Mini Me Geology Blog

Hi everyone, today we have another great ask-a-geologist question from one of our fabulous readers. Peter wrote to us and asked:  Granite, pumice, and scoria are igneous rocks, but why doesn’t granite have airholes, but the other two do?

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. 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. The are medium to large grains and you can tell the difference in the dark micas, clearer quartz and the pink or white feldspar.

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 hole 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. 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.

Remember to keep your questions coming in through the dig into geology section on our website or email us at rockinfo@minimegeology.com. Please comment below and subscribe to our channel so that you get the next installment of ask a geologist. Until next time, rock on everybody.

Filed under: Ask-a-Geologist Q&A, Mini Me Geology Videos, Rocks - Igneous, Sedimentary & Metamorphic on February 27th, 2014