21/11/2020
- How much sand is at Great Sand Dunes? Are there more sand grains than stars?
The first is a common visitor question at Great Sand Dunes. The second question may be pondered by those visitors that camp and gaze into the spectacular night skies on a moonless night at Great Sand Dunes.
In an effort to serve our visitors, the NPS will attempt to answer how much sand is in the dunefield. Keep in mind that the visible dunefield is just a small part of extensive sand deposits on this side of the valley, and these deposits also extend over 100 meters (300 feet) into the subsurface.
In 2011, the US Geological Survey mapped Great Sand Dunes using a laser scanner (lidar) mounted on an airplane. The lidar data is metric, so we'll use those units. It produced a map with accurate elevation, something that didn't exist prior, and the National Park Service can use this data to calculate sand volume. From that we get the area of the dunefield as 69 million square meters (27 square miles) and an average sand thickness above the surrounding valley floor as 95 meters (310 feet). If we multiply area by thickness we get 6.5 billion cubic meters (1.6 cubic miles) of sand. When you gaze at the dunefield from a distance, if you can imagine piling all that sand into a cube, each side would be 1.6 miles long.
We can calculate a value for sand grains, but it takes some assumptions. The average sand size has been measured to have a diameter of 0.25 mm. If the sand were perfectly round and perfectly packed, there would be a sand grain every 25 mm, but we know that's not true. The sand has irregular, rounded shapes and aren't perfectly packed. The porosity of the sand is about 35%, providing lots of air space, and we'll assume that there is an increase of grain to grain spacing of about 35%. That means there is a sand grain every 0.34 mm and 2,963 sand grains per linear meter. If we cube 2,963, there are 26 billion sand grains in a cubic meter.
If there are 26 billion grains of sand in a cubic meter and 6.5 billion cubic meters of sand in the dunefield, then we multiply these figures and get 170 Quintillion (1.7 x 1020) grains of sand in the dunefield of Great Sand Dunes. As for how many stars are in the universe, we'll let the astronomers at NASA - National Aeronautics and Space Administration calculate that!
Photo: Michael Curtis, used by permission
Image description: Aerial photo of dunefield, looking east to Medano Pass and Mount Zwischen
