importance of rock cycle essay

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The Importance of Rock Cycle

Importance of rocks: introduction, why is the rock cycle important: rock formation, importance of rock cycle: rock formation experiment, importance of rocks: conclusion.

One of the most important substances found on earth is rocks. There are different types of rocks and each one can be used to serve different kinds of purposes. Rocks can be used as a building material, to build tools, to enhance the beauty and utility of homes, and even to clean surfaces. Humans must always have a steady supply of rocks. It is therefore a good thing that the rock cycle ensures that the people on this planet will always have access to rocks. The following pages will explain the rock cycle as well as the experiment conducted to prove that there is a scientific basis to the rock cycle thesis.

The main argument behind the rock cycle is the assertion that rocks are not destroyed but undergo a transformation that after a long process will result in the creation of new rocks. Thus, if one can see broken rocks and rocks pulverized by erosion and the effects of wind, water, and human activity one should not be worried because these rock particles – after undergoing a process – will return to their previous rock-solid shape.

The following information will help explain the importance of rocks in general and the rock cycle in particular. The following is a list of rocks with their corresponding uses:

• Basalt (Igneous rocks) – can be sued in building roads.
• Granite (Igneous rocks) – can be used to build beautiful tombstones and monuments.
• Marble (Metamorphic rocks) – Floors made of marble, and counters made of marble are not only about utility but also for aesthetics purposes.
• Pumice (Igneous rocks) – this can be used as a scrubbing material or as a cleaning agent.
• Slate (Metamorphic rocks) – these durable rocks can be used as roofing material for roofs.

Underneath the earth’s surface are superheated rocks that are called magma. It is common knowledge that heat, especially high temperatures causes substances to move. Rock matter subjected to this level of heat can be transformed into a liquid state and therefore can be moved with ease. The very hot magma will move up from below. If magma finds its way underneath a volcano, the pressure builds up, and finally, at a critical point, the volcano will erupt, spewing magma. When magma cools down it will form igneous rocks.

Since magma contains rock matter and minerals that was mixed by the high temperatures when magma cools down and form igneous rocks one can also discover the different kinds of minerals contained in the said igneous rocks. When igneous rocks are exposed to the elements they will undergo a process called “weathering” and if it breaks down it will experience erosion. Rainwater as well as wind and earthquakes can speed up the weathering process as well as erosion.

Igneous rocks broken down by natural forces can then be transported and deposited by streams, wind, glacier water, or ocean currents. If sedimentary rocks are deposited in rivers and in places where it is exposed to wind and water a different type of sedimentary rocks are formed – clastic sedimentary rocks. But when sedimentary rocks are deposited deep in the ocean non clastic sedimentary rocks are formed and characterized by the minerals found in them. Examples of nonclastic sedimentary rocks are limestone, rock salt, rock gypsum, and Chert.

Sedimentary rocks can be broken down even further and then hundreds of years of the “weathering” and erosion process will result in the piling up of rocks. The weight of the rock and the heat coming from the earth’s molten core will cause heat and pressure to change the composition of the rocks, specifically the mineral content of the rocks. The result is a new type of rock called metamorphic which comes from the term metamorphosis or transformation. If metamorphic rocks are exposed to greater pressure and more heat then the rocks can melt and transform into magma. And the cycle goes on and on.

To fully understand the rock cycle an experiment was conducted using white and black chocolate chips to resemble rock matter. The proponent of the experiment began to cut the chocolate chips into pieces and then pulverize them using hands and fingers. This action mimics the “weathering” process experienced by exposed igneous rocks. The process of cutting and breaking down the chocolate chips continues until the chips are simplified further.

There are two kinds of chocolate chips one is white and the other is black. This represents the different mineral compositions found in rocks. The white and black chocolate chips also help in visualizing a feature of the rock cycle process where minerals are mixed or moved from one place to the next. Going back to the experiment the constant cutting and breaking of the chocolate chips will reduce it into something that is of finer consistency.

At the beginning of the experiment, the chocolate chips are more stable and solid that it requires a knife to cut through and break it into pieces. But in the latter part, the chips are reduced to an almost fine powder. Then the black and white chocolate powder – it could no longer be called chocolate chips because of the fine consistency of the chocolate – are mixed. One could easily see the effect of the mixture. Part of the mixture is colored black or dark brown and the other is colored white.

Then the result of the process is layered on top of a tin foil. The layering process mimics mother nature’s erosion where rock particles are moved from the mountains down into river beds, streams, oceans, etc. The tinfoil is then folded to mash the chocolate together. Then a burner is used to heat the tin foil. The burner mimics the heat coming from the earth’s molten core while folding the tin foil mimics the pressure exerted by tons of rocks piled on top of each other.

