November 24 – Volcano-no

Today’s factismal: Many of the words for lava are Hawaiian.

If you ask any geo-geek about molten rock and the first question that he’ll ask is “inside the ground or on top of it?” That’s because geology has different names for molten rock depending on where it is. When the molten rock is inside the ground, it is called magma. When it is exposed on the surface, it is called lava. And, unlike many other pedantries, this distinction actually makes sense. That’s because lava meets the air (or the water) on one side, which allows it to cool more quickly which means that it tends to have smaller crystals but magma is all wrapped up in a blanket of rock, which means it cools more slowly which means it tends to have bigger crystals.

This Hawai'i volcano makes very small crystals (Image courtesy USGS)

A Hawai’i volcano making a’a
(Image courtesy USGS)

And that isn’t the only place where volcanic terminology gets down and detailed. For example, when basaltic lava erupts at a low temperature (just about 2000°F), it is stiff and moves slowly; as a result, the outer surface breaks off in a layer of small pebbles known as clinkers. When the mass cools, it is called a’a, from the Hawai’ian for “stony lava”. But when the lava is nice and hot, it is very thin and moves quickly. This means that the upper surface, which forms a skin like that on hot milk, The skin cools into a distinctive form known as pahoehoe or “smooth lava”. And if the basaltic lava erupts under water, it forms small little pillows that we call (wait for it) pillow basalts.

Two geologists standing near a lava flow (Image courtesy USGS)

Two geologists standing near a pahoehoe lava flow
(Image courtesy USGS)

If you’d like to learn more about the weird and wonderful world of volcanoes, why not head on over to the USGS Volcano web site?

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