IGNEOUS ROCK  Identification

February 09, 2001

This exercise is intended for use with the CSI rock set of 9 specimens. Written by Dr. Alan I. Benimoff and last revised 02/09/01.

Photos  by Donald E. Clark

Please report any computer program problems to Dr. Alan I. Benimoff E-mail: benimoff@postbox.csi.cuny.edu

Igneous rocks are classified on the basis of texture and mineral composition. Examine the rock to determine its texture.Click on the texture that you observed and then observe the overall color of the igneous rock as you determine the rock name.

Determine the texture of the rock:

Very fine grained - you can't see the individual grains without the aid of a microscope. Petrologists(those who study rocks) refer to this as an aphanitic texture.

Coarse Grained -You can see the individual grains without the aid of a microscope. Petrologists(those who study rocks) refer to this as an phaneritic texture.

Very coarse grained - You can see the individual grains without the aid of a microscope and the grain size is greater than 0.25"(1.16cm).Petrologists(those who study rocks) refer to this as a pegmatitic texture.

Two different grain sizes -You can see large(coarse) grains embedded in a fine grained rock. Petrologists(those who study rocks) refer to this as a porphyritic texture. The large(coarse) grains are called phenocrysts and they are embedded in a fine grained matrix.

The rock looks glassy or contains frothlike fibers of entangled glass

The rock is usually fine grained and has holes in it.
You can't see the individual grains without the aid of a microscope and there are holes(vesicles) in it. Petrologists(those who study rocks) refer to this as an vesicular texture.

The rock contains fragments of rock extruded explosively from a volcano or fissure.
Petrologists(those who study rocks) refer to this as a pyroclastic texture if the pyroclasts(fragments) are welded together.They also classify these rocks on the basis of garin size. If the fragments are loose the deposit is called tephra.