The three basic types of rock
All rocks can be put into one of three (3) fundamentally different types. This should always be the first determination attempted whenever a new rock is encountered. In most cases, it’s not all that complicated:
Igneous Rocks
Igneous rocks are crystalline solids that form directly from the cooling of magma. This is an “exothermic” process (it loses heat) and involves a phase change from the liquid to the solid phase.
The earth’s crust is made of igneous rock — at least at the surface where our planet is exposed to the bitter coldness of space. Igneous rocks are assigned names based upon two things: composition (what they are made of) and texture (how big the crystals are).
Click here for more on igneous rock composition and texture.
Click here for more on elements and minerals common in igneous rocks.
Click here for more on magma and igneous rocks.
Click here for more on plate tectonics and the formation of magma.
Click here for a chart summarizing the main divisions of igneous rocks.
Click here for more on basalt and granite.
Sedimentary Rocks
At most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment. The rocks that are made as layers of this debris get compacted and cemented together are called sedimentary rocks. There are three (3) main types of sedimentary rocks:
Clastic: Your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken rock which have piled up and been “lithified” by compaction and cementation. Conglomerate, sandstone, and shale are common examples.
Chemical: Most chemical sediments form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal “playa lakes” occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time. Other common chemical sedimentary rocks include limestone and chert.
Organic: Any accumulation of sedimentary debris caused by organic processes. Coal, limestone, and chert are common examples. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into thick layers that form “organic” sedimentary rocks.
Click here for more on sedimentary processes and rocks.
Metamorphic Rocks
The metamorphics get their name from “meta” (change), “morph” (form), and “ick,” which relates to how difficult they can be to study and understand. Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions.
In many cases this involves burial that leads to a rise in temperature and pressure, but in GeoMan’s version a reduction in heat and pressure can also result in a true metamorphic change.
But whether a rise or fall in temperature and/or pressure, the metamorphic changes in the minerals ALWAYS move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble.
