Believe it or not the Earth is always changing. Sometimes the evidence is easy to see, like when a volcano blows or an earthquake rips the Earth's crust apart. More often the evidence can only be seen and measured over thousands of years. The constant motion of the building blocks of Earth means they are always being recycled, changing from one type of rock to another, in what is called the rock cycle.
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From Molten to Igneous Rock
Rocks begin in liquid form underneath the Earth's crust in what is called the mantle. This molten rock solidifies as it reaches the crust and is pushed up to form mountains or islands. These rocks are called igneous rocks. Volcanic igneous rocks are formed from molten rock that cooled quickly at the Earth's surface and plutonic igneous rocks are formed from molten rock cooled slowly at Earth's surface. Rocks such as obsidian and granite are examples of volcanic and plutonic igneous rocks respectively.
Erosion Creates Sedimentary Rock
As wind, rain and sun break down mountains into smaller particles, these particles are transported to lower ground and eventually deposited as sediment. Sedimentary rock forms over thousands of years as layer upon layer of sediments are deposited and compacted. Sandstone is an example of a sedimentary rock.
Movement and Metamorphic Rock
When exposed to the changes in pressure, temperature or chemistry that result from mountain building or when the plates that make up the Earth's crust shift, igneous and sedimentary rocks can change form. They are then called metamorphic rocks. Marble, for example, is formed when the sedimentary rock limestone is put under immense heat and pressure.
The Earth's crust is made up of plates that shift and move as they float on the molten mantle below them. In some cases the edge of one plate is stuck underneath the edge of another plate. As the bottom plate is sucked under the plate above it, the rock it is made of once again becomes part of the mantle. This molten rock will be cooled and made into mountains or islands, continuing the rock cycle.
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