3.1 Earth as a Planetary Body
Earth is a terrestrial planet in the solar system, and it is very much like the other inner planets, at least in its size, shape, and composition. However, many features make Earth very different from the planets and any other planet that we know of so far.
Earth is a sphere or, more correctly, an oblate spheroid, which is a sphere that is a bit squished down at the poles and bulges a bit at the equator. Alternatively, to be more technical, the minor axis (the diameter through the poles) is smaller than the major axis (the diameter through the equator). When the earth is cut into equal halves, each half is called a hemisphere. North of the equator is the northern hemisphere and south of the equator is the southern hemisphere. Eastern and western hemispheres are also designated.
Even the ancient Greeks knew that Earth was round by observing the arc shape of the shadow on the Moon during a lunar eclipse. The Sun and the other planets of the solar system are also spherical. Larger satellites, those that have enough mass for their gravitational attraction to have made them round, are as well.
Earth has a magnetic field that behaves as if the planet had a gigantic bar magnet inside of it. Earth’s magnetic field also has a north and south pole and a magnetic field that surrounds it. The magnetic field arises from the convection of molten iron and nickel metal in Earth’s outer liquid iron core. Earth’s magnetic field extends several thousand kilometers into space. The magnetic field shields the planet from harmful radiation from the Sun.
Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an axis. Earth spins around its axis, just as a top spins around its spindle. This spinning movement is called Earth’s rotation. At the same time that the Earth spins on its axis, it also orbits or revolves around the Sun, called a revolution.
A pendulum set in motion will not change its motion, and so the direction of its swinging should not change. However, Foucault observed that his pendulum did seem to change direction. Since he knew that the pendulum could not change its motion, he concluded that the Earth, underneath the pendulum was moving. An observer in space will see that Earth requires 23 hours, 56 minutes, and 4 seconds to make one complete rotation on its axis. However, because Earth moves around the Sun at the same time that it is rotating, the planet must turn just a little bit more to reach the same place relative to the Sun. Hence the length of a day on Earth is 24 hours. At the equator, the Earth rotates at a speed of about 1,700 km per hour, but at the poles, the movement speed is nearly nothing.
For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year. The gravitational pull of the Sun keeps Earth and the other planets in orbit around the star. Like the other planets, Earth’s orbital path is an ellipse, so the planet is sometimes farther away from the Sun than at other times. The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd, and the furthest is at aphelion (152 million km) on July 4th. Earth’s elliptical orbit has nothing to do with Earth’s seasons.
During one revolution around the Sun, Earth travels at an average distance of about 150 million km. Earth revolves around the Sun at an average speed of about 27 km (17 mi) per second, but the speed is not constant. The planet moves slower when it is at aphelion and faster when it is at perihelion.
The reason the Earth has seasons is that Earth is tilted 23.5 degrees on its axis. During the Northern Hemisphere summer the North Pole points toward the Sun, and in the Northern Hemisphere winter, the North Pole has tilted away from the Sun.
3.2 Geologic Time and Deep Time
In 1788, after many years of geological study, James Hutton, one of the early pioneers of geology, wrote the following about the age of the Earth: “The result, therefore, of our present inquiry is, that we find no vestige of a beginning, — no prospect of an end.” Although he was not precisely correct (there was a beginning, and there will be an end to planet Earth), he was trying to express the vastness of geological time that humans have a hard time perceiving. Although Hutton did not assign an age to the Earth, he was the first to suggest that the planet was very old. Today we know Earth is approximately 4.54 ± 0.05 billion years old, an age first calculated by Caltech professor Clair Patterson in 1956 by radiometrically dating meteorites with uranium-lead dating.
On a geologic scale, the lifespan of a human is very short, and we struggle to comprehend the depth of geologic time and slow geologic processes. Studying geologic time, also known as deep time, can help us overcome our limited view of Earth during our lifetime. For example, the science of earthquakes only goes back about 100 years; however, geologic evidence shows that large earthquakes have occurred in the past and will continue to occur in the future. Thus, human perspective of time does not always overlap with geologic timescales.