The heat and the pressure allowed the white and dark chocolate substances to melt and mix. The result is dark brown chocolate with white color presumably coming from the milky substance of the white chocolate. If more heat and more pressure are added the chocolate will melt and this mimics the transformation of metamorphic rocks into magma and the cycle goes on and on.

Rocks are an integral part of human existence. Without rocks, mankind would have no building materials to create sturdy and beautiful homes, buildings, monuments, and even tombstones. But aside from constructing materials rocks are also part of the earth’s geography. Soil comes from weathered rocks and therefore without it there would be no vegetation on earth and no living things either.

Concern over the sustainability of rocks and continuous access to them is a legitimate concern. But there is no reason to worry because rocks do not disappear they merely change from one form to another. Although this process takes years to complete the rock cycle will ensure the sustainable supply of rocks on the planet. The rock cycle is a helpful way of understanding why there are igneous rocks, sedimentary rocks, and metamorphic rocks. This also helps explains why there are minerals embedded in rocks. Some of the minerals are the reason why there are precious stones buried underneath the earth while other minerals are the source of beneficial materials that are needed in various industries.

But the rock cycle can be a highly theoretical proposition. An experiment is required to help understand the rock cycle process. The experiment described in the preceding pages is a simple demonstration of how rocks transform from one form to another and ensures how future generations will always have their steady supply of rock material. The use of chocolate chips is a wise choice because it can easily be broken down and the shape can be further simplified. The availability of white and dark chocolate is also very helpful because it can help demonstrate that different rocks have different mineral compositions.

Moreover, the use of the chocolate chips enables the proponents of the experiment to heat it while retaining consistency and mimics the behavior of rocks when under tremendous heat and pressure. The mixture of white and dark chocolate is a good visual aid when it comes to understanding how rocks are melted to become magma before they can be spewed out from the mouth of a volcano. And the cycle goes on and on.

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The Rock Cycle – Diagram and Explanation

The rock cycle is the natural, continuous process that forms, breaks down, and reforms rock through geological, chemical, and physical processes. Through the cycle, rocks convert between igneous, metamorphic, and sedimentary forms. It is a dynamic system that recycles Earth’s materials in different forms, from molten magma deep below the surface to solid rock formations and sediments. Understanding the rock cycle is not only crucial for geologists but also provides insight into Earth’s history, climate change, and the availability of natural resources.

Importance of the Rock Cycle

The rock cycle is an integral aspect of Earth sciences that sheds light on Earth’s age, history, and the forces that shape it. Understanding the rock cycle is key to discerning how rocks change forms, contributing to soil fertility, and providing resources like minerals and fossil fuels. It also has practical applications in industries like construction, where rock characteristics must be understood for structural integrity.

Types of Rocks

The three types of rocks are igneous, sedimentary, and metamorphic rocks:

Igneous Rocks

Igneous rocks form from the cooling and solidification of molten magma or lava. They have a crystalline structure.

• Intrusive Igneous Rocks : These rocks form when magma cools slowly beneath Earth’s crust , allowing for larger crystals to develop. Examples include granite, which is commonly used in countertops and is known for its coarse-grained structure.
• Extrusive Igneous Rocks : These rocks form when lava erupts from a volcano and cools quickly on Earth’s surface. This rapid cooling results in small or even microscopic crystals. Basalt is a common extrusive rock often found in oceanic crust.

Sedimentary Rocks

Sedimentary rocks form through the layering, compression, and cementation of mineral and organic matter. These rocks often have a layered appearance and are softer than most igneous and metamorphic rocks.

• Clastic Sedimentary Rocks : These rocks, such as sandstone and shale, form from the mechanical breakdown of other rocks and are classified by grain size and composition.
• Organic Sedimentary Rocks : Limestone and coal are examples of organic sedimentary rocks. Limestone typically comes from shells and skeletal fragments of marine organisms, while coal forms from the accumulation of plant debris.
• Chemical Sedimentary Rocks : Halite and gypsum are examples of rocks formed through evaporation or chemical reactions. Halite, or rock salt, forms when saltwater evaporates, while gypsum forms in various evaporative contexts including desert ponds and lake beds.

Metamorphic Rocks

Subjecting either igneous or sedimentary rocks to high heat and pressure alters their physical or chemical composition, forming metamorphic rocks .

• Foliated Metamorphic Rocks : These rocks, like slate and schist, have a layered or banded appearance from exposure to heat and directed pressure.
• Non-foliated Metamorphic Rocks : These rocks lack layers. Examples of non-foliated metamorphic rocks include marble and quartzite. Marble forms from the metamorphism of limestone, while quartzite forms from the metamorphism of quartz sandstone.

