rock cycle essay pdf

ENCYCLOPEDIC ENTRY

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

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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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Related Resources

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.

• National Park Service: Rocks and Minerals
• National Park Service: Igneous Rocks
• National Park Service: Sedimentary Rocks
• National Park Service: Metamorphic Rocks

The Rock Cycle

Many of Earth’s key processes function in cycles and rock cycle is no exception. The rock cycle is a web of processes that outlines how each of the three major rock types—igneous, metamorphic, and sedimentary—form and break down based on the different applications of heat and pressure over time. For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss. If it is heated further, the rock will melt completely and reform as an igneous rock.

Earth Science, Geology, Geography, Physical Geography

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Chapter 3 Intrusive Igneous Rocks

3.1 The Rock Cycle

The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle (Figure 3.2). The rock cycle is driven by two forces: (1) Earth’s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun.

The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is relatively thick, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.

In describing the rock cycle, we can start anywhere we like, although it’s convenient to start with magma. As we’ll see in more detail below, magma is rock that is hot to the point of being entirely molten. This happens at between about 800° and 1300°C, depending on the composition and the pressure, onto the surface and cool quickly (within seconds to years) — forming extrusive igneous rock (Figure 3.3).

Magma can either cool slowly within the crust (over centuries to millions of years) — forming intrusive igneous rock, or erupt onto the surface and cool quickly (within seconds to years) — forming extrusive igneous rock. Intrusive igneous rock typically crystallizes at depths of hundreds of metres to tens of kilometres below the surface. To change its position in the rock cycle, intrusive igneous rock has to be uplifted and exposed by the erosion of the overlying rocks.

Through the various plate-tectonics-related processes of mountain building, all types of rocks are uplifted and exposed at the surface. Once exposed, they are weathered, both physically (by mechanical breaking of the rock) and chemically (by weathering of the minerals), and the weathering products — mostly small rock and mineral fragments — are eroded, transported, and then deposited as sediments . Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean.

Exercise 3.1 Rock around the Rock-Cycle clock

Referring to the rock cycle (Figure 3.2), list the steps that are necessary to cycle some geological material starting with a sedimentary rock, which then gets converted into a metamorphic rock, and eventually a new sedimentary rock.

A conservative estimate is that each of these steps would take approximately 20 million years (some may be less, others would be more, and some could be much more). How long might it take for this entire process to be completed?

Unless they are re-eroded and moved along, sediments will eventually be buried by more sediments. At depths of hundreds of metres or more, they become compressed and cemented into sedimentary rock . Again through various means, largely resulting from plate-tectonic forces, different kinds of rocks are either uplifted, to be re-eroded, or buried deeper within the crust where they are heated up, squeezed, and changed into metamorphic rock .

Physical Geology Copyright © 2015 by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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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

The rock cycle is a natural process that describes how rocks are formed, broken down, and transformed into different types of rocks over time. It involves various geological processes such as weathering , erosion, deposition, compaction, cementation, melting, crystallization, and uplift. The rock cycle is a continuous process that occurs over millions of years and is driven by the Earth’s internal heat, tectonic activity, and external factors such as weather and climate.

Rock Cycle Processes

Igneous rock cycle process, metamorphic rock cycle process, sedimentary rock cycle process.

When rocks are pushed deep under the surface, they can melt into magma. If the conditions for the magma to remain liquid are no longer present, they are cooled and incorporated into an igneous rock. A rock that cools in the earth is called intrusive or plutonic, and it cools very slowly to produce a coarse-grained texture, such as rock granite . As a result of volcanic activity, the magma (called lava when it reaches the Earth’s surface), which is called extruded or volcanic rocks, can cool down very quickly while on the surface where the Earth is exposed to the atmosphere. These rocks are fine grained and sometimes so fast that no crystals form and do not result in a natural glass like obsidian , but the most common fine grained rock is known as basalt . Any of the three main rock types (igneous, sedimentary and metamorphic rocks) can melt into magma and cool down to igneous rocks .

Crystallization : The magma cools underground or on the surface and cures to a rickety rock. As the magma cools, different crystals form at different temperatures that undergo crystallization. For example, mineral olivine crystallizes at temperatures much higher than quartz than magma. The cooling rate determines how much time the crystals must form. Slow cooling produces larger crystals.

