Exploring lava dome eruptions

Within the broad spectrum of volcanic activity that is possible on Earth, the eruption of lava domes is a common and frequently hazardous phenomenon. Volcanic domes are formed when sticky, high-silica lava piles up around a vent instead of flowing rapidly away, as in the case of low-silica basaltic lava erupted in Hawai`i.

Erupting domes are commonly associated with fast-moving pyroclastic flows, which form when either blocks of fresh lava break off from the dome and move rapidly downhill or when a lava dome is shattered during strong explosive activity.

At the present time, there are 4 major lava dome eruptions taking place in the world – Augustine and Mount St. Helens volcanoes in the U.S., Soufriere Hills Volcano on the Caribbean island of Montserrat, and Santa Maria in Guatemala.

The activity is not making headline news now, but the volcanoes deserve close watching, because each is capable of producing powerful explosive activity and dangerous pyroclastic flows. Of the four volcanoes, only Santa Maria is not intensely monitored by scientists.

Augustine is a beautiful cone-shaped volcanic island rising steeply 1,260 m (4134 ft) above sea level in southern Cook Inlet about 290 km (180 mi) southwest of Anchorage, Alaska. The volcano’s summit consists of several domes, and the lower slopes are made chiefly of rock debris shed from these domes as pyroclastic flows and large landslides.

Augustine’s current eruption began on January 11, 2006, with a series of explosions that generated ash columns as high as 15 km (50,000 ft). A few days later, viscous lava began erupting at the summit and eventually spread partway down the volcano’s northern flank.

As the new lava crept down the steep side of the volcano, hot lava blocks from the leading edges broke away, generating pyroclastic flows that spread as far as about 3 km (2 mi) from the summit. Billowing volcanic ash rising from these pyroclastic flows has turned the usually brilliant white, snow-covered cone a dull brown. Photographs showing the red glow and pathways of the lava flows and pyroclastic flows can be seen at the Alaska Volcano Observatory‘s Web site.

At Mount St. Helens, a lava dome continues to grow inside the volcano’s horseshoe-shaped crater. This dome began forming 17 months ago on the south side of an earlier dome that had erupted between 1980 and 1986.

In late February, scientists placed a new monitoring station called a “spider” on the active part of the dome to track its movement. For the past two weeks, the spider has moved westward nearly a meter (3 feet) per day. A year ago, the active part of the dome moved 7 to 10 m (20-30 feet) per day! Close-up photographs and time-lapse sequences from cameras located on the volcano show how the steady extrusion of lava is building the dome. (See the Cascades Volcano Observatory Web site for current images.)

The current dome of Soufriere Hills Volcano began growing in August 2005, the third such episode during an eruption that began in 1995. The active part of the dome is shedding frequent rockfalls and small pyroclastic flows down the sides of the volcano (see glowing avalanches and dome at the Montserrat Volcano Observatory Web site.

The largest recorded collapse of an active volcanic dome occurred at Soufriere Hills in 2003, when about 210 million cubic meters (275 million cubic yards) of dome rock slid away over an 18-hour period. This event triggered large pyroclastic flows that swept into the sea, and caused a local tsunami and an underwater explosion that sent hot water and debris back onshore.

The longest historical dome-building eruption is ongoing at Santiaguito dome, which is erupting on the southwest flank of Santa Maria Volcano in Guatemala. The eruption began in 1922 after a large explosive eruption in 1902 carved a 1.5 km- (1 mi) wide crater in the side of the volcano and killed more than 1,000 people.

Since early March, several moderate explosions produced pyroclastic flows down the active dome and formed an ash plume about 3 km (10,000 feet) above the volcano, typical of the activity for the past 84 years.

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. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with scattered surface flows breaking out of the tube. In the past week, surface flows were on the coastal plain below Paliuli, less than 0.75 km (0.5 mi) inland of the coast at Kamoamoa, about 5.5 km (3.4 mi) from the end of Chain of Craters Road.

As of March 16, 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 900 m long by 250 m wide. 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 felt earthquakes beneath Hawai`i Island reported within the past week.

Mauna Loa is not erupting and earthquake activity remained low beneath the volcano’s summit. Inflation continues, but at a rate that has slowed since early October 2005.

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

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