Volcanic Rock Identification

Volcanic Rock Identification

Today, we’re going to talk just a little bit about volcanic rocks. And the goal of this is to help you with volcanic rock identification. Coupled with any of our Volcano books or kits, you’ll be set for an outstanding rockhounding experience. If, however, you would rather watch the complete Geo Talk video and see the samples, scroll to the end of this article.

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The Main Differences Between Volcanic Rocks and Plutonic Rocks

The main difference between volcanic rocks and plutonic rocks is that volcanic rocks are what we call fine-grained rocks. In other words, you cannot see the mineral makeup of the rock—only the general color.

The Division of Volcanic Rocks: Lavas, Pyroclastic, and Glassy

We can divide lava rocks into roughly three different categories:

  • The dark lavas. Basalt is a dark lavas. So you cannot see the mineral makeup of the rock—just simply the general color. And you’ll notice it’s dark or very black. The dark color comes from the dark minerals that form the rock: iron, biotite mica, amphibole, pyroxene, olivine, and calcium feldspar.
  • The light lavas. Rhyolite is one of the light-colored lavas. This rock contains primarily light-colored rock-forming minerals: quartz and potassium feldspar.
  • The medium lavas. Like plutonic rocks, volcanic rocks have their intermediate varieties as well. For example, dacite lava is a light gray color and fine-grained.

Pyroclastic Rocks

In addition to the lavas, you have pyroclastic (or fire-broken) rocks. These are made up of bits and pieces of volcanic rock that were blasted in very hot steam and ash. They were then embedded – fused together – in a matrix of ash.  As they cooled, they fused together.  They can come in a rhyolite color (pinkish). But they can also be a light gray.

Glassy Rocks

(Obsidian shown in the video)  Obsidian is classified as light-colored, even though it is usually quite dark.  This is because it is made up of pure quartz. But its color comes from iron, which gives it a dark color. Another trait of glassy rocks is that they have what we call a conchoidal fracture, which means it fractures like glass. (See the video!)

In the future, we’ll cover more about volcanic rocks.

Watch the Volcanic Rock Identification Geo Talk with Patrick Nurre


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