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USGS Geology in the Parks

Coasts: Sand and Dunes

Dunes

Dunes - Getting Started

Dunes provide a buffer to the coastal system - they protect the regions behind them from wind and waves, and provide a substrate, or ground, for plants to form communities upon. Three things are required for dune formation to occur: a large supply of sand, wind speeds capable of moving it, and an ideal location for its accumulation.

Sand Supply

A sandy beachSand comes from many locations and environments, and may be derived from either rock or biotic sources. It can have a source as close as rockfalls from seacliffs, or be transported hundreds to thousands of miles by streams and rivers. A portion is even carried as dust and sand in the air. Once the sand is has been washed into the ocean longshore drift moves and collects it in locations - prompting dune formation.

Sand Type

Sand grainsThe majority of the sand you'll see in temperate regions is composed of minerals eroded from bedrock. It is generally a combination of many mineral types and might look something like the sample at right. The primary component of rock-derived sand is silica, what both quartz and glass are made of, and is very resistant to erosion. It remains after other minerals have been broken down mechanically and altered chemically by their environments. Another mineral can also be found in sand - calcium carbonate, or CaCO3, in higher percentages the closer to the equator the sample comes from. Calcium carbonate is a biotic by-product - it is grown as a shell or skeleton by marine organisms suck as molluscs and corals. Many other minerals are also present in beach sand; among these black sand and green sand are explored in the links below.

Dig deeper - silica sand
Dig deeper - calcareous sand
Dig deeper - green sand
Dig deeper - black sand

Wind

Diagram showing deposition of sand

Once sand begins to pile up, ripples and dunes can form. Wind continues to move sand up to the top of the pile until the pile is so steep that it collapses under its own weight. The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle, usually about 30-34°, is called the angle of repose. Every pile of loose particles has a unique angle of repose, depending upon the properties of the material it's made of, such as the grain size and roundness. Ripples grow into dunes with increase of wind and sand input.

Sand beds exposed
Fort Funston, Golden Gate NRA.

The repeating cycle of sand inching up the windward side to the dune crest, then slipping down the dune's slip face allows the dune to inch forward, migrating in the direction the wind blows. As you might guess, all of this climbing then slipping leaves its mark on the internal structure of the dune. The image on the right shows fossil sand dune structure preserved in the Merced Formation at Fort Funston, Golden Gate National Recreation Area. The sloping lines or laminations you see are the preserved slip faces of a migrating sand dune. This structure is called cross-bedding, and can be the result of either wind or water currents. The larger the cross-bedded structure, however, the more likely it is to be formed by wind, rather than water.

A breeze of 10 mi/hr, or 16 km/hr will put fine sand in motion, but will not form dunes. Dune formation relies upon the weight and size of the sand grains in relation to the prevailing, or normal, wind. If the wind blows too hard, any existing dunes will actually be destroyed, as can happen in hurricanes. Once the wind drops back down to normal the dunes start to form again.

Dig deeper - hurricane effects on dune sand

Sand Trap

Waves on a beachWaves move the sand provided by longshore drift - the migration of sand along the coast by tide and wave action. Waves push the sand up on the beach, and then pull it back out, however the current will cause the overall movement to be in a certain direction. This can be seen in the image at left, the waves are breaking towards the viewer, pushing sand in that direction. When the landscape changes and the longshore drift is disrupted, beaches and dune fields can be cut off from their supply of sand, and shrink. For this reason engineers are very careful of where channels and sea-walls, rip-rap, and groins are placed.

Dig deeper - limited sand resources; the loss of beach sand and dunes

Dune Stability

Dune stabilized with vegitationBecause they are entirely composed of loose, or unconsolidated, sand, dunes tend to be very fragile, mobile, and susceptible to deterioration and erosion. They act as a buffer zone, protecting the land behind them from the force of ocean wind and waves. Plants that colonize the dunes act to stabilize the dunes and entrain blowing sediment. These "pioneer" species also act to give other species a foothold, enabling a progression of species that act to further stabilize and enlarge the dune field. Animals, humans and severe weather can all impact negatively on this stabilization. The image to the above right is an experimental plot showing the impact of grazing by horses on dune stability at Assateague National Seashore. For more on this study, click the link below.

Dig deeper - the impact of horse grazing on the dunes at Assateague National Seashore


Parks with Coastal Dunes | Parks with Terrestrial Dunes

 

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