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3: Rocks and the Rock Cycle - Geosciences


In this section we will learn about the rock cycle. This is how soil forms, through the breakdown of rocks. We need soil to survive—imagine trying to grow vegetables without it. This is an immediate connection to the food chain. The rock cycle also gives scientists and engineers an idea on where energy sources (mainly fossil fuels, which are found only in sedimentary rock) and building materials such as marble or granite may be located. We will see throughout the course how this cycle plays into just about every aspect of geology.

Thumbnail: The rock cycle. Legendary: magma; crystallization (freezing of rock); igneous rocks; erosion; sedimentation; sediments and sedimentary rocks; tectonic burial and metamorphism; metamorphic rocks; melting. (Public Domain; ).


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.

Figure 3.2 A schematic view of the rock cycle. [SE]

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).

Figure 3.3 Magma forming pahoehoe basalt at Kilauea Volcano, Hawaii [SE]

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.

Figure 3.5 Metamorphosed and folded Triassic-aged limestone, Quadra Island, B.C. [SE]


The Rock Cycle

The rock cycle is a series of processes that create and transform the types of rocks in Earth&rsquos 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&mdashsuch as melting, cooling, eroding, compacting, or deforming&mdashthat are part of the rock cycle.

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&mdashfor instance, calcium carbonate, salt, and silica&mdashthat forms when the solution it is dissolved in, usually water, evaporates and leaves the compound behind. This occurs as water travels through Earth&rsquos crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates.

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 &ldquonice&rdquo).

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 (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&rsquos 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 &ldquoholey&rdquo 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.

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


The Rock Cycle

The rock cycle is a concept used to explain how the three basic rock types are related and how Earth processes, over geologic time, change a rock from one type into another. Plate tectonic activity, along with weathering and erosional processes, are responsible for the continued recycling of rocks.

Rocks are classified into three basic types based on how they are formed.

  • Igneous - A rock formed by the cooling and crystallization of magma (molten rock) at or below the Earth's surface.
  • Sedimentary - A rock formed as a result of the weathering process, either by compaction and cementation of rock mineral fragments, or the precipitation of dissolved minerals.
  • Metamorphic - These rocks form as existing rocks are subjected to intense heat and/or pressure, usually over long periods of time.

The rocks in display are meant to be viewed in a clockwise direction. As you walk, keep in mind that existing rocks may change through natural processes over geologic time, or event melt to form new rocks.


What is 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. , 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. , which can cause accelerated rates of erosion due to flooding or wave action.

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.


The Rock Cycle Isn't Circular

Notice that all these changes have left out the essence of a cycle, because there is no overall direction to the circle. With time and tectonics, the material of Earth's surface moves back and forth in no particular pattern. The diagram is no longer a circle, nor is it limited to rocks. Therefore the "rock cycle" is poorly named, but it's the one we're all taught.

Notice another thing about this diagram: Each of the five materials of the rock cycle is defined by the one process that makes it. Melting makes magma. Solidification makes igneous rock. Erosion makes sediment. Lithification makes sedimentary rock. Metamorphism makes metamorphic rock. But most of these materials can be destroyed in more than one way. All three rock types can be eroded and metamorphosed. Igneous and metamorphic rocks can also be melted. Magma can only solidify, and sediment can only lithify.

One way to see this diagram is that rocks are way stations in the flow of material between sediments and magma, between burial and upheaval. What we really have is a schematic of the material cycle of plate tectonics. If you understand the conceptual framework of this diagram, you can translate it into the parts and processes of plate tectonics and bring that great theory to life inside your own head.


6.3 The Rock Cycle

Now that you have practiced identifying all three major categories of rock, let’s examine how these rocks are slowly but constantly being changed from one form to another. The processes involved in the constant changing of these components of the Earth’s crust are summarized in the rock cycle (Figure 6.3.1). 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.

Figure 6.3.1: A schematic view of the rock cycle.

In describing the rock cycle, we can start anywhere we like, although it’s convenient to start with magma. As you learned in Lab 4, magma is rock that is hot to the point of being entirely molten, with a temperature of between about 800° and 1300°C, depending on the composition and the pressure.

Magma can either cool slowly within the crust (over centuries to millions of years)—forming intrusive igneous rocks , or erupt onto the surface and cool quickly (within seconds to years)—forming extrusive igneous rocks (volcanic rocks). Intrusive igneous rocks typically crystallize 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 then 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.

Practice Exercise 6.4 Rock around the rock-cycle clock

Referring to the rock cycle (Figure 6.3.1), 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 rocks . 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 .

Media Attributions

the series of processes through which rocks are transformed from one type to another

an igneous rock that has cooled slowly beneath the surface

igneous rock that cooled at surface

unconsolidated particles of mineral or rock

rock that has formed by the lithification of sediments

a rock formed by metamorphic processes that change the composition, texture or both of a preexisting parent rock (protolith)


3: Rocks and the Rock Cycle - Geosciences

In this section we will learn about the rock cycle and the different types of rocks. Please watch this short video for an introduction:


As you can see, the rock cycle is never ending. The video explained how rocks change from one rock type to another, and—just as important—it showed the processes that cause those changes.

Learning the rock cycle and understanding the processes involved helps all of us. For example, you saw in the video how all rocks are eroded into fine particles. This is how soil forms, through the breakdown of rocks. We need soil to survive—imagine trying to grow vegetables without it. This is an immediate connection to the food chain. The rock cycle also gives scientists and engineers an idea on where energy sources (mainly fossil fuels, which are found only in sedimentary rock) and building materials such as marble or granite may be located. We will see throughout the course how this cycle plays into just about every aspect of geology.

Here’s a visual representation of the rock cycle:

As you continue through the module, refer back to this image. Remember that all the processes of the rock cycle are interconnected.


Watch the video: 3 Types of Rocks and the Rock Cycle: Igneous, Sedimentary, Metamorphic - FreeSchool (October 2021).