UM field students climbing the Boulder Batholith. (Photo by Rick and Susie Graetz)

The Boulder Batholith

A batholith is a large mass of igneous rock covering an area of at least 40 square miles. The Boulder Batholith, small by world standards, stretches from the Helena area south to Dillon and covers about 1900 sq. miles. It was named for the rounded boulders that typify the terrain it occupies, a result of weathering of fractured granite.

Through volcanic eruptions of 60 – 70 million years ago, magma (its magma when in the earth and lava when the molten rock flows to the surface) rose to within a few miles of the surface before cooling stopped it and caused cracks and fissures to occur.

In the case of the Boulder Batholith the material consisted of quartz monzonite and granite. The earth that covered the now solid rock long ago eroded exposing the piles of boulders we see today. These displays are especially noticed along the roads ways ascending and descending Pipestone and Homestake passes near Butte.

These fissures served as a conduit for mineral laden hot water to flow between the rock and deposit precious metals such as copper, silver and gold. This is called a contact zone or vein of minerals. They can be 50 feet wide and thousands of feet long.

Rick Graetz | UM Geography Department