Does it ever feel like geologists are speaking a different language?
With so many different terms to describe and categorize rocks, learning the lexicon of a geologist can be a bit daunting. To help learn some basic geo lingo, here’s a (far from comprehensive) guide of some of the most important classification and terms geologists employ.
THE BIG THREE
All rocks can be classified as either igneous, sedimentary or metamorphic.
- Igneous rocks are formed by the cooling and solidification of molten materials (magma or lava).
- Sedimentary rocks are formed by the deposition and subsequent solidification of material derived from preexisting rock.
- Metamorphic rocks are modified in the solid state (from preexisting igneous, sedimentary or even older metamorphic rocks) by some combination of heat, pressure and deformation
IGNEOUS
The term igneous is derived from from the Latin word ignis, which means fire. Igneous rocks are broadly classified as either intrusive (plutonic)—formed below the surface—or extrusive (volcanic)–formed at the Earth’s surface. Crystal size is a useful indicator as to whether an igneous rock is intrusive or extrusive. Phaneritic rocks have crystals large enough to be seen by the naked eye and imply an intrusive origin. Aphanitic rocks have crystals too small to be seen by the naked eye and imply an extrusive origin. In terms of chemical composition, a range between mafic to felsic is used to describe where a rock falls on the spectrum from primitive to evolved. Mafic rocks (like basalt) are more primitive and magnesium-rich, but silicon-poor. Felsic rocks (like granite) are more evolved and magnesium-poor, but silicon-rich.
SEDIMENTARY
Sedimentary rocks are evidence of Earth’s weathering and erosion at work, as they are recycled material derived from preexisting rock. Sedimentary rocks are either grouped as clastic or chemical. Clastic rocks (also called detrital) are composed of material deposited as physical particles by water, wind or ice. These rocks are subdivided according to the dominant size of their constituent grains. Those with primarily gravel-sized particles (>2 mm diameter) are termed conglomerates, then there are sandstones with sand-sized grains (2 mm-63 μm diameter) and finally mudstones composed of clay and silt (<63 μm). Chemical sedimentary rocks (like limestone) are precipitated either biochemically or inorganically from water supersaturated with mineral constituents in solution.
METAMORPHIC
Metamorphic rocks record at least two distinct geologic settings: their original emplacement/deposition and their recrystallization in the solid state. The original un-metamorphosed rock is termed a protolith. Metamorphic rocks are generally formed by either contact metamorphism—where magma is injected into the surrounding solid rock/country rock, increasing temperature but not pressure—or regional metamorphism—where great masses of rock over a large area often experience deep burial and are subject to different pressure/temperature conditions. Certain minerals will form under specific pressure and temperature conditions and are termed index minerals. There are a wide range of metamorphic classifications based on the temperatures and pressures that these index minerals record.
Emily Zawacki, Ph.D. 