The types of rocks on planet Earth: What to know about the 3 major groups

We highlight Earth's three main rock types, how they are formed and their common engineering uses.
Sade Agard
Magma, which is molten rock, is the source of igneous rocks
Magma, which is molten rock, is the source of igneous rocks

Vintei/ Wikicommons  

  • The rock cycle is a prime example of how Earth is truly dynamic. The three main rock types that compose the Earth's lithosphere undergo continual transformation into one another due to geologic processes.
  • Some rocks on Earth remain unchanged for millions or tens of millions of years. Others, close to the margins of the tectonic plates, change considerably faster.
  • All types of rocks have uses in a range of industries — including construction engineering.

If there's one fact we know about planet Earth, it's that it is dynamic. The rock cycle is a prime example of this- the three main rock types that compose the Earth's lithosphere undergo continual transformation into one another due to geologic processes.

It's true; some rocks on Earth remain unchanged for millions or tens of millions of years. We know this from the ancient rocks home to the many fossil discoveries that keep enlightening us about Earth's evolutionary past. Others, particularly those close to the margins of the tectonic plates that make up the Earth's lithosphere (the crust and upper mantle making up Earth's solid exterior), change considerably faster.

Still, the fact that the rocks can alter in any number of ways is one aspect of the rock cycle that sets it apart from other cycles.

Here, we will highlight the three main rock types, how they are formed and their common engineering uses if any.

How many types of rocks are there on Earth?

There are three main types of rock involved in the rock cycle: igneous, sedimentary, and metamorphic.

What are igneous rocks?

The types of rocks on planet Earth: What to know about the 3 major groups
Magma, which is molten rock, is the source of igneous rocks

Magma, which is molten rock, is the source of igneous rocks (from the Latin, ignis, meaning 'fire'). When igneous rocks are formed deep underground, they are called intrusive, or plutonic, igneous rocks. If they are formed on top of Earth’s crust, they are called extrusive, or volcanic, igneous rocks.

They are typically challenging to break since they are primarily crystalline (consisting of interlocking crystals).

Lava is the term used to describe magma that rises to the Earth's crust and erupts from volcanoes at the surface (i.e., it is no longer underground).  These molten materials eventually cool, harden, and may crystallize to form minerals. Igneous rocks are the solid byproducts of magma, lava, or volcanic activity.

Intrusive igneous rocks have a coarse texture and large mineral grains, formed over thousands or millions of years, cooling down slowly inside the Earth. In contrast, extrusive igneous rocks have very small grains and a relatively fine texture because the magma cools more quickly on the surface, giving crystals less time to form.

Importantly, igneous rocks are not only formed from mantle material. Any type of rock buried deep within the lithosphere can come in contact with magma, leading to the molten rock's composition changing into an igneous rock.

Types of igneous rocks

The types of rocks on planet Earth: What to know about the 3 major groups
Basalt is a type of igneous rock

Basalt, an extrusive igneous rock, comprises tightly packed crystals (about 1mm in size). Vesicles — gas bubbles that were caught as the lava cooled — are frequently found in this igneous rock. Small chunks of basalt are typically used as rail track 'ballast' which serves as strength for heavy loads and drainage.

When gas-rich magma froths up to create a sort of "mousse," pumice is created. The lava is so bubbly and glassy-looking that it floats on water. Have hard skin on the bottom of your feet? Well, you can use a rough pumice stone.

The types of rocks on planet Earth: What to know about the 3 major groups
Basalt is commercially used as rail track ballast

Granite is another igneous rock. It is an intrusive rock — created when viscous magma crystallizes and slowly cools before reaching the Earth's surface. It has been used in construction for thousands of years as a coarse-grained stone and is one of the world's strongest building materials.

What are sedimentary rocks?

The types of rocks on planet Earth: What to know about the 3 major groups
'Strata' of sedimentary rocks (sandstones and mudstones) at Bryce Canyon, Utah

All types of rocks are eroded by the natural forces of Earth; common examples of forces of weathering include wind, rain, sunlight, and frost. When smaller pieces of a larger rock are chipped off, they are transported by wind, water, ice, or biological activity to a new location. This sediment settles and collects, and the lowest layers become compacted by the pressure of the layers on top that which form solid rock.

There are three different types of sedimentary rocks: clastic, organic (or biological), and chemical. Classic sedimentary rocks are formed from pieces of other rock called clasts. Organic sedimentary rocks are formed from materials like plants, shells, and bones.

