summarize the rock cycle

The Rock Cycle

The rock cycle is a series of processes that create and transform the types of rocks in Earth’s crust.

Chemistry, Earth Science, Geology

Reunion Island Volcano

Active volcanoes like this one on Reunion Island—east of Madagascar, in the Indian Ocean—forms a type of igneous rock. Extrusive, or volcanic, igneous rocks are formed when molten hot material cools and solidifies.

Photograph by Steve Raymer

There are three main types of rocks: sedimentary, igneous, and metamorphic. Each of these rocks are formed by physical changes—such as melting , cooling , eroding, compacting , or deforming —that are part of the rock cycle . Sedimentary Rocks Sedimentary rocks are formed from pieces of other existing rock or organic material. There are three different types of sedimentary rocks : clastic , organic (biological), and chemical . Clastic sedimentary rocks , like sandstone, form from clasts , or pieces of other rock. Organic sedimentary rocks , like coal, form from hard, biological materials like plants, shells, and bones that are compressed into rock. The formation of clastic and organic rocks begins with the weathering , or breaking down, of the exposed rock into small fragments. Through the process of erosion , these fragments are removed from their source and transported by wind, water, ice, or biological activity to a new location. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form solid rock. Chemical sedimentary rocks , like limestone, halite, and flint, form from chemical precipitation. A chemical precipitate is a chemical compound—for instance, calcium carbonate, salt, and silica—that forms when the solution it is dissolved in, usually water, evaporates and leaves the compound behind. This occurs as water travels through Earth’s crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates. Metamorphic Rocks Metamorphic rocks are rocks that have been changed from their original form by immense heat or pressure. Metamorphic rocks have two classes: foliated and nonfoliated. When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, creating foliation . Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the direction of pressure that is applied. An example of this transformation can be seen with granite, an igneous rock . Granite contains long and platy minerals that are not initially aligned, but when enough pressure is added, those minerals shift to all point in the same direction while getting squeezed into flat sheets. When granite undergoes this process, like at a tectonic plate boundary, it turns into gneiss (pronounced “nice”). Nonfoliated rocks are formed the same way, but they do not contain the minerals that tend to line up under pressure and thus do not have the layered appearance of foliated rocks. Sedimentary rocks like bituminous coal, limestone, and sandstone, given enough heat and pressure, can turn into nonfoliated metamorphic rocks like anthracite coal, marble, and quartzite. Nonfoliated rocks can also form by metamorphism, which happens when magma comes in contact with the surrounding rock. Igneous Rocks Igneous rocks (derived from the Latin word for fire) are formed when molten hot material cools and solidifies. Igneous rocks can also be made a couple of different ways. When they are formed inside of the earth, they are called intrusive, or plutonic, igneous rocks . If they are formed outside or on top of Earth’s crust, they are called extrusive, or volcanic, igneous rocks . Granite and diorite are examples of common intrusive rocks. They have a coarse texture with large mineral grains, indicating that they spent thousands or millions of years cooling down inside the earth, a time course that allowed large mineral crystals to grow. Alternatively, rocks like basalt and obsidian have very small grains and a relatively fine texture. This happens because when magma erupts into lava, it cools more quickly than it would if it stayed inside the earth, giving crystals less time to form. Obsidian cools into volcanic glass so quickly when ejected that the grains are impossible to see with the naked eye. Extrusive igneous rocks can also have a vesicular, or “holey” texture. This happens when the ejected magma still has gases inside of it so when it cools, the gas bubbles are trapped and end up giving the rock a bubbly texture. An example of this would be pumice.

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Understanding Global Change

Discover why the climate and environment changes, your place in the Earth system, and paths to a resilient future.

The rock cycle describes the processes through which the three main rock types (igneous, metamorphic, and sedimentary) transform from one type into another. The formation, movement and transformation of rocks results from Earth’s internal heat , pressure from tectonic processes , and the effects of water , wind , gravity, and biological (including human) activities.  The texture, structure, and composition of a rock indicate the conditions under which it formed and tell us about the history of the Earth.

On this page:

What is the rock cycle, earth system model of the rock cycle, explore the earth system, links to learn more.

