There are only three basic types of rock found on Earth. The rocks are classified based on how they form. Even though rocks look like they will last forever, rocks change over time. The rock cycle explains how rocks change.
Igneous rock is formed when magma cools. Magma is liquid rock. It exists under the crust of the Earth in the mantle and within volcanoes. When magma cools under the surface of the Earth it forms intrusive igneous rock.
When magma cools above the surface of the Earth it forms extrusive igneous rock. Extrusive igneous rock cools much faster than intrusive igneous rock. Since it cools so quickly, extrusive igneous rock can have air bubbles in it. Crystals found within extrusive rock are very small because they haven’t had much time to form. Examples of extrusive igneous rock include basalt and obsidian.
Intrusive igneous rocks take much longer to cool than extrusive igneous rocks. As a result, the crystals found in intrusive igneous rock are much larger than those found in extrusive igneous rock. Granite is an example of intrusive igneous rock.
When a rock is sitting on the surface of the Earth is it exposed to water and wind. Water and wind can weather and erode rock. Weathering means breaking rock down into smaller parts. Eroding means moving these small pieces from one location to another. So, the rock is broken down into smaller pieces and those pieces are carried away. Many interesting features of rocks are created by weathering and erosion.
The small pieces of rock eventually settle on the ground. These small pieces of rock are called sediments. Sand is an example of a sediment. Sediments pile on top of each other in layers. Eventually, so many sediments are piled on top of each other that the bottom layers get squeezed together and become sedimentary rock. Sedimentary rock is layered with the oldest parts of the rock at the bottom and the newest rock at the top. Fossils are only found in sedimentary rock. An example of sedimentary rock is sandstone.
As sediments settle on the surface of the Earth, rocks below the surface are pushed deeper and deeper. Deep under the surface of the Earth, there is great heat and pressure. This high heat and pressure form the last type of rock, metamorphic rock.
There are two types of metamorphic rock. The first type is called foliated metamorphic rock. This rock is formed deep underground where the high pressure is unequal. That means one direction has greater pressure than the others. An example of unequal pressure would be putting all of your weight on a book. The top of the book is under much more pressure than the sides of the book. When this happens to rock, the minerals that make up the rock align perpendicularly to the direction of the pressure. This will make the rock looked striped. An example of foliated metamorphic rock is slate.
When the pressure around the metamorphic rock is equal, nonfoliated metamorphic rock forms. This rock is not striped. Marble is an example of nonfoliated metamorphic rock.
Each of the rocks can be transformed into the other kinds of rock. When any kind of rock melts, it becomes magma. Magma can only become igneous rock. Similarly, when any kind of rock is weathered and eroded, it becomes sediment. Sediments can only become sedimentary rock. Finally, when any rock is forced deep underground the heat and pressure will turn it into metamorphic rock.
Rocks can change quickly or very slowly. For example, a volcano can erupt and form igneous rock very quickly. On the other hand, it may take millions of years for sedimentary rock in the middle of a continent to either become sediments or get pushed deep enough under the surface of the Earth to become metamorphic rock.
The rock at the bottom of the ocean is much younger than the rock that makes up the continents. This is because the rock on the bottom of the ocean is continually recycled. Rock is forced into the mantle and eventually returns to the surface as igneous rock. New rock is formed at a rate of between two and six inches a year. That is a little faster than your fingernails grow.
The rocks of the Earth are changing all of the time, but these changes are often too slow for us to notice.