How Are Rocks Formed? The Cycle of Nature’s Building Blocks

Rocks, the building blocks of our planet, hold vital clues to Earth’s history. These seemingly ordinary formations, composed of minerals and sediments, serve as geological archives.

Each rock and pebble holds the story of our planet’s evolution over millions of years.

But how are rocks formed? The processes behind rock formation are as intricate as they are intriguing, offering us a window into the dynamic forces that have shaped and continue to shape our world.

There are three main processes that lead to the formation of rocks: the cooling and solidification of magma, the deposition of sediment, and extreme temperatures and pressures.

Igneous rocks, sedimentary rocks, and metamorphic rocks all originate from these processes.

The Rock Cycle

The rock cycle is an idealized process whereby the three rock types continuously transform into each other.

Any type of rock can eventually become any other type of rock. These transformations can and do happen. The rock cycle diagram shows a very condensed version of this.

Theoretically, the same rock can form into three different types of rock. It can then become the rock it was originally.

• An igneous rock can convert into a metamorphic rock when put under immense heat and pressure.
• Metamorphic rock can break up and form sedimentary rock.
• A sedimentary rock can sink into the earth’s magma and turn into liquid rock, ready to be extruded somewhere else.

There are three types of rock that form on Earth today. These are:

Type of Rock	Process of Formation
Igneous	Formed by cooling and solidification of magma or lava
Sedimentary	Formed by deposition and compression of mineral or organic particles
Metamorphic	Formed by exposure to extreme temperature and pressure

We will look at each of these in more detail below.

Igneous Rocks

Igneous rocks are formed by molten rock cooling and solidifying. They are said to be “formed by fire.” Molten rock is also known as magma or lava. There are two types of igneous rock: extrusive and intrusive.

Intrusive igneous rocks

Intrusive igneous rocks form very slowly as hot magma cools under the Earth’s crust. The slow rate of cooling allows the crystals within to grow large. This causes the rocks to look coarse-grained.

Examples of these are:

• Diorite
• Granite
• Gabbro
• Peridotite

Below is an abstract micrograph of peridotite rock.

Extrusive igneous rocks

Extrusive igneous rocks form above the earth’s crust when magma escapes. You’ll find these rocks at volcanic sites. Extrusive rocks cool very quickly, so the crystals they form are very small. The rocks often trap bubbles of gas within them as they solidify so quickly.

Due to how quickly they form, extrusive igneous rocks have a smooth texture. Below is obsidian. As it is so smooth and glassy, it has been used to make knives and jewelry for thousands of years.

Some great examples are:

• Obsidian
• Pumice
• Rhyolite
• Tuff
• Basalt
• Dacite

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Sedimentary Rocks

Sedimentary rocks are formed when either pieces of rock or organic material are compressed together. Over time, the compressed material hardens into rock.

The material comes from eroded or dissolved minerals or organic matter that has been carried by weathering processes like wind and water.

There are three types of sedimentary rock:

• Clastic
• Organic
• Chemical

Clastic sedimentary rocks

These form from pieces of other rock, called “clasts.” They form mechanically from the accumulation of debris. A good example is sandstone.

Wind and water transport these pieces of rock or dust, moving them to a new location where they build up and are compressed into rock.

Examples of clastic sedimentary rocks are:

• Breccia
• Siltstone
• Shale

Organic sedimentary rocks

These are made from organic material derived from living organisms. These rocks offer a glimpse into ancient ecosystems.

Most importantly, they provide valuable information about the Earth’s past climate and environmental conditions. Here’s how organic sedimentary rocks are formed:

Organic Material Accumulation: The process begins with the accumulation of organic material. In aquatic environments, they can be dead plants, algae, and the remains of marine organisms like shells, corals, and plankton.

They may accumulate on the ocean floor, lake bottoms, or in swampy areas.

Burial and Compaction: Over time, the accumulating organic material becomes buried by additional sediment layers.

As more sediment accumulates on top, the weight of the overlying sediments causes compaction and squeezes out water and gases from the organic material.

This compaction reduces the volume of the organic matter and increases its density.

Chemical Transformation: As the organic material continues to be buried and compacted, it undergoes chemical transformations.

The pressure and heat from the overlying sediments cause the organic material to undergo diagenesis.

Diagenesis is a process where organic molecules are altered and reorganized. This transformation is a key step in the formation of organic sedimentary rocks.

Lithification: Over millions of years, the organic material may become lithified, meaning it turns into solid rock.

