Sand dunes on Kīlauea are related to past explosive eruptions

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Many people visit the Footprints Area on the southwest rift zone of Hawai‘i 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 Kīlauea? What is this sand, and how does it relate to Kīlauea'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 Kīlauea, 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 Kīlauea sand is different from that in most of the world's deserts and beaches.

Much of Kīlauea'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 Kīlauea 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 Kīlauea. 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.


Volcano Activity Update

A breakout on the coastal plain that began on Wednesday, November 24, reached Kalapana Gardens subdivision and destroyed one residence on Saturday, November 27. Just two days later, a vigorous breakout above the pali diverted lava out of the tube system that was supplying lava to the coast and ocean entry. Coastal flow field activity had stalled by Tuesday, November 30, as a low shield began to grow over the new breakout point above the pali. In addition, a small flow was active at the top of the pali.

At Kīlauea's summit, the circulating lava lake in the collapse pit deep within the floor of Halema‘uma‘u Crater, has been visible via Webcam throughout the past week. The circulation pattern was interrupted sporadically by abrupt increases in the height of the lava surface. These periods of high lava level have been short-lived, lasting up to several hours, and each ended with a sudden drop of the lava surface back to its previous level. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Eight earthquakes beneath Maui, Kaho‘olawe, and Hawai‘i Islands were reported felt during the past eight days. Five of these quakes were located in the same area about 19 km (12 miles) southwest of Makena, Maui, at a depth of 16-17 km (10 miles). Three of these Maui earthquakes occurred on Tuesday, November 23 with a magnitude of 4.7 at 6:34 p.m., 3.9 at 6:51 p.m., and 3.4 at 11:15 p.m. Earthquakes of magnitude 2.6 occurred at 2:41 a.m. on Sunday, November 28 and 2.9 occurred at 4:25 p.m. on Monday, November 29. A magnitude-2.3 earthquake occurred at 7:32 p.m. on Wednesday, November 24, 2010, and was located 5 km (3 miles) north of Pāhala at a depth of 36 km (22 miles). Two earthquakes occurred on Monday, November 29, at locations south of Kīlauea summit: at 1:16 a.m., a magnitude-2.1 quake was located 4 km (2 miles) southeast and, at 4:58 a.m., a magnitude-1.7 quake, was located 1 km (1 mile) southwest.

A magnitude-2.1 earthquake occurred at 1:16 am on Monday, November 29, and was located 4 km (2 miles) southeast of Kīlauea Summit at a depth of 25 km (16 miles). A magnitude-1.7 earthquake occurred at 4:58 am on Monday, November 29, and was located 1 km (1 mile) southwest of Kīlauea summit at a depth of 1 km (1 mile).