A diagram of the geological time scale.The largest division of time is the Eon—Hadean, Archean, Proterozoic (sometimes combined together as the Precambrian), and Phanerozoic. Although life appeared more than 3,800 million of years ago (Ma), during most of Earth history from 3,500 Ma to 542 Ma (88 percent of geologic time), life forms consisted mainly of simple single-celled organisms such as bacteria. Only in more recent geologic time have more biologically complex organisms appeared in the geologic record.
The Phanerozoic Eon, the last 542 million years, is only 12 percent of geological time and is named for the time during which visible (phaneros-) life (-zoic), i.e., abundant fossils, appeared in the geological record. Although simple life forms had been around for billions of years, the Phanerozoic marks the beginning of abundant multicellular animals having preservable hard parts such as shells. Animals have been on land for only 360 million years, or 8 percent of geological time. Mammals have dominated since the demise of the dinosaurs around 65 Ma, or 1.5 percent of geological time, and the genus Homo has existed since approximately 2.2 Ma, or 0.05% (1/2,000th) of geological time.
Additionally, the Phanerozoic is divided into three Eras: Paleozoic, Mesozoic, and Cenozoic. Life of the Paleozoic (meaning “ancient life”) consisted of invertebrate animals, fish, amphibians, and reptiles. The Mesozoic (meaning “middle life”) is known as the Age of Reptiles popularized by the dominance of dinosaurs, which evolved into birds, and Cenozoic (meaning “new life”) is the Age of Mammals in which mammals evolved to be the dominant form of animal life on land following the mass extinction of the dinosaurs and other apex predator reptiles at the end of the Mesozoic. Early humans (hominids) appear in the rock record only during the last few million years of the Cenozoic.
3.3 Evolution of Life on Earth
It turns out life may have gotten its start pretty early in Earth’s history, and while the first couple billion years saw several important developments, the period was still dominated by very simple life forms.
Right at the beginning of the Paleozoic, there was a huge explosion of more complex life. And that’s when things started to get really interesting.
The Great Dying hit life hard, but the species that survived took over the planet and diversified into many interesting forms, including the dinosaurs.
With the non-avian dinosaurs extinct, it was time for mammals to take over. Finally, in the tiniest sliver of the history of life, humans emerge.
3.4. Structure of the Earth
The fundamental unifying principle of geology and the rock cycle is the Theory of Plate Tectonics. Plate tectonics describes how the layers of the Earth move relative to each other. Specifically, the outer layer divided into tectonic or lithospheric plates. As the tectonic plates float on a mobile layer beneath called the asthenosphere, they collide, slide past each other, and split apart. At these plate boundaries, significant landforms are created, and rocks comprising the tectonic plates move through the rock cycle.
The following is a summary of the Earth’s layers based on chemical composition (or the chemical makeup of the layers). Earth has three main geological layers based on chemical composition – crust, mantle, and core. The outermost layer is the crust and is composed of mostly silicon, oxygen, aluminum, iron, and magnesium. There are two types of crust, continental and oceanic crust. Continental crust. is about 50 kilometers (30 miles) thick, represents most of the continents, and is composed of low-density igneous and sedimentary rocks. Oceanic crust is approximately 10 kilometers (6 miles) thick, makes up most of the ocean floor, and covers about 70 percent of the planet. Oceanic crust is high-density igneous basalt-type rocks. The moving tectonic plates are made of crust, and some of the next layers within the earth called the mantle. The crust and this portion of the upper mantle are rigid and called the lithosphere and comprise the tectonic plates.
The oldest continental rocks are billions of years old, so the continents have had a lot of time for things to happen to them. Constructive forces cause physical features on Earth’s surface known as landforms to grow. Crustal deformation – when crust compresses, pulls apart, or slides past other crust – results in hills, valleys, and other landforms. Mountains rise when continents collide, when one slab of ocean crust plunges beneath another or a slab of continental crust to create a chain of volcanoes. Sediments are deposited to form landforms, such as deltas.
Volcanic eruptions can also be destructive forces that blow landforms apart. The destructive forces of weathering and erosion modify landforms. Water, wind, ice, and gravity are important forces of erosion.