This process can result in the formation of two main types of organic sedimentary rocks: coal and chalk.

This chalk rock below contains the fossilized shells of Belemnites, an extinct sea creature.

Chemical sedimentary rocks

Chemical sedimentary rocks form from the precipitation (rain) of dissolved compounds and minerals. The dissolved material travels in water through the earth’s crust. It settles in a new location once the water evaporates.

Limestone is the most common chemically formed sedimentary rock. Chert and banded iron are also formed this way. These chert layers are from Rainbow Rock in Oregon, US.

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Metamorphic Rocks

Metamorphic rocks are formed when a rock is heated to a very high temperature or put under vast pressure.

This can happen at places like tectonic plate boundaries, where vast land masses force each other under or over one another. It can happen without heat when rocks are deeply buried under other rocks.

There are two classes of metamorphic rock:

• Foliated metamorphic rock
• Non-foliated metamorphic rock

Foliated metamorphic rock

These rocks contain minerals that have a long or flat shape. When under pressure, these minerals form layers. Foliation is when these long, flat minerals line up perpendicular to the direction the pressure is coming from.

These rocks often have colored bands running through them. You can tell the pressure was applied from above or below if the rock has horizontal lines running through it.

Non-foliated metamorphic rock

As these rocks don’t contain layer-forming minerals, they form in a different way than foliated rocks. Coal is an example of this. Bituminous coal turns into anthracite coal. Limestone turns into marble. Sandstone turns into quartzite.

Limestone creates a non-foliated metamorphic rock: marble. Marble will not form foliated flat crystals, no matter how much heat or pressure is applied.

There’s contact metamorphism too, which happens when hot molten rock pours inside another rock and “bakes” it from within. This changes the structure of the outer rock without any pressure being involved at all.

How is Coal Formed?

Coal is formed from long-dead plants. Millions of years ago, ancient plants such as cycads fell into a bog.

They sank and got compacted into layers of peat. After that, they were compressed further under immense pressure and heat.

The stages of coal creation are laid out here, from first to last:

• Dead Trees and Plants: Organic matter accumulates in wet environments like swamps, bogs, or marshes.
• Peat Formation: Accumulated organic material begins to decompose slowly but remains waterlogged, becoming peat.
• Lignite (Brown Coal): Peat is compressed and loses moisture, transforming into soft, brown lignite with a relatively high water content.
• Sub-Bituminous Coal: Increasing burial pressure and temperature further compact lignite into sub-bituminous coal, which is black and has improved energy density.
• Bituminous Coal: Intense heat and pressure turn sub-bituminous coal into harder, black bituminous coal, suitable for electricity generation and industrial processes.
• Anthracite: Under extreme heat and pressure, bituminous coal becomes anthracite, the hardest and most energy-dense type, used in specialized industries and residential heating.

We now know how harmful coal is to the environment and are switching to alternatives. Despite this, coal has played an essential part in human progress.

How is Chalk Formed?

Chalk is formed from the bodies of tiny marine animals called plankton. The plankton involved are known as coccoliths. The coccoliths extracted calcite out of the seawater to create their shells.

As they died, their bodies sank to the bottom of a vast inland sea. They were compressed into a sedimentary type of white limestone now known as chalk.

The seas eventually receded, and the chalk formed into soft, porous rocks. This is why the White Cliffs of Dover in the UK are so famous, as the whole of the South of the UK was once underwater.

Chalk is now used for drawing, to raise pH in agricultural soils, and in paints.

How are Fossils Formed?

Fossils are formed when sediment turns into rock. Biological matter from animals or plants is trapped in layers of sediment.

Shells and bones last long enough to turn into rock along with the surrounding substrate.

Soft parts are not often preserved, but sometimes we can see their outline in the rock. Fossils are only brought to light once layers of sedimentary rock are eroded or cracked open.

How is Marble Formed?

Marble is made up of calcium carbonate, or calcite, which is why it is white when pure.

Below is the process limestone goes through to become marble. This happens most often at geological plate boundaries. Here, heat and pressure combine to create favorable conditions for marble.

• In warm tropical seas, limestone is formed from the calcite shells of tiny plankton, which then sink to the seabed.
• The layers of limestone are compressed further and sink further into the earth’s crust.
• The increasing heat from the underlying magma makes the rock pliable.
• Calcite is created when these forces affect the original sedimentary rock. This process is called recrystallization.
• Shifting tectonic plates and crust movements twist and squeeze the limestone, creating the swirling shapes we see in marble. Which color we see depends on the substances involved.
• Tectonic plate movement, or underlying magma, forces the marble toward the surface. It is then found and quarried.

