Types of rocks and the rock cycle
Rocks are the solid material made out of minerals that make up the interior of the Earth. The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed.
- Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of (once organic) material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock due to pressure and other processes. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone.
- Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include marble.
- Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). Igneous rocks that form outside the Earth are called extrusive while those formed inside the Earth (i.e. from cooling magma) are intrusive. When lava cools very quickly, no crystals form and the rock looks shiny like glass. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian. Igneous rocks that are low in silicates are mafic, and igneous rocks high in silicates are felsic. Lodestone is an example of a magnetic igneous rock.
The Rock Cycle
Just like there is a water cycle, there is also a much slower rock cycle:
- Igneous rocks are erupted outside the Earth and carried to the sea by rivers.
- The igneous rocks are eroded and layered by type, into sedimentary rocks.
- The sedimentary rocks are exposed to heat and pressurized, turning them into metamorphic rocks.
- The metamorphic rocks melt into magma and again become igneous rocks.