The ocean basins are all younger than 180 million years. Although the ocean basins begin where the ocean meets the land, the continent extends downward to the seafloor, so the continental margin is made of continental crust.
The ocean floor itself is not totally flat. The most distinctive feature is the mountain range that runs through much of the ocean basin, known as the mid-ocean ridge. The deepest places of the ocean are the ocean trenches, many of which are located around the edge of the Pacific Ocean. Chains of volcanoes are also found in the center of the oceans, such as in the area of Hawaii. Flat plains are found on the ocean floor with their features covered by mud.
The mantle is below the crust and is the most significant layer by volume, extending down to about 2,900 km (1,800 miles). The mantle is mostly solid and made of peridotite, a high-density rock composed of silica, iron, and magnesium. The upper part of this solid material is so hot that it is flexible and allows the tectonic plates floating on it to move relative to each other. Under the mantle is the 3,500 km (2,200 mi) thick core made of iron and nickel. The outer core is liquid, and the inner core is solid. Rotations within the solid and liquid metallic core generate Earth’s magnetic field.
3.4 Rocks and Minerals
In disciplines like geology, geosciences, or physical geography, the classic definition of a mineral is: 1) naturally occurring, 2) inorganic, 3) solid (at room temperature), 4) regular crystal structure, and 5) defined chemical composition. Some natural substances technically should not be considered minerals, but are exceptions to the rule. For example, both water (ice) and mercury are liquid at room temperature, but both have been considered minerals because they were classified as minerals before the definition that excludes them was accepted. Calcite is quite often formed by organic processes, but is still considered a mineral because it is widespread and geologically significant. Because of these discrepancies, the International Mineralogical Association in 1985 amended the definition to: “A mineral is an element or chemical compound that is normally crystalline, and that has been formed as a result of geological processes.” This means that the calcite in the shell of a clam is not considered a mineral. However, once that clamshell undergoes burial, diagenesis, or other geological processes, then the calcite is considered a mineral.
Typically, substances like coal, pearl, opal, or obsidian that do not fit the definition of a mineral are called mineraloids. A rock is a naturally formed, non-living earth material composed of one or more minerals or mineraloids. The mineral grains in a rock may be so tiny that they can only see them with a microscope, or several centimeters in size.
There are three main types of rocks composed of minerals: igneous (rocks crystallizing from molten material), sedimentary (rocks composed of products of mechanical weathering (sand, gravel, etc.) and chemical weathering (things precipitated from solution), and metamorphic (rocks produced by alteration of other rocks by heat and pressure. Click here to learn more about rocks and minerals from the Utah Geologic Survey.
Atoms and Isotopes
A chemical element is a substance that cannot be made into a simpler form by ordinary chemical means. The smallest unit of a chemical element is an atom. An atom has all the properties of that element. These are the parts of an atom:
- At the center of an atom is a nucleus made up of subatomic particles called protons and neutrons.
- Protons have a positive electrical charge. The number of protons in the nucleus determines what element the atom is.
- Neutrons are about the size of protons but have no charge.
- Tiny electrons, each having a negative electrical charge, orbit the nucleus at varying energy levels in a region known as the electron cloud.
Because electrons are minuscule compared with protons and neutrons, the number of protons plus neutrons gives the atom its atomic mass. All atoms of a given element always have the same number of protons but may differ in the number of neutrons found in its nucleus. Atoms of an element with differing numbers of neutrons are called isotopes. For example, carbon always has 6 protons but may have 6, 7, or 8 neutrons. This means there are three isotopes of carbon: carbon-12, carbon-13, and carbon-14. How many protons and neutrons make up carbon-12? Carbon-13? Carbon-14?
Ions and Molecules
Atoms are stable when they have a full outermost electron valence shell. For an atom to fill its outermost shell, it will give, take, or share electrons. When an atom either gains or loses electrons, this creates an ion. Ions have either a positive or a negative electrical charge. What is the charge of an ion if the atom loses an electron? An atom with the same number of protons and electrons has no overall charge, so if an atom loses the negatively charged electron, it has a positive charge. What is the charge of an ion if the atom gains an electron? If the atom gains an electron, it has a negative charge.
