Information Entries for Uncompahgre Peak and Geology

Geology (Uncompahgre Peak)

Title: Lake City, where the ash flows like water

Entered by: WSC_Geologist12

Added: 01/31/2012, Last Updated: 01/31/2012

Sources: Lipman, P.W., 1976, Geologic map of the Lake City Caldera area, western San Juan Mountains, southwestern Colorado: U.S. Geological Survey, Miscellaneous Investigations Series Map I-962, scale 1:48000.

The title says it all. The rocks in the area of Uncompahgre Peak, Nellie Creek, and Ultimately the Henson Creek drainage are what I like to think of as a volcanic nightmare. If you only want to hear about the rock types, then scroll through my history story to the bottom, where I describe the rock types.
Picture it this way: 32 Million Years ago the San Juan region was dominated by volcanic activity, think Mordor from Lord of the Rings (maybe less rugged, but there is no way to know). Each time a volcano erupted (there were quite a few) it left a deposit in the form of ash, lava, or breccia. Some volcanoes held their eruptions to the last minute before blowing their tops in an extremely violent manner. After eruption, the volcano's magma chamber is left empty and the weight of the volcano collapses in on the magma chamber likely rattling everything around (I've always wondered if this is an instantaneous process, if so it would be extreme. Unfortunately no humans were around 32 Million Years ago to tell me). At this point all the rocks and soils are shifted and fault as a result of the ever changing topography. The resulting landform is a "caldera" which we see now as a ring shaped fault that outlines the flanks of the volcano that erupted. This happened a total of 13 times in the San Juan Mountains. There are 4 separate calderas in the Lake City Region, the Uncompahgre Caldera, the Silverton Caldera, the Lake City Caldera, and the San Juan Caldera. Every time these calderas collapsed they shifted the landscape which causes the rock layers to tilt (dip), and overturn to whatever position they rested.
Today the mountains do NOT reflect the shape of the ancient volcanoes and it is difficult to distinguish what rocks came from what volcanoes, let alone determine where the center of the volcanoes are. The majority of the mountains in the region are composed of the ash, lava, or breccia that was deposited by these volcanoes. That brings us to Uncompahgre.
Uncompahgre Peak has been mapped as a sequence of 3 separate lava flow units that are unique to the peak. All other evidence of these flows has been eroded.
Take a look at the images attached I have drawn approximate lines where the contact between these units exists (extrapolated from the map cited below). The bottom layer of the flows is a sequence of Andesite. The next layer, observed by the mappers only on the west face is essentially sedimentary, originated from mudflows that flowed down the flanks of a volcano. The composition of this unit is volcanic clasts bound together by ash and mud (weathered/eroded ash). The upper unit which makes up the most of the peak is a Quartz Latite which for general terms is Ryolite. These can be distinguished from the lower andesite by composition. These three units lie on top of the Carpenter Ridge Tuff, which is a very thick unit composed of ash that likely flowed down the flanks of an ancient volcano. Most of the units near Lake City have this origin.
For more information see a local geologist, or check out the geologic map of the area: Geolgic map of the Lake City Caldera area 1976.
Thanks for reading!

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