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Scoria {InstaScience}

January 11, 2016 2 min read

Scoria is an igneous rock formed by extrusion, very similar to pumice and basalt. Learn how this type of rocks forms and see how it differs from pumice.

A few years back, we took a very long road trip out to Arizona to see the Grand Canyon. It was spectacular and I still can’t really put into words how amazing it is to see this canyon in person. But a surprising highlight of our trip out there was a last minute addition, Sunset Crater National Monument.

We were giddy at the chance to see a cinder cone volcano up close. Not only did we had the chance to see lava tubes and lava flows, but we got the chance to study several types of igneous rocks. It was simply beautiful. One of the rocks we found was scoria, a.k.a., lava rock or rather those red landscaping rocks you see in the garden department.

Scoria is an igneous rock formed by extrusion. These rocks are black when the first form, but over time they can take on a brownish or reddish hue as the iron present in the rock oxidizes. Scoria rock is blown out of the top of a cinder volcano as it explodes. It cools relatively quick, which leads to nooks and crannies all throughout the rock.

Underground molten rock has a good bit of dissolved gas in it. Once it reaches the surface, the dissolved gas present in the molten rock bubbles up and escapes as the lava cools. If the lava cools before all of the gas has escaped, the bubbles are trapped and igneous rocks like scoria and pumice are formed.

Scoria differs from pumice in a few ways. Pumice is much lighter in color, its normally light grey or tan. Pumice rock has a higher concentration of trapped bubbles, which means that it is much lighter than scoria. Because of this, scoria won’t float in water the way pumice can. Despite their differences, scoria and pumice are both known as basaltic rocks due to their similarities with basalt in chemical make-up.

Fun Fact – The Wupatki people use scoria rock when building their homes.


Want to learn more about igneous rocks? Check out the following articles:


Keep the learning going with these science activities!

  • Socria Observation – Get a bit of lava rock from your local store to add to your rock collection. Have the students observe how the rock feels and how it looks. You can use a magnifying glass or a palm-sized microscope to look a bit closer.
  • Volcanic Eruption – These activity uses an empty yogurt container, dirt, and toothpaste to show how magma erupts from the surface of the earth.
  • Igneous Rock Nature Study from the Handbook of Nature Study blog.

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