The picture above shows the Sand Dunes themselves, as well as the San Juan mountain range in the background. The creation of these mountain is assumed to have take place around 30-35 million years ago. Based on information given by the NPS (National Park Service), the eruption of the La Garita caldera, the largest volcanic explosion in noted history was the source. After the initial explosion and several years of volcanic activity, such as lava flow took place, which created the flat-rounded San Juan mountains. The volcano eruption needed to create the mountain range is associated with that of a strato-volcano (Highly explosive). The strato volcano varies from cone and shield volcanoes due to its composition, number of vents, and the type of landforms it creates. There are many forms associated with strato volcanoes, which can be both beautiful and dangerous. Lava flow, pyroclastic flows (flying rocks), lahars (volcanic mudslides), and debris flow are just a few forms associated with the strato. With volcanic eruptions not being seen in thousands of years, the land forms created are evidence to show that activity is still present.
The San Luis Valley is a small portion of the Rio Grande Rift, which was created by diverging plate boundaries around 35-40 million years ago. The rift is very large in size and stretches from central Colorado to northern Texas. As the Rio Grande Rift separated it created Grabens (depressed blocks of land) and Horsts (raised blocks of land) in the San Luis Valley. As depicted in the image above, facing west towards the San Juan Mountains, you have the Monte Vista Graben; and to the east facing the Sangre de Cristo Mountains you have the Baca Graben. Between the two Grabens is the Alamosa Horst, which was formed by the extension of the earth's crust. Due to the location of the rift, geologists expect little to no action occurring the in the near future.
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