When atoms chemically bond, they form compounds. The smallest unit of a compound with all the properties of that compound is a molecule. When two or more atoms share electrons to form a chemical bond, they form a molecule. The molecular mass is the sum of the masses of all the atoms in the molecule.
Ions come together to create a molecule so that electrical charges are balanced; the positive charges balance the negative charges, and the molecule has no electrical charge. An atom will balance its electrical charge by sharing an electron with another atom, giving it away, or receive an electron from another atom.
The joining of ions to make molecules is chemical bonding. There are three main types of chemical bonds:
- Ionic bond: Electrons are transferred between atoms. An atom of a metal will give one or more electrons to a non-metallic atom.
- Covalent bond: An atom shares one or more electrons with another atom. The sharing of electrons is not always evenly distributed within a molecule. If one atom has the electrons more often than another atom in the molecule, the molecule has a positive and a negative side. It is a polar molecule because it acts a little bit as if it had poles, similar to a magnet.
- Hydrogen bond: These weak, intermolecular bonds are formed when the positive side of one polar molecule is attracted to the negative side of another polar molecule.
What is a Mineral?
Minerals are categorized based on their chemical composition. Owing to similarities in composition, minerals within the same group may have similar characteristics. Geologists have a precise definition of minerals. A material is characterized as a mineral if it meets all of the following traits:
- Inorganic, crystalline solid.
- Formed through natural processes and has a definite chemical composition.
- Identified by their characteristic physical properties such as crystalline structure, hardness, density, flammability, and color.
Minerals are crystalline solids. A crystal is a solid in which the atoms are arranged in a regular, repeating pattern. The pattern of atoms in different samples of the same mineral is the same. Is glass a mineral? Without a crystalline structure, even natural glass is not a mineral.
Organic substances are the carbon-based compounds made by living creatures and include proteins, carbohydrates, and oils. Inorganic substances have a structure that is not characteristic of living bodies. Coal is made of plant and animal remains. Is it a mineral? Coal is a classified as a sedimentary rock but is not a mineral.
Minerals are made by natural processes, those that occur in or on Earth. A diamond created deep in Earth’s crust is a mineral. Is a diamond created in a laboratory by placing carbon under high pressures a mineral? No. Do not buy a laboratory-made “diamond” for jewelry without realizing it is not technically a mineral. But also be careful you do not purchase blood diamonds or other minerals that were mined to fund war, mass murders, or genocides.
Nearly all (98.5 percent) of Earth’s crust is made up of only eight elements – oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium – and these are the elements that make up most minerals.
All minerals have a specific chemical composition. The mineral silver is made up of only silver atoms and diamond is made only of carbon atoms, but most minerals are made up of chemical compounds. Each mineral has its own chemical formula. Halite is NaCl (sodium chloride). Quartz is always made of two oxygen atoms bonded to a silicon atom, SiO2. If a mineral contains any other elements in its crystal structure, it is not quartz.
A hard mineral containing covalently bonded carbon is diamond, but a softer mineral that also contains calcium and oxygen along with carbon is calcite. Some minerals have a range of chemical composition. Olivine always has silicon and oxygen as well as iron or magnesium or both.
Their physical characteristics can identify minerals. The physical properties of minerals are related to their chemical composition and bonding. Some characteristics, such as a mineral’s hardness, are more useful for mineral identification. Color is readily observable and undoubtedly visible, but it is usually less reliable than other physical properties.
- Color: Diamonds are popular gemstones because the way they reflect light makes them very sparkly. Turquoise is prized for its striking greenish-blue color. Notice that specific terms are being used to describe the appearance of minerals. Color is rarely very useful for identifying a mineral. Different minerals may be the same color. Real gold, is very similar in color to the pyrite. The same mineral may also be found in different colors. A tiny amount of iron makes the quartz purple. Many minerals are colored by chemical impurities.
