Sedimentary rocks are classified in three groups, inorganic (clastic), chemical (crystalline), and organic (bioclastic). Inorganic or Clastic rocks are formed through compaction and cementation of fragments from running water merging with calm water. These rocks are classified according to their grain size. Beds or strata are the fragments that are deposited in horizontal layers. Chemicla or Crystalline rocks are formed through evaporation and precipitation. These rocks formwhen the minerals in water setle, and that water evaporates, leaving the rocks and materials behindThis is the process of monominerallic. Monominerallic means composed of one mineral. The last class is organic or bioclastic. Organic sedimentary rocks are formed from the remains of something that was living at one point then died.
In the articles that i read they all say sedimentary rocks are found covering most of Earth's crust. While they form they start out under the water. But once that water is evaporated, the rocks are left on the Earth's surface.
Sediments that make the sedimentary rocks are form by weathering and erosion. Once they become sediments they go through deposition and burial, then compaction and/or cementation, now you have sedimentary rocks.
Another form of rocks is Igneous. Igneous means fire formed. These rocks are formed from molten rock that hardens, or solidifies, after awhile. There are two type of igneous rocks, intrusive and extrusive. Intrusive igneous rocks have larger crystals and it cools slower under the surface. Extrusive igneous rocks cool faster because the magma hardens when it reaches the Earth's surface. Most of these rocks have small crystals.
Since igneous rocks are formed from lava or magma, they are found around volcanoes. If lava flowed into water, the rock could look very glassy. Which means the crystals don't have time to grow since the rock cooled very quickly. If the igneous rock is coarse, that means the crystals are large. If the rock is fine that means the crystals are small.
The last type of rock is a metamorphic rock. The word metamorphic means to change. These rocks form from excising rocks, like igneous, sedimentary and already existing metamorphic rocks. Extreme heat and/or pressure from underground makes these rocks form. Metamorphic rocks don't melt, they recrystallize, which means to form or cause to form to crystallize again. There are two types of metamorphism, regional and contact.