For the classroom:

• Teaching Resources

Global Change Infographic

The rock cycle is an essential part of How the Earth System Works.  Click the image on the left to open the Understanding Global Change Infographic . Locate the rock cycle icon and identify other Earth system processes and phenomena that cause changes to, or are affected by, the rock cycle.

Rocks can be: (1) made of minerals, each of which has a specific crystal structure and chemical composition; (2) made of pieces of other rocks; (3) glassy (like obsidian); or, (4) contain material made by living organisms (for example coal, which contains carbon from plants). Different types of rocks form in Earth’s different environments at or below the Earth’s surface. For example, igneous rocks form when molten rock from the mantle or within the crust (see plate tectonics ) cools and either hardens slowly underground (e.g., granite), or hardens quickly if it erupts from a volcano (e.g., basalt). Rocks that experience sufficient heat and pressure within the Earth, without melting, transform into metamorphic rocks.  Rock exposed by mountain building or even modest uplift weathers and erodes and the resulting sediments can form sedimentary rocks. The formation and transformation of the various rock types can take many paths through the rock cycle depending on environmental conditions, as shown in the diagram below.

A simplified diagram of the rock cycle highlighting some of the UGC concepts related to this process

Molten lava cooling to form igneous rocks forming in Hawai’i National Park (left) metamorphic rocks in Death Valley National Park (right). Source: NPS Igneous Rocks and NPS Metamorphic Rocks

The rock cycle is affected by various human activities and environmental phenomena, including:

Sedimentary rocks along the California coast. Source: Explore Sediments Story Map

• The Earth’s internal heat and pressure, which can cause rock to melt completely or transform it into a metamorphic rock.
• The uplift of land caused by tectonic processes , which exposes rock that was underground to weathering and erosion .
• The rate of weathering, which is affected by climatic conditions such as precipitation and temperature . The rate at which the chemical reactions of weathering break down minerals often increases in the presence of water and under warmer temperatures. Plant growth , especially roots can physically break up rocks and also change the environmental chemistry (for example, increase acidity), increasing the rate of chemical weathering. In turn, the kind of rock that is weathered determines soil quality , nutrient levels (especially nitrogen and phosphorus levels), and local biodiversity .
• Rates of erosion caused by water , wind , ice , or gravity, which are driven by the water cycle, atmospheric and ocean circulation patterns, and regional topography (the structure of the landscape).
• The size and depth of the bodies of water, such as lakes, rivers, or the ocean, where sediment is deposited. Slower rates of water flow lead to the deposition of finer grained sediments and to slower rates of deposition.
• The extraction of rocks and fossil fuels , which in turn can destabilize soils , increase erosion , and decrease water quality by increasing sediment and pollutants in rivers and streams.
• Urbanization , which involves paving land with concrete, which can increase water runoff, increasing erosion and decreasing soil quality in the surrounding areas.
• Hydraulic fracking to remove oil and gas, which uses water, sand, and chemicals to create new or expand existing cracks in rocks that allow oil and gas to flow into drill holes for extraction .
• Human land and water use , including deforestation and agricultural activities .  Removing trees and other plants, plowing fields, and overgrazing by livestock destabilizes soils and can increase rates of erosion by 10 to 100 times.
• Damming rivers and extracting water from freshwater ecosystems for human use changes where and how much sedimentation occurs, which affects soil quality and causes changes in habitats .
• Plants and other organisms, such as those that build coral reefs, can trap sediment that otherwise might be deposited elsewhere.
• Extreme weather events , which can cause accelerated rates of erosion due to flooding or wave action.

The Earth system model below includes some of the processes and phenomena related to the rock cycle.  These processes operate at various rates and on different spatial and temporal scales. For example, urbanization and industrialization of many agricultural activities has occurred over the last 300 years, and especially over the last 70 years, while tectonic processes and mountain building occur over millions of years. Can you think of additional cause and effect relationships between the parts of the rock cycle and other processes in the Earth system?

Click the icons and bolded terms (e.g. plate tectonics , Earth’s internal heat, and erosion ) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you.

