How Are Rocks Formed?
The Earth’s crust and upper mantle are collectively known as the lithosphere, from which the tectonic plates are composed. It is in the lithosphere that rocks are formed and reformed. And depending on the type of rock, the process through which they are created varies. In all, there are three types of rocks: igneous, sedimentary, and metamorphic. Each type of rock has a different origin.
Metamorphic Rocks – Metamorphic rocks are formed by great heat and pressure. They are generally found inside the Earth’s crust where there is enough heat and pressure to form the rocks. Metamorphic rocks are often made from other types of rock. For example, shale, a sedimentary rock, can be changed, or metamorphosed, into a metamorphic rock such as slate or gneiss. Other examples of metamorphic rocks include marble, anthracite, soapstone, and schist.
Igneous Rocks – Igneous rocks originate either from volcanic action as molten lava, which hardens, or from the slow cooling of molten masses beneath the earth’s surface, which are exposed after a volcanic eruption or after much erosion. Many important minerals, notably uranium, have been found enveloped in igneous rocks. Examples of igneous rocks include basalt and granite.
Sedimentary Rocks – Sedimentary rocks are formed from the sediment, or broken pieces of the earth’s rock structure worn down by weather and erosion. The fragmented pieces become compacted and in time much of it is cemented in to form rock. Generally, something like a stream or river will carry lots of small pieces of rocks and minerals to a larger body of water. These pieces will settle at the bottom and over a really long time (perhaps millions of years), they will form into solid rock. The chief sedimentary rocks are sandstone, shale, dolomite and limestone.
Interesting Facts about Rocks
The word “igneous” comes from the Latin word “ignis” which means “of fire.”
Ores are rocks that include minerals that have important elements such as metals like gold and silver.
Sedimentary rocks form layers at the bottoms of oceans and lakes.
Marble is a metamorphic rock formed when limestone is exposed to high heat and pressure within the Earth.
Layers of sedimentary rocks are called strata.
Rocks are constantly changing in what is called the rock cycle. It takes millions of years for rocks to change.
There are actually some rocks that come from space called meteorites. They may have different elements or mineral make up than a typical earth rock. Typically they are made up mostly of iron.