reverse fault.

Strike-slip faults have a side to side motion. In the pure strike-slip motion, crustal blocks on either side of the fault do not move up or down relative to each other. There is left-lateral, called sinistral, and right-lateral, called dextral, strike-slip motion. In left-lateral or sinistral strike-slip motion, the opposite block moves left relative to the block that the observer is standing on. In right-lateral or dextral strike-slip motion, the opposite block moves right relative to the observer’s block. Strike-slip faults are most commonly associated with transform boundaries and are prevalent in fracture zones adjacent to mid-ocean ridges.

Diagram showing a strike-slip fault fault with two plates moving in opposite directions.

Bends in strike-slip faults can create areas where the sliding blocks create compression or tension. Tensional stresses will create transtensional features with normal faults an2d basins like California’s Salton Sea, and compressional stresses will create transpressional features with reverse faults and small-scale mountain building, like California’s San Gabriel Mountains. The faults that play off of transpression or transtension features are known as flower structures.

An example of a right-lateral strike-slip fault is the San Andreas Fault, which denotes a transform boundary between the North American and Pacific plates. An example of a left-lateral strike-slip fault is the Dead Sea fault in Jordan and Israel.

5.3 Causes of Earthquakes

People feel approximately 1 million earthquakes a year. Few are noticed very far from the source. Even fewer are major earthquakes. Earthquakes are usually felt only when they are greater than a magnitude 2.5. The USGS Earthquakes Hazards Program has a real-time map showing the most recent earthquakes. Most earthquakes occur along active plate boundaries. Intraplate earthquakes (not along plate boundaries) are still poorly understood.

Earthquake energy is known as seismic energy, and it travels through the earth in the form of seismic waves. To understand some of the basics of earthquakes and how they are measured, consider some of the fundamental properties of waves. Waves describe a motion that repeats itself in a medium such as rock or unconsolidated sediments. The magnitude refers to the height, called amplitude, of a wave. Wavelength is the distance between two successive peaks of the wave. The number of repetitions of the motion over time, called cycles per time, is the frequency. The inverse of frequency, which is the amount of time for a wave to travel one wavelength, is the period. When multiple waves combine, they can interfere with each other. When the waves are in sync with each other, they will have constructive interference, where the influence of one wave will add to and magnify the other. If the waves are out of sync with each other, they will have destructive interference. If two waves have the same amplitude and frequency and they are ½ wavelength out of sync, the destructive interference between them can eliminate each wave.

The elastic rebound theory explains the release of seismic energy. When rock is strained to the point that it undergoes brittle deformation, built-up elastic energy is released during displacement, which in turn radiates away as seismic waves. When the brittle deformation occurs, it creates an offset between the fault blocks at a starting point called the focus. This offset propagates along the surface of rupture, which is known as the fault plane.

The fault blocks of persistent faults like the Wasatch Fault of Utah are locked together by friction. Over hundreds to thousands of years, stress builds up along the fault. Eventually, stress along the fault overcomes the frictional resistance, and slip initiates as the rocks break. The deformed rocks “snap back” toward their original position in a process called elastic rebound. Bending of the rocks near the fault may reflect this build-up of stress, and in earthquake-prone areas like California, strain gauges that measure this bending are set up in an attempt to understand more about predicting an earthquake. In some locations where the fault is not locked, seismic stress causes continuous movement along the fault called fault creep, where displacement occurs gradually. Fault creep occurs along some parts of the San Andreas Fault.

The release of seismic energy occurs in a series of steps. After a seismic energy release, energy begins to build again during a period of inactivity along the fault. The accumulated elastic strain may produce small earthquakes (on or near the main fault). These are called foreshocks and can occur hours or days before a massive earthquake, but they may not occur at all. The main release of energy occurs during the major earthquake, known as the mainshock. Aftershocks may then occur to adjust strain that built up from the movement of the fault. They generally decrease over time.

5.4 Earthquake Zones

Nearly 95 percent of all earthquakes take place along one of the three types of tectonic plate boundaries, but earthquakes do occur along all three types of plate boundaries. About 80 percent of all earthquakes strike around the Pacific Ocean basin because it is lined with convergent and transform boundaries. Called the Ring of Fire, this is also the location of most volcanoes around the planet. About 15 percent take place in the Mediterranean-Asiatic Belt, where convergence is causing the Indian Plate to run into the Eurasian Plate creating the largest mountain ranges in the world. The remaining 5 percent are scattered around other plate boundaries or are intraplate earthquakes.

Transform Plate Boundarie

Transform plate boundaries occur where two tectonic plates are grinding parallel to each other rather than colliding or subducting. Deadly earthquakes occur at transform plate boundaries, creating strike-slip faults because they tend to have shallow focuses where the rupture occurs. The faults along the San Andreas Fault zone produce around 10,000 earthquakes a year. Most are tiny, but occasionally one is massive. In the San Francisco Bay Area, the Hayward Fault was the site of a magnitude 7.0 earthquake in 1868. The 1906 quake on the San Andreas Fault had a magnitude estimated at 7.9.

During the 1989 World Series, a magnitude 7.1 earthquake struck Loma Prieta, near Santa Cruz, California, killing 63 people, injuring 3,756, and cost $6 billion. A few years later in Northridge, California, a magnitude 6.7 earthquake killed 72 people, injured 12,000 people, and caused $12.5 billion in damage. This earthquake occurred on an unknown fault because it was a blind thrust fault near Los Angeles, California.

Although California is prone to many natural hazards, including volcanic eruptions at Mt. Shasta or Mt. Lassen, and landslides on coastal cliffs, the natural hazard the state is linked with is earthquakes. New Zealand also has strike-slip earthquakes, about 20,000 a year, but only a small percentage of those are large enough to be felt. A 6.3 quake in Christchurch in February 2011 killed about 180 people.

Convergent Plate Boundaries

Earthquakes at convergent plate boundaries mark the motions of subducting lithosphere as it plunges through the mantle, creating reverse and thrust faults. Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin. The Philippine Plate and the Pacific Plate subduct beneath Japan, creating a chain of volcanoes and produces as many as 1,500 earthquakes annually.

In March 2011 an enormous 9.0 earthquake struck off of Sendai in northeastern Japan. This quake, called the 2011 Tōhoku earthquake, was the most powerful ever to strike Japan and one of the top five known in the world. Damage from the earthquake was nearly overshadowed by the tsunami it generated, which wiped out coastal cities and towns. Two months after the earthquake, about 25,000 people were dead or missing, and 125,000 buildings had been damaged or destroyed. Aftershocks, some as large as major earthquakes, have continued to rock the region. A map of aftershocks is seen here. Recently, the New York Times created an interactive website of the Japan earthquake and tsunami.

The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time. Subduction of the Juan de Fuca plate beneath North America produces active volcanoes, but large earthquakes only hit every 300 to 600 years. The last was in 1700, with an estimated magnitude of around 9.0. The elastic rebound theory as applied to subduction zones can be viewed here.

Massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge. The 2001 Gujarat earthquake in India was responsible for about 20,000 deaths, and many more people became injured or homeless. In Understanding Earthquakes: From Research to Resilience, scientists try to understand the mechanisms that cause earthquakes and tsunamis and the ways that society can deal with them.

