Saturday, June 23, 2001

Volcano fumes affect
vegetation downwind

Eruption Update

U.S. Geological Survey
Hawaiian Volcano Observatory

Acid rain, opal and contrasts in vegetation -- thanks to Halemaumau.

Few landscape changes are as extreme as that between the windward and leeward sides of Kilauea's caldera. Simply drive from the Visitor Center in Hawaii Volcanoes National Park to the Southwest Rift pullout along Crater Rim Drive. In a road distance of four miles and straight-line distance of three miles, one goes from a lush forest to a nearly barren surface. Why?

The volcano, mainly.

Precipitation drops rapidly in that distance, from a yearly average of about 110-120 inches on the windward side to about 60-70 inches at Hawaii Volcanoes Observatory and somewhat less at the Southwest Rift pullout.

Such a drop certainly has an effect on the degree of vegetation. Still, plenty of rain falls, no matter what side of the caldera you're on.

You might guess that the difference in cover is due to different ages of volcanic deposits. But that's not right. Most of the lava flows on either side of the caldera were erupted from the shield volcano that once formed the summit of Kilauea before it collapsed to form the caldera.

These flows were erupted mainly during the 14th century. That's plenty of time for thick vegetation to grow: Witness the marvelous forest from the park entrance to Kilauea Iki.

Thick deposits of several explosive eruptions between about 1500 and 1790 A.D. cover the lava flows on both sides of the caldera. This tends to slow the return of vegetation. A deposit of volcanic ejecta has fewer cracks than does the surface of a lava flow, which shrinks as it cools and fractures thoroughly.

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U.S. Geological Survey
Hawaiian Volcano Observatory
P.O. Box 51, Hawaii National Park, HI 96718
Phone (808) 967-7328 Fax (808) 967-8890

Cracks are where vegetation often gets started -- for example, note the ferns and ohia in cracks of the Mauna Ulu flows along Chain of Craters Road. So, it often takes longer for vegetation to return to a surface blanketed by ash than to one underlain by cracked pahoehoe and aa.

There's a more important distinction between the windward and leeward sides, though: acid rain. Thanks to Halemaumau and its surroundings, about 100 tons of sulfur dioxide are emitted into the air daily. Tradewinds blow this gas into the Kau Desert. When it rains, the water combines with the sulfur dioxide to form dilute sulfuric acid. Acid rain results.

Several measurements of the acidity just below the ground surface have been made downwind of Halemaumau, in the Sand Hill area. Many of the measurements show a soil pH (a measure of how acidic the soil is) of about 3.5. That's similar to diluted vinegar. Few plants like such acidic conditions.

The acid rain has another effect. As water percolates into the ground, it dissolves silicon dioxide from volcanic glass in the explosive deposits. Chemically, silicon dioxide makes up about half of the deposits, so there's a lot available.

If the ground water then encounters air, such as at the ground surface, it evaporates, leaving the silicon dioxide behind as a form of the mineral opal. This mineral is what forms the hard crust (hardpan) on much of the ground surface southwest of the caldera. Vegetation doesn't like hardpan.

Acid rain, opal crust, gravely deposits, reduced precipitation -- these factors are mostly responsible for making the leeward side of Kilauea's caldera more barren than the lush upwind side.


Eruptive activity at
Kilauea continues, but
surface flows of lava
are seldom seen

Eruptive activity of Kilauea Volcano continued unabated at the Pu'u 'O'o vent during the past week.

Lava moves away from the vent area toward the ocean in a network of tubes and descends Pulama pali in two separate areas.

The coastal flats tube system has developed and matured to a stage where breakouts with surface flows are now seldom seen. The decrease of surface breakouts may also be attributed to a possible decline in the volume of lava in the tube system. Lava continued to enter the ocean in the area east of Kupapa'u throughout the week.

There were no earthquakes reported felt during the week ending on June 21.


The Star-Bulletin introduces "Volcano Watch," a weekly column written by scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory. The writers will address the science and history of volcanos in Hawaii and elsewhere. It also will include a short, separate update on eruption activity on the Big Island.

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