The Rock Cycle: Igneous, Sedimentary and Metamorphic Rocks

The Rock Cycle

The rock cycle describes the formation of rocks over a period of time. Unlike the water cycle, it is difficult to see the rock cycle steps happening. The process occurs very slowly hence taking years to complete. However, factors such as volcanic eruptions and floods speed up the process of rock formation. There are three types of rocks; igneous rocks, sedimentary rocks and metamorphic rocks (Gill 22). Igneous rocks are formed with the rapid cooling of hot magma. Sedimentary rocks on the other hand are formed when the layers of dirt and rock particles are pilled and compressed over a period of time. Finally, metamorphic rocks a combination of rock types which are formed by high pressure and heat.


The rock cycle happens in the following steps:

  • Weathering and erosion
  • Transportation
  • Deposition
  • Compaction and cementation
  • Metamorphism
  • Rock melting

Weathering and Erosion

The following section describes the weathering process of the rocks through chemical weathering. There are four types of weathering; freeze-thaw, exfoliation, chemical and biological weathering (Tang Zhi-yong & Hong-tao Zheng 65). Chemical weathering refers to the disintegration of rocks through a chemical reaction. The reaction occurs between the rocks, water and other substances dissolved in water resulting in the disintegration of the rocks. When this happens, new minerals are formed which combine with other chemical conditions at the surface of the earth. Chemical weathering is essential in the formation of soil which facilitates the growth of plants.

The chemical reactions involved in the disintegration of rock involves components such as acid, oxygen and water. Weathering of rocks happens by the action of rain water, extreme temperatures and biological activities (Tang Zhi-yong & Hong-tao Zheng 67). The following section describes the stages of chemical weathering in which rocks are broken down through hydrolysis, oxidation, carbonation, hydration and dehydration.

Hydrolysis refers to the chemical breakdown of substance with the aid of water. The reaction between the rock and water leads to a breakdown of the rock.  Over a long period of time, the water infiltrates the rock causing weak fragments that eventually disintegrate to tiny pieces. Oxidation in the chemical weathering process occurs when the rock reacts with oxygen. As a result of the oxidation, the iron in the rocks begin to rust. The presence of moisture further speeds up the reaction causing the disintegration.

Carbonation occurs as a result of a reaction between water and carbon dioxide to form carbonic acid. Carbonation mainly occurs in limestone and it results in the formation of hydrogen carbonate which is soluble in water. On the other hand hydration refers to the absorption of water molecules into the mineral composition of the rock (Tang Zhi-yong & Hong-tao Zheng 69). As a result of this absorption, the volume of the rock which eventually leads to deformation of the rock. The pressure exerted inside the rock builds up overtime hence it disintegrates the structure of the rock causing cracks which eventually become soil particles. Finally, dehydration refers to the removal of water from the rock. The process leads the breakdown of rocks over periods of time.


            When the igneous, sedimentary and metamorphic rocks are broken down into tiny pieces the particles are washed away. The rushing water transports the particles into streams, rivers and oceans. Wind also facilitates the transportation of the small rock particles.


The flow of water carrying the rock particles into the ocean causes a buildup of the materials. The rock particles mixed with soil combine to form a layer of sediment. The buildup of the sediments creates little islands within the river which eventually cause deltas. The deltas form water channels in the river.

Figure 1: The process of rock formation
Figure 1: The process of rock formation

Figure 1: The process of rock formation

Compaction and Cementation

            The continued accumulation of soil and rock particles cause a compact mass caused by pressure exerted by the buildup of the materials. The dissolved minerals in the water fill the gaps between the particles acting as cement (Tyburczy & Du Frane 54). Over several years of the compaction and sedimentation, the sediment turns into a sedimentary rock.


            The movement of tectonic plates below the earth’s surface buries the sedimentary and igneous rocks below the surface. Hence, the rocks are exposed to high heat and pressure changing them to metamorphic rock.

Rock Melting

The increased pressure from the movement of the tectonic plates cause the rocks to melt forming magma. The accumulation of magma causes a volcanic eruption releasing the magma to the earth’s surface. When the magma gets to the earth, it is referred to as lava. With time, the lava cools and hardens forming igneous rock. The formation of the new igneous rocks starts the process of weathering and erosion hence repeating the cycle.


In the rock cycle, igneous rocks undergo weathering and erosion which is then transported to oceans and seas. The accumulation of the particles causes the formation of sedimentary rocks. Overtime the sedimentary rocks are buried underground, the heat and pressure forces the rocks to melt forming metamorphic rocks (Hunt 301). The melting of the rocks causes formation of magma which is converted into lava. The condensation of lava causes the formation of igneous rocks and the process begins again.

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