Divergent Plate Boundaries

Many earthquakes occur where tectonic plates are moving apart or where a tectonic plate is tearing itself apart. Earthquakes at mid-ocean ridges are small and shallow because the plates are young, thin, and hot. On land where continents split apart, earthquakes are larger and stronger. A classic example of normal faulting along divergent boundaries is the Wasatch Front in Utah and the entire Basin and Range through Nevada.

Intraplate Boundaries

Intraplate earthquakes are the result of stresses caused by plate motions acting in solid slabs of the lithosphere. In 1812, a magnitude 7.5 earthquake struck near New Madrid, Missouri. The earthquake was strongly felt over approximately 50,000 square miles and altered the course of the Mississippi River. Because very few people lived there at the time, only 20 people died. Many more people live there today. A similar earthquake today would undoubtedly kill many people and cause a great deal of property damage.

5.5 Focus and Epicenter

The focus, also called a hypocenter, of an earthquake, is the point of initial breaking or rupturing where the displacement of rocks occurs. The focus is always at some depth below the ground surface in the crust, and not at the surface. From the focus, the displacement propagates up, down, and laterally along the fault plane. The displacement produces shock waves, create seismic waves. Generally speaking, the larger the displacement and the further it propagates, the more significant the seismic waves and ground shaking. More shaking is usually the result of more seismic energy released. The epicenter is the location on the Earth’s surface vertically above the point of rupture (focus). The epicenter is also the location that most news reports give because it is the center of the area where people are affected. The focus is the point along the fault plane from which the seismic waves spread outward.

5.6 Seismic Waves

Seismic waves are an expression of the energy released after an earthquake in the form of body waves and surface waves. When seismic energy is released, the first waves to propagate out are body waves that pass through the body of the planet. Body waves include primary waves (P waves) and secondary waves (S waves). Primary waves are the fastest seismic waves. They move through rock via compression, very much like sound waves move through the air. Particles of rock move forward and back during the passage of the P waves. Primary waves can travel through both fluids and solids. Secondary waves travel slower and follow primary waves, propagating as shear waves. Particles of rock move from side to side during the passage of S waves. Because of this, secondary waves cannot travel through fluids, including liquids, plasma, or gas.

When an earthquake occurs at a location in the earth, the body waves radiate outward, passing through the earth and into the rock of the mantle. A point on this spreading wavefront travels along a specific path which reaches a seismograph located at one of the thousands of seismic stations scattered over the earth. That specific travel path is a line called a seismic ray. Since the density (and seismic velocity) of the mantle increases with depth, a process called refraction causes earthquake rays to curve away from the vertical and bend back toward the surface, passing through bodies of rock along the way.

Surface waves are produced when P and S body waves strike the surface of the earth and travel along the Earth’s surface, radiating outward from the epicenter. Surface waves travel more slowly than body waves. They have complex horizontal and vertical ground movement that creates a rolling motion. Because they propagate at the surface and have complex motions, surface waves are responsible for most of the damage. Two types of surface waves are Love waves and Rayleigh waves. Love waves produce horizontal ground shaking and, ironically from their name, are the most destructive. Rayleigh waves produce an elliptical motion of points on the surface, with longitudinal dilation and compression, like ocean waves. However, with Raleigh waves rock particles move in a direction opposite to that of water particles in ocean waves.

Earth is like a bell, and an earthquake is a way to ring it. Like other waves, seismic waves bend and bounce when passing from one material to another, like moving from a dense rock to rock with even higher density. When a wave bends as it moves into a different substance, it is known as refraction, and when waves bounce back, it is known as reflection. Because S waves cannot move through a liquid, they are blocked by the liquid outer core, creating a shadow zone on the opposite side of the planet to the earthquake source.

5.7 Measuring Earthquakes


Seismographs are instruments used to measure seismic waves. They measure the vibration of the ground using pendulums or springs. The principle of the seismograph involves mounting a recording device solidly to the earth and suspending a pen or writing instrument above it on a spring or pendulum. As the ground shakes, the suspended pen records the shaking on the recording device. The graph resulting from measurements of a seismograph is a seismogram. Seismographs of the early 20th century were essentially springs or pendulums with pens on them that wrote on a rotating drum of paper. Digital ones now use magnets and wire coils to measure ground motion. Typical seismograph arrays measure vibrations in three directions: north-south (x), east-west (y), and up-down (z).

To determine the distance of the seismograph from the epicenter, seismologists use the difference between the times when the first P waves and S waves arrive. After an earthquake, P waves will appear first on the seismogram, followed by S waves, and finally body waves, which have the largest amplitude on the seismogram. Surface waves do lose energy quickly, so they are not measured at great distances from the focus. Seismograph technology across the globe record the arrival of seismic waves from each earthquake at many station sites. The distance to the epicenter can be determined by comparing arrival times of the P and S waves. Electronic communication among seismic stations and connected computers used to make calculations mean that locations of earthquakes and news reports about them are generated quickly in the modern world.

Locating Earthquakes

Each seismograph gives the distance from that station to the earthquake epicenter. Three or more seismograph stations are needed to locate the epicenter of an earthquake through triangulation. Using the arrival-time difference from the first P wave to first S wave, one can determine the distance from the epicenter, but not the direction. The distance from the epicenter to each station can be plotted as a circle, the distance being equal to the circle’s radius. The place where the circles intersect demarks the epicenter. This method also works in three dimensions with spheres and multi-axis seismographs to locate not only the epicenter but also the depth of the focus of the earthquake.

Seismograph Network

The International Registry of Seismograph Stations lists more than 20,000 seismographs on the planet. Seismologists can use and compare data from sets of multiple seismometers dispersed over a wide area, which is a seismograph network. By collaborating, scientists can map the properties of the inside of the earth, detect detonation of large explosive devices, and predict tsunamis. The Global Seismograph Network, a set of world-wide linked seismographs that distribute real-time data electronically, consists of more than 150 stations that meet specific design and precision standards. The Global Seismograph Network helps the Comprehensive Nuclear-Test-Ban Treaty Organization monitor for nuclear tests. The USArray is a network of hundreds of permanent and transportable seismographs within the United States. The USArray is being used to map the subsurface through a passive collection of seismic waves created by earthquakes.

Determining Earthquake Magnitudes

Magnitude is the measure of the intensity of an earthquake. The Richter scale is the most well-known magnitude scale devised for an earthquake and was the first one developed by Charles Richter at CalTech. This was the magnitude scale used historically by early seismologists. The Richter scale magnitude is determined from measurements on a seismogram. Magnitudes on the Richter scale are based on measurements of the maximum amplitude of the needle trace measured on the seismogram and the arrival time difference of S and P waves which gives the distance to the earthquake.

The Richter scale is a logarithmic scale, based on powers of 10. The amplitude of the seismic wave recorded on the seismogram is ten times greater for each increase of 1 unit on the Richter scale. That means a magnitude six earthquake shakes the ground ten times more than a magnitude 5. However, the actual energy released for each 1 unit magnitude increase is 32 times greater. That means energy released for a magnitude six earthquake is 32 times greater than a magnitude 5. The Richter scale was developed for distances appropriate for earthquakes in Southern California and on seismograph machines in use there. Its applications to larger distances and very large earthquakes are limited. Therefore, most agencies no longer use the methods of Richter to determine the magnitude, but generate a quantity called the Moment Magnitude, which is more accurate for large earthquakes measured at the seismic array across the earth. As numbers, the moment magnitudes are comparable to the magnitudes of the Richter Scale. The media still often give magnitudes as Richter Magnitude even though the actual calculation was of moment magnitude.