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The rock cycle is illustrated in Figure . Igneous rocks are produced when molten rock cools and solidifies. When exposed at the earth's surface, the rock is broken down into tiny particles of sediment by weathering and erosion. This weathered material is carried by water or wind to form sedimentary deposits such as beaches, sand bars, or deltas. The sediment is gradually buried by more sediment and subjected to higher pressure and temperature. It eventually hardens into sedimentary rock ( lithifies ). If burial continues, the increasing pressure and temperature at depth recrystallizes the sedimentary rock into a metamorphic rock. The rock cycle is completed when the metamorphic rock becomes so hot that it melts and forms a magma again. Igneous and sedimentary rocks can become metamorphic rocks if they are buried deeply enough or are affected by plate tectonic processes. Metamorphic rocks exposed at the surface will also weather to form sedimentary deposits.

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What is the Rock Cycle

The rock cycle is the process that describes the gradual transformation between the three main types of rocks : sedimentary, metamorphic, and igneous. It is occurring continuously in nature through geologic time.

What Causes the Rock Cycle

It occurs due to:

• Plate tectonic activity
• Erosional processes

Steps of the Rock Cycle: How does it Work

1) Formation of Igneous Rock – Melting, Cooling, and Crystallization

Magma, the molten rock present deep inside the earth, solidifies due to cooling and crystallizes to form a type of rock called igneous rocks . Cooling of igneous rocks can occur slowly beneath the surface of the earth or rapidly at its surface.

2) Formation of Sedimentary Rock – Weathering, Erosion, Sedimentation, and Compaction

Due to weathering and erosional activities, the igneous rocks are broken down to form sediments in the form of gravel, sand, silt, and clay, which gets mixed and pressed together for extended periods to form sedimentary rocks .

3) Formation of Metamorphic Rocks – Metamorphism

Over a very long period of time, sedimentary and igneous rocks end up being buried deep underground the soil, usually because of the movement of tectonic plates. Deep below the surface, these rocks are exposed to high heat and pressure, which change them into a different type of rock called metamorphic rock.

4) Weathering

Igneous, sedimentary, and metamorphic rocks present on the surface of the earth are constantly being broken down by wind and water over a long time.

5) Transportation

Carrying away of broken rocks by rain, streams, rivers, and oceans to a distant place from their origin.

6) Deposition

During the carriage of rocks by rivers, the rock particles (mixed with soil) sink and become a layer of sediment. Often the sediments build up and form small accumulations, which over time and pressure turn into sedimentary rock.

Melting of underground metamorphic rock forms magma, which on crystallization forms igneous rock, thus continuing the cycle.

Why is the Rock Cycle Important

• Helping in the formation of soil thus sustaining every life forms on earth
• Forming life-sustaining minerals such as sodium, iron, potassium, and calcium into the biosphere
• Forming the energy reserves of the earth like fossil fuels and radioactive sources
• Providing the building materials used to build structures such as iron, limestone, marble, granite, and basalt
• Providing raw materials for currency, investments, and adornments such as gold, diamonds, rubies, and emeralds

Ans. The two main forces that provide energy for the earth’s rock cycle are the sun and the internal heat of the earth. While the sun provides energy for weathering, erosion, and transportation, the earth’s internal heat helps in the processes like subduction, melting, and metamorphism.

Ans. The concept of the rock cycle was first suggested by James Hutton, the 18th-century founder of modern geology.

Ans. Since the rock cycle is a continuous process, the cycle does not stop after the formation of quartzite. Eventually, the quartzite rock could change into a sedimentary or an igneous rock to continue the cycle.

Ans. Compaction is the process in which sediment is squeezed to reduce the pore space between the grains due to the weight and pressure of overlying layers. Cementation is the process in which sediments are glued together by minerals that are deposited by water. Both compaction and cementation help in the formation of sedimentary rocks.

Article was last reviewed on Monday, November 2, 2020

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The rock cycle is the long, slow journey of rocks down from Earth’s surface and then back up again. Rocks often change during this process. During the rock cycle, rocks form deep in the Earth, move and sometimes change, go up to the surface, and eventually return below the ground. The three main kinds of rock are igneous, sedimentary, and metamorphic. Each type of rock moves around the cycle in different ways. 

