The relation between earth movements and volcanism in the San Juan Mountains of Colorado

Abstract

The late Tertiary volcanism in the San Juan Mountains of southwestern Colorado followed a long interval during which the crust was stable. The volcanic rocks have been divided into four main groups, (1) those of pre‐Potosi (?) age in the eastern area, (2) the Lake Fork quartz latite, San Juan tuff, and Silverton volcanic series, (3) the Potosi volcanic series and the Fischer quartz latite, and (4) the Hinsdale formation separated by long intervals without eruptions. Each of the groups is made up of rocks from basalt to rhyolite with chemical and other peculiarities. During the eruption of a group there was little deformation but there was subsidence after the eruption of two of the groups and doming after the eruption of the last group. The magma moved into the area from the sides and was erupted about as rapidly as it moved.in. For the group named the Potosi volcanic series, the first eruptions had a composition near that of the primary magma ‐ dark quartz latites. Their eruption was followed by a short time without eruptions during which the magma became layered by crystal settling. The next eruptions were rhyolites followed abruptly, probably because of active movement of magna by dark quartz latites. The process was repeated three times. Subsidence caused by withdrawal of magma followed, and after a long time a new and different magma moved into the area and yielded the rocks of the Hinsdale formation. After the Hinsdale, a magma intruded the area with subsequent doming, but without eruptions.