- Streak: Streak is the color of a mineral’s powder. Streak is a more reliable property than color because streak does not vary. Minerals that are the same color may have a different colored streak. Many minerals, such as the quartz do not have a streak. To check streak, scrape the mineral across an unglazed porcelain plate. Yellow-gold pyrite has a blackish streak, another indicator that pyrite is not gold, which has a golden yellow streak.
- Luster: Luster describes the reflection of light off a mineral’s surface. Mineralogists have special terms to describe luster. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. Minerals such as quartz have a non-metallic luster. Different types of non-metallic luster are described in Table below.
- Specific Gravity: Density describes how much matter is in a certain amount of space: density = mass/volume. Mass is a measure of the amount of matter in an object. The amount of space an object takes up is described by its volume. The density of an object depends on its mass and its volume. For example, the water in a drinking glass has the same density as the water in the same volume of a swimming pool. Gold has a density of about 19 g/cm3; pyrite has a density of about 5 g/cm3 – that is another way to tell pyrite from gold. Quartz is even less dense than pyrite and has a density of 2.7 g/cm3. The specific gravity of a substance compares its density to that of water. Substances that are denser have higher specific gravity.
- Hardness: Hardness is a measure of whether a mineral will scratch or be scratched. Mohs Hardness Scale is a reference for mineral hardness. With a Mohs scale, anyone can test an unknown mineral for its hardness. Imagine you have an unknown mineral. You find that it can scratch fluorite or even apatite, but feldspar scratches it. You know then that the mineral’s hardness is between 5 and 6. Note that no other mineral can scratch diamond. Breaking a mineral breaks its chemical bonds. Since some bonds are weaker than other bonds, each type of mineral is likely to break where the bonds between the atoms are weakest. For that reason, minerals break apart in characteristic ways.
- Cleavage is the tendency of a mineral to break along specific planes to make smooth surfaces. Halite breaks between layers of sodium and chlorine to form cubes with smooth surfaces. One reason gemstones are beautiful is that the cleavage planes make an attractive crystal shape with smooth faces.
- Fracture is a break in a mineral that is not along a cleavage plane. Fracture is not always the same in the same mineral because the structure of the mineral does not determine fracture. Minerals may have characteristic fractures. Metals usually fracture into jagged edges. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, form smooth curved surfaces when they fracture.
Other Identifying Characteristics
- Mineral Formation: Minerals form under an enormous range of geologic conditions. There are probably more ways to form minerals than there are types of minerals themselves. Minerals can form from volcanic gases, sediment formation, oxidation, crystallization from magma, or deposition from a saline fluid, to list a few.
- Formation from Hot Material: A rock is a collection of minerals. Imagine a rock that becomes so hot it melts. Many minerals start out in liquids that are hot enough to melt rocks. Magma is melted rock inside Earth, a molten mixture of substances that can be hotter than 1,000oC. Magma cools slowly inside Earth, which gives mineral crystals time to grow large enough to be seen clearly. When magma erupts onto Earth’s surface, it is called lava. Lava cools much more rapidly than magma when it is below the surface. In cooling lava, mineral crystals do not have time to form and are very small. The chemical composition will be the same as if the magma cooled slowly. Existing rocks may be heated enough so that the molecules are released from their structure and can move around. The molecules may match up with different molecules to form new minerals as the rock cools, a process called metamorphism.
- Formation from Solutions: Water on Earth, such as the water in the oceans, contains chemical elements mixed into a solution. Various processes can cause these elements to combine to form solid mineral deposits.
- Minerals from Salt Water: When water evaporates, it leaves behind a solid precipitate of minerals. Water can only hold a certain amount of dissolved minerals and salts. When the amount is too great to stay dissolved in the water, the particles come together to form mineral solids, which sink. Halite readily precipitates out of the water, as does calcite. Some lakes, such as Mono Lake in California or The Great Salt Lake in Utah, contain many mineral precipitates.
- Minerals from Hot Underground Water: Magma heats nearby underground water, which reacts with the rocks around it to pick up dissolved particles. As the water flows through open spaces in the rock and cools, it deposits solid minerals. The mineral deposits that form when a mineral fills cracks in rocks are called veins. When minerals are deposited in open spaces, large crystals form.