Moment Magnitude Scale

The Moment Magnitude scale depicts the absolute size of earthquakes, comparing information from multiple locations and using a measurement of actual energy released calculated from the cross-sectional area of rupture, amount of slippage, and the rigidity of the rocks. Because of the unique geologic setting of each earthquake and because rupture area is often hard to measure, estimates of moment magnitude can take days to months to calculate.

Like Richter magnitude, the moment magnitude scale is logarithmic. Both scales are used in tandem because the estimates of magnitude may change after a quake. The Richter scale is used as a quick determination immediately following the quake (and thus is usually reported in news accounts), and the moment magnitude is calculated days to months later. Magnitude values of the two magnitudes are approximately equal except for very large earthquakes.

Modified Mercalli-Intensity Scale

The Modified Mercalli Intensity Scale is a qualitative scale (I-XII) of the intensity of ground shaking based on damage to structures and people’s perceptions. This scale can vary depending on the location and population density (urban vs. rural). It was also used for historical earthquakes which occurred before quantitative measurements of magnitude could be made. The Modified Mercalli Intensity maps show where the damage is most severe based on questionnaires sent to residents, newspaper articles, and reports from assessment teams. Recently, USGS has used the internet to help gather data more quickly.

Shakemaps (written ShakeMaps by the USGS) use high-quality seismograph data from seismic networks to show areas of intense shaking. They are the result of rapid, computer-interpolated seismograph data. They are useful in crucial minutes after an earthquake, as they can show emergency personnel where the most significant damage likely occurred and locate areas of possible damaged gas lines and other utilities.

5.8 Earthquake Risk

Determining Ground Shaking

In general, the larger the magnitude, the stronger the shaking and the longer the shaking will last. However, other factors influence the level of shaking as described in the following paragraphs. Table and descriptions from https://earthquake.usgs.gov/learn/topics/mag_vs_int.php

Location and Direction

Closer earthquakes will inherently cause more shaking than those farther away. The location about epicenter and direction of rupture will influence how much shaking is felt. The direction that the rupture propagates along the fault influences the shaking. The path of greatest rupture can intensify shaking in effect known as directivity.

The nature of the ground materials affects the properties of the seismic waves. Different materials respond differently to an earthquake. Think of shaking jello versus shaking a meatloaf; one will jiggle much more to the same amount of shaking. The response to shaking depends on their degree of consolidation; lithified sedimentary rocks, and crystalline rocks shake less than unconsolidated sediments and landfill. This is because seismic waves move faster through consolidated bedrock, move slower through unconsolidated sediment, and move slowest through unconsolidated materials with high water content. Since the energy is carried by both velocity and amplitude, when a seismic wave slows down, its amplitude increases, which in turn increases seismic shaking. Energy is transferred to the vertical motion of the surface waves.

Depth of Focus

The focus is the place within the Earth where the earthquake starts, and the depth of earthquakes influences the amount of shaking. Deeper earthquakes cause less shaking at the surface because they lose much of their energy before reaching the surface. Recall that most of the destruction is caused by surface waves which are caused as the body waves reach the surface.

5.9 What Determines Destruction?

Building Materials

Building material choices can influence the amount of damage caused by earthquake shaking. The flexibility of building materials relates to their resistance to damage by earthquake waves. Unreinforced Masonry (URM) is the most devastated by ground shaking. Wood framing held together with nails which can bend and flex with wave passage are more likely to survive earthquakes. Steel also can deform elastically before brittle failure. The Salt Lake City campaign “Fix the Bricks” has useful information on URMs and earthquake safety.

Shaking Intensity and Duration

Greater shaking and duration of shaking will cause more destruction than less shaking and shorter shaking.


Resonance is when the frequency of seismic energy matches a building’s natural frequency of shaking, determined by properties of the building, and intensifies the amplitude of shaking. This famously happened in the 1985 Mexico City Earthquake, where buildings of heights between 6 and 15 stories were especially vulnerable to earthquake damage. Skyscrapers designed with earthquake resilience have dampers, and base isolation features to reduce resonance.

Changes in the structural integrity of a structure could alter its resonance. Conversely, changes in measured resonance can indicate potential changes in structural integrity.

Earthquake Recurrence

Geologists dig earthquake trenches across some faults to measure ground deformation and estimate the frequency of occurrence of past earthquakes. Trenches are effective for faults with relatively long recurrence intervals (100s to 10,000s of years), which is the period between significant earthquakes. In areas with more frequent earthquakes and more measured earthquake data, trenches are less necessary. A long hiatus in earthquake activity could indicate the buildup of stress on a specific segment of a fault with strain held in place by friction, which would indicate a higher probability of an earthquake along that segment. This hiatus of seismic activity along a length of a fault (i.e., a fault that is locked and not having any earthquakes) is known as a seismic gap.

5.10 Secondary Hazards Caused by Earthquakes


Liquefaction is when saturated unconsolidated sediments (usually silt or sand) is liquefied from shaking. Shaking causes loss of cohesion between grains of sediment, reducing the effective stress resistance of the sediment. The sediment flows very much like the quicksand presented in movies. Liquefaction creates sand volcanoes, which is when liquefied sand is squirted through an overlying (usually finer-grained) layer, creating cone-shaped sand features. It may also cause buildings to settle or tilt.

Earthquake-induced tsunamis have caused many of the more recent devastating natural disasters. Tsunamis form when the sea floor is offset by earthquakes in the ocean subsurface. This offset can be caused by fault movement or underwater landslides and lifts a volume of ocean water generating the tsunami wave. Tsunami waves travel fast with low amplitude in deep ocean water, but are significantly amplified as the water shallows as they approach the shore. When a tsunami is about to strike land, the water in front of the wave along the shore will recede significantly, tragically causing curious people to wander out. This receding water is the drawback of the trough in front of the tsunami wave which then crashes on shore as a wall of water upwards of a hundred feet high. Warning systems have been established to help mitigate the loss of life caused by tsunamis.


Shaking can trigger landslides (see landslide section for more information). One example is the 1992 magnitude 5.9 earthquake in St. George Utah. This earthquake caused the Springdale landslide, having a scarp that offset and destroyed several structures in the Balanced Rock Hills subdivision.


Seiches are waves on lakes generated by earthquakes, which cause sloshing of water back and forth and, sometimes, even changes in elevation of the lake. A seich in Hebgen Lake during the 1959 earthquake caused significant destruction to structures and roads around the lake.

Land Elevation Changes

Significant subsidence and upheaval of the land can occur about the slippage that causes earthquakes. Land elevation changes are the result of the relaxation of stress and subsequent movement along the fault plane. The 1964 Alaska earthquake is an excellent example of this. Where the fault cuts the surface, elevation of one side causes a fault scarp that may be a few feet to 20 or 30 feet in height. The Wasatch Mountains represent an accumulation of fault scarps of a couple of dozen feet at a time over a few million years.