Cycle begins again

The igneous rock gets eroded by weather, and the cycle begins again.

The magma (hot, liquid rock) bursts through Earth’s surface in the form of a volcano. It turns into solid rock, called igneous rock.

The rock cycle

The rock cycle is a never-ending process in which rocks continually shift and change over millions of years.

Heat and pressure deep in the Earth can make rock change into a different type, called metamorphic rock.

Intense heating

If the heat is very intense, both sedimentary and metamorphic rock can get so hot they turn into magma.

Rock pieces settle as sand, mud, or pebbles on the coast. The pieces then get carried into the sea by rivers and slowly settle in the bottom of the sea.

Rivers and streams carry the pieces of rock away, while breaking them down further. Glaciers (large rivers of ice and rock) remove bits of rock from mountainsides and carry them long distances.

Erosion is the movement or carrying away of rock pieces by a river, glacier, or wind.

Rain, wind, frost, chemicals, heat, and living things all break down rocks.

The rock particles at the bottom of a sea or lake get squashed and packed together. They gradually harden to form solid sedimentary rock.

Rock cycle in motion ›

Watch how the effects of changing weather on Earth cause rocks to form and change from one type to another in a never-ending cycle.

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What Are the Steps of the Rock Cycle?

How to smooth pebbles and stones

The rock cycle is the ongoing process of the continually changing states of earth minerals. Much like the water cycle, which consists of the way water changes to become steam, clouds, rain, then collects into bodies of water again, the rock cycle explains the way the minerals in the earth change. Once the rock cycle is understood, geological patterns and phenomena such as mountains, volcanoes and stream beds can be better understood and studied.

It's hard to determine which step in the rock cycle is the first, since the cycle is an ongoing process that virtually never ends. However, for the purposes of explaining the cycle, we'll start with what we see all around us on a daily basis: rocks. As time goes by, rocks are worn down by wind, rain, rivers, streams, freezing and thawing ice and other forces of nature. Rocks can break up and even slowly turn into small particles like sand that are collectively called sediment.

Sediments are blown all over by the wind and carried by streams. Many particles end up at the bottom of riverbeds where they are compacted and eventually become what is known as sedimentary rock. Sandstone is a type of sedimentary rock. When the earth's tectonic plates shift, these rocks can be pulled underneath the ground where it is extremely warm.

When rocks are pushed deep enough under the earth's surface, the heat can literally melt them, and magma is created. When magma comes out of the ground, it is known as lava, but not all magma makes it above ground. Some magma is pushed upward where it is slightly cooler, and since different minerals take different amounts of time to cool, the minerals in the magma separate and rocks such as granite are formed underground. These rocks eventually make their way to the surface through movements in the earth's plates. Other rocks are made when lava comes forth from a volcano. This way, the lava cools much more quickly, not giving the minerals time to separate. This process usually forms lava rocks and other such stones.

Now the rocks have found their way back to Step 1, where they will once again become sediment, sedimentary rocks, magma, and then rocks again.

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Module 3: Rocks and the Rock Cycle

Putting it together: rocks and the rock cycle.

In this section, you learned the following:

• The three rock types and how they are classified
• How each of the different rock types form
• The techniques geologist use to identify different rocks
• How to use these techniques to identify common rocks
• How the rock cycle works

In the rock cycle, illustrated in figure 1, the three main rock types—igneous, sedimentary, and metamorphic—are shown. Arrows connecting the three rock types show the processes that change one rock type into another. The cycle has no beginning and no end. Rocks deep within the Earth are right now becoming other types of rocks. Rocks at the surface are lying in place before they are next exposed to a process that will change them.

Figure 1. The Rock Cycle.

The rock cycle is a continuous and dynamic cycle that has no starting or stopping point and no set progression. Rocks can move through different paths within the cycle. The rock cycle explains how each rock type forms and the processes involved. We saw how the processes within the cycle influence everything from soil formation to recording the history of the Earth to the role deformation plays in rocks. It is easy to see how the rock cycle influences our dynamic and ever changing earth.

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Earth Cycles

The Rock Cycle: Uniformitarianism and Recycling

by Anne E. Egger, Ph.D.

