The Rock Cycle
The Earth’s rocks do not stay the same forever. They are continually changing because of processes such as weathering and large earth movements, and are gradually recycled over millions of years in a process called the rock cycle.
An Overview of Rocks
The three main types, or classes, of rock are:
- Sedimentary
- Metamorphic
- Igneous
The primary differences among them have to do with how they are formed.
Sedimentary Rocks
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment.
Gradually, sediment accumulates in layers (called strata) in a process called stratification.
The weight of the sediments on top squashes sediments at the bottom. This is called compaction. Water is squeezed out from between the pieces of rock and crystals of different salts form. These crystals form a sort of glue that sticks or cements the pieces of rock together in a process called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
Metamorphic Rocks
Metamorphic rocks are formed under the surface of the earth due to intense heat and pressure (squeezing) exerted on sedimentary or igneous rocks.
The rocks that result from these processes often have ribbon-like layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface.
Examples of this rock type include gneiss and marble.
Igneous Rocks
Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens.
Sometimes the magma cools inside the earth, resulting in the formation of intrusive igneous rocks. Other times it erupts onto the surface from volcanoes (in this case, it is called lava) and cools, forming extrusive igneous rocks.
The rate that igneous rock cools affects its texture.
If the magma cools quickly, small crystals form in the rock, forming rocks with smooth textures. This can happen if the magma erupts from a volcano. Obsidian and basalt are examples of this type of rock. They are extrusive igneous rocks.
If the magma cools slowly, large crystals form in the rock, forming rocks with coarse grains and texture. This can happen if the magma cools deep underground. Granite and gabbro are examples of this type of rock. They are intrusive igneous rocks.
The Rock Cycle: Changing Rocks
As we discuss the rock cycle, we will be doing so starting with igneous and sedimentary rocks at the surface of the earth, looking at the geologic process that wear them down, then discussing the processes that cause them to form and reform into different types of rocks.
Wearing Rocks Down
The first steps of the rock cycle actually involve the destruction of existing rocks on the surface of the earth. There are 3, simple processes that change existing sedimentary and exposed igneous rocks into sediment, which itself is the foundation for the rock cycle (as we will discuss it):
- Weathering
- Erosion
- Deposition
Weathering
Weathering is the process where rock on the surface of the earth (like that in mountains, plateaus, etc.) gets broken down into small pieces.
There are two main kinds of weathering:
- Chemical Weathering
- Mechanical Weathering
Chemical Weathering changes the chemical makeup of rocks on the surface of the earth.
The effects of rain on rock are a great example of this; rainwater is naturally slightly acidic because carbon dioxide from the air dissolves in it, forming carbonic acid. Minerals in rocks may react with the rainwater, causing the rock to be weathered.
Some types of rock are easily weathered by chemicals. For example, limestone and chalk are made of a mineral called calcium carbonate. When acidic rainwater falls on limestone or chalk, a chemical reaction happens. New soluble substances are formed in the reaction. These are washed away and the rock is weathered. Chemical weathering can make caves form and cliffs fall away.
When fossil fuels such as coal, oil and natural gas are burned, carbon dioxide and sulphur dioxide escape into the air. These dissolve in the water in the clouds and make the rainwater more acidic than normal. When this happens, we call the rain ‘acid rain’. Acid rain makes chemical weathering happen more quickly. Buildings and statues made from rock are damaged as a result. This is worse when the rock is limestone rather than granite. Acid rain also kills trees and fish.
Mechanical Weathering, on the other hand, occurs when rocks break down due to physical processes.
One example of this is ice wedging (also known as freeze-thaw weathering). In ice wedging, water seeps into the cracks of rocks because of rain, etc. It then freezes, expands and cracks the rock.
Temperature changes can also cause a rock to expand and contract, causing it to break over time.
Further examples of mechanical weathering include the effects of wind, rain and water, all of which wear away rock over time.
A subset of mechanical weathering (sometimes considered its own category of weathering) is biological weathering.
Types of biological weathering include weathering caused by plants and animals. For example, burrowing animals such as rabbits can dig into a crack in a rock, making it bigger and splitting the rock.
You may have seen weeds growing through cracks in the pavement. If you have gone for a walk in the countryside, you may even have seen bushes or trees growing from cracks in rocks or disused buildings. This is because plant roots can grow in cracks. As they grow bigger, the roots push open the cracks and make them wider and deeper. Eventually pieces of rock may fall away.
People can even cause biological weathering just by walking. Over time, paths in the countryside become damaged because of all the boots and shoes wearing them away.
Erosion: Carrying It All Away
A close friend of weathering is erosion.
Erosion is the movement of the broken pieces away from the site of weathering.
Weathering and erosion are often confused, so be careful when answering questions about them. For example, a basalt cliff may be weathered by freeze-thaw, a type of physical weathering. This means that pieces of the cliff may break away. Erosion happens when these pieces of rock fall away down the cliff.
Weathering and erosion in combination are responsible for some of the most amazing geological features on the surface of the earth. The Grand Canyon, below, is a perfect example of the ability of weathering and erosion to shape the landscape over long periods of time.
Deposition
Deposition occurs when eroded particles settle back down on the surface of the earth.
These particles usually settle in bodies of water or other low lying areas such as valleys or stream beds.
Creating Rocks: The Rock Cycle
After weathered and eroded sediments are deposited in low lying areas, they begin to be covered by other, newer layers of eroded sediment. The deposited sediments build up in layers, called strata. This process is called stratification.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. Water is squeezed out from between the pieces of rock and crystals of different salts form. The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
Over time, these processes make sedimentary rock.
Sedimentary rock is eventually pushed downward due to the pressure from the rocks above it (et al) and is eventually turned into metamorphic rocks
As metamorphic rock progresses downward, it eventually melts, becoming magma. As this magma cools – either inside the earth or as it is expelled in the form of lava – it becomes igneous rock.
Once igneous rock is exposed to the surface of the earth, the process of weathering, erosion and deposition begins again, starting the rock cycle anew.
An exceptional overview of this process can be found here and here
* It is important to note that, though we discuss the rock cycle as a one way process, this is not the case in nature. Sedimentary rock can become metamorphic, as can igneous. The processes are interchangeable.
Tags: Deposition, Erosion, Geology, Igneous Rock, Metamorphic Rock, Rock Cycle, Sedimentary Rock, Weathering
Posted in Earth Science
