Volcano Watch — Acid rain, opal, and vegetation contrasts-thanks to Halema‘uma‘u

In a road distance of 7 km (4 miles) and straight-line distance of 5 km (3 miles), one goes from a lush tree fern (hapu`u)-`ohi`a forest to a nearly barren surface. Why? The volcano, mainly.

Precipitation drops rapidly in that distance, from a yearly average of about 280-300 cm (110-120 inches) on the windward side to about 150-180 cm (60-70 inches) at HVO 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 Kīlauea 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 Kīlauea Iki.

Thick deposits of several explosive eruptions between about A.D. 1500 and 1790 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. Cracks are where vegetation often gets started-for example, note the ferns and `ohi`a 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 `a`a.

An important factor affecting vegetation is the grain size of the surface explosive deposits. Those on the windward side tend to be fine-grained material that holds water and quickly weathers to release nutrients favorable for plant growth. The surface deposits on the leeward side are coarse, commonly gravel in size, don't hold water, and don't weather nearly as fast. Other things being equal, the fine volcanic ash would therefore be a more favorable substrate than the coarse gravel.

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. Trade winds blow this gas into the Ka`u 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 acid the soil is) of about 3.5. That's about like dilute vinegar. Few plants like such acid 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, gravelly deposits, reduced precipitation-these factors are mostly responsible for making the leeward side of Kīlauea's caldera more barren than its lush upwind side.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lavamoves 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.