The Elkhorn Mountains

January 07, 2019

Marble (1)

Photographer: Rod Benson
Summary Author: Rod Benson 

The Elkhorn Mountains south of Helena, Montana are remnants of volcanoes that were active some 74 to 81 million years ago. During that period, a tectonic plate was subducting beneath western North America, allowing magma to rise to the surface. As a result, the Elkhorns are made up primarily of extrusive igneous rocks. The volcanic rocks that make up the Elkhorns (lots of andesite) formed when lava poured onto the surface and cooled.

Despite the volcanic origin of the Elkhorns, the outcropping shown in the photo is made of marble - a metamorphic rock formed as limestone was changed by heat and/or pressure. Sometime during the late Cretaceous, magma melted its way into the area, coming close enough for its associated heat (over 1,300 F or 700 C) to change the limestone into marble - a process known as contact metamorphism. Evidence for this is the presence of granite (formed as that magma cooled), located not far below the marble.

Limestone is usually formed by sediment deposited in a shallow tropical sea, so how did limestone form in a center of volcanic activity? One possibility is that hot springs existed here when the area was volcanically active. An unusual variety of limestone called travertine can form on the surface around hot springs by the rapid precipitation of calcium carbonate.

In the millions of years that followed, the igneous rocks and marble were deformed (folded, faulted) by tectonic forces that built the Rocky Mountains (80 to 55 million years ago). As the formations were pushed up, they were also shaped by erosion, including glaciation. In fact, the marble in the photo marks the top of a cirque formed by a glacier that once (or multiple times) flowed from Elkhorn Peak toward the present-day location of the town of Elkhorn. To access a blog and photo tour of the hike to the summits of Elkhorn Peak, and also Crow Peak, click here. Photo taken on September 15, 2018.