What are the Different Types of Rocks?

Michael Anissimov
Michael Anissimov

There are three types of rocks: igneous, metamorphic, and sedimentary, in order of abundance. Within these classifications are many hundreds of types. The first two are formed under conditions of extreme heat and pressure. Scientists now know enough about rocks to produce some artificially — for example ruby and diamond.

A piece of slate.
A piece of slate.

Igneous rocks are formed when magma cools into solid form. This can happen on the surface with volcanic discharge, but primarily takes place beneath the earth’s crust. Over 700 varieties of igneous rock have been described, some with crystals and some not. Igneous is derived from the Latin word for "fire," ignis. A majority — about 90% — of igneous rocks are silicate minerals, which are rich in silicon and oxygen. Quartz is among the most familiar and abundant of this type; clays and feldspar are other examples.

A piece of feldspar, an igneous rock.
A piece of feldspar, an igneous rock.

Metamorphic rocks are formed when a preexisting rock, called a protolith, is under conditions of high heat and pressure, causing it to metamorphose chemically, structurally, or both. The protolith might be an igneous, sedimentary, or another metamorphic rock. Slate, marble, and quartzite are some examples of this type. Most have a structural characteristic called foliation, which means that the rock is composed of many tiny compressed layers. The process of metamorphosis is usually accompanied by complex chemical reactions.

A piece of sandstone, a sedimentary rock.
A piece of sandstone, a sedimentary rock.

Sedimentary rocks are less abundant than the other two varieties, composing only 5% or so of the earth’s crust. They are formed in one of three ways: when bits of a larger rock chip off and settle to the ground, when the remains of plants or animals build up in quantity, or when a solution containing a mineral leaves deposits over time. They are named for their origin — sediment.

Limestone is an example of sediment rock.
Limestone is an example of sediment rock.

This type of rock is formed over long periods of time as tiny grains of material are pressed against each other and join loosely. The process by which sedimentary rocks are formed is delicate enough that fossils can be preserved within them. Common examples include sandstone, chalk, and limestone.

Quartz makes up over 10 percent of the planet's crust.
Quartz makes up over 10 percent of the planet's crust.
Pumice igneous rocks have a sponge-like appearance and a low density.
Pumice igneous rocks have a sponge-like appearance and a low density.
Michael Anissimov
Michael Anissimov

Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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Discussion Comments

anon335134

Are there any rocks that have value to them? If yes, let me see them to match what I have.

anon318527

Why isn't it reliable to date igneous rocks relatively?

healthy4life

I never could remember all the names of rocks I learned in school. However, marble is fairly easy to remember.

It seems that everybody these days has either marble countertops or marble floors in their home. Marble is pretty and durable, which makes it ideal for this.

I love the swirls of color. I see a lot of black and green marble, but I've also seen some more natural looking colors that resemble the actual rock more.

I have a feeling that the black and green has been altered in order to sell better. I've never seen a rock with those colors and that degree of shine.

cloudel

I remember studying all kinds of rocks and minerals in my geology classes, both in school and in college. I really loved learning how to identify the different ones, and I thought I might one day want a career as a geologist.

There is a lot more involved than what you learn in high school geology, though. Even mere rock identification can get tricky. You can't always tell just by appearance what a rock type is.

JackWhack

@lighth0se33 – Clay isn't a rock itself. It makes up part of the composition of many other kinds of rocks, though.

Just like sand particles compose sandstone rocks, bits of clay here and there get stuck in other rocks as they form. I think it's a matter of them being in the right place at the right time.

So, those chunks of clay that you pick up out of a riverbed are not technically rocks. They might one day compose a rock, if the conditions are right, though.

lighth0se33

I did not know that clay is considered a type of rock. It is so breakable and easily crumbled that I would not associate it with something known to be hard and nearly unbreakable.

calabama71

@dill1971: There are certain properties of minerals that held to identify them. Some of the characteristics to look for are: color, luster, crystal form, cleavage, hardness, and transparency.

There are several websites that you can go to that list all of the characteristics of rocks and minerals in detail. They have great pictures of different kinds of rocks.

PurpleSpark

@dill1971: It is not always an easy task to identify rocks and minerals. There are so many different kinds that it can take years of study to be able to accurately identify them. There are differences that can help distinguish the two.

One interesting fact is that all rocks are made of at least two minerals, but minerals are not at all made of rocks.

There are around 3000 known minerals on earth. There are field guides that you can use to help identify different rocks and minerals. The field guides have great pictures of rocks and minerals.

dill1971

How can you tell the difference between rocks and minerals?

sevenseas

Limestone, a type of sedimentary rock is found in many parts of the world including Mediterranean region, Adriatic coast and other.

The landscape with this type of rock is also called karst.

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