Chemical sedimentary rocks begin when a chemical compound, such as calcium carbonate, is dissolved in solution with water. Over time, the water evaporates, leaving the hard compound behind. This process is constantly occurring as water weathers existing rock, dissolving the minerals it contains and transporting them elsewhere.

Fossils frequently exist inside the layers, known as "beds" or "strata," of sedimentary rocks.

Types of sedimentary rock

The types of rocks on planet Earth: What to know about the 3 major groups
Limestone packed with small ammonite fossils

Calcium carbonate, or CaCO3, is the main component of limestone - a chemical sedimentary rock; pour acid over it, and watch it fizz! Small, rounded ooliths (sand-size spheres of CaCO3) that form in tropical lagoons and shell fragments commonly compose limestones. 

Limestones are extensively used in agriculture for neutralizing soils as well as in construction for making cement, concrete, and steel. Limestone was incorporated in buildings such as the Empire State building, and was also used in ancient buildings - fine limestone blocks once formed the outer casing of the Great Pyramid at Giza —one of the Seven Wonders of the Ancient World.

Sandstone is another type of sedimentary rock and is, basically, compacted sand. There are many different forms of sandstone since it is created from sediment deposits by rivers, the sea, and the wind.

The types of rocks on planet Earth: What to know about the 3 major groups
Limestone is incorporated in the Empire State building, U.S.

A mudstone is formed when fine-grained clay particles are compacted. Clay that has settled out in still water, such as lakes, lagoons, or the deep sea, gives rise to mudstones. A flaky version of mudstone is known as shale, which, when crushed and heated with limestone, can be used to make cement for the construction industry.

Round pebbles can be compressed together to form a conglomerate. They are created from sediment deposited by rapidly moving rivers or waves on beaches deposit. Crushed versions of conglomerate rocks can be used to make aggregate for the production of concrete.

What are metamorphic rocks?

The types of rocks on planet Earth: What to know about the 3 major groups
Mountain building pulls deeply buried metamorphic rock toward the surface

Rocks are continuously dragged below the Earth's surface due to movements of the lithosphere. As you go deeper beneath the surface, there is a rise in temperature- around 25 degrees every kilometer.

As the heat and pressure increase, the chemical properties of the rock are changed (they are only stable under a specific range of temperatures and pressures), known as metamorphism.

This process will cause new minerals to grow, and in some rocks, with flat or elongated minerals, the pressure will squeeze and stretch minerals in patterns of lines or waves called foliation. In foliation, the minerals line up in layers perpendicular to the direction of pressure that is applied Rocks that are formed this way are known as foliated metamorphic rocks.

Metamorphic rocks may also be nonfoliated. There are formed the same way as foliated rocks, but they do not contain minerals that tend to line up under pressure and do not have a layered appearance.

Metamorphic rocks are only formed underground, which begs the question: why do we see metamorphic rock formations at the surface of the Earth?

This is due to mountain-building processes, or the abrupt weathering and erosion of underlying rocks, which can sometimes pull the deeply buried layers of metamorphic rock toward the surface. The process by which these rocks emerge explains why we can witness a range of rocks on the surface that date back millions of years.

Types of metamorphic rocks

The types of rocks on planet Earth: What to know about the 3 major groups
Crystals in schists (metamorphic) line up to give the rock wrinkly layers, known as foliation

Marble is actually limestone that has undergone metamorphosis. It has many uses in various industries, from construction to cosmetics, for example, it was used extensively to build the Taj Mahal.

Slate is typically formed after mudstone has been subjected to increased heat and pressure. The rock's ease of splitting at specific planes (lines) is a defining attribute of slate. This is due to its crystals' fine-grained interlocking structure, which lay flat in one direction. Some uses of slates are as gravestones and the tiles on houses.

The types of rocks on planet Earth: What to know about the 3 major groups
Marble (metamorphic) was used extensively to build the Taj Mahal

Gneiss, another metamorphic rock, develops from the sedimentary rock shale. Gneisses have a banded structure because of their interconnecting, coarse-grained crystals that line up in distinct pale and dark layers. These rocks are useful as a building material for flooring, decorative stones, and work surfaces.

Schist can form from many different types of rocks. Its shiny appearance is due to the mineral mica, whose crystals line up to give the rock wrinkly layers of foliation. As attractive as schists may be, they are rarely used as a building material due to their lack of strength.

A billion-year-old geological process, one common misconception

The rock cycle has generally been in motion for billions of years, and there isn't a particular point in time when it "begins" or "ends." A popular misconception is that Earth's rocks change from igneous to sedimentary to metamorphic (i.e., in that order). However, this is untrue. Any rock on the surface of the Earth can undergo a geologic transition into any other rock, in any direction.

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