Montana Earth Science Picture of the Week

The red and gray areas on the map show where igneous rock can be found at the surface, or just beneath the soil. Igneous rock is formed when magma or lava cools. The red areas are where lava erupted onto the surface and hardened to form a category of igneous rock that geologists refer to as “volcanic” (a.k.a. extrusive). The gray areas are where the magma hardened beneath the surface, making a kind of igneous rock is called “plutonic” (a.k.a. intrusive). Basalt is the most common specific type of volcanic rock and granite is the most common specific type of plutonic rock.

Plutonic, or volcanic? How can they tell? . . .
One thing that helps geologists determine whether an igneous rock is plutonic or volcanic is the size of the crystals, or grains, that makes up the rock. When lava erupts onto the surface it tends to cool quickly, whereas magma beneath the surface may take centuries to harden. As a result the various minerals in lava have little time to organize themselves into crystals. Consequently, volcanic rocks tend to be fine-grained and display a more uniform color than plutonic rocks. On the other hand, as magma cools slowly beneath the surface, minerals have more time to form crystals. The crystals eventually run out of space as they grow into each other, forming interlocking “grains” of various colors, such as the quartz (white), biotite (black), and feldspars (pink, gray) that can be seem in a typical granite.

Below: This photo was taken 25 miles southeast of Havre. The battlefield where Chief Joseph surrendered is shown in the foreground with the eastern edge of the Bear Paw Mountains (a.k.a. Bear Paws or Bear's Paw Mtns.) in the distance. Although the mountains in the photo are made up of plutonic rock, most of the Bear Paws consist of volcanic rock, which is more fine-grained than the plutonic rock that makes up the mountains between Butte and Helena. As the map at the top of this page shows, most of the mountains in central mountains are made up of plutonic and/or volcanic igneous rock.

Terms: igneous rock, plutonic rock