Inside Aspen Mountain |

Inside Aspen Mountain

Tim Willoughby
This diagram shows the number of vertical faults that slice through Aspen Mountain. (USGS Spurr Monograph 1898)

The inside of Aspen Mountain moves constantly, and that is good news.

It is no coincidence that the mountain containing nature’s biggest stash of silver is also the mountain with the most geologic faults. The tumultuous forces that shaped Aspen Mountain created fractures and mineralization ensued. Sedimentary rock layers fractured and tilted toward town as they were pressed between the harder igneous rock that defines the flanking ridges ” Shadow Mountain and the Ute Trail cliffs. A geologist’s term for this occurrence is an inclined syncline.

For a visual image of the processes that shaped Aspen Mountain, stack alternating sheets of thin cardboard and uncooked lasagna noodles to represent layers of sedimentary rock. Hold the stack between your hands. Your hands represent hard igneous rocks as you push hard toward the middle with each hand. The layers bend toward the middle. Then tilt the bent layers toward you. The layers now form a bend (syncline) that is tilted (inclined). The cracks in the lasagna created from your squeezing are the fault lines.

Now pour something precious like honey over the top. As it seeps into the lasagna cracks, you will simulate how the mineralization process left precious metals in the fault fracture zones, except in Aspen’s case, precious metal oozed from the bottom up. It later dissolved and redistributed as water seeped and flowed through the faults from above.

Miners who tunneled through the mountain searched for the fault lines and could predict with some accuracy where they would encounter the same fault line at different depths. The forces that created the mountain, still at work, were the main cause of tunnel cave-ins. The area around faults is unstable, not like the surrounding solid rock. Miners did not timber in self-supporting solid rock, but they constructed reinforced sides and ceilings where tunnels passed through faulted sections. They carefully interlocked wood fir timbers as thick as eight inches for protection.

When man and mountain battle, mountain eventually wins. As the inside of the mountain moved, the thick timbers would snap like matchsticks. Replacing broken timbers was a constant chore. When movement was slow, a rectangular box of reinforcement would bend to the point where an ore car could not pass (see diagram). Underground measurements showed movement of about half a foot each year. Over a period of 10 years, shafts that passed through fault areas no longer lined up from top to bottom. Straightening them was almost as much work as digging a new one.

The constant, slow, underground movement of large blocks, up and down along vertical faults, may be a life-saver. Longitudinal faults such as the San Andreas in California move in spurts and cause great harm. Aspen experiences earthquakes, but forces do not build as greatly with the constant movement of the vertical blocks. Here earthquake magnitude is minimal and rarely felt.

When you ski over an Aspen Mountain fault you are likely more concerned about avalanches than earthquakes. Most of us understand only an inkling of geologic forces and are not even aware of the faults that lay beneath us. Aspen’s miners had daily encounters with nature’s “faults.” It must have been quite different spending your day on the inside of a moving mountain.


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