The Rock Cycle | How Are Rocks Formed?

Let us learn the concept that describes the transition among the three main rock types: igneous, sedimentary and metamorphic, also known as the rock cycle. 

The rock cycle is a series of processes in which rocks are continuously being transformed through physical processes such as melting, cooling, weathering, eroding, compacting and deforming.

 

Beneath the Earth’s crust is molten material called magma that travels to the Earth’s surface usually through volcanoes and fissures. When this molten rock crystallizes and solidifies, an igneous rock is formed. Plutonic rocks are formed when magma cools and solidifies underground while volcanic rocks are formed from lava that flows on the surface then cools and solidifies.  The Jetafe andesite  and Talibon diorite, both collected in Bohol, are examples of volcanic and plutonic rocks respectively. 

On the other hand, sedimentary rocks are easier to identify because as the name suggests, they are composed of sediments. When rocks are uplifted and exposed to the surface, they undergo a process of disintegration and decomposition (weathering) to produce sediments. These sediments are then carried away (erosion) usually by wind and water and gather overtime in marine or terrestrial basin (deposition). If the sediments are buried under further layers of sediments, they can become lithified to produce sedimentary rocks. The sandstone rock is an example of a sedimentary rock. 

Lastly, when pre-existing rocks have substantially changed as a result of intense heat and/or pressure within the Earth’s crust, metamorphic rocks are formed. This process is referred to as metamorphism which means “change in form”. Alicia schist, which constitutes the basement of the island of Bohol is a perfect example of a foliated metamorphic rock produced by regional metamorphism related to extreme change in temperature and pressure.

 

Once we are authorized to open, we would like to welcome you and show you the amazing rock collections on display at the National Museum of Natural History. 

Text by Jiles Arvin Vergara and poster by Jaan Nogot / NMP GPD

© National Museum of the Philippines (2020)