The three types of rocks

It’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 then 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 based off of organic remains (such as limestone), or just form from the cementing of other rocks.

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.

The key element for igneous rocks is magma; magma is a mixture of molten or semi-molten rock, as well as some gases and other volatile elements. The deeper you go underground, the hotter it gets, and if you go deep enough, you reach the mantle – which can be thought of as a huge layer of magma surrounding the Earth’s core.

As you probably know, when magma cools, it turns into rock; if it cools in the underground, at higher temperatures (but still lower than that of the magma), the cooling process will take slowly, and crystals will have time to develop. That’s why you see rocks such as granite with big crystals – because 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, then you get what is called a volcanic rock – not really igneous, but also originating from lava. The classical example here is basalt – 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 equal though – different magmas can have different chemical compositions, different quantities of gases and different temperatures; different magmas -> different rocks. That’s why you get this incredible variety. There are over 700 hundred types of igneous rocks, and they are generally the hardest and heaviest (densest) 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”. This happens 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

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 termic metamorphism); the 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 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 that they often become elongated and the original features are destroyed. 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 both from the initial rocks, and 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, and they change into other crystals).

Marble is a non-foliated metamorphic rock.

They can be divided into many categories, but they are typically split into:

  • Foliated metamorphic rocks; where pressure squeezes or elongates the crystals and they have a clear preferential allignment.
  • Non-foliated metamorphic rocks; where the crystals have no preferential allignment. Some rocks, such as limestone are made of minerals that simply don’t elongate, no matter how much stress you apply.

Metamorphic rocks can form in different conditions, in different temperatures (up to 200 °C) and pressures (up to 1500 bars). Simply by being buried deep enough for a long enough time, a rock will become metamorphic. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion; they are also formed when the rock is heated up by the intrusion of hot molten rock called magma from the Earth’s interior.

The most common metamorphic rocks are:

  • amphibolite
  • schist (blueschist, greenschist, micaschist, etc)

    MINOLTA DIGITAL CAMERA

    A micaschist. The dark brown rounded minerals are garnet, and everything you see with a whiteish tint is the mica. The reddist areas are rusty mica. Image modified from Willowleaf Minerals.

  • eclogite
  • gneiss
  • hornfels
  • marble
  • migmatite
  • phyllite
  • quartzite
  • serpentinite
  • slate

Sedimentary Rocks

Sedimentary rocks are called that because they were once sediment. Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently naturally transported (or not). Sedimentary rocks from through deposition of material at the Earth’s surface and within bodies of water.

A conglomerate – a rock made from cemented gravel. Image via Earth Physics Teaching.

Sedimentary rocks are quite difficult to classify, as you can look at the situation from many different angles (the chemical make-up, the sedimentation process, organic/anorganic material), but the most common classification is the following:

  • clastic sedimentary rocks; basically small rock fragments (many silicates) transported and deposited by fluids (water, bed flows). These rocks are further classified on the size and composition of the clastic crystals included in the sedimentary rocks (most often quartz, feldspar, mica and clay).
  • conglomerates (and breccias); conglomerates are dominantly composed of rounded gravel, while breccias are composed of angular (sharper) gravel.
  • sandstones; as the name says it, it’s a rock made from many-sand-sized minerals and rock grains. The most dominant mineral in sandstones is quartz, because it is the most common mineral in the Earth’s surface crust.

    An old, red sandstone. Image via Ian Hopkinson.

  • mudrocks; again, the name says it all – rocks made from solidified mud. They typically contain very fine particles and are transported as suspended particles by turbulent flow in water or air, depositing once the flow settles.
  • biochemical rocks; you’ll probably be surprised to find out that most limestone on the face of the Earth comes from biological sources. In other words, most limestone you see today comes from the skeletons of organisms such as corals, mollusks, and foraminifera. Coal is another example of biochemical rock.
  • chemical rocks; these rocks include gypsum and salt (halite) – formed mostly through water evaporation

Yes, salt is a mineral – and it can be quite beautiful. The rock is called halite in this context and it’s a sedimentary rock.

There are also other type of specific sedimentary rocks, like for example the ones formed in hot springs. Most of the solid surface of our planet (very roughly 70%) is represented by sedimentary rocks – but if you start to go deep enough, they are replaced by igneous and metamorphic.

As I mentioned with the biochemical rocks, fossils can become rocks in time. You can actually have entire mountains made up from reefs, like you can see below.

This entire mountain in Romania was formed based on a coral reef. Image via MP Interactiv

Some common sedimentary rocks are:

  • argilite
  • breccia
  • chalk
  • chert
  • claystone
  • coal
  • conglomerate
  • dolomite
  • limestone
  • gypsum
  • greywacke
  • mudstone
  • shale
  • siltstone
  • turbidite

This is just scratching the surface when it comes to rocks – you could spend a lifetime studying them and still be surprised. But I hope that for your general knowledge or if you want to impress some friends (or if you’re considering starting geology), the information here was useful and interesting to you. Feel free to send any questions and comments my way and I’ll do my best to answer them!

Enjoyed this article? Join 40,000+ subscribers to the ZME Science newsletter. Subscribe now!

Like us on Facebook