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All About Rhyolite Rocks

Last month we went deep on the topic of Granites, their origin, qualities and uses.

This month, let’s dive into another type of rock: Rhyolites. When you think of rhyolites, think of a rock that forms by slow lava flowing on the Earth’s surface. These are igneous rocks (rocks formed from lava/magma).

The word

The etymology (word origin) of the word ‘rhyolite’ is rather interesting and unexpected. It is rumored that the World War 1, German fighter pilot Ferdinand von Richthofen (also known as the Red Baron, gave the rock it’s name. Von Richthofen was a geologist, and he used the Greek

word ‘rhyax’, which means ‘stream of lava’, and then added an ‘ite’ on at the end, a suffix that many rocks have.

How are rhyolites created?

If you recall, granites are created under the Earth crusts, when magma slowly cools over the course of a million or more years! These are called intrusive rocks (forming inside the crust).

In the case of rhyolites, they are formed volcanic eruptions, thick blocks of lava, that is too thick to flow like a river, is ejected from the exploding volcano, thrown high into the air, and then landing on the ground, to cool into rock. (Lava is simply magma that has broken through the crust). These are called extrusive rocks (forming outside the crust).

Rhyolites cool a lot faster than granites. While granites have millions of years to cool down, as soon has rhyolites are exposed to our atmosphere, they begin cooling right away, therefore the crystal size in rhyolites are smaller, since there is less time for them to form.

It is worth nothing that fast moving ‘thin’ lava does not form rhyolites, but into other types of rock.

What are rhyolites made of?

They have a significant amount of silicon/silica (sand). But that does not mean they are sandstone!

Surprisingly, the composition of a rhyolite is almost exactly the same as a granite. However, one of the key differences is that rhyolites may contain more potassium, and little muscovite, whereas granites contain muscovite.

Both come from magma, just magmas that have different mineral composition.

What do rhyolites look like?

Like granite, they come in a wide rainbow of colors. But, depending on how fast they cool, they can either be smooth or rough. From glassy black obsidian to gray porous pumice, rhyolites do not come in a ‘set’ form, and are quite variable.  

Some rhyolites will have a grainy ‘peppered’ appearance, while the majority of them are rather plan looking and lacking in distinguishing features, just looking like ‘plain rock’, which is in great contrast when you think of granite and it’s unmistakable pattern.

What are some different rhyolite rocks?

There are a great many varieties, but some of the most well-known ones are:

  • Pumice
  • Obsidian
  • Tuff

How hard is rhyolite?

Their hardness is around 6 on the Mohs scale. Granite ranges from 6 to 7. To put that in perspective, quartz is 7, and steel nails are about 6.5, whereas a Swiss Army knife blade is around 5.5, and copper is about 3.5.  

How do people use rhyolites?

Unlike granite, rhyolites are not a popular building block material.

However, pumice (a type of rhyolite) is used in creating concrete, and also used in dentistry (teeth polishing). Additionally, rhyolites are VERY popular as used as an aggregate in construction of roads, and for landscape rock, in essence being a ‘filler’ rock.

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