|Information Entries for Sunlight Peak and Geology|
Geology (Sunlight Peak)
Title: Geologic History of the Needle Mountains
Entered by: ztop
Added: 11/17/2010, Last Updated: 11/17/2010
Sources: Blair, Rob; 1996, Origin of the Landscapes of the San Juan Mountains; in The Western San Juan Mountains: Their Geology, Ecology and Human History; University of Colorado Press, Boulder, Colorado. Faure, Gunter, 2001, Origin of Igneous Rocks:the Isotopic Evidence; Springer-Verlag, Berlin. Fielder, John, 2004 , Mountain Ranges of Colorado; Westcliffe Publishers, Boulder, Colorado Lipman, Peter; Thomas A Steven, and Harald H Mehnert, 1970, Volcanic History of the San Juan Mountains, Colorado, as Indicated by Potassium–Argon Dating; GSA Bulletin; August 1970; v. 81; no. 8; p. 2329-2352; Lipman, Peter W., 2006, Geologic Map of the Central San Juan Caldera Cluster, Southwestern Colorado, USGS Geological Investigations Series I-2799
The Needle Mountain 14ers--Eolus, Sunlight and Windom--are an island of ancient igneous basement rock in the midst of the San Juan volcanic field. Surrounded by young volcanic rocks and sediments that once covered the peaks, they are composed of Precambrian granite.
Sediments were deposited in the area through the quiet times of the early Paleozoic Era. During the Pennsylvanian (~250 million years ago) the area was uplifted as part of the Uncompahgre highland, which raised much of western Colorado. Both the Uncompahgre and Front Range uplifts were a product of plate tectonic activity as Africa collided with North America to the southeast. At the end of this tectonism, the mountains were eroded, leaving a thin layer of sedimentary rocks. Gradually the sea crept over the area. Marine and then fluvial sediments were deposited during Jurassic and Cretaceous time as the seaway that covered the area was filled in. These sediments covered the entire San Juan Mountain area at the start of volcanic activity.
Volcanic flows in the San Juan Mountains began during the Oligocene Epoch, 30-35 million years ago. The earliest volcanic flows were made up of lavas that flowed across the landscape punctuated by occasional explosions of ash and rock. These rocks covered much of the southern Rocky Mountains, although a lot of it has since been eroded. As the eruptions continued, magma moved closer to the surface, finally collapsing the volcanoes above and creating huge craters called calderas. The broken rock of the calderas were places where hot, mineral-rich solutions could move through the rocks and deposit the gold, silver, lead, copper and zinc ores which were mined in Silverton, Creede, Ouray, and Telluride.
The Needle Mountains are composed of resistant Precambrian granite which stands high above the sedimentary rocks to the south and volcanics to the north and east. The rock contains abundant pink feldspar which gives these peaks their distinctive color. The large crystal size indicates that the magma cooled slowly, allowing time for the crystals to grow large. This granite is a competent (hard) rock, broken by a few fractures. The large blocks on the summits of these peaks are a product of this fracturing.
During the Pleistocene these mountains were extensively glaciated creating the sharp ridges, steep walls and deep cirques that are so distinctive of the Needle Mountains. Glaciers sitting against Sunlight and Windom would have come together to form one large glacier as the ice moved down the valley toward Chicago Basin.