Calderas form during massive volcanic eruptions where large portions of the surface collapse into the emptying magma chamber below. This type of eruption generates hundreds of cubic miles of ash, which in turn creates tremendous amounts of volcanic rock.
Pictured right: Upper image shows the initial phase of an idealized caldera ash flow eruption. Lower image shows subsequent collapse of overlying rock into the magma chamber below forming a depression-like feature called a "caldera".
Colorado has experienced at least 20 caldera eruptions. The San Juan volcanic field is one of the most notable areas in Colorado where this type of eruption occurred. Hundreds of square miles of Colorado's southwest are covered by ash flow tuff generated by these eruptions. The world's largest caldera, La Garita, is also located in the San Juan volcanic field. This caldera extruded over 1,200 cubic miles of ash flow, which is also the world's largest ash flow deposit.
Below: Locations of several Calderas (beige) along with other volcanics (red) in Colorado. Note the large cluster of calderas in the southwest which lie within the San Juan Volcanic Field. Within La Garita's outline, multiple smaller calderas were active post La Garita's eruption.