• Glossary Terms

We all see changes in the landscape around us, but your view of how fast things change is probably determined by where you live. If you live near the coast, you see daily, monthly, and yearly changes in the shape of the coastline. Deep in the interior of continents, change is less evident – rivers may flood and change course only every 100 years or so. If you live near an active fault zone or volcano, you experience infrequent but catastrophic events like earthquakes and eruptions.

Throughout human history, different groups of people have held to a wide variety of beliefs to explain these changes. Early Greeks ascribed earthquakes to the god Poseidon expressing his wrath, an explanation that accounted for their unpredictability. The Navajo view processes on the surface as interactions between opposite but complementary entities: the sky and the Earth. Most 17th century European Christians believed that the Earth was essentially unchanged from the time of creation. When naturalists found fossils of marine creatures high in the Alps, many devout believers interpreted the Old Testament literally and suggested that the perched fossils were a result of the biblical Noah's flood.

Uniformitarianism

In the mid-1700s, a Scottish physician named James Hutton began to challenge the literal interpretation of the Bible by making detailed observations of rivers near his home. Every year, these rivers would flood, depositing a thin layer of sediment in the floodplain . It would take many millions of years, reasoned Hutton, to deposit a hundred meters of sediment in this fashion, not just the few weeks allowed by the Biblical flood. Hutton called this the principle of uniformitarianism: Processes that occur today are the same ones that occurred in the past to create the landscape and rocks as we see them now. By comparison, the strict biblical interpretation, common at the time, suggested that the processes that had created the landscape were complete and no longer at work.

Hutton argued that in order for uniformitarianism to work over very long periods of time, Earth materials had to be constantly recycled. If there were no recycling, mountains would erode (or continents would decay , in Hutton's terms), the sediments would be transported to the sea, and eventually the surface of the Earth would be perfectly flat and covered with a thin layer of water. Instead, those sediments once deposited in the sea must be frequently lifted back up to form new mountain ranges. Recycling was a radical departure from the prevailing notion of a largely unchanging Earth. As shown in Figure 1, Hutton first conceived of the rock cycle as a process driven by Earth's internal heat engine. Heat caused sediments deposited in basins to be converted to rock, heat caused the uplift of mountain ranges, and heat contributed in part to the weathering of rock. While many of Hutton's ideas about the rock cycle were either vague (such as "conversion to rock") or inaccurate (such as heat causing decay), he made the important first step of putting diverse processes together into a simple, coherent theory .

Hutton's ideas were not immediately embraced by the scientific community, largely because he was reluctant to publish. He was a far better thinker than writer – once he did get into print in 1788, few people were able to make sense of his highly technical and confusing writing (to learn more about Hutton and see a sample of his writing, visit the Resources for this module). His ideas became far more accessible after his death with the publication of John Playfair's "Illustrations of the Huttonian Theory of the Earth" (1802) and Charles Lyell 's "Principles of Geology" (1830). By that time, the scientific revolution in Europe had led to widespread acceptance of the once-radical concept that the Earth was constantly changing.

A far more complete understanding of the rock cycle developed with the emergence of plate tectonics theory in the 1960s (see our Plate Tectonics I module). Our modern concept of the rock cycle is fundamentally different from Hutton's in a few important aspects: We now largely understand that plate tectonic activity determines how, where, and why uplift occurs, and we know that heat is generated in the interior of the Earth through radioactive decay and moved out to the Earth's surface through convection . Together, uniformitarianism , plate tectonics, and the rock cycle provide a powerful lens for looking at the Earth, allowing scientists to look back into Earth history and make predictions about the future.

Comprehension Checkpoint

If Earth's materials were not recycled then

The Rock Cycle

The rock cycle consists of a series of constant processes through which Earth materials change from one form to another over time. As within the water cycle and the carbon cycle, some processes in the rock cycle occur over millions of years and others occur much more rapidly. There is no real beginning or end to the rock cycle, but it is convenient to begin exploring it with magma . You may want to open the rock cycle schematic in Figure 2 and follow along in the sketch; click on the diagram to open it in a new window.