- Mining and Mineral Use: Some minerals are beneficial. An ore is a rock that contains minerals with useful elements. Aluminum in bauxite ore is extracted from the ground and refined to be used in aluminum foil and many other products. The cost of creating a product from a mineral depends on how abundant the mineral is and how much the extraction and refining processes cost. Environmental damage from these processes is often not figured into a product’s cost. It is crucial to use mineral resources wisely.
Finding and Mining Minerals
Geologic processes create and concentrate minerals that are valuable natural resources. Geologists study geological formations and then test the physical and chemical properties of soil and rocks to locate possible ores and determine their size and concentration. A mineral deposit will only be mined if it is profitable. A concentration of minerals is only called an ore deposit if it is profitable to mine. There are many ways to mine ores.
Surface mining allows extraction of ores that are close to Earth’s surface. Overlying rock is blasted, and the rock that contains the valuable minerals is placed in a truck and taken to a refinery. Surface mining includes open-pit mining and mountaintop removal. Other methods of surface mining include strip mining, placer mining, and dredging. Strip mining is like open pit mining but with material removed along a strip.
Placers are valuable minerals found in stream gravels. California’s nickname, the Golden State, can be traced back to the discovery of placer deposits of gold in 1848. The gold weathered out of hard metamorphic rock in the western Sierra Nevada, which also contains deposits of copper, lead, zinc, silver, chromite, and other valuable minerals. The gold traveled down rivers and then settled in gravel deposits. Currently, California has active mines for gold and silver and for non-metal minerals such as sand and gravel, which are used for construction.
Underground mining is used to recover ores that are deeper into Earth’s surface. Miners blast and tunnel into rock to gain access to the ores. How underground mining is approached – from above, below, or sideways – depends on the placement of the ore body, its depth, concentration of ore, and the strength of the surrounding rock. Underground mining is costly and dangerous. Fresh air and lights must also be brought into the tunnels for the miners, and accidents are far too frequent.
The ore’s journey to becoming a useable material is only just beginning when the ore leaves the mine. Rocks are crushed so that the valuable minerals can be separated from the waste rock. Then the minerals are separated out of the ore. A few methods for extracting ore are:
- Heap leaching: the addition of chemicals, such as cyanide or acid, to remove ore.
- Flotation: the addition of a compound that attaches to the valuable mineral and floats.
- Smelting: roasting rock, causing it to segregate into layers so the mineral can be extracted.
To extract the metal from the ore, the rock is melted at a temperature greater than 900°C, which requires much energy. Extracting metal from rock is so energy intensive that if you recycle just 40 aluminum cans, you will save the energy equivalent of one gallon of gasoline.
Although mining provides people with many necessary resources, the environmental costs can be high. Surface mining clears the landscape of trees and soil, and nearby streams and lakes are inundated with sediment. Pollutants from the mined rock, such as heavy metals, enter the sediment and water system. Acids flow from some mine sites, changing the composition of nearby waterways.
U.S. law has changed so that in recent decades a mine region must be restored to its natural state, a process called reclamation. This is not true of older mines. Pits may be refilled or reshaped and vegetation planted. Pits may be allowed to fill with water and become lakes or may be turned into landfills. Underground mines may be sealed off or left open as homes for bats.
Some minerals are valuable because they are beautiful. Jade has been used for thousands of years in China. Diamonds sparkle on many engagement rings. Minerals like jade, turquoise, diamonds, and emeralds are gemstones. A gemstone, or gem, is a material that is cut and polished for jewelry. Gemstones are usually rare and do not break or scratch easily. Most are cut along cleavage faces and then polished so that light bounces back off the cleavage planes. Light does not pass through gemstones that are opaque, such as turquoise. Gemstones are not just used in jewelry. Diamonds are used to cut and polish other materials, such as glass and metals, because they are so hard. The mineral corundum, of which ruby and sapphire are varieties, is used in products such as sandpaper.
Minerals are used in much less obvious places. The mineral gypsum is used for the sheetrock in homes. Window glass is made from sand, which is mostly quartz. Halite is mined for rock salt. Copper is used in electrical wiring, and bauxite is the source for the aluminum used in soda cans.
