What is the Difference Between Igneous Sedimentary and Metamorphic Rocks

The main difference between igneous sedimentary and metamorphic rocks is that igneous rocks form from magma or lava and sedimentary rocks form from accumulation or deposition of small particles whereas metamorphic rocks form from the transformation of an existing rock type into a new form.

Rocks are hard solid materials that occur on Earth – beneath the surface and above the surface. Basically, there are three major types of rocks classified based on the method of formation; igneous, sedimentary and metamorphic rocks.

Key Areas Covered

1. What are Igneous Rocks
     – Definition, Features, Formation
2. What are Sedimentary Rocks
     – Definition, Features, Formation
3. What are Metamorphic Rocks
     – Definition, Scope, Formation
4. What is the Difference Between Igneous Sedimentary and Metamorphic Rocks
     – Comparison of Key Differences

Key Terms

Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

What are Igneous Rocks

Igneous rocks are a type of rocks that form due to the solidification of lava or magma. Therefore, these are also called magmatic rocks. These rocks form via solidification followed by cooling of lava or magma. Generally, magma forms from the partial melting of rocks that lies in either the mantle of Earth or its crust. There are basically three major processes that cause the melting of these rocks: increased temperature, decreased pressure, or changes in composition. The solidification of this magma into igneous rocks can happen either beneath the surface of Earth or on the top of the surface. Moreover, if this solidification occurs below the surface, we call it intrusive igneous rocks, and if it occurs on the surface, we call it an extrusive rock.

Figure 1: Igneous Rock

Igneous rocks may form via crystallization. This forms granular, crystalline rocks. But sometimes, these rocks form without crystallization. Then the rocks formed are known as natural glasses. We can observe the occurrence of igneous rocks at a wide range of landforms such as shields, platforms, orogens, basins, extended crusts, ocean crusts, etc. Generally, metamorphic and igneous rocks cover about 90% of the surface of the Earth’s crust. Moreover, most of the oceanic crusts on Earth are made of igneous rocks.

What are Sedimentary Rocks

Sedimentary rocks are a type of rocks that form via accumulation or deposition of sediment materials. This formation involves the deposition or accumulation followed by cementation with minerals or organic matter. These rocks form on the surface of the Earth. The process is called “sedimentation”.

Figure 2: Sedimentary Rock

Furthermore, rocks that form from sedimentation are “sediments”. The minerals for the cementation process are carried to a particular location by water, wind, ice, or mass movement. Organic matter for cementation is carried to the location via water flow in the form of piles. In addition, small scale sedimentation may occur due to the precipitation of sediment materials from water solution. Moreover, the coverage on the Earth surface with sedimentary rocks lies between 10-8% by volume.

What are Metamorphic Rocks

Metamorphic rocks are a type of rocks that form from the transformation of an existing rock type into a new rock type. We call this process metamorphism. We call the original rock protolith. Protolith undergoes high temperature and high-pressure processes, which also cause a physical and chemical change in the rock. Moreover, a protolith can be a sedimentary rock, igneous rock or an existing metamorphic rock.

Figure 3: A Foliated Structure

Moreover, we can observe a layered pattern in a cross-section of the metamorphic rock. We call this foliation. This foliation occurs when a rock is shortened on one axis during the process of recrystallization. However, there are non-foliated structures as well. The rocks that undergo uniform pressure and temperature on all the sides form non-foliated rocks.

Difference Between Igneous Sedimentary and Metamorphic

Definition

Igneous rocks are a type of rocks that form due to the solidification of lava or magma while sedimentary rocks are a type of rocks that form via accumulation or deposition of sediment materials. In contrast, metamorphic rocks form from the transformation of an existing rock type into a new rock type.

Formation

Igneous rocks form from magma or lava while sedimentary rocks from sediment materials and minerals or organic matter, and metamorphic rocks form from igneous rocks, sedimentary rocks or an existing metamorphic rock.

Occurrence

Moreover, igneous rocks cover about 80% of Earth’s crust while sedimentary rocks cover about 80% of Earth’s crust. In addition, metamorphic rocks cover about 12% of Earth’s surface. 

Conclusion

There are three basic types of rocks as sedimentary, igneous and metamorphic rocks which are classified according to the method of formation. The main difference between igneous sedimentary and metamorphic rocks is that igneous rocks form from magma or lava and sedimentary rocks form from accumulation or deposition of small particles whereas metamorphic rocks form from the transformation of an existing rock type into a new form.

Reference:

1. “Igneous Rocks.” National Geographic Society, 5 Sept. 2019, Available here.
2. “Igneous Rock.” Wikipedia, Wikimedia Foundation, 27 May 2020, Available here.

Image Courtesy:

1. “Kanaga Volcano (22432739869)” By Michelle Combs, USGS – (Public Domain) via Commons Wikimedia
2. “Chalus road – ninara 02” By Ninara – (CC BY 2.0) via Commons Wikimedia
3. “The stones of the Dutch – Lleida Pyrenees 04” By PePeEfe – Own work (CC BY-SA 3.0) via Commons Wikimedia