Great Sand Dunes National Park and Preserve
Sand Avalanche and Ripples
NPS/Andrew Valdez
#ScienceSaturday - How do giant dunes move? Very slowly!
As predominate southwesterly wind bounces sand across a dune, it often forms ripples on the windward side and a 'slipface' or avalanche slope on the other side. Sand will continue to accumulate near the dune's crest, and the slipface's angle will try to increase. A gentle avalanche will occur, bringing the 'angle of repose' back to 34 degrees. (No dunes slope at Great Sand Dunes is greater than that angle, even though they may appear steeper when viewed face-on.)
"Normally, avalanches migrate to the top of the slope," said park geologist Andrew Valdez. "But in the slope pictured here, wind has blown across the slipface, creating ripple bedding that stopped the avalanche from pro-grading upslope." The more densely-packed ripples, usually found on the windward side, were firm enough to stop this avalanche from continuing. The dunes slowly continue to shift, although it takes more than a century to notice even minor changes in the dunefield.
The resulting scene of a very slowly migrating dune is one that appears alien and otherworldly to our eyes, without a typical landscape point of reference. The Great Sand Dunes landscape invigorates the imagination!
Learn more about Great Sand Dunes' geology on the park's geology pages: www.nps.gov/grsa/learn/nature/sanddunes.htm
#GreatSandDunes #Dunes #Sand #Avalanche #SandAvalanche #Ripples #Geology #Science
Sand Avalanche and Ripples
NPS/Andrew Valdez
#ScienceSaturday - How do giant dunes move? Very slowly!
As predominate southwesterly wind bounces sand across a dune, it often forms ripples on the windward side and a 'slipface' or avalanche slope on the other side. Sand will continue to accumulate near the dune's crest, and the slipface's angle will try to increase. A gentle avalanche will occur, bringing the 'angle of repose' back to 34 degrees. (No dunes slope at Great Sand Dunes is greater than that angle, even though they may appear steeper when viewed face-on.)
"Normally, avalanches migrate to the top of the slope," said park geologist Andrew Valdez. "But in the slope pictured here, wind has blown across the slipface, creating ripple bedding that stopped the avalanche from pro-grading upslope." The more densely-packed ripples, usually found on the windward side, were firm enough to stop this avalanche from continuing. The dunes slowly continue to shift, although it takes more than a century to notice even minor changes in the dunefield.
The resulting scene of a very slowly migrating dune is one that appears alien and otherworldly to our eyes, without a typical landscape point of reference. The Great Sand Dunes landscape invigorates the imagination!
Learn more about Great Sand Dunes' geology on the park's geology pages: www.nps.gov/grsa/learn/nature/sanddunes.htm
#GreatSandDunes #Dunes #Sand #Avalanche #SandAvalanche #Ripples #Geology #Science