Geologists classify rocks based on how they are formed and the elements they contain. There are three main categories of rocks: igneous, sedimentary and metamorphic.Continue Reading
Each of the three main groups may be broken down into several categories, depending on a variety of factors.
Igneous rocks are formed when volcanic magma cools and crystallizes into rock. When magma cools beneath the Earth's surface, the resulting rocks are considered plutonic, while rocks that form when lava cools above the Earth's crust is volcanic rock.
Sedimentary rocks are developed from materials that were deposited on the Earth's surface. Although the materials are deposited on the surface, the resulting rocks can be formed either directly on the surface or deep within the planet. Most sedimentary rocks are formed over millions of years under the surface, due to the high pressure underneath the earths crust.
Metamorphic rocks are formed when either igneous or sedimentary rocks undergo a chemical transformation deep within the earth. This transformation occurs as a result of heat and pressure, which chemically alters the original rock, known as the protolith, into a new form. In some cases, the protolith can be another metamorphic rock, but at some point it came from either an igneous or sedimentary rock.Learn more about Geology
Stratigraphic columns are illustrations made by geologists to represent the relative composition, thickness and scope of layers of rock, such as in a canyon, a volcano or a cliff. Some drawings are black and white with labeled layers, while others have colors that more closely resemble layers of rock described in the column. Each layer is distinct, and geologists label the general composition of each portion.Full Answer >
Rocks are important because geologists use evidence from them to learn about what the Earth was like in the past. They allow scientists to build a historical record of the planet to learn what events occurred before people lived.Full Answer >
More than any other type of fossil, index fossils define geological time periods for geologists by providing the most useful information about the age of the rock they are found in. Index fossils are also called key fossils or type fossils.Full Answer >
As of 2014, geologists contend that granite is a plutonic rock resulting from deeply buried shale and sandstone that melted, partially melted or metamorphosed, depending on the category of granite in question. Igneous granite forcefully ripped through the Earth's crust, while metamorphic has no evidence of fluidity.Full Answer >