Ask-a-Geologist Video #11: Why do pumice and scoria have holes and granite does not?

We received a great question about igneous rocks through our Ask-a-Geologist feature. Peter wrote to us and asked, “Granite, pumice and scoria are igneous rocks, but why doesn’t granite have air holes, but the other two do?” We love this question, so let’s get into the answer!

Hi Peter, these three rocks that you asked about are all igneous rocks that form from volcanic magma. The reason that pumice and scoria have holes in the rock and granite does not is due to the way they form.

How Granite Rock Forms

This rock is a piece of granite, which is an intrusive igneous rock. It formed inside the volcano when magma was allowed to cool underground. Sometimes, magma will inject into cracks in the rock surrounding the magma chamber and over time will cool very slowly because it isn’t exposed to air. One way to tell if a rock cooled slowly is the grain size of the rock. Granite has mineral grains that are very easy to see. There are medium to large grains and you can tell the difference in the dark micas, clearer quartz, and the pink or white feldspar.

How Pumice and Scoria Rocks Form

Pumice and scoria are extrusive rocks and form outside of the volcano usually on top of lava flows. The top of these lava flows become very frothy and when they cool the gasses in the lava expand and escape forming air holes or vesicles in rock. These rocks are slightly different.

Pumice is usually a light-colored rock and forms on slow-moving, viscous lava flows, and is light and floats on water. The Scoria sample is a dark red color and forms on a more liquid lava flow, has larger holes than pumice, is slightly heavier, and does not float on water.

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One Response to “Ask-a-Geologist Video #11: Why do pumice and scoria have holes and granite does not?”

  1. Do crystals form in pumice?

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