Day 1: Rock Cycle by 3/4 (A) and 3/3 (B)
Introduction
There are three types of rocks: igneous, sedimentary and metamorphic. Each of these types is part of the rock cycle. Through changes in conditions one rock type can become another rock type. Or it can become a different rock of the same type.
What Are Rocks?
A rock is a naturally formed, non-living earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass (Figure below).
There are three types of rocks: igneous, sedimentary and metamorphic. Each of these types is part of the rock cycle. Through changes in conditions one rock type can become another rock type. Or it can become a different rock of the same type.
What Are Rocks?
A rock is a naturally formed, non-living earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass (Figure below).
Three Main Categories of Rocks
Rocks are classified into three major groups according to how they form. Rocks can be studied in hand samples that can be moved from their original location. Rocks can also be studied in outcrop, exposed rock formations that are attached to the ground, at the location where they are found.
Igneous Rocks
Igneous rocks form from cooling magma. Magma that erupts onto Earth’s surface is lava, as seen in Figure below. The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallize.
Rocks are classified into three major groups according to how they form. Rocks can be studied in hand samples that can be moved from their original location. Rocks can also be studied in outcrop, exposed rock formations that are attached to the ground, at the location where they are found.
Igneous Rocks
Igneous rocks form from cooling magma. Magma that erupts onto Earth’s surface is lava, as seen in Figure below. The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallize.
Intrusive and Extrusive Igneous Rocks
Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see Figure below for an example).
Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see Figure below for an example).
Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought igneous rocks to the surface. Figure below shows a landscape in California’s Sierra Nevada made of granite that has been raised to create mountains.
Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks(Figure below).
Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (Figure below).
Sedimentary Rocks
Sedimentary rocks form by the compaction and cementing together of sediments, broken pieces of rock-like gravel, sand, silt, or clay (Figure below). Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates.
Sedimentary rocks form by the compaction and cementing together of sediments, broken pieces of rock-like gravel, sand, silt, or clay (Figure below). Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates.
Sediments
Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:
Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:
- fragments of other rocks that often have been worn down into small pieces, such as sand, silt, or clay.
- organic materials, or the remains of once-living organisms.
- chemical precipitates, which are materials that get left behind after the water evaporates from a solution.
Streams carry huge amounts of sediment (Figure below). The more energy the water has, the larger the particle it can carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles.
Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps. Avalanches drop large piles of sediment. Glaciers leave large piles of sediments, too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present. Darker sediments form when the environment is oxygen poor.
Sedimentary Rock Formation
Accumulated sediments harden into rock by lithification, as illustrated in the Figure below. Two important steps are needed for sediments to lithify.
Sedimentary Rock Formation
Accumulated sediments harden into rock by lithification, as illustrated in the Figure below. Two important steps are needed for sediments to lithify.
- Sediments are squeezed together by the weight of overlying sediments on top of them. This is called compaction.
- Fluids fill in the spaces between the loose particles of sediment and crystallize to create a rock by cementation.
When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the layers are preserved.
Metamorphic Rocks
Metamorphic rocks form when the minerals in an existing rock are changed by heat or pressure within the Earth. See Figure below for an example of a metamorphic rock.
Metamorphic Rocks
Metamorphic rocks form when the minerals in an existing rock are changed by heat or pressure within the Earth. See Figure below for an example of a metamorphic rock.
In the large outcrop of metamorphic rocks in Figure below, the rocks’ platy appearance is a result of the process metamorphism. Metamorphism is the addition of heat and/or pressure to existing rocks, which causes them to change physically and/or chemically so that they become a new rock. Metamorphic rocks may change so much that they may not resemble the original rock.
Metamorphism
Any type of rock – igneous, sedimentary, or metamorphic - can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of different chemical composition.
Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (Figure below). Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated.
Any type of rock – igneous, sedimentary, or metamorphic - can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of different chemical composition.
Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (Figure below). Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated.
The Rock Cycle
Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly; many take place below the Earth’s surface, so we may not even notice the changes. Although we may not see the changes, the physical and chemical properties of rocks are constantly changing in a natural, never-ending cycle called the rock cycle.
The concept of the rock cycle was first developed by James Hutton, an eighteenth century scientist often called the “Father of Geology” (shown in Figure below). Hutton recognized that geologic processes have “no [sign] of a beginning, and no prospect of an end.” The processes involved in the rock cycle often take place over millions of years. So on the scale of a human lifetime, rocks appear to be “rock solid” and unchanging, but in the longer term, change is always taking place.
Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly; many take place below the Earth’s surface, so we may not even notice the changes. Although we may not see the changes, the physical and chemical properties of rocks are constantly changing in a natural, never-ending cycle called the rock cycle.
The concept of the rock cycle was first developed by James Hutton, an eighteenth century scientist often called the “Father of Geology” (shown in Figure below). Hutton recognized that geologic processes have “no [sign] of a beginning, and no prospect of an end.” The processes involved in the rock cycle often take place over millions of years. So on the scale of a human lifetime, rocks appear to be “rock solid” and unchanging, but in the longer term, change is always taking place.
In the rock cycle, illustrated in Figure below, 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.
Processes of the Rock Cycle
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.
Crystallization
Magma cools either underground or on the surface and hardens into an igneous rock. As the magma cools, different crystals form at different temperatures, undergoing crystallization. For example, the mineral olivine crystallizes out of magma at much higher temperatures than quartz. The rate of cooling determines how much time the crystals will have to form. Slow cooling produces larger crystals.
Erosion and Sedimentation
Weathering wears rocks at the Earth’s surface down into smaller pieces. The small fragments are called sediments. Running water, ice, and gravity all transport these sediments from one place to another by erosion. During sedimentation, the sediments are laid down or deposited. In order to form a sedimentary rock, the accumulated sediment must become compacted and cemented together.
Metamorphism
When a rock is exposed to extreme heat and pressure within the Earth but does not melt, the rock becomes metamorphosed. Metamorphism may change the mineral composition and the texture of the rock. For that reason, a metamorphic rock may have a new mineral composition and/or texture.
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.
Crystallization
Magma cools either underground or on the surface and hardens into an igneous rock. As the magma cools, different crystals form at different temperatures, undergoing crystallization. For example, the mineral olivine crystallizes out of magma at much higher temperatures than quartz. The rate of cooling determines how much time the crystals will have to form. Slow cooling produces larger crystals.
Erosion and Sedimentation
Weathering wears rocks at the Earth’s surface down into smaller pieces. The small fragments are called sediments. Running water, ice, and gravity all transport these sediments from one place to another by erosion. During sedimentation, the sediments are laid down or deposited. In order to form a sedimentary rock, the accumulated sediment must become compacted and cemented together.
Metamorphism
When a rock is exposed to extreme heat and pressure within the Earth but does not melt, the rock becomes metamorphosed. Metamorphism may change the mineral composition and the texture of the rock. For that reason, a metamorphic rock may have a new mineral composition and/or texture.
Questions
- What are the three main types of rocks and how does each form?
- What is the visible difference between an intrusive and an extrusive igneous rock?
- How does the difference in the way intrusive and extrusive rocks form lead to the differences in how the rocks appear?
- If you see a sedimentary rock outcrop with layers of red sandstone on top of layers of tan sandstone, what do you know about the ages of the two layers?
- Describe the conditions that lead to foliated versus non-foliated metamorphic rocks.
- Describe how an igneous rock changes into a metamorphic rock.
- Describe how an igneous rock changes into a sedimentary rock.
- In which rock type do you think fossils, which are the remains of past living organisms, are most often found?