IGNEOUS ROCKS by Richard (Dick) Gibson

Courtesy USGS

Igneous comes from the Latin word ignis, meaning fire—the same root from which we get the word “ignite”.  Igneous rocks are born of fire, starting their existence as molten material—magma—deep within the earth.  Depending on their chemical composition and how long they took to cool, different igneous rocks can form.

Lava flows and volcanic ash erupt onto the earth’s surface. Such materials cool very quickly—so fast, in fact, that they have little time for crystals to grow.  These rocks have very tiny mineral grains.  If a body of magma does not reach the surface, but solidifies underground, it may take millions of years for it to cool.  That gives plenty of time for individual crystal grains to grow, sometimes as large as an inch across.  Both fine and coarse grained igneous rocks have grains that are tightly interlocking, a result of all the original molten material solidifying like a mush of nuts and cereal flakes and blueberries freezing in a bowl of milk in the freezer.

The most common source of igneous rocks is the melting associated with subduction—when rocks are carried deep enough within the earth, high temperatures eventually melt them. The more buoyant magma rises toward the surface.  Sometimes it makes it, sometimes it does not, but even bodies of formerly molten rock deep within the earth are often exposed by uplift and erosion.

Igneous rocks are classified according to where they solidified and their composition. The list below only includes some of the commonest varieties. 

Igneous Rock Types 

Granite—light colored, forms within the earth (big crystals) 

Rhyolite—light colored, forms near or at the earth’s surface (tiny crystals) 

Gabbro—dark (iron-rich), forms within the earth (big crystals) 

Basalt—dark (iron-rich), forms near or at the earth’s surface (tiny crystals)   

Narrow shoots of molten magma may be forced into pre-existing rocks, forming dikes (if the shoot cuts across the pre-existing rock) or sills (if the magma forces its way between layers).  Much larger bodies of granite can form as a result of subduction along long continental margins.  In western North America, the Sierra Nevada, Idaho, and Boulder Batholiths are examples of this.

Dikes

Granite in the Boulder batholith

The Boulder Batholith extends from the Highland Mountains to Helena, and is the host for the extremely rich mineralization (copper, silver, gold, and more) at Butte and elsewhere in southwest Montana. The core of the Tobacco Roots is also a batholith, a satellite of the Boulder Batholith, and smaller granite bodies occupy parts of the Pioneer Mountains.