By Robert Hazen, Ph.D., George Mason University
Geologists recognize three major groups of rock, and they differ by the processes by which they’re formed. These are the igneous rocks, formed from molten material; sedimentary rocks, formed in layers; and finally, metamorphic rocks, formed by the transforming influence of temperature and pressure.
The igneous rocks include all the rocks that form from a hot, molten state. Geologists make a further subdivision, between volcanic rocks—those are rocks that solidify on the surface—and intrusive rocks that solidify underground.
There’s really no difference, compositionally, between these two; it’s just that one forms on the surface and the other forms underneath the surface.
The igneous rocks cool very quickly and are likely to be very fine-grained. For example, basalt from Hawaii is a very fine-grained rock.
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Because of all the gas that’s contained in the rock, we often see lots of little pockets; air pockets and channels where the gas has expanded, puffing out the rock and making it porous. But if a rock cools very quickly, we might even get a volcanic glass, like the vitreous, black, glass material called obsidian.
So important was obsidian, because of its sharp edge when it breaks, that it was used by Native Americans for most efficient tools, for weapons. Indeed, all over the North American continent, one can find various pieces of obsidian spear points and other sorts of tools that were made out of the very distinctive obsidian that comes from Yellowstone Park.
This indicates that there must have been a vast Native American trading route in obsidian, because of the value of this volcanic material.
Learn more about the rigidity of rocks.
Intrusive rocks, as mentioned before, have basically the same composition as basalts and obsidian; but they cool much slower, and so they have much larger crystals.
If one looks at a Pikes Peak granite—or any granite for that matter—one will see individual crystals. This implies that the rock cooled slowly enough for those crystals to form.
The second great class of rocks are called sedimentary rocks, and these are rocks that are deposited in layers. They can be either deposited as layers of particles—as in layers of sand, or layers of silt and mud—or they can be chemical precipitates.
Some sedimentary rocks, for example, are salt deposits, layers of salt that evaporated out, or maybe layers of calcium carbonate, called limestone.
Limestones are formed from the gradual build-up of calcium carbonate, sometimes just by dead animals—shellfish falling down and forming layers of shells in the bottom of the ocean—and sometimes by the gradual build-up from chemical precipitation of calcium carbonate.
Sandstones are sedimentary rocks formed from sand. They can accumulate at ocean margins, at beaches, and even in deserts. Shales are formed from mud and silt, layers of very fine particles; and very often one’ll find thick deposits of shale that come from old ocean basins, where thousands of feet of shale may have been deposited over millions of years.
A very common feature of sedimentary rocks is that they hold fossils, the evidence of past life. Needless to say, fossils provide invaluable information about the evolution and the development of life, over millions of years of Earth’s history.
An important idea in sedimentary rocks is called the law of superposition: it’s a fairly obvious idea that the younger layers are always deposited on top of the older layers.
This is very important, again, in understanding the evolution of life, because as one looks at layer after layer after layer, one can see that the lifeforms change; when lifeforms change, that’s evolution.
Third, we come to metamorphic rocks. Metamorphic rocks include all the rocks whose mineralogy is altered by temperature and pressure, sort of like a pressure cooker. Some rocks go through phase transitions, such as the transition of graphite to diamond under temperature and pressure; that occurs deep within the Earth.
Most other minerals, if they’re subjected to high enough temperature and pressure, too will transform and change their form.
As we take a sediment and bury it deeper and deeper, it’ll go through a series of transformations called metamorphism. There are wonderful sequences of metamorphic rocks, as we go from flat-lying sediments to more and more deeply buried materials that have then been uplifted and exposed.
One can see this in New England, for example. We can start with a nice mudstone or shale, and after it’s been baked and pressurized and cooked, it eventually turns into a slate; a hard slate that can be used, for example, for roofing materials or for flagstones in the garden.
Learn more about the rock cycle.
Micas and Schist
If we take a slate, and we squeeze it and heat it more, eventually those minerals will transform largely to micas, and we will form a schist. Some might have seen this in New England, or other parts of the country: the shiny mica surfaces reflecting the sunlight as one drives by outcrops of this metamorphic rock.
When we subject a schist to higher and higher temperatures and pressures, it eventually can transform to the layered gneiss, where one can still see some of those sparkling minerals; but this is a denser, harder rock of different minerals, higher-pressure and higher-temperature minerals.
Common Questions about Rocks and Their Classification
Igneous rocks include all the rocks that form from a hot, molten state. Geologists make a further subdivision, between volcanic rocks—those are rocks that solidify on the surface—and intrusive rocks that solidify underground.
Sedimentary rocks are rocks that are deposited in layers. They can be either deposited as layers of particles—as in layers of sand, or layers of silt and mud—or they can be chemical precipitates.
An important idea in sedimentary rocks is called the law of superposition: it’s a fairly obvious idea that the younger layers are always deposited on top of the older layers. This is very important in understanding the evolution of life, because as one looks at layer after layer, one can see that the lifeforms change; when lifeforms change, that’s evolution.