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Igneous
rocks are formed from molten magma or lava. The word, igneous
means "fire". All igneous rock starts deep in the earth as hot, molten
magma. If the magma cools and hardens inside the earth it is called
"intrusive" rock. These rocks cool slowly and have large crystals. When the
magma comes out of the earth's crust through a volcano, it is called
"extrusive". It cools off quickly, and the crystals that form are very
small. Molten, or hot, liquid rock is called MAGMA when it is still inside
the earth, but once it comes out through a volcano it is called LAVA. Much
of the earth's surface was formed by volcanic activity.
Every minute of every day, rocks are being worn down by wind and rain. Tiny grains of dirt, sand, mud and clay are worn off and washed into streams, rivers, lakes and oceans. This is called EROSION. When these tiny bits of sand and dirt settle to the bottom of the water, they are called sediment. Minerals in the water and microscopic, or very tiny sea animals also get mixed in with the dirt and sand to form the sediment. Every day more sediment piles on top of what is already there. After thousands and millions of years we end up with a really deep pile of sediment. The weight and pressure from all the stuff on top turns the sediment on the bottom into sedimentary rock!
Metamorphic rocks are formed when other kinds of rocks are changed by great heat and pressure inside the earth. The word "metamorphic" means changed. Think of metamorphic rocks as recycled rocks. When igneous, sedimentary or even metamorphic rocks get buried deep beneath the surface of the earth, over millions of years the heat and pressure inside the earth change them into something else. Limestone can be changed to marble, sandstone can be changed into quartzite, and shale can be changed to slate. It's just another example of how the earth is constantly changing!
The Rock Cycle is a group of changes. Igneous rock can change into sedimentary rock or into metamorphic rock. Sedimentary rock can change into metamorphic rock or into igneous rock. Metamorphic rock can change into igneous or sedimentary rock. The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock. The rock cycle never stops.
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