How were the Great Sand Dunes formed?
The story of how the Great Sand Dunes were formed is continually evolving, as new research discoveries occur each year. Below is a basic summary of what most geologists currently understand to be the broad series of events that took place in the formation of these massive dunes. A detailed scientific paper, published in 2007, is available as a .pdf file: On the Age and Origin of the Great Sand Dunes, Colorado. However, even newer research from 2015 indicates that Lake Alamosa is not the primary source of sand, but rather later, smaller lakes on the eastern side of the valley.
You may also learn about geological components of the Great Sand Dunes system, hydrology of Great Sand Dunes, and the variety of dune types in the park.
Basic Geological Story
Through the breaking apart and movement (rifting) of large surface plates on Earth's surface, the Sangre de Cristo Mountains were uplifted in the rotation of a large plate. Fossils from the bottom of an ancient sea are now preserved in high layers of rock in the Sangre de Cristos. The San Juan Mountains were created through extended and dramatic volcanic activity. With these two mountain ranges in place, the San Luis Valley was born, covering an area roughly the size of the state of Connecticut.
Sediments from both mountain ranges filled the deep chasm of the valley, along with huge amounts of water from melting glaciers and rain. The presence of larger rocks along Medano Creek at the base of the dunes, elsewhere on the valley floor, and in buried deposits indicates that some of the sediment has been washed down in torrential flash-flood events.
In 2002, geologists discovered lakebed deposits on hills in the southern part of the valley, confirming theories of a huge lake that once covered much of the
Smaller lakes still covered the valley floor, including two broad lakes in the northeastern side of the valley. Large amounts of sediment from the volcanic San Juan Mountains continued to wash down into these lakes, along with some sand from the Sangre de Cristo Mountains. Dramatic natural climate change later significantly reduced these lakes, leaving behind the sand sheet. Remnants of these lakes are still found today, in the form of sabkha wetlands.
Sand that was left behind after these lakes receded blew with the predominant southwest winds toward a low curve in the Sangre de Cristo Mountains. The wind funnels toward three mountain passes here - Mosca, Medano, and Music Passes - and the sand accumulates in this natural pocket. The winds blow from the valley floor toward the mountains, but during storms the winds blow back toward the valley. These opposing wind directions cause the dunes to grow vertically. See an animation showing how reversing dunes are formed.
Two mountain streams, Medano and Sand Creeks, also capture sand from the mountain side of the dunefield and carry it around the dunes and back to the valley floor. The creeks then disappear into the sand sheet, and the sand blows back into the dunefield. Barchan and transverse dunes form near these creeks. Learn more about the hydrology of Great Sand Dunes.
This combination of opposing winds, a huge supply of sand from the valley floor, and the sand recycling action of the creeks, are all part of the reason that these are the tallest dunes in North America. There are other dunes in Colorado, and in most western states in the US, but none as tall (750 feet) and none as dramatic. Here giant dunes rise in front of the alpine Sangre de Cristo Mountains, while streams flow across the sand seasonally, making for an unusual and unexpected sight.
Are the dunes still growing? How much do they change over time?
Currently, there is enough vegetation on the valley floor that there is little sand blowing into the main dunefield from the valley. However, even today there are still some small parabolic dunes that originate in the sand sheet and migrate across grasslands, joining the main dunefield. At other times, some of these migrating dunes become covered by grasses and shrubs and stop migrating. Thus, the dunes system is currently fairly stable.
Compare the two photos below showing the Great Sand Dunes' stability over 138 years. Opposing wind directions balance each other out over time. Also, the main dunefield is moist beneath the thin layer of dry surface sand. In windstorms, the top few inches of sand blows around, and the moist sand remains largely in place.
How old are the dunes?
Scientists don’t yet know a precise age. According to the 2007 scientific paper referenced at the top of this page, the dunes were thought to date from when Lake Alamosa began to disappear, estimated at 440,000 years ago.
However, newer research in 2015 seems to indicate that the Great Sand Dunes date from younger sediments above the Lake Alamosa deposits. A layer of pink granitic sand apparently separates Lake Alamosa deposits from higher deposits which are the source of Great Sand Dunes' sand.
The source of the pink granitic sand layer is still unknown, because it does not seem to match the rock types observed in the San Luis Valley. The closest known deposits of this type of pink granite are in mountains above the Arkansas River, near Buena Vista, Colorado. Some geologists theorize that the Arkansas River flowed into the San Luis Valley at one time, but the time frame for this doesn't match the currently estimated ages for the layer of pink sand. More research is needed to clarify these unusual findings.
A relatively new dating process, Optically-Stimulated Luminescence (OSL), is still in development. This method takes core samples of sand from deep within a dune, and attempts to measure how long quartz grains have been buried in the dark. If the deepest sand deposits can be accurately dated, the age of the dunes could be determined. Samples of sand from deep in the dunes have returned OSL dates varying between a few hundred years to tens of thousands of years old.
On the Age and Origin of the Great Sand Dunes (Madole, Romig, et al, 2007)
2007 Rocky Mountain Section Friends of the Pleistocene Field Trip: Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7–9, 2007 (USGS online publication) Michael N. Machette, Mary-Margaret Coates, and Margo L. Johnson
General Geology of the Northern San Luis Valley, Colorado (.pdf) James P. McCalpin
Tertiary Stratigraphy and Tectonic Development of the Alamosa Basin (Northern San Luis Basin, Rio Grande Rift, South-Central Colorado (.pdf) Brian S. Brister and Robbie R. Gries
Geoindicator: Dune Formation and Reactivation (web page, Global Change Research and Information Office)