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If you’ve never thought about them before, you could easily think that all rocks are the same. By this point in the course, you’ll know that is not the case! Rocks are aggregates of minerals. A mineral is an inorganic, crystalline solid. This means that it has a regular, repeating series of molecules that ultimately determine its form. Quartz is an example of a very common mineral. Another is halite, commonly known as table salt! You can have very large quantities of a particular mineral, and even very large single crystals of minerals. Once you have more than one mineral together though, you’ve got a rock. There are three broad categories of rocks. They are Igneous, Sedimentary, and Metamorphic.
Igneous rocks start as molten material that cools and solidifies. If you’ve ever seen granite, it was once super heated liquid! When this liquid is beneath the surface of the earth, it’s called magma. Once it reaches the surface, perhaps as an eruption, it is called lava. Lava cools very quickly, since the temperature of the surface (let’s say 32 degrees Fahrenheit, or zero degrees Celsius) is significantly cooler than the lava, which can be more than 1000 degrees F! This rapid cooling is called quenching, and is how glass-like rocks, like obsidian, form. Bringing material as hot as lava to the surface is somewhat like sticking a red-hot iron into a pot of water. Igneous rocks can (and often do) cool beneath the surface of the earth, the molten material moving up from the mantle but never making it to the surface. Other times they extrude at the surface, either at mid-oceanic ridges or hotspots.
The igneous rocks in this picture form what is called a pahoehoe (pronounced pah-hoy-hoy) flow, or a smooth, undulating, ropey texture.
Sedimentary rocks are made up of grains that break off of other rocks through a process called weathering. When rocks are exposed to rain, wind, temperature changes, roots, and some chemicals, they can be broken down into their basic components. Physical weathering breaks off grains of rock, while chemical weathering breaks rocks down into more basic elemental components. Putting vinegar onto baking soda is a good example of this. These grains can come from other sedimentary rocks, from igneous rocks, or from metamorphic rocks. Grains are transported downstream, eventually settling in a basin, or low energy environment (for example a lake or an ocean). Over time, as more sediment is deposited, layers of sediment are buried deep enough to be lithified, or turned to stone. Because features of the environment are preserved in the sediments, we can tell something about where the rock was deposited. For example, a rock with leaf fossils preserved must have been formed in a slow, stagnant environment - one in which the leaves would not be disturbed or moved away. Sedimentary rocks are all about what remains. Black shales, organic rich sediments from which we extract oil and gas, form in deep marine settings, where there is very little energy and there is very little oxygen to decompose the organic material that gets deposited. More on that in a bit!
The sedimentary rocks in this picture have been tilted.
Metamorphic rocks are rocks that have been deformed. They have been heated or squashed or buried (which often means they are both heated and squashed), which can cause minerals in the rocks to recrystallize. Metamorphic rocks form in places where change is happening: they can form in mountain belts (collision zones) where rock is compressed or buried, along fault planes, along subduction zones, next to magma pockets as the neighboring rock is cooked, to name a few. Marble is metamorphosed limestone (a chemical sedimentary rock). Slate was once used as the backing of chalkboards. Any type of rock can be metamorphosed, including metamorphic rocks!
It’s hard to tell why a rock is considered metamorphic from a photo and out of context. In this picture, the rocks in the foreground were baked by a large igneous intrusion, and bleached by superheated water.
Each of these rock types relies on the others in some way.
Take a few minutes to watch this fun video that describes the three types of rock and how they are connected.
Geology Kitchen: The 3 Types of Rocks (8:35)
Activity: Make a concept map!
Think about the interaction between igneous, sedimentary, and metamorphic rocks, and spend a few minutes drawing a picture that draws attention to relationships between these rocks. After you’ve given it a go, check out the concept map below! What is different between your concept map and this one? Do these differences surprise you?
- Click here for solution.