Back

IGNEOUS ROCKS
Lesson 3 - Page 5

Next 

However, what do you do if the rock is aphanitic and you canít see the minerals? In this case, you can use color as a guide. Rhyolites are usually light colored, ranging from white to pink to light gray. In contrast, basalts are dark colored, usually dark green to black. Andesites are in between, for example gray as opposed to white or black.

A more precise method is to make a thin section of rock and count the minerals in a given space and then calculate percentages. You can use this method for both aphanitic and phaneritic rocks.

There are many other types of igneous rocks caused by many different situations. The cooling rate can create bizarre looking rocks. Some rocks start cooling inside and then are "coughed" up. The rock will have a background (matrix) of fine grained volcanic rocks with larger crystals that were cooling slowly. This texture is called porphyritic


Thin section of granite


Porphyritic andesite

   Back

[Back to Igneous Rocks]  [Earth Science Grid]

Next