Steps of the Rock Cycle

Molten rock called magma is the source material for rocks. Igneous rocks form both under the surface and above it when magma becomes lava. Heat and pressure changes igneous and sedimentary rocks into metamorphic rocks. Erosion and weathering break igneous and sedimentary rocks up, which compact into sedimentary rocks. Sediments from organic sources also contribute to sedimentary rocks. Tectonic forces drive some rocks back below the surface, where they can change forms or melt and become magma once again.

Melting : Rocks beneath Earth’s crust melt due to high pressure and temperature, forming magma.

Cooling and Solidification : Magma cools and solidifies either beneath the Earth’s surface (intrusive) or upon reaching the surface as lava through volcanic activity (extrusive). Magma and lava form igneous rocks.

Mechanical and Chemical Weathering : Igneous, metamorphic, and sedimentary rocks on Earth’s surface undergo mechanical disintegration and chemical decomposition.

Erosion : Natural forces like wind, water, and ice erode rocks. Temperature changes also play a role, making rocks expand and contract and sometimes break.

Deposition : Eroded materials from all rock types as well as organic sources settle in basins.

Compaction and Cementation : Layers of sediment are compacted and cemented together, forming sedimentary rocks.

Heat and Pressure : Existing rocks undergo changes in physical or chemical composition due to high heat and pressure, leading to the formation of metamorphic rocks.

Remelting : Metamorphic rocks may melt again, forming magma, and the cycle continues.

Forces Driving the Rock Cycle

Several forces driving the rock cycle, including internal forces within the Earth, surface actions, and even gravity:

Earth’s Internal Heat

• Radioactive Decay : The decay of radioactive isotopes like uranium provides heat, facilitating the melting of rocks.
• The Earth’s surface cools magma and lava, forming rocks. Seasonal changes and variations due to latitude cause temperature stresses in rocks and impact the rate at which weathering occurs.

Tectonic Forces

• Subduction Zones : Subduction zones occur where tectonic plates meet and one gets pushed beneath another, leading to high heat and pressure.
• Rift Zones : Plates moving apart form spreading rift zones that allow magma to rise, creating new crust.

Surface Processes

• Weathering Agents : Water, ice, and wind play roles in mechanical weathering.
• Chemical Agents : Acid rain and oxidization cause chemical weathering. Water dissolves soluble compounds.

Gravitational Forces

• Gravity : Aids in the settling of sedimentary particles during the deposition phase.
• Blatt, Harvey; Tracy, Robert J. (1996). Petrology; Igneous, Sedimentary, and Metamorphic (2nd ed.). W. H. Freeman. ISBN 0-7167-2438-3.
• Bucher, Kurt; Grapes, Rodney (2011). Petrogenesis of Metamorphic Rocks . Springer Science & Business Media. ISBN 978-3-540-74169-5.
• Plummer, Charles; McGeary, David; Carlson, Diane (2005). Physical Geology . Mc Graw Hill. ISBN 0-07-293353-4.
• Vigneresse, Jean Louis; Barbey, Pierre; Cuney, Michel (1996). “Rheological Transitions During Partial Melting and Crystallization with Application to Felsic Magma Segregation and Transfer”. Journal of Petrology . 37 (6): 1579–1600. doi: 10.1093/petrology/37.6.1579
• Wyllie, Peter J. (August 1988). “Magma Genesis, Plate Tectonics, and Chemical Differentiation of the Earth”. Reviews of Geophysics . 26 (3): 370–404. doi: 10.1029/RG026i003p00370

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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 Explained: How Rocks Are Made

The rock cycle is a continuous process that involves the formation, alteration, and destruction of rocks. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks are formed from the cooling and solidification of magma or lava. Sedimentary rocks are formed from the accumulation of sediments, such as sand, silt, clay, and organic matter. Metamorphic rocks are formed from the alteration of existing rocks due to heat, pressure, or chemical reactions.

What is the Rock Cycle

The rock cycle is a continuous process that describes how rocks are transformed between three main types: igneous, sedimentary, and metamorphic. Each rock type can change into another through various geological processes, driven by forces like heat, pressure, and erosion.

The Rock Cycle: How It Works

The rock cycle is a continuous process that has been happening for billions of years. It involves the formation of three main types of rocks: igneous, sedimentary, and metamorphic.

Igneous rocks are formed when molten rock (magma or lava) cools and solidifies. Magma can form when rocks in the Earth's crust melt, or when hot, molten material from the Earth's mantle rises up to the surface. Once magma cools and solidifies, it becomes an igneous rock.

