The Three Rock Types

Even though there’s much more to geology than the study of rocks, rocks do play a major role in the field.

In the following video, we’ll learn a bit about the three different types of rocks.

Transcript

Rock Formations and Rock Types – Hilary Olson – The University of Texas at Austin

Most people who have gone through a K through 12 education have some memory of the three classifications that geologists use to describe rocks: Igneous, Metamorphic and Sedimentary.

Petroleum geologists are most interested in sedimentary rocks, but anyone working in the subsurface should also understand the other two types because they encounter them from time to time.

Igneous rocks are formed when magma comes up from the mantle and cools into a solid. These are rocks like granite and basalt. The reasons that magma doesn’t always form the same kind of rock have to do with its chemical composition and cooling history.

Some rocks, like granite, cool slowly underground and form large crystals. These are called ‘intrusive’ igneous rocks. When a volcano erupts, the lava forms an ‘extrusive’ igneous rock – an example would be a basalt. As these hot rocks are exposed to air they cool quickly, which doesn’t give them enough time to form large or well-organized crystals.

Igneous rocks are never hydrocarbon source rocks because they don’t have organic matter in them during formation, but they do sometimes act as reservoir rocks for migrating hydrocarbons.

Metamorphic rocks are any existing rock types that were subjected to great pressures and heat – substantially changing them, but short of melting them. The process of metamorphism is a long one, and it should be noted that a single parent rock can be metamorphosed into several different named rock types depending on how long, and to what degree, it undergoes the process.

Shale, for example, which is sedimentary rock, under the right conditions moves through stages of metamorphism to become slate, phyllite and eventually schist.

Lastly, we come to sedimentary rocks. This is where the action is for petroleum geologists. One type of sedimentary rock can be made of small particles which accumulate after transport by wind, water or ice.

The particles could be mineral grains, remains of animals or plants or other rock fragments. For example rivers and streams scrape away sand and soil particles as they flow, and aquatic plants and animals live and die in the water, dropping organic matter to the bottom, to be incorporated into the formation of a sandstone, or if the particles are very small – a shale.

Another type of sedimentary rock may form from the precipitation or extraction of dissolved substances over time. For example, through cyclical exposure of saturated water to air, salt may precipitate out of solution to form a special kind of sedimentary rock called an evaporite.

Minerals such as calcium carbonate can be extracted from seawater by organisms to produce microscopic skeletons, eventually forming a rock called limestone.

All these sediments have to end up somewhere. Those ‘somewheres’ are called depositional environments, and they might be part of a large desert, on the banks of slow-moving rivers or quite often they are on the bottoms and shores of lakes, seas and oceans.

Over time they get compacted by overlying sediments and cemented together by minerals such as quartz or calcite carried in the water flowing through the sediments. Once they’re cemented, that’s when they’re called sedimentary rocks.

If they don’t get cemented together, they are called ‘unconsolidated’ sediments. You’ll hear people talk about ‘unconsolidated sands.’ That is just sand that hasn’t been cemented into a rock. Examples of common sedimentary rocks include shale, sandstone and limestone. Sedimentary rocks make up all of the hydrocarbon source rocks in the world, and most of the reservoir rocks too.

If you were able to watch the Earth for millions of years, you could see all of these rock types transforming from one to another.

Molten magma might erupt out of a volcano and cool into an ‘igneous rock’ like rhyolite. Later, that rhyolite gets worn away by a river that carries it as tiny grains all the way down the mountain and deposits those grains in an ocean basin.

When the grains cement together, they become a ‘sedimentary rock.’ If that sedimentary rock is buried deeply enough and subjected to enough heat and pressure, it might turn into a metamorphic rock like a quartzite.

Then, if that metamorphic rock were to enter a subduction zone and get pushed down far enough into the mantle, it might melt completely back into magma and start the rock cycle over again.

In summary, there are three types of rocks:

Igneous

Igneous rocks are rocks that have solidified from a molten state. Some igneous rocks form when magma erupts at the earth’s surface and solidifies. Others are injected deep underground and solidify over thousands or millions of years.

Metamorphic

Metamorphic rocks form when igneous rocks, sedimentary rocks, or even other existing metamorphic rocks are chemically altered under high pressure or temperature conditions. During the process of metamorphism, the rock remains mostly solid, but gradually recrystallizes to a new mineral composition or rock texture (for example, size, shape, appearance, arrangement and sorting of grains, particles or minerals).

Sedimentary

Sedimentary rocks are formed when non-molten materials are deposited on the Earth’s surface in a particular depositional environment and squeezed together to form solid rock, a process known as lithification.

Petroleum geologists are most interested in sedimentary rocks because these rocks contain organic matter that, under the right conditions, can form oil and gas.

Sedimentary rocks also make up most reservoirs if the hydrocarbons have migrated after being produced.

Types of Sedimentary Rocks

When we break down the earth’s crust by rock type, we find that only about 8% of the crust is composed of sedimentary rocks. Despite this relatively low percentage, the volume of all the world’s sedimentary rocks is still enormous.

Sedimentary rocks are further broken down into different types.

We mentioned before that most sedimentary rocks are made up of clasts that have been cemented together. These grainy sedimentary rocks are described as clastic.

Clastic sedimentary rocks are typically named by grain size. As you can see below, grain sizes range from microscopic to pebble-sized and larger.

Fine Grained

Fine-grained rocks are shales, mudstones, siltstones, and claystones. These rocks are made up of tiny clay particles and other minerals. Grain sizes range from 0.001 to 0.06 mm. These grains are so small that they can only be seen using a microscope.

Fine-grained sedimentary rocks are important to petroleum geologists for two reasons. First, they are considered potential source rocks, rocks capable of generating oil and gas under the right conditions. Second, they are often impermeable, which means they are capable of trapping oil and gas beneath them.

Shale is one of the most important fine-grained sedimentary rocks for the formation of oil and gas.

Medium Grained

Medium-sized clasts, ranging from 0.06 to 2 mm, form sandstones. These grains can be felt and seen.

Why are sandstones important? Well, unlike fine-grained rocks, sandstones can store water, oil, and gas within their pore spaces.

Large Grained

Rocks made up of larger grains such as pebbles and even boulders are called conglomerates. Conglomerates are rare compared to fine-grained sedimentary rocks.

Chemical

Unlike clastic sedimentary rocks, which are made up of grains cemented together, chemical sedimentary rocks are named based on their composition.

A limestone consists primarily of calcium carbonate deposited by marine organisms. Grain size varies widely. Limestones can both store and impede the movement of oil and gas.

Salt Beds

A salt bed is literally a layer of salt that has precipitated and crystallized as the salty water containing it has dried up.  Salt beds exhibit very low permeability, meaning that oil and gas often become trapped beneath them.

Images: “Volcanic Eruption in Holuhraun Iceland” by GISBA via iStock; “Granite Stone” by Joe_Potato via iStock; “Isolated Sample of Gneiss” by frenchmen77 via iStock; “Sedimentary Rock” by Rinelle via iStock; “Shale Rock Cross-Section” by CedarWings via iStock; “Sunny Day at Rock Formation The Wave in Arizona, USA” by sara_winter via iStock; “Conglomerate of Sedimentary Deposit Plus Lichens” by Pi-Lens via iStock; “White Cliffs of Dover in Kent, England” by stockcam via iStock; “Deu Atom Asse” by Joerg Sarbach via Associated Press