What is the Difference Between Gabbro and Basalt

The main difference between gabbro and basalt is that gabbro forms deep beneath the Earth’s surface, whereas basalt forms at or very near the surface of the planet.

Both gabbro and basalt are types of igneous rocks. Igneous rocks are usually found in two forms as intrusive and extrusive igneous rocks, based on the method of formation. Gabbro is a type of intrusive rock while basalt is a type of extrusive rock.

Key Areas Covered

1. What is Gabbro
     – Definition, Features, Formation
2. What is Basalt
     – Definition, Features, Formation
3. What is the Difference Between Gabbro and Basalt
     – Comparison of Key Differences

Key Terms

Basalt, Gabbro, Igneous Rocks

What is Gabbro

Gabbro is a type of intrusive igneous rock that forms deep beneath the Earth’s crust. This rock type forms due to the slow cooling of magnesium-rich and iron-rich magma. The final product of this cooling is a holocrystalline mass. Chemically, slow-cooling, coarse-grained gabbro is equal to rapid-cooling, fine-grained basalt.

Most of the oceanic crust on Earth is made up of gabbro rocks. This rock type forms at mid-ocean ridges. We can also find gabbro in plutons that are associated with continental volcanism.

Figure 1: Gabbro

Generally, gabbro is a dense, greenish-black or dark-coloured rock containing pyroxene, plagioclase, and trace amounts of amphibole. Gabbro is coarse-grained. It contains crystals that are 1 mm or greater in size. Moreover, this rock type is equigranular in its texture. However, it can sometimes be porphyritic. This type of rock forms as a massive and uniform in-situ crystallization product. Furthermore, gabbro is an essential component in the ocean crust. This rock type often contains important minerals such as chromium, nickel, gold, silver, platinum, and copper sulfides.

What is Basalt

Basalt is a type of an extrusive igneous rock that forms at or near the surface of a Planet’s crust. This means this rock type not only forms in Earth but also in other planets such as Mars or moon. These rocks form from the rapid cooling of magnesium and iron-rich lava. Almost 90% of volcanic rocks on Earth’s crust falls under the category of basalt.

Due to the presence of low silica content, basalt lava has a low viscosity. This can cause rapid lava spreading over a great area before it gets cooled and solidified. Generally, basalt is fine-grained material with very low amounts of silica and feldspar. Typically, this rock appears in grey or black colour. It can easily undergo weathering into brown coloured basalt due to the oxidation of its mafic minerals.

Figure 2: A Columnar Basalt

We mainly use the term basalt to describe intrusive rocks. There are different types of basalt, including tholeiitic basalt, high-alumina basalt, alkali basalt, boninite or high magnesium basalt, lunar basalt, etc.

Difference Between Gabbro and Basalt

Definition

Gabbro is a type of intrusive igneous rock which forms deep beneath the Earth’s crust while basalt is a type of an extrusive igneous rock that forms at or near the surface of a Planet’s crust.

Texture

Gabbro is coarse-grained while basalt is fine-grained.

Appearance

While gabbro is usually green in colour, basalt is grey to black. 

Type

Moreover, gabbro is an intrusive rock while basalt is an extrusive rock. 

Formation

Gabbro is deep beneath the Earth’s surface while basalt is at or near the surface of the Earth. 

Conclusion

Both gabbro and basalt are types of igneous rocks. Igneous rocks are usually found in two forms as intrusive and extrusive igneous rocks, based on the method of formation. Gabbro is a type of intrusive rock while basalt is a type of extrusive rock. The main difference between gabbro and basalt is that gabbro forms deep beneath the Earth’s surface, whereas basalt forms at or very near the surface of the planet.

Reference:

1. “Gabbro.” Geology, Available here.
2. “Gabbro.” Wikipedia, Wikimedia Foundation, 3 May 2020, Available here.
3. “Basalt.” Wikipedia, Wikimedia Foundation, 19 May 2020, Available here.

Image Courtesy:

1. “GabbroRockCreek1” By Wilson44691 (Mark A. Wilson, Department of Geology, The College of Wooster). – Own work (Public Domain) via Commons Wikimedia
2. “Szentgyörgyhegy03” By Darinko – Own work (Public Domain) via Commons Wikimedia