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How to Test a Lava Rock to See What It Is Made of

Lava rocks come from cooled molten lava after eruptions. The volcanic glass that forms the rocks can be found in several types of rock, notably obsidian and pumice. Pahoehoe and a'a lava have completely different appearances and are easy to tell apart. Lava can look chunky, airy, bubbly, glassy and shiny. You can identify lava rocks simply using images and descriptions from geology and volcanology books and articles online. If you get a Mohs scale kit and a chemistry set, you'll be able to confirm your findings.

Instructions

- 1
  Place your rocks into a bucket or large bowl of water. This is one of the easiest tests for pumice. Those that are pumice rock will float. It is light, fibrous and spongy.
- 2
  Purchase a small Mohs scale kit to test the rocks for hardness. Lava, or volcanic glass will typically have a hardness of 5.5 - 6.0, depending on its particular chemical composition. A steel knife blade may or may not cut it, so this will give you some basic information.
- 3
  Look at the rocks closely for bubbles and shininess. Obsidian is a deep, rich black that is smooth and glassy looking. If you have rocks that have thin edges, hold it to the light and you'll be able to see through it as the black glass is transparent. Some pahoehoe lava has what appears to be oil slicks or rainbow-like covering over the dark bubbly stones. Bubbles can be very tiny, but can be seen with the naked eye.
- 4
  Buy or borrow a chemistry set or take the rocks in for analysis at a chemist's. An article about volcanoes on Oregon State University's website states, "Chemically lava is made of the elements silicon, oxygen, aluminum, iron, magnesium, calcium, sodium, potassium, phosphorus, and titanium -- plus other elements in very small concentrations. Your experiment results should be comparable.

Rock Collecting