Volcanic ash builds 'sand dunes'

Volcanic sand dunes? Don’t worry, these aren’t the next disaster waiting to happen. They’re merely another neat feature of volcanic landscapes. Although rare in the State of Hawai`i, volcanic sand dunes can be found in the Ka`u Desert.

To understand volcanic sand dunes, it helps to refresh our thinking about dunes in general. Dunes form whenever there is sufficient wind and a sustained source of sand. The wind must whip along faster than about 20 kilometers per hour (13 mph) to get sand into motion. The sand must be fine grained—smaller than 1 millimeter (less than 1/32 in.)—but not dust, which will stay in suspension and be blown far beyond the dune region.

Most Hawaiian sand dunes are light-colored beach-sand hills that have formed downwind from our white-sand beaches, a prodigious source of sand. The sand consists chiefly of coral and shell fragments, which are rich in calcium carbonate. The dunes advance inland from the shoreline unless stabilized by vegetation or blocked by a cliff or other large topographic obstruction.

The beach-sand dunes range widely in age. Small, active dunes are found on all the islands. Some are more than 10,000 years old, such as those that underlie Kahului, Maui. Groundwater moving through the sand dissolves and redeposits the carbonate, converting the soft dunes into hard rock. On Kaua`i, some old dune deposits have been mined for making cement. A lava flow sandwiched between the oldest Kaua`i dunes yielded an age of 375,000 years.

Explosive eruptions, which produce ash, are the quickest way to make sand-size particles of volcanic rock. At Kilauea, in the years from about A.D. 1500 to 1790, numerous explosive eruptions buried the summit beneath blocks and ash in deposits that exceed 12 meters (40 ft) thickness. To get ash redistributed into sand dunes, bring on the trade winds.

Kilauea’s sand dunes form a narrow, discontinuous, irregular band on the southwest rift zone, crossing the Ka`u Desert at an altitude of 760-790 meters (2500-2600 ft). A few can be seen from the highway, but most are in the roadless part of Hawai`i Volcanoes National Park, downslope from the Footprints area and Mauna Iki. They consist chiefly of glassy basaltic particles smaller than 0.3-0.5 mm in size. Other components are tiny rock chips and crystal fragments, mainly olivine.

The most extensive Kilauean dunes form ridges several meters high (20-30 ft). They are irregular in form, but many are oriented preferentially along the predominant wind direction, the northeasterly trades. Another factor that may enhance their elongation is the trend of faults and gaping cracks along the rift zone, coincidentally in line with wind direction. The majority of the dunes are vegetated. In some cases the anchoring vegetation grows preferentially in these cracks.

Most of the dunes are stabilized, showing little or no change in position during the past 50 years. A few are advancing, but the rate of advance is slow—a few centimeters to as much as 0.5 meters per year (max. 1.6 ft), judging from comparison of 1954 air photos and current mapping. Perhaps the most vivid sign of sand on the march is the way it buries familiar features. Volcanic sand has nearly completely buried some rock walls built in the early 1900s to keep cattle from national park land.

The age of Kilauean sand dunes extends from today back to the time of the explosive eruptions, or about A.D. 1500. Found within many dunes are thin ash layers that show the evidence of deposition by ash falling from volcanic clouds. Thus the current dune field grew initially by two processes: abundant glassy ash of fine-sand size was being blown downwind between eruptions and then mantled by new ash from an ensuing eruption, a process repeated many times until the end of that spate of Kilauean explosive activity at about A.D. 1790.

The dunes’ long-term fate? Unless future explosions renew abundant supplies of ash, or unless lava flows bury them, the current dune field will inch downslope, to be taken by the sea.

Activity Update

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone.

Lava continues to flow through the PKK lava tube from its source near Pu`u `O`o to the ocean, with very few surface flows breaking out of the tube. Small flows are visible intermittently on the steep slope of Pulama pali. Two ocean entries, at East Lae`apuki and East Kamoamoa, were active as of August 4. Several partial collapses of the East Lae`apuki bench have occurred during the last month. Access to the ocean entries and the surrounding area has been closed due to significant hazards. No easily accessible surface flows are currently present. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

During the week ending August 3, only one earthquake was felt on Hawai`i Island. At 5:52 p.m. on July 30, a magnitude-3.3 earthquake located 39 km (24 miles) west of Kailua occurred at a depth of 5.4 km (3.4 miles); this earthquake was felt at Kailua.

Mauna Loa is not erupting. During the week ending August 3, twelve earthquakes were recorded beneath the summit area. Seven were deep long-period events; the others were short-period events at various depths. Inflation continues.

This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by HawaiiNews.com with permission.

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