At least two class periods. Igneous rocks are those that cool from molten rock materials known as magma. Igneous rocks are classified on the basis of their texture and their mineralogical composition. Texture is the overall size, shape, and arrangement of the mineral grains that make up the rock. There is a crude correlation between the color of the rock and its composition.
Rocks containing more silica, such as granite and rhyolite, are generally lighter in color.
A simplified igneous rock classification chart is included as well as a percentage composition chart. These may be reproduced for student use. Maine is rich in igneous rock bodies. These rocks have provided the basis for much of the mining and quarrying activity that has taken place in the state over the years. Gem tourmalines, aquamarines, topaz, and other gem materials are found in Maine's igneous rocks.