Magma, or molten rock, forms only at certain locations within the Earth, mostly along plate boundaries. (It is a common misconception that the entire interior of the Earth is molten, but this is not the case. See our Earth Structure module for a more complete explanation.) When magma is allowed to cool, it crystallizes, much the same way that ice crystals develop when water is cooled. We see this process occurring in places like Iceland, where magma erupts out of a volcano and cools on the surface of the Earth, forming a rock called basalt on the flanks of the volcano (Figure 3). But most magma never makes it to the surface and it cools within Earth's crust . Deep in the crust below Iceland's surface, the magma that doesn't erupt cools to form gabbro . Rocks that form from cooled magma are called igneous rocks; intrusive igneous rocks if they cool below the surface (like gabbro), extrusive igneous rocks if they cool above (like basalt).

Uplifting, weathering, and erosion

Rocks like basalt are immediately exposed to the atmosphere and weather. Rocks that form below the Earth's surface , like gabbro , must be uplifted and all of the overlying material must be removed through erosion in order for them to be exposed. In either case, as soon as rocks are exposed at the Earth's surface, the weathering process begins. Physical and chemical reactions caused by interaction with air, water, and biological organisms cause the rocks to break down. Once rocks are broken down, wind, moving water, and glaciers carry pieces of the rocks away through a process called erosion. Moving water is the most common agent of erosion – the muddy Mississippi, the Amazon, the Hudson, the Rio Grande, all of these rivers carry tons of sediment weathered and eroded from the mountains of their headwaters to the ocean every year. The sediment carried by these rivers is deposited and continually buried in floodplains and deltas . In fact, the US Army Corps of Engineers is kept busy dredging the sediments out of the Mississippi in order to keep shipping lanes open (see Figure 4).

Erosion is caused primarily by

Sedimentary rocks

Under natural conditions, the pressure created by the weight of the younger deposits compacts the older, buried sediments . As groundwater moves through these sediments, minerals like calcite and silica precipitate out of the water and coat the sediment grains. These precipitants fill in the pore spaces between grains and act as cement, gluing individual grains together. The compaction and cementation of sediments creates sedimentary rocks like sandstone and shale, which are forming right now in places like the very bottom of the Mississippi delta .

Because deposition of sediments often happens in seasonal or annual cycles, we often see layers preserved in sedimentary rocks when they are exposed (Figure 5). In order for us to see sedimentary rocks, however, they need to be uplifted and exposed by erosion . Most uplift happens along plate boundaries where two plates are moving towards each other and causing compression. As a result, we see sedimentary rocks that contain fossils of marine organisms (and therefore must have been deposited on the ocean floor) exposed high up in the Himalaya Mountains – this is where the Indian plate is running into the Eurasian plate.

Most uplift happens

“Cooked” rocks

If sedimentary rocks or intrusive igneous rocks are not brought to the Earth's surface by uplift and erosion , they may experience even deeper burial and be exposed to high temperatures and pressures. As a result, the rocks begin to change. Rocks that have changed below Earth's surface due to exposure to heat , pressure, and hot fluids are called metamorphic rocks. Geologists often refer to metamorphic rocks as "cooked" because they change in much the same way that cake batter changes into a cake when heat is added. Cake batter and cake contain the same ingredients, but they have very different textures, just like sandstone, a sedimentary rock, and quartzite, its metamorphic equivalent. In sandstone, individual sand grains are easily visible and often can even be rubbed off; in quartzite, the edges of the sand grains are no longer visible, and it is a difficult rock to break with a hammer, much less rubbing pieces off with your hands.

Some of the processes within the rock cycle, like volcanic eruptions, happen very rapidly, while others happen very slowly, like the uplift of mountain ranges and weathering of igneous rocks. Importantly, there are multiple pathways through the rock cycle. Any kind of rock can be uplifted and exposed to weathering and erosion ; any kind of rock can be buried and metamorphosed. As Hutton correctly theorized, these processes have been occurring for millions and billions of years to create the Earth as we see it: a dynamic planet.

All processes in the rock cycle take millions of years.

A North American example

The rock cycle is not just theoretical; we can see all of these processes occurring at many different locations and at many different scales all over the world. As an example, the Cascade Range in North America illustrates many aspects of the rock cycle within a relatively small area, as shown in Figure 6.

