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Roger Weller, geology instructor                     

San Juan Mountains, Colorado
Yvonne Carter
Physical Geology
Spring 2009


                                     Wheeler Geologic Area
                     San Juan Mountains, Colorado


The central San Juan Mountain Range in Colorado holds elegant proof true beauty has evolved from a catastrophic supervolcano.  Roughly thirty-one miles southeast of Creede is the Wheeler Geologic Area.  To get there, one must endure 14 miles of an extremely rough road.  The Forest Service warns only high clearance 4 wheel drive vehicles stand a chance on this road. It seems only avid 4 wheelers or hikers traverse the high altitude terrain these days to view the picturesque landscape which Wheeler has to offer.


             Photo courtesy of: Aaron Johnson



Once claiming the status of Colorado’s first National Monument in 1908, Wheeler National Monument was changed in 1950 to Wheeler Geologic Area because of the difficulty of getting there.  The area was named in honor of Captain George Wheeler, who conducted geographical explorations between 1869 and 1879.




                                    Photos Courtesy of: Aaron Johnson


            To understand how Wheeler came to be we have to go back approximately 40- 25 million years.  It was during this time a massive ignimbrite flare-up occurred in the area of what is now Colorado, Utah, and Nevada.  Ignimbrite calderas are large volcanic depressions which are made when gas charged viscous magma bodies eject huge columns of ash that cover the surrounding area with a thick blanket of welded ash.  The shallow roof of the chamber usually then collapses.  

drawing from:


The ash at Wheeler is a fine-grained, crystal-rich dacite volcanic rock comprising mainly of welded shards of sanidine, plagioclase, biotite, quartz, magnetite, hornblende, ilmenite, and sphene, better known as the Fish Canyon Tuff.  The Fish Canyon Tuff was the result of the largest single pyroclastic eruption known on Earth, the La Garita Caldera, which happened around 28 million years ago.  The size of the caldera was difficult to map because of erosion and its vast scale.  It took geologists 30 years to actually determine that the size was 22 by 47 miles because inside the La Garita Caldera is several smaller calderas.  These include the San Luis, Bachelor, and Creede Calderas. 

          Below shows the State of Colorado, the dark red areas represent smaller calderas formed while the yellow area is the La Garita Caldera.




          The volume of the volcanic deposit from the La Garita Caldera was approximately 5,000 cubic kilometers (1,200 cubic miles).  That is enough material to bury the state of California to a depth of almost 39 feet.  Mt. St. Helens in 1980 only had about 1.2 cubic kilometers (0.3 cubic miles) of ash deposits.

                  In 1982, the Volcanic Explosivity Index (VEI) was devised by Chris Newhall of the U.S. Geological Survey and Steve Self at the University of Hawaii.  The VEI is a scale used to describe the relative size or magnitude of explosive volcanic eruptions.  The numbers 0-8 represent the increase of the volume of erupted material, the duration of the eruption in hours, the height of the column of materials erupting out, and the explosiveness, all around a factor of ten.  As one can see, the La Garita Caldera would have been off the chart, thus earning the description of a supervolcano.  However, geologists do not use the term “supervolcano”, they use the term “mega-colossal”.



gGraph from: U.S.G.S.

                        Over the space of a few million years, nine major ash flow sheets erupted from the scattered central volcanoes in the area, thus adding to the already existing tuff and varying in depth over the area as deep as five hundred feet.



                                    Photos Courtesy of: Aaron Johnson


            As time went on, the area was buried and compressed into rock, then raised up  thus allowing weathering to take place.  Since the amount of compaction varied, the rate of erosion differed also.  The varying rates of erosion resulted in alien-looking formations.  One will see balancing rocks, water chutes, pinnacles, towers, and mounds all within one square mile.  Wheeler Geologic Area is just one of the many beautiful places which Colorado claims as her own.




Creede, Colorado                                       La Garita Mountains, Colorado

Photos by: Yvonne Carter


Bachelor, Colorado                                    Natural Arch, La Garita, Colorado

Photos by: Yvonne Carter




The La Garita Mountains.   Photos Courtesy of:  Rod Holloway


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