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Volcano Watch: Sand dunes related to past explosive eruptions

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Many people visit the Footprints Area on the southwest rift zone of Hawaii Volcanoes National Park. If lucky, they find human footprints indenting a gray ash bed that was deposited during the 1790 eruption. The ash was wet when it fell, and people walked on a muddy surface probably within a few hours after the eruption.

Sometimes, a favorite footprint can’t be found during a later visit. Sand has blown in and covered the print. Disappointed, the visitor looks up and sees high sand dunes just south of the shelter. Sand dunes, on Kilauea? What is this sand, and how does it relate to Kilauea’s volcanic activity?

Most sand dunes are formed by wind blowing fine material that has been eroded off preexisting rocks. The sand grains may have come from rocks millions of years old, or rarely, from coral a few hundred to a few thousand years old. No matter what the source, it was water that eroded the source material, grinding it down to sand-size grains. Wind then takes over and blows the sand into dunes, such as in the Sahara Desert.

But in the higher parts of Kilauea, lava flows are only a few hundred years old, too young to be much eroded. And, close examination of the sand grains shows that they don’t look like material in a lava flow.

They are almost pure glass, unlike the rather crystalline rock of most flows. So, the Kilauea sand is different from that in most of the world’s deserts and beaches.

Much of Kilauea’s sand comes ready-made, or at least is well along toward becoming the grains found in dunes. Explosive eruptions produce huge volumes of sand-size grains in the form of volcanic ash, as well as finer and coarser particles. Rather than water grinding away at rocks to produce sand, Pele does it straight from the vent.

Most of the explosive eruptions take place at the summit, so that is where most of the dunes should be found, right? Maybe in theory, but, in practice, coarse material falls in the summit region, too, covering the sand-size ash and keeping it from being blown around by the wind.

It is only beyond the area in which the coarse material falls that sand-size ash can be at the surface for decades or centuries and remain susceptible to wind blowing. This distance at Kilauea is 3-5 km (2-3 miles) from the caldera.

Beyond there, one finds a few dunes, though the largest are still farther out, 9 km (5.5 miles) in the Footprints Area and 10 km (6 miles) in another spectacular dune field mauka of the end of the Hilina Pali Road.

But the story is more complex. The grain size of the dune sand is typically too coarse for its distance from the summit. That is, fallout during eruptions would have dropped finer particles where we now find coarser sand.

So, the dune sand is not derived from wind blowing around ash-fall deposits in the immediate vicinity. Instead, the sand-size particles fell farther upslope and were, after eruption, brought into the general area of the dunes before becoming incorporated in the dunes.

Two processes account for this. One is obvious: dunes can migrate downwind, southwestward at Kilauea. Sand-size material that originally fell nearer the vent can be moved by repeated windstorms into areas where it is too coarse to have been originally deposited during the eruption.

In addition, the largest dune fields are near large drainages, down which flash floods pour during heavy rains. Such floods pick up loose ash particles and transport them downslope, which happens to be the direction of the trade wind. When the flood ends, the sand-size grains are deposited. The sun comes out, the deposits dry, and the surface is then available for wind to pick up some of the grains and blow them farther downwind to form dunes.

Whether or not water is involved, sand migration is a slow, step-by-step process. But slow steps add up. The glassy dune sand in the Footprints Area was erupted only 300-500 years ago. It has migrated several kilometers (miles) during that time and will continue to move southwest as long as the trade winds blow.

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