Sedimentary rocks are formed from the accumulation of sediment. Sediment is material that has been weathered and eroded from other rocks. When sediment is transported by wind, water, or ice, it is eventually deposited in layers. Over time, the layers of sediment can be compacted and cemented together, forming a sedimentary rock.

Metamorphic rocks are formed when existing rocks are subjected to heat, pressure, or both. The heat and pressure can cause the rocks to change their mineral composition, texture, and structure. Metamorphic rocks can form from any type of rock, including igneous, sedimentary, and even other metamorphic rocks. 

The rock cycle is a vital part of the Earth's system. It helps to recycle the Earth's materials, and it also plays a role in the formation of mineral deposits. The rock cycle is a complex process, but it is essential for understanding the Earth's geology.

Processes in the Rock Cycle

The rock cycle describes how rocks change from one type to another over geologic time, driven by Earth’s internal heat and surface processes. Here’s an overview of each stage:

Cooling and Solidification: When molten rock, either magma from beneath the Earth's surface or lava after a volcanic eruption, cools down, it solidifies into igneous rock. The formation of these rocks depends on the location of cooling. Intrusive igneous rocks like granite form when magma cools slowly underground, allowing for the development of large crystals. In contrast, extrusive igneous rocks , such as basalt or pumice, arise from lava cooling swiftly at or on the Earth's surface, often resulting in small or no crystals due to the rapid cooling rate.

Weathering and Erosion: These processes break down rocks that have been exposed to the Earth's surface. Weathering can be physical, where rock is fragmented into smaller pieces without chemical change, through actions like freeze-thaw cycles, wind abrasion, and plant root expansion. Chemical weathering, on the other hand, involves the alteration of rock minerals through reactions with water, acids, or oxygen, as seen when feldspar turns into clay. Biological weathering also plays a role, where organisms contribute to rock breakdown by secreting acids or through physical disruption. Erosion follows, where these weathered materials, now sediments, are transported by natural forces like wind, water, ice, or gravity.

Deposition, Compaction, and Cementation: Sediments settle out of the transport medium when its energy wanes, leading to deposition in places like river deltas, ocean beds, or lake bottoms. Over time, these layers of sediment compact under their own weight, expelling water and air. Cementation then binds the grains together as minerals precipitate from groundwater, transforming the loose sediment into solid rock. This process forms sedimentary rocks such as sandstone, shale, and limestone, which often show visible layering from successive deposition events.

Heat and Pressure (Metamorphism): Rocks can be subjected to intense heat and pressure when buried deep within the Earth, leading to metamorphism without melting. Contact metamorphism happens when rocks are heated by magma intrusions, typically creating non-foliated rocks like marble. Regional metamorphism occurs over larger areas due to tectonic forces and deep burial, often producing foliated rocks like schist or gneiss where minerals realign into layers. These changes reflect the transformation of the rock's mineral content and texture due to the extreme conditions.

Melting again: If rocks are driven deep into the mantle or are near a heat source like a mantle plume, they can melt back into magma. This can happen in subduction zones or under areas of mantle upwelling. When this magma cools, it crystallizes again into igneous rock, thus completing the rock cycle. The type of igneous rock formed depends on the magma's composition and the rate at which it cools.

These processes collectively illustrate the rock cycle's role in reshaping Earth's surface over geological time, with each rock type undergoing transformations due to the planet's dynamic internal and external forces.

In addition to the key processes listed above, there are a number of other factors that can influence the rock cycle. These include:

• The composition of the rocks involved
• The temperature and pressure conditions
• The presence of fluids
• The time frame over which the processes occur

The rock cycle is a dynamic process that is constantly changing. The rate at which rocks are cycled through the system depends on a number of factors, including the climate, tectonic activity, and the presence of water.

Facts About the Rock Cycle

• The rock cycle is a closed system, meaning that the total amount of rock on Earth remains constant.
• The rock cycle takes millions of years to complete.
• The rock cycle is driven by the Earth's internal heat and by the forces of plate tectonics.
• The rock cycle plays a role in the formation of mineral deposits, such as coal, oil, and diamonds.
• No set order: Rocks can transition between any of the three types, and the cycle doesn’t follow a single pathway.
• Recycling of materials: The rock cycle demonstrates how Earth's materials are constantly being recycled and transformed.

The rock cycle is a fundamental concept in geology. It helps us to understand the origin of rocks, the distribution of minerals, and the evolution of the Earth's crust. The rock cycle is also a reminder that the Earth is a dynamic planet, and that its surface is constantly changing.

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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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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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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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