5.11 Human-Induced Earthquakes

Can humans create earthquakes? Maybe not intentionally, but the answer is yes and here is why. If a water reservoir is built on top of an active fault line, the water may lubricate the fault and weaken the stress built up within it. This may either create a series of small earthquakes or potentially create a massive earthquake. Also, the sheer weight of the reservoir r’s water can weaken the bedrock causing it to fracture. Then the obvious concern is if the dam fails. Earthquakes can also be generated if humans inject other fluids into a fault such as sewage or chemical waste. Finally, nuclear explosions can trigger earthquakes. In fact, one way to determine if a nation has tested a nuclear bomb is by monitoring the earthquakes and energy released by the explosion.

5.12 Volcanism

A volcano occurs where lava erupts at the surface and solidifies into rock. This section describes volcano location, type, hazards, and monitoring.

Magma Generation

Magma and lava contain three components – melt, solids, and volatiles (dissolved gases). The liquid part, called melt, is made of ions from minerals that have already melted. The solid part, called solids, are crystals of minerals that have not melted (higher melting temperature) and are floating in the melt. Volatiles are gaseous components dissolved in the magma such as water vapor, carbon dioxide, sulfur, and chlorine. The presence and amount of these three components affect the physical behavior of the magma.

Geothermal Gradient

Although it is scorching under the Earth’s surface, the crust and mantle are mostly solid. This heat inside the Earth is caused by residual heat left over from the original formation of Earth and radioactive decay. The rate at which temperature increases with depth is called the geothermal gradient. The average geothermal gradient in the upper 100 kilometers of the crust is generally about 25 degrees Celsius per kilometer (km). So, for every kilometer of depth, the temperature increases by about 25 degrees Celsius.

Pressure-temperature diagrams illustrate the geothermal gradient together with the behavior of rock by graphing depth (pressure) and temperature (see figure). The figure shows the geothermal gradient changing with depth through the crust into the upper mantle. The diagram shows the geothermal gradient as a red line, and at 100 km depth, the temperature is about 1,200°C. Also, the pressure at the bottom of the crust (shown here as depth at 35 km deep) is about 10,000 bars. Bar is a measure of pressure, 1 bar being normal atmospheric pressure at sea-level. At these pressures and temperatures in the Earth, the crust and mantle rocks are solid. On the P-T diagram, the green solidus line shows the pressures and temperatures at which rocks start to melt. Since the geothermal gradient (red line) is always left of the solidus (green line) to a depth of 150 km, the rocks of the upper mantle are solid. This relationship continues through the mantle to the core-mantle boundary at about 2880 km. The solidus line slopes to the right because the melting temperature of any substance depends on the pressure. Higher pressure at greater depth requires a higher temperature to melt rock. In another example, water boils at 100°C at an atmospheric pressure close to 1 bar. However, if the pressure is lowered, as shown in the video below, then water boils at a much lower temperature.

The P-T diagram shows that there are three main ways that pressure and temperature conditions can change to cause rock to cross the green solidus line to the right to induce melting and create magma: 1) lower the pressure (decompression melting), 2) add volatiles (flux melting),and 3) increase the temperature (heat). Bowen’s work and the Bowen’s Reaction Series diagram show that minerals melt at different temperatures, so one can visualize that the green solidus line is a fuzzy zone in which some minerals are melting, and some remain solid. This is called partial melting and represents real magmas containing solid, liquid, and volatile components.

The figure below uses P-T diagrams to show how melting can occur at three different plate tectonic settings. Setting A is a typical situation in the middle of a stable plate in which no magma is generated. Setting B is at a mid-ocean ridge (decompression melting). Setting C is a hotspot (decompression melting plus the addition of heat), and setting D is a subduction zone (flux melting).

Decompression Melting

Magma is created at the mid-ocean ridge by decompression melting. The mantle is solid but is slowly flowing under enormous pressure and temperatures due to convection. Rock is not a good conductor of heat so as mantle rock rises, the pressure is reduced along with the melting point (the green line) but the rock temperature remains about the same and the rising rock begins to melt. Pressure changes instantaneously as the rock rises but temperature changes slowly because of the low heat conductivity of rock. On the figure above, setting B: mid-ocean ridge shows a mass of mantle rock at a pressure-temperature location X on the P-T diagram as well as its geographical location on the cross section under a mid-ocean ridge. At this location, the P-T diagram shows the red arrow increasing to the right. Thus, hotter rock is now shallower, at a lower pressure, and the new geothermal gradient (red line) shifts past the solidus (green line) and melting starts. As this magma continues to rise at divergent boundaries and encounters seawater, it cools and crystallizes to form new lithospheric crust.

Flux Melting

Another way that rocks melt is when volatiles gases (e.g., water vapor) are added to mantle rock from a descending subducting slab in a process called flux melting (or fluid-induced melting). The subducting slab contains oceanic lithosphere and hydrated minerals. As the slab descends and slowly increases in temperature, volatiles is expelled from these hydrated minerals, like squeezing water out of a sponge. The volatiles then rises into the overlying asthenospheric mantle lowering the melting point of the peridotite minerals (olivine and pyroxene). The pressure and temperature of the overlying mantle rock do not change, but the addition of volatiles lowers the melting temperature. This is analogous to adding salt to an icy roadway. The salt lowers the melting/crystallization temperature of the solid water (ice) so that it melts. Another example is welders adding flux to lower the melting point of their welding materials.

Flux melting is illustrated in setting D: island arc (subduction zone) of the P-T diagram above. Volatiles added to mantle rock at location “Z“ act as a flux to lower the melting temperature. This is shown in the P-T diagram by the solidus (green line) shifting to the left. The solidus line moves past the geothermal gradient (red line) and melting begins. Magmas producing the volcanoes of the Ring of Fire, associated with the circum-Pacific subduction zones are a result of flux melting. As introduced in the minerals chapter, water ions can bond with other ions in the crystal structures of amphibole (and other silicates), and this is important in considering how magmas form in subduction zones by “flux melting.” Such hydrated minerals in subducting slabs contribute water to the flux melting process.

Magma Composition

In 1980, Mount St. Helens blew up in the costliest and deadliest volcanic eruption in United States history. The eruption killed 57 people, destroyed 250 homes and swept away 47 bridges. Mount St. Helens today still has minor earthquakes and eruptions, and now has a horseshoe-shaped crater with a lava dome inside. The dome is formed of viscous lava that oozes into place.

It should first be noted that magma is molten material inside the earth, whereas lava is molten material on the surface of the earth. The reason for the distinction is because lava can cool quickly from the air and solidify into rock rapidly, whereas magma may never reach the earth’s surface. Volcanoes do not always erupt in the same way. Each volcanic eruption is unique, differing in size, style, and composition of the erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines (1) the eruption style, (2) the type of volcanic cone that forms, and (3) the composition of rocks that are found at the volcano.

Different minerals within rocks melt at different temperatures, and the amount of partial melting and the composition of the original rock determine the composition of the magma. Magma collects in magma chambers in the crust at 160 kilometers (100 miles) beneath the surface of a volcano.

The words that describe the composition of igneous rocks also describe magma composition. Mafic magmas are low in silica and contain more dark, magnesium, and iron-rich mafic minerals, such as olivine and pyroxene. Felsic magmas are higher in silica and contain lighter colored minerals such as quartz and orthoclase feldspar. The higher the amount of silica in the magma, the higher is its viscosity. Viscosity is a liquid’s resistance to flow.

