What Kind of Rock Is This?
ES0610  Volcanic Rocks
Matthew Nyman, TERC

Is the rock dark? If yes, your sample is probably basalt. Basalts may also contain vesicles (holes where volcanic gases were trapped) and small, light-colored, rectangular-shaped feldspar crystals.

Matthew Nyman, TERC

Is the rock light in color (cream-colored or tan, or with an orange tint)? If yes, your sample is probably rhyolite. Rhyolites may also show banding that formed as viscous lava flowed. Small crystals of quartz may be present.

Matthew Nyman, TERC

Is the rock medium in color, between dark and light (gray, pink or green colored)? If yes, your sample is probably andesite. Andesite may contain small visible crystals embedded within fine-grained material.

If you have identified your sample, return to the beginning of the rock identification key, select another rock sample, and begin the process over again.

If your rock sample does not have any of the features described on this page, reexamine the rock with a hand lens or magnifying glass. Look at the sample from all angles. Concentrate on fresh surfaces where you might be able to better determine the texture. Return to the beginning of the rock identification key and start over again.