Week 2: The Igneous Faces of Mount Rushmore

The Presidents of Mount Rushmore.

When I was younger, and also as a result of being part of an active, Coloradan family, my parents loved to take road trips to various nearby monuments, memorials, or places of interest. One of those places included perhaps one of the more famous formations of igneous rocks around, Mount Rushmore, in Keystone, South Dakota. Being an intrusive igneous rock formation, Mount Rushmore is mostly made of granite, which is believed to have formed about 1.6 billion years ago when magma rose from deep within the earth's crust. This molten rock then cooled in two portions - one quickly to form the fine grained granite rock (feldspar and granite) that would eventually become the carving foundation, and another of coarse grained pegmatite created by slow cooling, both of which help to give the memorial the relatively light color due to the quartz and feldspar they contain. At the same time, the molten rock also came into contact with sandstone and shale and created metamorphic rock, mostly in the form of mica schists, which can still be seen in the memorial. Mount Rushmore is certainly a sight to behold, and given that granite is slow to erode, will continue to be so for thousands of years to come.

Igneous rocks underneath the faces with veins of granite/