Viscosity determines what the magma will do. Mafic magma is not viscous and will flow easily to the surface. Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form intrusive igneous rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move, and so it gets stuck. Dissolved gases become trapped by thick magma, and the magma chamber begins to build pressure.

Explosive Eruptions

The type of magma in the chamber determines the type of volcanic eruption. A massive explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II in which more than 40,000 people died. A large explosive volcanic eruption is 10,000 times as powerful. Felsic magmas erupt explosively because of hot, gas-rich magma churning within its chamber. The pressure becomes so great that the magma eventually breaks the seal and explodes, just like when a cork is released from a bottle of champagne. Magma, rock, and ash burst upward in an enormous explosion creating volcanic ash called tephra. It should be noted that when looked under a microscope, the volcanic “ash” is actual microscopic shards of glass. That is why it is so dangerous to inhale the air following an eruption.

Scorching hot tephra, ash, and gas may speed down the volcano’s slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000oC (1,800 degrees F).

Before the Mount St. Helens eruption in 1980, the Lassen Peak eruption on May 22, 1915, was the most recent Cascades eruption. A column of ash and gas shot 30,000 feet into the air. This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again. Mt. Shasta, the other active volcano in California, erupts every 600 to 800 years. An eruption would most likely create a large pyroclastic flow, and probably a lahar. Of course, Mt. Shasta could explode and collapse like Mt. Mazama in Oregon.

Volcanic gases can form toxic and invisible clouds in the atmosphere that could contribute to environmental problems such as acid rain and ozone destruction. Particles of dust and ash may stay in the atmosphere for years, disrupting weather patterns and blocking sunlight.

Effusive Eruptions

Mafic magma creates gentler effusive eruptions. Although the pressure builds enough for the magma to erupt, it does not erupt with the same explosive force as felsic magma. People can usually be evacuated before an effusive eruption, so they are much less deadly. Magma pushes toward the surface through fissures and reaches the surface through volcanic vents. Click here to view a lava stream within the vent of a Hawaiian volcano using a thermal camera.

Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in lava types like a ropy form pahoehoe and a chunky form called aa. Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building, road, or infrastructure.

5.13 Distribution of Volcanic Activity

Most volcanoes are located at active plate boundaries called interplate volcanism. The prefix “inter-“ means “between.” In contrast, some volcanoes are not associated with plate boundaries, but rather are located within the plate far from plate boundaries. These are called intraplate volcanoes, and many are formed by hotspots and fissure eruptions. The prefix “intra-“ means “within.” The following discusses the location of volcanism in more detail with mid-ocean ridges, subduction zones, and continental rifts representing interplate volcanism, and hot spots representing intraplate volcanism.

Volcanoes along Mid-Oceanic Ridges

Although most volcanism occurs on the ocean floor along the mid-ocean ridge (a type of divergent plate boundary), they are also the least observed since most are under 10,000 to 15,000 feet of ocean, an exception being Iceland. As the oceanic plates diverge and thin, hot mantle rock is allowed to rise, pressure from depth is released which causes the ultramafic mantle rock (peridotite) to melt partially. The resulting magma is basaltic in composition based on the concept of partial melting discussed earlier. Because most volcanoes on the ocean floor are basaltic, most of the oceanic lithosphere is also basaltic near the surface with phaneritic gabbro and ultramafic peridotite forming underneath. Icelandic volcanism is an example of this, but lying above sea level.

An underwater volcanic eruption occurs when basaltic magma erupts underwater forming pillow basalts and/or in small explosive eruptions. Lava erupting into seawater forms pillow-shaped structures (see figure) hence the name. In association with these seafloor eruptions, an entire underwater ecosystem thrives in parts of the mid-ocean ridge. This ecosystem exists around tall vents emitting black, hot mineral-rich water called deep-sea hydrothermal vents (also known as black smokers).

This hot water, up to 380 °C (716 °F), is heated by the magma and dissolves many elements, which support the ecosystem. Deep underwater where the sun cannot reach, this ecosystem of organisms depends on the heat of the vent for energy and vent chemicals as its foundation of life called chemosynthesis. The foundation of the ecosystem is hydrogen sulfide-oxidizing bacteria that live symbiotically with the larger organisms. Hydrogen sulfide (H2S, the gas that smells like rotten eggs) needed by these bacteria is contained in the volcanic gases emitted from the hydrothermal vents. The source of most of this sulfur and other elements is the Earth’s interior. Below are three short videos regarding a deep-sea submersible submarine and deep-sea hydrothermal vents.

Volcanoes along Convergent Boundaries

Volcanoes are a vibrant manifestation of plate tectonics processes. Volcanoes are common along convergent and divergent plate boundaries, but are also found within lithospheric plates away from plate boundaries. Wherever mantle can melt, volcanoes may be the result.
Volcanoes erupt because mantle rock melts. The first stage in creating a volcano is when mantle rock begins to melt because of extremely high temperatures, lithospheric pressure lowers, or water is added.

Subduction Zones

During the process of subduction, water is expelled from the hydrated minerals causing partial melting by flux melting in the overlying mantle rock. This creates a mafic magma that rises through the lithosphere and can change composition by interacting with surrounding continental crust as well as by magma differentiation. These changes then evolve basaltic magma into more silica-rich rock in volcanoes and plutons. These silica-rich rocks are felsic to intermediate rocks such as andesite, rhyolite, pumice, and tuff. The “Ring of Fire” surrounding the Pacific Ocean is dominated by subduction and contains volcanoes with silica-rich magma. These volcanoes are discussed in more detail in the stratovolcano section.

Large earthquakes are extremely common along convergent plate boundaries. Since the Pacific Ocean is rimmed by convergent and transform boundaries, roughly 80 percent of all earthquakes occur around the Pacific Ocean basin, called the Ring of Fire. A description of the Pacific Ring of Fire along western North America is below:

  • Subduction at the Middle American Trench creates volcanoes in Central America.
  • The San Andreas Fault is a transform boundary.
  • Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes like Mount St. Helens, Mount Rainer, Mount Hood and more.
  • Subduction of the Pacific plate beneath the North American plate in the north creates the long chain of the Aleutian Islands volcanoes near Alaska.

Continental Rifts

In addition to volcanoes at the mid-ocean ridge and subduction zones, some volcanoes are at continental rifts where the lithosphere is diverging and thinning such as in the Basin and Range Province in North America and the East African Rift Basin in Africa. The thinning allows for some of the lower crustal rocks or upper mantle rocks to rise releasing some pressure and causing partial melting. The magma generated is less dense than the surrounding rock and rises through the crust to the surface erupting as basalt. These basaltic eruptions are usually in the form of flood basalts, cinder cones, and basaltic lava flows. For example, relatively young cinder cones are located in south-central Utah, the Black Rock Desert Volcanic Field, which is part of the Basin and Range crustal extension. The 1-minute video (below) illustrates volcanism in the Basin and Range Province. These Utah cinder cones and lava flows started erupting 6 million years ago with the last volcanic eruption 720 years ago.


