How Are Rocks Formed?

Nature has a way of creating the most beautiful and majestic things. Mountains, canyons, and even the simple pebble at your...

How Are Rocks Formed?

Nature has a way of creating the most beautiful and majestic things. Mountains, canyons, and even the simple pebble at your feet remind us of our Earth's power and mystery. But have you ever wondered how these natural wonders are formed?

The rock cycle is perhaps one of the most important geological processes that shape our planet. It is a never-ending and constantly changing process that involves the transformation of one type of rock into another.

The formation, movement, and transformation of rocks result from Earth's internal and external processes. Internal processes include things like crystal growth, melting, and recrystallization, while external processes encompass weathering and erosion.

Let's take a closer look at the rock cycle and how various rocks are formed.

Three Types of Rock

 There are three main types of rocks: Igneous, sedimentary, and metamorphic.

1.   Igneous Rocks

Igneous rocks are born deep within the Earth's mantle, where extreme heat and pressure create unique crystalline structures. Over time, these rocks may be thrust up to the surface through volcanic activity or tectonic processes, and begin to cool and harden.

But this isn't the end of the igneous rock's journey. Depending on where the molten rock solidifies, these rocks can be classified as either intrusive or extrusive.

Intrusive, or plutonic, igneous rocks form when magma (molten rock) cools and hardens deep inside the earth. This process happens slowly, allowing large, coarse crystals to form.

Extrusive, or volcanic, igneous rocks, on the other hand, are formed when magma exits on or near the earth's surface and cools rapidly. Because there's not enough time for large crystals to form, extrusive igneous rocks have a much finer texture. They also tend to be glassy in appearance and have a vesicular texture due to trapped gas bubbles.

Magma is thought to be generated within the plastic upper mantle at depths of around 40 miles to as much as 120 miles. This molten rock is composed of iron, magnesium, silicon, and other elements that have been partially melted by the earth's heat.

Usually, erupting molten rock has a low viscosity that allows it to flow easily. This means that magma can travel long distances before it solidifies. This ends up forming some of the world's great plateaus, such as the Deccan Traps in India and the Columbia River plateau of Washington, Oregon, and Idaho.

Igneous processes have been active throughout the history of the earth, and they continue to create new landforms today. Some of the recent lava flows that have made headlines include the 1980 eruption of Mount St. Helens in Washington and the 2018 eruption of Kilauea in Hawaii.

2.   Sedimentary Rocks

Sedimentary rocks are some of the most common rocks you'll find on Earth's surface. They cover about 75% of the planet and are formed when smaller rocks, minerals, shells, and organic matter are deposited in layers.

Over time, the weight of the accumulating sediment causes the lower layers to compress and harden. This process can take place over long periods of time, sometimes millions of years.

The type of sedimentary rock that forms depends on the composition of the materials that are being deposited. For example, sandstone is formed from sand grains that have been cemented together, while limestone forms from the accumulation of shell, coral, algal, fecal, and other organic debris.

Sedimentary rocks are often classified according to how they were formed. The three main categories are clastic, chemical, and organic.

Clastic sedimentary rocks

Clastic rocks are the most common type of sedimentary rock. They are formed from the weathering and erosion of preexisting rocks, which results in the breaking up of those rocks into smaller pieces.

The smaller fragments, or clasts, are then transported away from the source rock by wind, water, or ice. Eventually, the clasts are deposited in a new location, where they accumulate and undergo further compaction and cementation to form a new sedimentary rock.

Common clastic sedimentary rocks include sandstone, shale, and conglomerate.

Chemical sedimentary rocks

Chemical sedimentary rocks form when dissolved minerals precipitate out of ion-rich water solutions. This process can happen when water evaporates, leaving behind the minerals that were dissolved in it.

Chemical sedimentary rocks are usually classified by the minerals that they contain. For example, rocks that are rich in calcite or aragonite are called carbonate rocks. Rocks that are rich in silica are called siliceous rocks. And rocks that are rich in iron oxide are called ferruginous rocks.

Organic sedimentary rocks

Organic sedimentary rocks form from the accumulation of organic debris, such as plant and animal remains. The most common type of organic sedimentary rock is coal, which forms from the compression and decomposition of plant matter over millions of years.

3.   Metamorphic Rocks

Metamorphic rocks are rocks that have been transformed by heat, pressure or both. The word "metamorphic" comes from the Greek word "metamorphosis," which means "change in form."

Metamorphic rocks typically form deep underground, where high temperatures and pressures can change the structure and composition of preexisting rocks. For example, heat can cause minerals to recrystallize, while pressure can cause them to deform or change shape.

The most common type of metamorphic rock is slate, which forms from the metamorphosis of shale. Slate has a layered appearance and can be split into thin sheets. Other common metamorphic rocks include marble, which forms from the metamorphosis of limestone, and quartzite, which forms from the metamorphosis of sandstone.


Mother Earth is constantly changing. Rocks are constantly being formed, weathered, eroded, and deposited. If you look around your environment, you'll notice that rocks are everywhere! They come in all shapes and sizes and are an important part of our planet.

Frequently Asked Questions

1.   How are rocks classified?

Rocks can be classified according to how they are formed. The three main categories are igneous, sedimentary, and metamorphic.

Igneous rocks are formed from the cooling and solidification of molten rock. Sedimentary rocks are formed from the accumulation of sediment. And metamorphic rocks are rocks that have been transformed by heat and pressure.

2.   What is the difference between a rock and a mineral?

A rock is an aggregate of one or more minerals. A mineral is a naturally occurring, inorganic substance with a definite chemical composition and a crystalline structure.

3.   What are the most common rocks?

The most common rocks are igneous rocks, which are formed from the cooling and solidification of molten rock. The most common igneous rocks are basalt and granite. 

4.   Why are igneous rocks called primary rocks?

Igneous rocks are called primary rocks because they are the first rocks to form on Earth. All other rocks, including sedimentary and metamorphic rocks, are derived from igneous rocks.

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