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!
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!
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.
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So basalt is distinguished by the rapid cooling of the lava (?) and the small pieces of crystal? Did I hit it right? Lol
So rocks are classified and named according to the cooling time, cryztal size, color and where they come from?
Igneous rocks are classified first by grain size (visible with the naked eye or not) and then by their relative proportions of different minerals.
Oh okay. That's something new for today. Basically, it's about proportion of sizes for igneous rocks from macro to microscopic relation of minerals? Thanks!
First comes the distinction between fine grained (volcanic) and coarse grained (plutonic) rocks. After that the classification is based on mineral proportions.
For example the coarse grained equivalent of a basalt is a gabbro.
Ahhh okay. I would love to get to know a bit more about rocls and make distinctions between them as volcanic or plutonic then igneous, sedimentary and metamorphic. That would be fun
Check out the Introduction by @mountainwashere
https://steemit.com/geology/@mountainwashere/rock-types-and-the-rock-cycle
Hey there, @josejirafa! I see your questions have been entertained by @sooflauschig, so I have no need to repeat them all! What he said are all true and I'm glad you are very curious to know more about geology. Just keep asking! :)
The fun part is really distinguishing these rocks from one another, even just among the three major classifications of igneous, sedimentary, and metamorphic!
Nice your post