The primary source of intraplate volcanism is hotspots. Hotspots occur when lithospheric plates glide over a hot mantle plume, which is an ascending column of hot rock (solid, not magma) originating from deep within the mantle. A chain of ancient volcanoes formerly active but now inactive for millions of years can be seen on the seafloor, or on continents, which leads to an active intraplate volcano, indicating hotspot volcanism. The Pacific oceanic plate overrode a hotspot mantle plume producing a long volcanic island chain beginning with the Emperor Seamounts in the northwest Pacific and terminating at the Hawaiian Islands with currently active volcanoes. When the North American continental plate overrode a mantle plume hotspot, a chain of ancient volcanic calderas formed extending from Southwestern Idaho to the Yellowstone caldera.

Once the ascending magma reaches the lithosphere, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across. Think of the Bowen’s Reaction Series and the temperatures of the magmas that contain the respective minerals. If hot mafic magma rises beneath felsic continental crust spreads into a head below the felsic boundary, the higher heat of the mafic magma may cause the felsic rock above it to melt. There may be mixing of the mafic material from below with the felsic above to form intermediate magmas, or the felsic magma may melt and rise higher forming granitic batholiths or even emerging as a felsic volcano. Such felsic (granitic) batholiths lie at the core of the Sierra Nevada Mountains and comprise the dramatic features of Yosemite. Since most mantle plumes are beneath the oceanic lithosphere, the early stages of volcanism typically take place on the seafloor. Over time, basaltic volcanoes may form islands like those in Hawaii. If the hotspot is under continental lithosphere, then magma of more felsic to intermediate (silica-rich) composition rises into an explosive volcano like Mt. St. Helens or the Yellowstone caldera. Two three-minute videos (below) illustrates hotspot volcanoes.

5.14 Volcano Features and Types

There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other aspects. The figure shows the main features of a typical stratovolcano: 1) magma chamber, 2) upper layers of lithosphere, 3) the conduit or narrow pipe through which the lava erupts, 4) the base or edge of the volcano, 5) a sill of magma between layers of the volcano, 6) a diapir or feeder tube to the sill, 7) layers of tephra (ash) from previous eruptions, 8 & 9) layers of lava erupting from the vent and flowing down the sides of the volcano, 10) the crater at the top of the volcano, 11) layers of lava and tephra on (12), a parasitic cone [A parasitic cone is a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its separate magma chamber, 13) the vents of the parasite and the main volcano, 14) the rim of the crater, 15) clouds of ash blown into the sky by the eruption; this settles back onto the volcano and surrounding land.

The most massive craters are called caldera, such as the Crater Lake Caldera in Oregon. Many volcanic features are produced by viscosity, a fundamental property of lava. Viscosity is the resistance to flowing by a fluid. Low viscosity magma flows easily more like syrup, the basaltic volcanism that occurs in Hawaii on shield volcanoes. High viscosity means a sticky magma, typically felsic or intermediate, that flows slowly, similar to toothpaste.

Shield Volcanoes

The largest volcano is a shield volcano and is characterized by broad, low-angle flanks, a small vent or groups of vents at the top, and basaltic magma. The name “shield” comes from the side view resembling a medieval warrior’s shield. They are typically associated with hotspots, midocean ridges, or continental rifts where upper mantle material rises, and build up slowly from many low-viscosity basaltic lava flows that can travel long distances, hence making the low-angle flanks. Because the magma is basaltic and low viscosity, the eruption style is not explosive but rather effusive, meaning that volcanic eruptions are small, localized, and predictable. Therefore, this eruption style is not typically much of a hazard.

Mauna Loa (info) and the more active Kilauea (info) in Hawaii are good examples of vents on a shield volcano. The eruption of Kilauea from fissures in Hawaii in 2018, while not explosive, produced viscous lavas that did considerable damage to roads and structures. Shield volcanoes are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift (USGS, 2011).

The most substantial volcanic edifice in the Solar System is Olympus Mons on Mars, a shield cone as large as the state of Arizona indicating little if any plate tectonic activity on Mars as the volcano erupted over the same hotspot for millions of years.

Basaltic magma can form several rock types and unique landforms. Based on magma temperature, composition, and content of dissolved gases and water vapor, there are two main types of basaltic volcanic rocks with Hawaiian names – pahoehoe and aa. Pahoehoe is a basaltic magma that flows smoothly into a “ropey” appearance. In contrast, aa (sometimes spelled a’a or ʻaʻā and pronounced “ah-ah”) has a crumbly blocky appearance [17]. (Peterson and Tilling 1980). Felsic silica-rich lavas also form aa flows.

In basaltic lava flows, the low viscosity lava can smoothly flow, and it tends to harden on the outside but continue to flow internally within a tube. Once the interior flowing lava subsides, the tube may be left as an empty lava tube. Lava tubes famously make caves (with or without collapsed roofs) in Hawaii, Northern California, the Columbia River Basalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho. Fissures, cracks that originate from shield-style eruptions, are also common. Magmas from fissures are typically very fluid and mafic. The volcanic activity itself causes some fissures, and some can be influenced by tectonics, such as the common fissures parallel to the divergent boundary in Iceland. See above for fissure flows from Kilauea in 2018.

Since basalt flows are thick accumulations of lava with a homogeneous composition that flows quickly when the lava begins to cool it can contract into columns with a hexagonal cross-section called columnar jointing. This feature is common in basaltic lava flows but can be found in more felsic lavas and tuffs as well.

Composite Volcanoes

Composite volcanoes, also called stratovolcanos, has steep flanks, a symmetrical cone shape, a distinct crater, and rises prominently above the surrounding landscape. The figure at the beginning of this section shows a stratovolcano. Examples include Mount Rainier in the Cascade Range in Washington and Mount Fuji in Japan. Stratovolcanoes can have magma with felsic to mafic composition. However, felsic to intermediate magmas are most common. The term “composite” refers to the alternating layers of pyroclastic materials (like ash) and lava flows. The viscous nature of the intermediate and felsic magmas in subduction zones results in steep flanks and explosive eruption styles. Stratovolcanoes are made of alternating lava flows and ash.

Lava Domes

Lava domes are a relatively small accumulation of silica-rich volcanic rocks, such as rhyolite and obsidian, that is too viscous to flow, and therefore, pile high close to the vent. The domes often form within the collapsed crater of a stratovolcano near the vent and grow by expansion from within. As it grows, its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. An excellent example of a lava dome is inside of a collapsed stratovolcano crater is Mount Saint Helens. Examples of a stand-alone lava dome are Chaiten in Chile and the Mammoth Mountain in California.


Calderas are usually large, steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an emptying magma chamber. Calderas are generally very large with a diameter up to 15 miles. Although the word caldera only refers to the vent, many use calderas as a volcano type, typically formed by high-viscosity felsic volcanism with high volatile content. Crater Lake, Yellowstone, and Long Valley Caldera are good examples. At Crater Lake National Park in Oregon, about 6,800 years ago Mount Mazama was a composite volcano that erupted in a large explosive blast ejecting massive amounts of volcanic ash. The eruption rapidly drained the underlying magma chamber causing the top to collapse into it forming a significant depression that later filled with water. Today a resurgent dome is found rising through the lake as a cinder cone, called Wizard Island.

The Yellowstone caldera erupted three times in the recent past, at 2.1, 1.3, and 0.64 million years ago. Each eruption created large rhyolite flows and pyroclastic clouds of ash that solidified into tuff. These extra large eruptions rapidly emptied the magma chamber causing the roof to collapse and form a caldera. Three calderas are still preserved from these eruptions, and most of the roads and hotels of Yellowstone National Park are located within the caldera. Two resurgent domes are located within the last caldera.

