Geology | What is Basalt?

Clue: Yes, it's a rock.

It may look like any ordinary rock you might find on the street... but believe it or not, it has its own differences from others, both macro- and microscopically.

So today we'll talk about the basalt.

To make things simpler, a basalt is a dark-colored, fine-grained rock you'll commonly find near the surface of volcanoes or in mid ocean ridges. When the lava flows out from volcanoes or ocean ridges, it cools rapidly and forms the rock. (If you didn't know, most of the Earth's oceanic crust is basaltic in composition!)

Those with untrained eyes may easily mistake basalt for any other dark-colored rock (such as obsidian, gabbro, or tektites/meteorites). Luckily, you can distinguish basalt by its fine-sized grains as well as by its dominant mineral composition of pyroxene and feldspars. But for added verification, looking at basalt under a microscope will be more than enough to confirm its identity.

The sample above is actually identified to be a porphyritic basalt. This simply means that it has some larger grains of plagioclase feldspars and pyroxenes called phenocrysts set in a matrix of smaller ones called the groundmass. In the photo, you'll easily distinguish the larger tabular grains from the smaller ones surrounding it.

In the groundmass are also the fine-grained brown-colored grains of volcanic glass. As accessory minerals, tiny irregularly shaped opaque minerals can also seen to be scattered in the rock sample.

For a brief history of the formation of this rock, or the petrogenesis, let me tell you that it required two stages of cooling. The first stage allowed the magma to cool slowly and produce the larger crystal grains of plagioclase and pyroxene. Then, the second stage happened more rapidly, which created smaller crystals of the groundmass because they had leser time to grow. Volcanic glass formed last, filling the empty spaces between the minerals that formed first.

Just remember that the petrogenesis I shared is specific for the sample above only! The history of the formation of a rock varies depending on numerous factors, such as texture, mineral composition, and grain size. There are a lot of basalts that can exhibit differences in those areas, and therefore a lot of stories that can be told.

Awesome, right? Who knew that rocks could be so interesting?

There's definitely more than what meets the eye!

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Curious to know more? Let me know if you have any questions, I'll try my best to answer them! And if you want me to feature any other rock or fossil, just hit me up and I'll see what I can do!

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Picture taken by myself using my iPhone 6S, as seen under a microscope. The thin section belongs to my university.

Information from my Geology 150: Petrography and Petrology notes, as taken from An Introduction to Igneous and Metamorphic Petrology (2001) by Ohn D. Winter.