History of the Museum

Butte Mining History

Orphan Girl Time Line

 Butte's mining history starts with its geology. Seventy-eight million years ago, western Montana was near the western edge of the North American Continent. Continental crust is lighter than dense oceanic crust, and when the two collide, oceanic crust usually descends (a process called subduction) beneath continental crust. Eventually, this descending crust reaches depths at which it melts. The more buoyant molten rock, called magma, ascends through the overlying continental crust. When it reaches the surface, it can flow or explode out as a volcano. Volcanoes were located near Butte about 78 million years ago - their remnants still exist in the Elkhorn Mountains southeast of Helena.

Subduction is illustrated on the right hand side of the U.S. Geological Survey image at left. This is the situation today in western Washington and Oregon, where the Cascade Volcanoes like Mt. St. Helens reflect the descent and melting of oceanic crust. The situation was similar in the area of Butte 78 million years ago - but all of the molten rock did not reach the surface.  Many of the huge "blobs" of molten rock solidified beneath the surface to form rocks we now call the Butte Granite - part of the Boulder Batholith, a body of rock about 90 miles long and 40 miles wide. On the geologic map at right (courtesy Montana Bureau of Mines and Geology), the Boulder Batholith is the red area in the circle in the southwest part of the state.

As the molten rock neared the surface, it cooled and the pressure became less, allowing the disolved minerals to solidify, or crystallize. It is not clear why there was such a great concentration of metals in the area that became Butte, but it was extraordinarlily rich. Over several periods of time spanning millions of years, cracks in the rock became lined with deposits of copper and lead sulfides, silver compounds, some gold, a lot of molybdenum and manganese and zinc, and many more elements. The mineral-filled cracks would be called veins of ore when miners enountered them in the late 1800s.

Rounded granitic rocks

of the Boulder Batholith