How does marble get its swirling colors?

Marble isn’t always white; sometimes it’s gray or black. It can also have swirling veins of red, green, or yellow. These colors come from impurities within the calcite.

For instance, larnite is a rare sort of impurity. Forsterite, on the other hand, creates crystals of quartz within the marble.

Clay, sand, and chert create the swirling patterns you see in marble. As the soft rock is twisted, these materials are compressed and folded into the calcite.

Graphite, pyrite, quartz, mica, and iron oxides can all affect the color of marble. Even water within the rocks can affect the colors and patterns present in marble.

This picture shows Breccia Capraia vein marble from Italy.

Where can deposits of marble be found?

Marble can be found in India, Italy, Greece, Brazil, and the UK, to name a few. It’s quarried from the surface. You can also find large deposits in the United States, Turkey, Spain, and China.

Carrara marble is from Italy’s Tuscany region. Notably, Italy has some of the highest-quality marble.

Greek marble comes from the Peloponnese, Thessaly, and Macedonia. The Greek Parthenon was made of this marble, as were many of their sculptures.

Vermont in the US also has deposits of high-quality marble.

How is Obsidian Formed?

Obsidian is one of the extrusive igneous rock types. This indicates that magma spilled out volcanically and quickly formed it. “Quickly” in geological time can mean a couple of hundred years.

Obsidian from Medicine Lake Volcano in California was found to be just 900 years old. For a rock, that’s more or less the duration of its childhood.

When obsidian cools too quickly, it doesn’t have time to form small crystals. When this happens, you get a form of glass. A rock like this is called a mineraloid.

Obsidian has the same composition as granite and rhyolite. It can even be formed from the same magma. It’s the speed of its formation and the fact that it forms on the exterior of the earth that create a completely different type of rock.

Where can obsidian be found?

Obsidian can be found in Japan, Canada, Chile, Ecuador, Kenya, New Zealand, Peru, and Russia. It can also be found in the Western US.

There are many more locations for obsidian deposits. The factor linking them all is that they have, or have had, a lot of volcanic activity.

It’s rare to find obsidian that’s more than a few million years old. This is because obsidian is vulnerable to weathering and fracturing. After this time period, it breaks up to become part of a different rock.

What has obsidian been used for?

Obsidian has been used for Aztec sacrificial knives due to its sharp edges. It was said to symbolize Tezcatlipoca, the God of the Smoking Mirror and of Night. Accordingly, it was also used to make mirrors.

Since prehistoric times, obsidian has been used for arrowheads and scrapers for hides. Now, it’s mainly used in jewelry.

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3 Best Places to See Rock Formations

Here’s a list of must-see destinations for those with an interest in geology. There’s plenty of wildlife, too, and some great places to hike.

1. Bryce Canyon National Park, Utah, US

Both Bryce Canyon and Arches national parks are home to Utah’s spectacular red sandstone formations. These were formed by sedimentation. Particles carried by the desert winds settled here and built up.

These formations are called “hoodoos” in Bryce Canyon, which are spires of eroded rock. They’re made by the erosion of the wind and not by people, however.

2. Giant’s Causeway, Ireland

The Giant’s Causeway in Ireland is made of over 40,000 interlocking hexagonal blocks of basalt. Basaltic volcanism formed it about 60 million years ago.

The legend goes that these were the works of two giants who fought one another.

3. Wave Rock, Hyden Wildlife Park, Australia

Weathering and chemical deposits have sculpted Wave Rock into a landmark. It was formed more than 2,700 million years ago out of granite rock. It is 45.9 ft (14 m) high and 361 ft (110 m) long.

Weathering and erosion by water have carved the base inward, leaving an overhanging wave of rock. The water has also redeposited dissolved chemicals in the rock.

This water comes from springs, which are seasonal. They carry carbonate and hydroxide chemicals, which attach to the iron in the granite.

Fun Facts

• Chalk is used in toothpaste as a mild abrasive.
• Obsidian is still used in very thin, finely honed scalpel blades for precise surgery.
• The fantastic red sandstone rock formations of Utah were made by blown sand, which hardened into sedimentary rock.
• Wave Rock is one of the oldest rocks in Australia. Some parts of it are 2.7 billion years old.

How Are Rocks Formed FAQs