Metamorphic rocks can be changed physically or chemically to form a different rock under the high pressures and temperatures. Regional metamorphism refers to effects on large rock masses over a large area, usually associated with mountain formation events in orogenic belts. These rocks exhibit different bands of different mineralogy and colors, often called foliation. Another main type of metamorphism occurs when a rock mass comes into contact with an igneous intrusion that heats up this surrounding country rock. This contact metamorphism results in an over temperature of the magma and / or a rock which is altered and recrystallized by the addition of liquids that add chemical material (metasomatism) to the surrounding rock. Any pre-existing rock species can be replaced by metamorphism processes.

Metamorphism: When a rock is exposed to extreme heat and pressure within the Earth but does not melt, the rock becomes metamorphosed. Metamorphism can change the mineral composition and the texture of the rock. Thus, a metamorphic rock can be a new mineral composition and / or texture.

Rocks exposed to the atmosphere are variably unstable and subject to weathering and erosion. Abrasion and erosion break down the original rock into smaller pieces and remove dissolved materials. This shredded material accumulates and is embedded by additional material. While an individual sandstone is still a member of the rock class from which it is formed, it is a rock sediment composed of mixed grains. Sedimentary rocks may consist of collection of these small fragments (plastic clastic rock), accumulation and lithification of living organisms, or removal of mineral sediment from biologically deposited material. evaporation (sedimentary sedimentary rock ). Due to processes such as plant residues, such as elastic or organic material, frangible fractions may form from fragments separated from larger rocks of any species. Biogenic and sedimentary rocks consist of accumulation of minerals from dissolved chemicals from all other rock types.

Erosion and Sedimentation: Attrition, rock glides into smaller pieces on the surface of the Earth. Small pieces are called sediments. Flowing water, ice and gravity transport these deposits from one place to another by erosion. During sedimentation, sediments are laid or deposited. In order to form a sedimentary rock, the accumulated sediment must be compacted and cemented together.

Several processes can turn one type of rock into another type of rock. The key processes of the rock cycle are crystallization, erosion and sedimentation, and metamorphism.

Where does the energy that drives Earth’s rock cycle come from? Processes driven by heat from Earth’s interior are responsible for creating igneous and metamorphic rocks. Weathering and erosion, external processes powered by energy from the Sun, produce the sediment from which sedimentary rocks form.

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Middle school Earth and space science

Course: middle school earth and space science   >   unit 4, the rock cycle.

• Understand: the rock cycle

Key points:

• The rock cycle describes how rocks on Earth form and change over time.
• When rocks are pushed deep below Earth’s surface, they can melt to form magma . Magma that reaches Earth’s surface through volcanic activity is called lava .
• Igneous rocks form when magma or lava cools and solidifies.
• Weathering breaks igneous and other types of rocks into smaller pieces called sediment . Erosion transports sediment from one place to another. Deposition drops sediment in a new location.
• Sedimentary rocks form when sediment is compacted and cemented. This process is called lithification .
• Metamorphic rocks form when existing rocks are exposed to intense heat and pressure. Metamorphic rocks can be formed from igneous, sedimentary, or even other metamorphic rocks.

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• Earth Science

We see rocks in our surroundings, from different shapes to sizes. Rocks never remain the same, and they are always changing with time. The Rock cycle is a basic concept in geology that describes the time-consuming transitions through geologic time.

Rocks are constantly being recycled and going through several processes of chemical and physical changes. There are three main rock types:

• Sedimentary
• Metamorphic

The diagram below shows that rocks are altered or destroyed when it is forced out of its equilibrium conditions. This cycle of rock formation and wearing out is constantly recycled the earth’s minerals. The rock cycle is nothing but a process by which rocks of one type with certain characteristics change into rocks of another kind.

Rock Cycle: Transition to Igneous

When rocks are pushed deep under the earth’s surface, they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it will solidify into an igneous rock.

Rock Cycle: Transition to Metamorphic

The rock exposed to high temperatures and pressures can be changed physically or chemically to form a different rock, called metamorphic.

Read More: Rock Types

For more information on the formation of metamorphic rocks, watch the below video

Rock Cycle: Transition to Sedimentary

Rocks exposed to the atmosphere are very unstable and subject to the processes of weathering and erosion. This process breaks the original rock down into smaller fragments and carries away dissolved materials. Sedimentary rocks form from deposits that accumulate on the Earth’s surface.

Read More: Weathering

Frequently Asked Questions – FAQs

What is a rock, what are the main three types of rocks, what is a rock cycle, explain the creation of igneous rock., how metamorphic rocks are formed, the video about the types of rock and rock formation.

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