Yellowstone volcanism started as a hot spot under the North American lithosphere about 17-million years ago near the Oregon/Nevada border. As the North American plate slid southwestward over the stationary hotspot, surface volcanism followed and helped form Idaho’s Snake River Plain, eventually arriving at its current location in northwestern Wyoming. As the plate moved to the southwest over the stationary hotspot, it left a track of past volcanic activities.

The Long Valley Caldera near Mammoth California is a massive explosive volcano that erupted 760,000 years ago and dumped a significant amount of ash throughout the United States, similar to the Yellowstone eruptions. This ash formed the large Bishop Tuff deposit. Like the Yellowstone caldera, the Long Valley Caldera contains the town of Mammoth Lakes, a major ski resort, an airport, and a major highway. Further, there is a resurgent dome in the middle and active hot springs.

Cinder Cones

Cinder cones are small volcanoes with steep sides, made of cinders and volcanic bombs ejected from a pronounced central vent. Typically, they come from mafic lavas that have high volatile content. Cinders form when hot lava is ejected into the air, cooling and solidifying before they reach the flank of the volcano. The largest cinders are called volcanic bombs. Cinder cones form in short-lived eruption events that are relatively common in the western United States.

A relatively recent and striking example of a short-lived cinder cone is the 1943 eruption near the village of Parícutin, Mexico. The cinder cone started with an explosive eruption shooting cinders out of a vent in the middle of a farmer’s field. Quickly, volcanism continued building the cone to a height of over 300 feet in a week and 1,200 feet in the first eight months. After the initial explosive gases and cinders were released, growing the cone, basaltic lava poured out around the base of the cone. This order of events is typical for cinder cones: first violent eruption, then the formation of cone and crater, followed by a low-viscosity lava flow from the base (the cone of cinders is not strong enough to support a column of lava rising to the top of the crater). The Parícutin cinder cone was built over nine years and covered about 100-square miles with ashes and destroyed the town of San Juan.

Flood Basalts

A rare volcanic eruption type, unobserved in modern times, is the flood basalt. Flood basalts are some of the largest and lowest viscosity types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still up for debate. Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been involved in the Earth’s largest mass extinction at the end of the Permian.

5.15 Predicting Volcanic Eruptions

Volcanologists attempt to forecast volcanic eruptions, but this has proven to be nearly as difficult as predicting an earthquake. Many pieces of evidence can mean that a volcano is about to erupt, but the time and magnitude of the eruption are difficult to pin down. This evidence includes the history of previous volcanic activity, earthquakes, slope deformation, and gas emissions.

History of Volcanic Activity

A volcano’s history, how long since its last eruption and the time span between its previous eruptions, is a good first step to predicting eruptions. If the volcano is considered active, it is currently erupting or shows signs of erupting soon. A dormant volcano means there is no current activity, but it has erupted recently. Finally, an extinct volcano means there is no activity and will probably not erupt again. Active and dormant volcanoes are heavily monitored, especially in populated areas.


Moving magma shakes the ground, so the number and size of earthquakes increase before an eruption. A volcano that is about to erupt may produce a sequence of earthquakes. Scientists use seismographs that record the length and strength of each earthquake to try to determine if an eruption is imminent.

Magma and gas can push the volcano’s slope upward. Most ground deformation is subtle and can only be detected by tiltmeters, which are instruments that measure the angle of the slope of a volcano. However, ground swelling may sometimes create considerable changes in the shape of a volcano. Mount St. Helens grew a bulge on its north side before its 1980 eruption. Ground swelling may also increase rock falls and landslides.

Gas Emissions

Gases may be able to escape a volcano before magma reaches the surface. Scientists measure gas emissions in vents on or around the volcano. Gases, such as sulfur dioxide (SO2), carbon dioxide (CO2), hydrochloric acid (HCl), and even water vapor can be measured at the site or, in some cases, from a distance using satellites. The amounts of gases and their ratios are calculated to help predict eruptions.

Remote Monitoring

Some gases can be monitored using satellite technology. Satellites also monitor temperature readings and deformation. As technology improves, scientists are better able to detect changes in a volcano accurately and safely.

Since volcanologists are usually uncertain about an eruption, officials may not know whether to require evacuation. If people are evacuated, and the eruption does not happen, the people will be displeased and less likely to evacuate the next time there is a threat of an eruption. The costs of disrupting business are significant. However, scientists continue to work to improve the accuracy of their predictions.

5.16 Volcanic Hazards and Monitoring

Volcanoes are responsible for a large number of deaths. Volcanic hazards have been famous for centuries, but recent eruptions are better documented. The most obvious hazard is the lava itself found within a lava flow, but the hazards posed by volcanoes go far beyond a lava flow. For example, on May 18, 1980, Mount Saint Helens erupted with an explosion and landslide that removed the upper 1,300 feet (400 m) of the mountain. This explosion was immediately followed by a lateral blast and pyroclastic flow [23] that covered 230 square miles of forest with ash and debris. The effects of the blast are shown on the before and after images (see figures). The pyroclastic flow (see below) moved at speeds of 50 – 80 miles per hour (80-130 km/hr), flattened trees and ejected a giant ash cloud into the air. Watch the 7-minute USGS video for an account of May 18, 1980, which killed 57 [24]. Pyroclastic flows are common in explosive eruptions of stratovolcanoes.

In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a pyroclastic flow over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered in an archeological expedition in the 18th century. Pompeii famously contains the remains (casts) of people suffocated by ash and covered by 10 feet (3 m) of ash, pumice lapilli, and collapsed roofs.

Pyroclastic Flows

The most dangerous volcanic hazard are pyroclastic flows (video). These flows are a mix of lava blocks, pumice, ash, and hot gases between 400 to 1,300 ℉. The turbulent cloud of ash and gas races down the steep flanks at high speeds up to 120 mph (much faster than people can run) into the valleys around composite volcanoes [24]. Most explosive, silica-rich, high viscosity magma volcanoes such as composite cones usually have pyroclastic flows. The rock tuff and welded tuff is often formed from these pyroclastic flows.

There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs [27]. Two short videos below document eyewitness video of pyroclastic flows. In the early 1990s, Mount Unzen erupted several times with pyroclastic flows. The pyroclastic flow shown in this famous short video killed 41 people. In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people in moments.

Landslides and Landslide-Generated Tsunamis

The flanks of a volcano are steep and unstable which can lead to slope failure and generate dangerous landslides. For example, the landslide at Mount St. Helens 1980 released a considerable amount of materials as the entire north flank collapsed. The landslide moved at speeds of 100-180 mph. These landslides can be triggered by movement of magma, explosive eruptions, large earthquakes, and heavy rainfall. In unique situations, the landslide material can reach water and cause a tsunami. In 1792 in Japan, Mount Unzen erupted causing a giant landslide that reached the Ariake Sea and made a tsunami that killed 15,000 people on the opposite shore.


A lahar is an Indonesian word for a mudflow that is a mixture of water, ash, rock fragments, and other debris moving down the flanks of a volcano (or other nearby mountains covered with freshly-erupted ash) and entering adjacent river valleys. They form from the rapid melting of snow or glaciers on volcanoes. They are similar to a slurry of concrete but can flow up to 50 mph while still on the steep flanks. Since lahars are slurry-like, they can travel long distances in river valleys almost like a flash flood.