The Cascade Range in the northwestern United States is located near a convergent plate boundary , where the Juan de Fuca plate, which consists mostly of basalt saturated with ocean water is being subducted, or pulled underneath, the North American plate. As the plate descends deeper into the Earth, heat and pressure increase and the basalt is metamorphosed into a very dense rock called eclogite . All of the ocean water that had been contained within the basalt is released into the overlying rocks, but it is no longer cold ocean water. It too has been heated and contains high concentrations of dissolved minerals , making it highly reactive, or volatile. These volatile fluids lower the melting temperature of the rocks, causing magma to form below the surface of the North American plate near the plate boundary . Some of that magma erupts out of volcanoes like Mt. St. Helens, cooling to form a rock called andesite , and some cools beneath the surface, forming a similar rock called diorite .

Storms coming off of the Pacific Ocean cause heavy rainfall in the Cascades, weathering and eroding the andesite . Small streams carry the weathered pieces of the andesite to large rivers like the Columbia and eventually to the Pacific Ocean, where the sediments are deposited. Continual deposition of sediments near the deep oceanic trench results in the formation of sedimentary rocks like sandstone. Eventually, some sandstone is carried down into the subduction zone, and the cycle begins again (see the Experiment! section in the Resources for this module).

The rock cycle is inextricably linked not only to plate tectonics , but to other Earth cycles as well. Weathering , erosion , deposition, and cementation of sediments all require the presence of water, which moves in and out of contact with rocks through the hydrologic cycle; thus weathering happens much more slowly in a dry climate like the desert southwest than in the rainforest (see our module The Hydrologic Cycle for more information). Burial of organic sediments takes carbon out of the atmosphere , part of the long-term geological component of the carbon cycle (see our module The Carbon Cycle module); many scientists today are exploring ways we might be able to take advantage of this process and bury additional carbon dioxide produced by the burning of fossil fuels (see News & Events in Resources). The uplift of mountain ranges dramatically affects global and local climate by blocking prevailing winds and inducing precipitation . The interactions between all of these cycles produce the wide variety of dynamic landscapes we see around the globe.

Earth’s materials are in constant flux. Some processes that shape the Earth happen quickly; others take millions of years. This module describes the rock cycle, including the historical development of the concept. The relationship between uniformitarianism, the rock cycle, and plate tectonics is explored in general and through the specific example of the Cascade Range in the Pacific Northwest.

Key Concepts

The rock cycle is the set of processes by which Earth materials change from one form to another over time.

The concept of uniformitarianism, which says that the same Earth processes at work today have occurred throughout geologic time, helped develop the idea of the rock cycle in the 1700s.

Processes in the rock cycle occur at many different rates.

The rock cycle is driven by interactions between plate tectonics and the hydrologic cycle.

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Chapter 7 Metamorphism and Metamorphic Rocks

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

• Summarize the factors that influence the nature of metamorphic rocks and explain why each one is important
• Describe the mechanisms for the formation of foliation in metamorphic rocks
• Classify metamorphic rocks on the basis of their texture and mineral content, and explain the origins of these differences
• Describe the various settings in which metamorphic rocks are formed and explain the links between plate tectonics and metamorphism
• Summarize the important processes of regional metamorphism, and explain how rocks that were metamorphosed at depths of 10 km or 20 km can now be found on Earth’s surface
• Summarize the important processes of contact metamorphism and metasomatism, and explain the key role hydrothermal fluids

Metamorphism is the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed. In most cases, but not all, this involves the rock being deeply buried beneath other rocks, where it is subjected to higher temperatures and pressures than those under which it formed. Metamorphic rocks typically have different mineral assemblages and different textures from their parent rocks (Figure 7.1) but they may have the same overall composition.

Most metamorphism results from the burial of igneous, sedimentary, or pre-existing metamorphic to the point where they experience different pressures and temperatures than those at which they formed (Figure 7.2). Metamorphism can also take place if cold rock near the surface is intruded and heated by a hot igneous body. Although most metamorphism involves temperatures above 150°C, some metamorphism takes place at temperatures lower than those at which the parent rock formed.

Physical Geology by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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