Rain clouds volcanic activity

Image of a raindrop

Visitors to Hawai`i from northern climates arrive with dreams of sunny beaches and gentle breezes. But this winter, rains of biblical proportions caused flooding, landslides, sewage spills, beach and park closures, and a catastrophic dam failure on Kaua`i.

The Big Island received a fair share of the deluge. Some areas on Kilauea, for example, recorded over 30 inches of rain in just two weeks in March. Yet we didn't hear any news stories about flooding and destruction on the Big Island. The main reason for this is the difference in age of the islands.

At the extreme ends of the age spectrum in the main Hawaiian islands, Kaua`i and Kilauea have soils that differ both in structure and just plain amount. Kaua`i has had a 5- million-year start on the Big Island in breaking down the porous and permeable volcanic rock into layers of clay-rich soils. Not only does water move more slowly downward through the upper layers of the old soil on Kaua`i, but it also hits clay-rich horizons that cause the water to move laterally along that shallow boundary instead of continuing to percolate downward.

The recent rains on the Big Island were obviously hard on crops and annoying to those of us whose homes were taken over by mold. The rains also made it slightly more difficult for us at HVO to assess what was going on at Kilauea Volcano's summit during a time of intense seismic and deformation activity.

Kilauea had been inflating since late 2003 - interesting and surprising in and of itself, since the eruption continued unabated from the Pu`u `O`o vent on the east rift zone - but in mid-January 2006, rates of extension and uplift increased dramatically. Seismic activity increased shortly afterwards, and, by February, we were recording more than 25 locatable earthquakes per day, many of them large enough to shake us up at the observatory.

In late January, the location of the maximum uplift moved to the southeastern caldera. This raised concerns that the volcano could be gearing up for a summit eruption there. Naturally, we were watching our continuously recording instruments carefully for signs that the magma might be shallowing, and doing whatever supplemental surveying the weather would allow (not much!).

Besides keeping us out of the field, the rain affected our ability to interpret data from two of our monitoring techniques that are particularly affected by severe rainfall: ground tilt and gas-emission measurements.

Measuring the tilt of the ground is a time-honored way to detect subsurface migration of magma. Unfortunately, the ground can tilt, not only in response to volcanic activity, but also from intense rainfall. Fortunately, the pattern of tilting at the various sites in the summit area was coherent enough to convince us that the signal from the inflating magma body was far larger than the rain-induced tilt. Also, we discovered that, after a certain amount of continuous rain, most locations that house tiltmeters become immune to spurious changes from additional rainfall.

Another valuable clue to what's going on beneath the surface is the abundance and composition of gases emitted by the volcano. One of the primary gases that we monitor is sulfur dioxide (SO₂), which dissolves very easily in water. When we get large amounts of rainfall, SO₂ is effectively "scrubbed" from the volcanic gases as they rise from the magma to the surface.

We already knew that emissions of SO₂, as well as carbon dioxide, had increased in the previous year. Combined with gravity measurements that showed mass was being added beneath the summit area and the lack of any obvious change in eruption rate at Pu`u `O`o, we were fairly sure that the magma supply to Kilauea had increased. Fortunately, the pressures did not rise to the point of creating new eruptive vents. Seismicity returned to normal levels by early April, but we continue to monitor the summit area closely.

While those of us on the windward side of the Big Island still haven't put away our raincoats, we hope the sunny weather elsewhere on the islands continues and wish the islanders who were hard-hit in the recent rains all the best in their recovery work.

Activity update

This past week, activity levels at the summit of Kilauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day). Extension of the summit caldera, indicating inflation, appears to have resumed after pausing earlier in April.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with occasional surface flows breaking out of the tube, mainly at the 2,300-ft elevation. In the past week, surface flows were active on the coastal plain in about the same location as the last several weeks: 0.4 km (0.3 mi) inland of the coast at Kamoamoa, about 5.5 km (3.4 mi) from the end of Chain of Craters Road.

As of April 20, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. The active lava bench continues to grow following the major collapse of November 28 and is now approximately 1,000 m (3,300 ft) long by 315 m (1,000 ft) wide, with a total area of 17 ha (42 acres). Access to the ocean entry and the surrounding area remains closed, due to significant hazards. 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.

There were no earthquakes beneath Hawai`i Island reported felt within the past week.

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (three earthquakes were located). Extension of lengths between locations spanning the summit, indicating inflation, continues to slow down.

Visit our web site (hvo.wr.usgs.gov) for daily Kilauea eruption updates and nearly real-time Hawai`i earthquake information.

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