Much of the sand at Great Sand Dunes is the result of tremendous volcanic activity. Across the valley from the dunes, the San Juan Mountains formed 35 to 15 million years ago as lava and ash erupted, creating a vast plateau that covers most of southwest Colorado. Some of the largest volcanic explosions in Earth’s history occurred there, with the evidence being the huge calderas (volcanic crater) that are common there. Once the volcanic activity ceased, the San Juans have been eroding, turning the volcanic rocks into sand size fragments. About 70% of the sand at Great Sand Dunes originated as volcanic rocks. We know this because a close of the sand reveals that its minerals are of volcanic origin, confirmed by age-dating the sand. This volcanic 70% of the sand is the same age as the volcanic activity in the San Juan Mountains.
The San Luis Valley itself was formed as part of ‘rifting’ across western North America. Rifting is the pulling apart and rotation of plates or blocks over the Earth’s hot mantle, forming wide basins and uplifted mountain ranges. Here, the San Luis Valley floor has tilted downward to the east while the Sangre de Cristo Mountain Range has risen to 14,000 feet (4,000 m). This allows for a perfect depositional basin at the base of the mountains where sediments can gather. Immense quantities of volcanic sediments eroded from the San Juan Mountains, mixed with some sand from the narrower Sangre de Cristo Mountains, have been carried by streams into large lakes that have periodically filled this basin. The streams that originate in the San Juans are larger and along with having more water, carry more sediment, making the San Juans the greater source of sediment. During drier times, these primarily volcanic sands have blown from the basin into a low curve of the Sangre de Cristo Mountains, forming the tallest dunes in North America. When viewed under a microscope, this sand has flecks of every color of the rainbow, revealing the dozens of minerals from where it originated in the fiery volcanic past. The clear quartz may have come from the volcanic ash blasted out of the giant caldera eruptions.
When you visit the park, take time to notice the unique geological features around you. Each tells a story of how the elements of this diverse landscape came together in just the right way, resulting in these dunes that have been a place of inspiration, wonder, and recreation for thousands of years.
