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November 5, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Water on volcanoes: heavy rain and crater lakes

The terrible tragedy in Nicaragua and Honduras from Hurricane Mitch's extraordinary rainfall was made worse by a volcano. The volcano didn't erupt, and it isn't even listed separately among the 1,511 volcanoes known to have been active in the past 10,000 years. But La Casita, a subsidiary cone of the San Cristobal volcanic complex in northwestern Nicaragua, is steep, stands some 1,200 m (4,000 feet) above its surroundings, and has lots of loose rubble on it. Rainfall amounting to perhaps 1 m (40 inches) in a day or two saturated and eventually mobilized the soupy rubble into a landslide and mudflow that rushed through coffee farms, killing 1,500-2,000 people.

Initial reports incorrectly suggested that torrential downpours caused a crater lake to overflow and breach the crater rim. La Casita actually has no crater lake, but so much water poured down the volcano that some people thought it must have come from a draining lake.

Craters and calderas are natural places for lakes to form, because they are closed depressions that collect rainfall and snowmelt. The best known to Americans is Crater Lake, Oregon, in a caldera indenting the summit of Mount Mazama. Newberry Volcano in Oregon has two lakes in its caldera, separated by a small cone built on the caldera floor.

There is one crater lake in Hawai`i. Green Lake occupies part of the floor of Kapoho Crater. This cone in lower Puna was formed by magma erupting through groundwater 300-350 years ago. The lake developed soon thereafter; it was known to Hawaiians as Ka Wai a Pele, according to Ellis' 1823 account.

Crater lakes have inherent beauty, perched on volcanoes and surrounded by rugged cliffs. Often a crater lake forms an oasis amid desolation.

But crater lakes also have inherent danger. Living below a volcano with a crater lake is like living downstream from a dam of unknown strength.

Kelut Volcano in Indonesia is infamous for the many times that its crater lake overflowed to form lethal mudflows. Tunnels were finally constructed to drain most of the lake and reduce the risk of overflow. Mount Pinatubo in the Philippines has a caldera lake that began forming after the gigantic eruption in 1991. Rainfall is inexorably raising lake level; before long water will spill over the caldera rim if nothing is done about it. The rim, made of loose ash, could erode quickly, unleashing a torrent.

Eruptions into crater lakes can be entirely benign or extremely violent. In 1971-72, slow extrusion of lava onto the floor of a crater lake at La Soufriere Volcano, on the Caribbean island of St. Vincent, heated and eventually evaporated 75 percent of the water, with no harm to anyone. Then, in 1979, rapidly rising magma mixed with some of the remaining lake water, resulting in strong explosions and eruption columns 18 km (11 miles) high. Magma rising rapidly into any crater lake can lead to such explosions; a fissure opening across the floor of Green Lake could do it.

Crater lakes can also trap gas emitted by fumaroles on the crater floor, with at times disastrous results. At Lake Nyos (Cameroon) in 1986, trapped carbon dioxide gas suddenly burped from the lake. The gas, heavier than air, silently flowed down a valley from the crater rim, suffocating 1,700 people and 3,000 cattle.

The October 30 tragedy at La Casita, and the treacherous history of crater lakes, show that water and volcanoes can be a deadly combination, even without an eruption.

(Late development: As if Mitch weren't bad enough for Nicaragua, Cerro Negro Volcano, 35 km (20 miles) southeast of La Casita, began erupting on November 3.)

Eruption and Earthquake Update

There is no change in the eruptive activity of Kilauea Volcano. Lava continues to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the seacoast. Active surface flows from breakouts of the tube system are often observed in the coastal flats. Lava is entering the ocean along a section west of Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam clouds are highly acidic and laced with glass particles.

There were no felt earthquakes since October 28.

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Updated: 16 Nov 1998