"The State of The World" Number 37
Complete Index Of All Issues
Rocks Of Fire, Dust And Change
There are many varieties of rocks on Earth (and everywhere else in the universe; see When Space Rocks Collide With Earth), but all are listed in three general classifications according to their formation: igneous rocks, sedimentary rocks and metamorphic rocks.
Igneous (from a Latin word that means fire; "ignite" is from the same word) rocks are created by the cooling of molten magma (molten rock beneath the surface of the Earth; see also The Greatest Islands Of Earth) or lava (magma that flows out or is ejected through cracks or openings in the Earth's crust).
Examples of igneous rocks are granite, basalt, obsidian, pumice and diorite.
Sedimentary rocks are formed from material that was eroded by wind or water, deposited, and hardened by great pressure.
Examples of sedimentary rock are sandstone, limestone, clay, chalk, coal, dolomite, and shale.
Metamorphic (from a Greek word that means to change in form) rocks are formed from igneous or sedimentary rock that has been exposed to great pressure and / or heat.
Examples of metamorphic rock are marble, slate, gneiss, anthracite (a variety of coal), quartzite and schist.