READ: The Rock Cycle
READ: The Rock Cycle
Will this rock be there forever?
Rocks may seem permanent, but they're not. Over time a rock will change into another type of rock. How this happens is known as the rock cycle. There are three main types of rocks. There are several processes that can change one type into another. First we need to learn what the rocks types and processes are.
What is a Rock?
A rock is a naturally formed, non-living Earth material. Rocks are made of minerals. A mineral is a solid material that forms by a natural process. A mineral can be made of an element or a compound. It has a specific chemical composition.
The minerals that form rocks may be so tiny that you can only see them with a microscope. The minerals may be really large. A rock may be made of only one type of mineral. More often rocks are made of a mixture of different minerals. A few types of rocks are made from materials that are not minerals. For example, coal is organic so it is not a mineral, nor is it made of minerals. Yet coal is a rock.(a) Granite has large crystals because it cools slowly. (b) Basalt has very small crystals because it cools quickly. (c) Coal is made up of organic material. (d) Jade is made of the mineral jadeite.
Rock Types
Geologists group rocks based on how they form. There are three major rock types. Each will be described in more detail in the coming lessons.
- Igneous rocks form when magma cools below Earth’s surface or lava cools at the surface.
- Sedimentary rocks form when sediments are compacted and cemented together.Sediments are pieces of rock. They may be gravel, sand, silt, or clay. Some sedimentary rocks form the solid minerals left behind after a liquid evaporates.
- Metamorphic rocks form when an existing rock is changed by heat or pressure. The minerals in the rock change but do not melt. The rock experiences these changes within the Earth.
Rocks can change from one type to another. The rock cycle describes how this happens.
The Rock Cycle
The rock cycle, illustrated in the figure below, depicts how the three major rock types – igneous, sedimentary, and metamorphic - convert from one to another. Arrows connecting the rock types represent the processes that accomplish these changes.
Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly. 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. Even at the surface, we may not notice the changes. The rock cycle has no beginning or end.
This video discusses how to identify igneous rocks:
This video discusses how to identify a metamorphic rocks:
The 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.
Summary
- The three main rock types are igneous, metamorphic and sedimentary.
- The three processes that change one rock to another are crystallization, metamorphism, and erosion and sedimentation.
- Any rock can transform into any other rock by passing through one or more of these processes. This creates the rock cycle.
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