During the 1980 Mount St. Helens eruption, lahars reached 17-miles (27 km) down the North Fork of the Toutle River. Prehistoric lahar flows have been mapped at significant volcanoes such as Mount Rainier near Tacoma, Washington (Rosi et al. 1999). Prehistoric lahars occupied river floodplains where large cities are located today as shown on the map. Similarly, Mount Baker poses a hazard as shown by this hazards map for Mount Baker north of Seattle, Washington. A recent scenario played out when a lahar from the volcano Nevado del Ruiz in Colombia buried a town in 1985 and killed an estimated 25,000 people.

Tephra and Ash

Volcanoes, especially composite volcanoes, eject large amounts of tephra (ejected rock materials) and ash (fragments less than 0.08 inches [2 mm]). Tephra is heavier and falls closer to the vent. Larger blocks and bombs pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier. Ash is fine and can be carried long distances away from the vent, and can cause building collapses and respiratory issues like silicosis. Hot ash can be dangerous to those close to the eruption and disrupt services such as airline transportation farther away [24]. For example, in 2010 the Eyjafjallajökull volcano in Iceland created a large ash cloud in the upper atmosphere that caused the most significant air travel disruption in northern Europe since a seven-day airline shut down during World War II. No one was hurt, but the cost to the world economy was estimated to be billions of dollars.

Volcanic Gases

Magma contains dissolved gases. As rising magma reaches the surface, the confining pressure decreases allowing gases to escape; similar to gases coming out of solution after opening a soda bottle. Therefore, volcanoes when not erupting release hazardous gases such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S) and hydrogen halides (HF, HCl, or HBr). Carbon dioxide can sink and accumulate in low-lying depressions on the earth’s surface. For example, Mammoth Mountain Ski Resort in Mammoth Lakes, California is located within the Long Valley Caldera. Therefore, the whole ski resort and town are within the caldera. In 2006, three ski patrol members were killed after skiing into snow depressions near fumaroles that had filled with carbon dioxide (info). Therefore, in volcanic areas where carbon dioxide emissions occur, avoid low-lying areas that may trap carbon dioxide. In rare cases, a volcano can suddenly release gases without warning. Called a limnic eruption, this commonly occurs in crater lakes as gases pour from the water. It infamously occurred in 1986 in Lake Nyos, Cameroon, killing almost 2,000 people due to carbon dioxide asphyxiation.

Volcanic Monitoring

Volcano monitoring requires geologists to use many instruments to detect changes that may indicate an eruption is imminent. Some of the main observations include regular monitoring for 1) earthquakes (including unique vibrational earthquakes called harmonic tremor, caused by magma movement), 2) changes in the orientation and elevation of the land surface, and 3) increase in gas emission. Concise videos (below) summarize how an increased frequency of earthquakes can show that magma is moving and that an eruption may occur soon. Another video (below) shows how gas monitoring is used to monitor volcanoes and predict an eruption. As the magma gets closer to the surface and pressure is released, the gases come out of solution in the magma. A rapid increase of gas emission can indicate an eruption is imminent. The last video (below) shows how a GPS unit and tiltmeter can detect movement of the land indicating that the magma is moving underneath.

5.17 Volcanic Landforms and Geothermal Activity

Volcanic Landforms and Vents

Volcanoes are associated with many types of landforms. The landforms vary with the composition of the magma that created them. Hot springs and geysers are also examples of surface features related to volcanic activity.

The most apparent landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes or eruptions that take place through fissures. The eruptions that created the entire ocean floor are essentially fissure eruptions. Magma intrusions ALSO can create landforms. The image on the right is of Shiprock in New Mexico, which is the neck of an old volcano that has eroded away

Lava Domes

When lava is viscous, it flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome. However, because the viscosity of the magma is so thick, the lava does not flow far from the volcanic vent. Lava flows often make mounds right in the middle of craters at the top of volcanoes.

Lava Plateaus and Land

A lava plateau forms when large amounts of fluid lava flow over an extensive area. When the lava solidifies, it creates a large, flat surface of the igneous rock. Lava creates new land as it solidifies on the coast or emerges from beneath the water. Over time the eruptions can create whole islands. The Hawaiian Islands are formed from shield volcano eruptions that have grown over the last 5 million years.

Hot Springs and Geysers

Water sometimes comes into contact with the hot rock. The water may emerge at the surface as either a hot spring or a geyser. Water heated below ground that rises through a crack to the surface creates a hot spring. The water in hot springs may reach temperatures in the hundreds of degrees Celsius beneath the surface, although most hot springs are much cooler.

5.18 Hazards and Benefits of Volcanic Activity

There are several hazards that volcanic activity can produce.

  • Eruption clouds occur when massive quantities of ash are ejected into the atmosphere where it can reach heights of 50,000 feet. Eruption clouds have proven to be very dangerous for aviation jets because the ash can shut down the engines. The ash cloud can also be very hazardous regarding air pollution.
  • Lahars are volcanic mudflows. Lahars are very dangerous because they do not require a volcanic eruption yet can travel hundreds of miles. All that is required is loose pyroclastic material on the volcano that mixes with precipitation or melting snow.
  • Lava flows are layers of molten rock that flows over the surface, later cooling and solidifying.
  • Lava bombs are large chunks of pyroclastic material ejected from a volcano. The larger pyroclastic material is called blocks.
  • Pyroclastic flows are some of the most dangerous hazards caused by composite volcanoes. Pyroclastic flows are superheated clouds of pyroclastic material (e.g., hot rock and tephra) ranging in size from small rocks to the size of houses that are over 1,000 degrees F traveling down a mountain at speeds up to 100 mph.
  • Tephra (or volcanic ash) is fine particles of pyroclastic material that can be carried thousands of miles away by prevailing winds. Regions hundreds of miles away could suffer collapsed buildings is the falling ash accumulates enough. Tephra can also cool the entire planet if enough is ejected into the atmosphere.
  • Poisonous gases such as carbon dioxide, carbon monoxide, and sulfur dioxide can travel down a volcano and asphyxiate (suffocating) wildlife and humans. In 1986, an invisible cloud of carbon dioxide traveled down a volcano in Africa asphyxiating 1,742 people and 3,000 cattle.

There are many benefits to volcanic activity. One of the major benefits is the fact that volcanic activity can create very fertile soil for agriculture. The problem is that many civilizations developed near volcanoes for this reason – with sometimes deadly effects. Volcanic activity can also create many mineral resources such as gold, silver, nickel, copper, and lead. Volcanic rock is often used for landscaping, tile, and cement.

Some of the most amazing landscapes are near volcanoes because volcanic activity builds land creating, breathtaking scenery. So volcanoes are economically vital for many regions because of the recreational activity and tourism they bring.

Finally, a new but essential trend is geothermal power. The heat generated by volcanoes can create electricity to power civilization. Geothermal power is an entirely renewable resource free of pollution and energy dependency on fossil fuels. Iceland – the surface manifestation of the mid-Atlantic ridge – has a goal of powering the entire nation on geothermal energy. Geothermal energy is also being used in California, Kilauea, Hawaii, and now Utah.


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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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