t’s the first thing you learn in a geology class — very briefly the three types of rocks are:
Igneous — they form from the cooling of magma deep inside the earth. They often have large crystals (you can see them with the naked eye).
Metamorphic — they are formed through the change (metamorphosis) of igneous and sedimentary rocks. They can form both underground and at the surface.
Sedimentary — they are formed through the solidification of sediment. They can be formed from organic remains (such as limestone), or from the cementing of other rocks.
Now the long story, which is much more interesting, is this:
Now, the long story, which is much more interesting, is this:
Igneous Rocks
Lava flow on Hawaii. Lava is the extrusive equivalent of magma. Image via Wiki Commons.
Magma is the heart of any igneous rock. Magma is composed of a mixture of molten or semi-molten rock, along with gases and other volatile elements. As you go deeper underground, the temperature rises; go further and you’ll eventually reach the Earth’s mantle — a huge layer of magma surrounding the Earth’s core.
As you probably know, when magma cools, it turns into rock; if it cools while still underground at high temperatures (but at temperatures still lower than that of the magma), the cooling process will be slow, giving crystals time to develop. That’s why you see rocks such as granite with big crystals — the magma had time to cool off. The crystals are also differentiated, as you can see below.
Note the white, almost rectangular feldspar crystals, the grey virtually shapeless quartz crystals, and the black crystals, which can be either black mica or amphibole. Image modified from Eastern Illinois University.Note the white, almost rectangular feldspar crystals, the grey virtually shapeless quartz crystals, and the black crystals, which can be either black mica or amphibole. Image modified from Eastern Illinois University.
However, if the magma erupts or is cooled rapidly, you instead get a volcanic rock –– not really igneous, but also originating from lava. The classical example here is basalt, which can have many small crystals or very few large ones. Volcanic rocks are also called extrusive igneous rocks, as opposed to intrusive igneous rocks. Some volcanic rocks (like obsidian) don’t have any crystals at all.
Basalt — note the almost complete lack of visible crystals. Now compare it to the granite. Image via Georgia State University.
Pumice.
Not all magma is made equally: different magmas can have different chemical compositions, different quantities of gases and different temperature — and different types of magma make different types of rocks. That’s why you get incredible variety. There are over 700 hundred types of igneous rocks, and they are generally the hardest and heaviest of all rocks. However, volcanic rocks can be incredibly lightweight –– pumice, for example, can even float, and was called by ancient sailors “the foam of the sea”. Pumice is created when a volcano violently erupts, creating pockets of air in the rock. The most common types of igneous rocks are:
andesite
basalt
dacite
dolerite (also called diabase)
gabbro
diorite
peridotite
nepheline
obsidian
scoria
tuff
volcanic bomb
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Metamorphic Rocks
Here, the name says it all. These are rocks that underwent a metamorphosis; they changed. They were either sedimentary or igneous (or even metamorphic), and they changed so much, that they are fundamentally different from the initial rock.
Different types of metamorphism. Image via Tankon Yvtar.
There are two types of metamorphism (change) that can cause this:
contact metamorphism (or thermal metamorphism) — rocks are so close to magma that they start to partially melt and change their properties. You can have recrystallization, fusing between crystals and a lot of other chemical reactions. Temperature is the main driver here.
regional metamorphism (or dynamic metamorphism) — this typically happens when rocks are deep underground and they are subjected to massive pressure — so much so that they often become elongated, destroying the original features. Pressure (often times with temperature) is the main driver here.
Folded foliation in a metamorphic rock from near Geirangerfjord, Norway. Image via Wiki Commons.
Metamorphic rocks can have crystals and minerals from the initial rocks as well as new minerals resulting from the metamorphosis process. However, some minerals are clear indicators of a metamorphic process. Among these, the most usual ones are garnet, chlorite, and kyanite.
Equally as significant are changes in the chemical environment that result in two metamorphic processes: mechanical dislocation (the rock or some minerals are physically altered) and chemical recrystallization (when the temperature and pressure changes, some crystals aren’t stable, causing them to change into other crystals).