3.5 The Rock Cycle
The most fundamental view of Earth materials is the rock cycle, which presents the primary materials that comprise the Earth and describes the processes by which they form and relate to each other. The rock cycle is usually said to begin with a hot molten liquid rock called magma or lava. Magma forms under the Earth’s surface in the crust or mantle and erupts on Earth’s surface as lava. When magma or lava cools, it solidifies by a process called crystallization in which minerals grow within the magma or lava. The rock that results from this is an igneous rock from the Latin word ignis, meaning “fire.”
Igneous rocks, as well as other types of rocks, on Earth’s surface, are exposed to processes of weathering and erosion to produce sediments. Weathering is the physical and chemical breakdown of rocks into smaller fragments and erosion is the removal of those fragments from their original location. Once igneous rocks are broken down and transported, these fragments or grains are considered sediments. Sediments such as gravel, sand, silt, and clay can be transported by water in the form of streams, ice in the form of glaciers, and air in the form of wind. Sediments ultimately come to rest in a process known as deposition. The deposited sediments accumulate in place, often under water such as a shallow marine environment, get buried.
Within the burial process, the sediments go through compaction by the weight of overlying sediments and cementation as minerals in groundwater glue the sediments together. The process of compacting and cementing sediments together is lithification, and lithified sediments are considered a sedimentary rock, such as sandstone and shale. Other sedimentary rocks, known as chemical sedimentary rocks, are not made of weathered and eroded sedimentary fragments. They are instead made by direct chemical precipitation of minerals.
Pre-existing rocks may be metamorphosed into a metamorphic rock, meta- means “change”, -morphos means “form” or “shape.” When rocks are subjected to extreme increases in temperatures or pressures, the minerals alter into enlarged crystals, or entirely new minerals with similar chemical make up. These high temperatures and pressures can occur when rocks are buried deep within the Earth’s crust or where they come into contact with hot magma or lava. In some cases, the temperature and pressure conditions can allow rocks to melt and create magma and lava, thus showing the cyclical nature of the rock cycle as new rocks are born.
Click here to learn more about various types of igneous, sedimentary, and metamorphic rocks from the Utah Geologic Survey (UGS).
3.6 Igneous Rocks
Igneous rocks form from the cooling and hardening of molten magma in many different environments. These rocks are identified by their composition and texture. More than 700 different types of igneous rocks are known.
The rock beneath the Earth’s surface is sometimes heated to high enough temperatures that it melts to create magma. Different magmas have different composition and contain whatever elements were in the rock that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust. Whether rock melts to create magma depends on:
- Temperature: Temperature increases with depth, so melting is more likely to occur at greater depths.
- Pressure: Pressure increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures.
- Water: The addition of water changes the melting point of rock. As the number of water increases, the melting point decreases.
- Rock composition: Minerals melt at different temperatures, so the temperature must be high enough to melt at least some minerals in the rock. The first mineral to melt from a rock will be quartz (if present), and the last will be olivine (if present).
As a rock heats up, the minerals that melt at the lowest temperatures will melt first. Partial melting occurs when the temperature on a rock is high enough to melt only some of the minerals in the rock. The minerals that will melt will be those that melt at lower temperatures. Fractional crystallization is the opposite of partial melting. This process describes the crystallization of different minerals as magma cools.
Bowen’s Reaction Series indicates the temperatures at which minerals melt or crystallize. An understanding of the way atoms joins together to form minerals leads to an understanding of how different igneous rocks form. Bowen’s Reaction Series also explains why some minerals are always found together, and some are never found together.
If the liquid separates from the solids at any time in partial melting or fractional crystallization, the chemical composition of the liquid and solid will be different. When that liquid crystallizes, the resulting igneous rock will have a different composition from the parent rock.
Intrusive Igneous Rock
Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive igneous rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Intrusive igneous rocks cool slower than extrusive igneous rocks, which allows for larger crystal structure to take develop.
Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought igneous rocks to the surface. Yosemite is a classic example of intrusive igneous rock. The molten magma never reached Earth’s surface, so the molten material had millions of years to cool down slowly to form granite. Later, geologic forces and erosion have caused those granite plutons to surface as they are today.
Extrusive Igneous Rock
Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks. Extrusive igneous rocks cool much more rapidly than intrusive rocks, reducing the time for crystal structure to form within the rocks.
Cooling rate and gas content create a variety of rock textures. Lavas that cool exceptionally rapidly may have a glassy texture. Those with many holes from gas bubbles have a vesicular texture.
Human Uses of Igneous Rock
Igneous rocks have a wide variety of uses. One significant use is as stone for buildings and statues. Granite is used for both of these purposes and is popular for kitchen countertops. Pumice is commonly used as an abrasive as household products or for smoothing skin. Ground up pumice stone is sometimes added to toothpaste to act as an abrasive material to scrub teeth. Peridotite is sometimes mined for peridot, a type of olivine that is used in jewelry. Diorite was used extensively by ancient civilizations for vases and other decorative artwork and is still used for art today.
3.7 Sedimentary Rock
Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:
- Fragments of other rocks that often have been worn down into small pieces, such as sand, silt, or clay.
- Organic materials, or the remains of once-living organisms
- Chemical precipitates, which are materials that get left behind after the water evaporates from a solution.
Rocks at the surface undergo mechanical and chemical weathering. These physical and chemical processes break rock into smaller pieces. Physical weathering breaks the rocks apart, whereas chemical weathering dissolves the less stable minerals. These original elements of the minerals end up in solution, and new minerals may form. Sediments are removed and transported by water, wind, ice, or gravity in a process called erosion.
Streams carry vast amounts of sediment. The more energy the water has, the larger the particle it can carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles.
Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps. Avalanches drop large piles of sediment. Glaciers leave large piles of sediments, too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present, whereas darker sediments form when the environment is oxygen-poor.
Sedimentary Rock Formation
Accumulated sediments harden into rock by a process called lithification. Two important steps are needed for sediments to lithify.
Sediments are squeezed together by the weight of overlying sediments on top of them, called compaction. Cemented, non-organic sediments become clastic rocks. If organic material is included, they are bioclastic rocks.
Fluids fill in the spaces between the loose particles of sediment and crystallize to create a rock by cementation. When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical sedimentary rocks.
Biochemical sedimentary rocks form in the ocean or a salt lake. Living creatures remove ions, such as calcium, magnesium, and potassium, from the water to make shells or soft tissue. When the organism dies, it sinks to the ocean floor to become biochemical sediment, which may then become compacted and cemented into solid rock.
Human Use of Sedimentary Rock
Sedimentary rocks are used as building stones, although they are not as hard as igneous or metamorphic rocks. Sedimentary rocks are used in construction. Sand and gravel are used to make concrete; they are also used in asphalt. Many economically valuable resources come from sedimentary rocks. Iron ore and aluminum are two examples.
3.8 Metamorphic Rock
Metamorphism is the addition of heat and/or pressure to existing rocks, which causes them to change physically and/or chemically so that they become a new rock. Metamorphic rocks may change so much that they may not resemble the original rock.
Any type of rock – igneous, sedimentary, or metamorphic – can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of the different chemical composition. Hornfels, with its alternating bands of dark and light crystals, is an excellent example of how minerals rearrange themselves during metamorphism.
Extreme pressure may also lead to foliation, the flat layers that form in rocks as pressure squeezes the rocks. Foliation forms typically when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated. Quartzite and limestone are nonfoliated. The two main types of metamorphism are both related to heat within Earth:
- Regional metamorphism: Changes in enormous quantities of rock over a wide area caused by the extreme pressure from overlying rock or compression caused by geologic processes. Deep burial exposes the rock to high temperatures.
- Contact metamorphism: Changes in a rock that is in contact with magma because of the magma’s extreme heat.
Quartzite is very hard and is often crushed and used in building railroad tracks. Schist and slate are sometimes used as building and landscape materials. Graphite, the “lead” in pencils, is a mineral commonly found in metamorphic rocks.