Types of Rocks
There are three different types of rocks: igneous, metamorphic, and sedimentary. Each of these types is related through the rock cycle. That said, they’re all unique. Rock types are actually classified by their separate formation processes. Just as well, how they form gives them their distinct properties.
Marble, forming a cave. Marble is a type of metamorphic rock.
Characteristics of Igneous Rocks
Igneous rocks are special in that they’re made by a process known as crystallization. They’re actually made out of a bunch of crystals! Intrusive igneous rocks tend to have large crystals. Oppositely, extrusive igneous rocks can have crystals so small, you can’t even see them. This gives the rock a glassy appearance.
How are Igneous Rocks Formed?
Igneous rocks form as hot magma cools down. This causes it to harden into solid rock. Yet, how fast it cools matters a lot. Cooling speed is what determines whether an igneous rock is intrusive or extrusive. Anytime that magma breaks through the earth’s surface (becoming lava), it will form extrusive rock. The relatively cold air on the surface rapidly cools the magma. Additionally, large crystals don’t have enough time to form in extrusive rock. It cools too fast! Oppositely, there are times when the magma in a plume never even reaches the earth’s surface. It pushes into the crust without breaking it. This creates volcanic landforms. Inside of them, you find intrusive igneous rock.
Igneous Rock Examples
Columns of basalt. Basalt is the rock that makes up oceanic crust.
Obsidian, an extrusive igneous rock. It almost looks like black glass.
Characteristics of Metamorphic Rocks
Metamorphic rocks are any rock that has been changed significantly by heat or pressure. They’re extremely common. Along with igneous rocks, they make up about 95% of the volume of the upper crust. Moreover, metamorphic rocks are classified into two types: foliated and non-foliated.
Foliated Metamorphic Rocks
Foliated metamorphic rocks are those which are ‘banded.’ This means they have flat bands of crystal in them. We call this effect ‘foliation.’ It gives the rock a distinct appearance, as you can see below.
Gneiss. Gneiss is a foliated metamorphic rock. If you look closely, you can see its color bands.
How are Metamorphic Rocks Formed?
Remember, metamorphic rocks are just changed sedimentary and igneous rocks. When either of these two types is heated or put under pressure, they can become metamorphic rock. We call this process metamorphism.
Usually, metamorphism takes a long time. Over thousands of years, pressure can deform rock layers deep within the earth. This converts them into metamorphic rock. Another way this can happen is through contact metamorphism. This is when magma rises through the earth and ‘bakes’ the rock it touches.
Characteristics of Sedimentary Rocks
Sedimentary rocks are a bit different from the two other kinds. For one, they’re only found on earth’s crust, where all of its sediment is. That’s because they’re made out of sediment. What’s more, this makeup gives them a grainy appearance. Some are literally made out of grains of sand.
Sandstone. You can see that this sedimentary rock has lots of rough grains.
As shown above, the sediment which makes up a sedimentary rock determines its type. Sand makes up sandstone, and mud makes up mudstone. This seems obvious. But, it’s not as clear for some kinds of sedimentary rocks. As a result, scientists have invented a classification system for them. You can read more about it here.
How are Sedimentary Rocks Formed
Finally, whatever sediment we’re looking at, we still have to ask: how do we turn small grains into hard rock? The answer: time and pressure.
Over long periods of time, sediment builds up in certain areas. For example, rocks might erode and have their sediment deposited at the bottom of the lake. As more and more sediment builds up, the grains at the bottom compact. If there’s enough pressure, they might turn into sedimentary rock. This process is cementation. It tends to produce rocks in large layers, called strata.
To clarify, cementation sounds a lot like metamorphism. But, it’s different in that it’s creating a rock, not changing one.