Monday, April 22, 2024
Volcano Watch

A taste of Kilauea's sand

You can find some unusual, “spicy” sand on Kilauea, if you know where to look. Drive into the kipuka along Chain of Craters Road beyond Muliwai a Pele and stop at an unpaved pullout just before the first left-hand curve in the road. Walk a few meters (yards) away from the road. Most of the ground surface is bare, stripped of loose debris by heavy rain, but look closely in small swales and you’ll see salt-and-pepper sand.

Salt and pepper? Well, not exactly, but the sand has that appearance. Heavy on the pepper, but it’s the salt that stands out. What is it?

An explosion from Kilauea’s caldera, probably in the 1700s and possibly about 1790, propelled a column of pulverized rocks high into the air. Most of the rocks are dark basalt, the common rock that one sees everywhere on Kilauea and Mauna Loa.

But some are light gray and gray-green and come from the interiors of solidified lava lakes. They have a light color because they contain many crystals of a white mineral called plagioclase feldspar, formed when the lava cooled slowly in the thick lakes.

When the pulverized rocks fell back to earth, they formed a deposit of sand-sized volcanic ash with a pepper-and-salt appearance: the normal basalt is the pepper, and the pieces of lava-lake interiors are the salt.

There’s another kind of “salt” in the sand, too. It contains much coarser crystals of plagioclase feldspar and shiny black crystals of the mineral pyroxene. You have to look closely to find it, but when you do, you’ll be seeing pieces of a rock from within Kilauea’s interior that crystallized so slowly that the crystals could grow to more than 5 mm (1/4 inch) across.

This rock is called a gabbro, or, more correctly, a microgabbro, because the grain size isn’t really up to that of a typical gabbro.

The location recommended above is simply one of many places to see the salt-and-pepper ash. Careful searching will reveal it everywhere along the Chain of Craters Road, and out to Pu`u `O`o, for that matter, except where younger lava flows cover it.

Discerning readers will recognize a problem. How can the salt-and-pepper ash be distributed southeastward from Kilauea’s caldera? The trade winds should have blown it southwestward from the summit and kona winds, northward.

To sprinkle salt and pepper southeast of the caldera, there must have been a strong west or northwest wind blowing. Such a wind is vanishingly rare at low altitudes in Hawai`i, but winds higher than about 10 km (30,000 feet) commonly blow from the west at 50 knots or more. Northwest winds seem more common in the winter. See the web site for daily forecasts of these winds.

The evidence suggests that the explosion column reached into the high-level wind and that the ash was consequently blown eastward or southeastward. As it eventually fell to lower altitudes, the trades would have twisted the trajectory a little toward the southwest. Thus, the explosive eruption was able to sprinkle salt and pepper far southeast of its vent in the caldera.

Probably the ash column rose much higher than 10 km (30,000 feet), because the salt and pepper is spread all the way to the coastal area of the Chain of Craters Road. To go so far, the ash must have been very high before it started to fall.

To see the salt-and-pepper sand along lower Chain of Craters Road, stop at any of the parking areas near the coastline and walk 150-250 m (yards) inland. With careful searching, you’ll find the salt-and-pepper sand in low areas between tumuli. The sand has been stripped away near the sea cliff by storm waves and tsunamis; in fact, one could map the limit of the tsunami swash by locating where the salt-and-pepper sand disappears.

Pele will certainly spice Kilauea in the future. Some of the explosions will be large enough for ash to rise to great heights and drift southeastward. Kilauea’s history tells of violent explosions, as well as far more numerous lava flows. Pele’s notorious temper may reflect the explosive nature of the volcano. Just as we throw things in anger, so Pele hurls rocks to drive off Kamapua`a.

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.

The PKK lava tube continues to produce intermittent surface flows from above the top of Pulama pali to the ocean. Three ocean entries were active as of May 26. The two largest are at East Lae`apuki and East Kamoamoa, with a much smaller entry halfway in between. Surface flows are active intermittently inland of the entries. The East Lae`apuki entry is the closest activity to the end of Chain of Craters Road, in Hawai`i Volcanoes National Park, and is located about 4.5 km (3 miles) from the ranger shed. Expect a 2-hour walk each way and bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Remember-the beaches that sometimes form next to an active bench are just as dangerous as the bench itself. Stay off both, and heed the National Park warning signs.

During the week ending May 25, there were two earthquakes reported felt on Hawai`i Island and a rare one on O`ahu. A magnitude-4.1 quake occurred 34 km (21 miles) northeast of Kahalu`u, O`ahu at a depth of 7 km (4 miles) at 5:52 a.m. on Friday, May 20; this earthquake was felt at Kailua and Kane`ohe, O`ahu. A magnitude-3.0 earthquake occurred 5 km (3 miles) southeast of Kapoho at a depth of 0.5 km (0.3 miles) at 9:10 a.m. on Sunday, May 22; this earthquake was felt at Leilani Estates, Nanawale, and Pohoiki. A magnitude-1.6 earthquake occurred 7 km (4 miles) south of Pu`u `O`o at a depth of 5 km (3 miles) at 11:32 p.m. on Wednesday, May 25; this earthquake was felt at Pahoa.

Mauna Loa is not erupting. During the week ending May 25, 9 earthquakes were recorded beneath the summit area. Four were deep and long-period in nature. Inflation has slowed beneath the summit and flanks over the last few weeks.

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

Hawaii Star Wire

Press releases, media advisories, and other announcements submitted to the Hawaii Star.

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