Indonesia pays heavy price for proximity to plate boundary

Release Date:

Merapi volcano, on the island of Java, began to make headlines in late March, when increasing seismicity led the Volcanological Survey of Indonesia (VSI) to issue warnings of an impending eruption.

Indonesia pays heavy price for proximity to plate boundary...

Merapi volcano, a stratovolcano in central Java. Photo by Jack Lockwood, U.S. Geological Survey, September 6, 1982.

(Public domain.)

To volcano watchers, Merapi is a familiar name with a long history of destructive eruptions. Its impact on humans is exacerbated by its location in an area with one of the highest population densities in the world. About 70,000 people live in the immediate vicinity of the steep-sided cone, and the ancient royal city of Yogyakarta is only 25 km (15 miles) to the south.

Merapi has frequent eruptions that can last for several years. These are characterized both by lava domes—mounds of viscous lava that pile up over the vent in the summit crater—and by pyroclastic flows, searing hot flows of ash, pumice, and gas that can travel up to 110 km/hour (70 mph). Many of the pyroclastic flows are generated when steep-sided domes collapse, releasing pressurized gas within. Pyroclastic flows have been responsible for numerous fatalities in past eruptions at Merapi.

Merapi is also prone to lahars--mud or debris flows that originate on the slopes of a volcano. "Lahar" is actually an Indonesian word, which gives us a clue to how common this hazard is on Indonesian volcanoes.

The VSI's warning in March was borne out when the first lava flow was spotted near Merapi's summit on April 10. By early May, a new lava dome was growing at the summit. Pyroclastic flows began in mid-May, prompting the reported evacuation of 22,000 residents.

During the last week, growth of the lava dome at Merapi has continued, accompanied by frequent pyroclastic flows, which have advanced as far as 4 km to the southeast of the summit. On June 6, another round of evacuations began, in response to VSI concerns that a major collapse of the lava dome could trigger much larger pyroclastic flows.

On May 27, as Merapi's activity was waxing, a magnitude-6.3 earthquake struck 20 km (10 miles) southeast of Yogyakarta, killing more than 5,500 people, injuring thousands more, and leaving 600,000 homeless.

How closely was the earthquake tied to Merapi's activity several tens of kilometers to the north? Both the volcanic and seismic activity of Indonesia are ultimately controlled by plate tectonics. As our readers probably know, the Earth's lithosphere (the crust and upper portion of the mantle) is divided into about a dozen large plates that drift slowly across the earth's surface.

The most dynamic areas in the world, geologically speaking, are along plate boundaries. Indonesia, which sits on the Sunda Plate, is on a collision course with the northeastward drifting Australian Plate. Just offshore of Java is the Sunda trench, where the Australian Plate is subducting, or sinking, beneath the Sunda Plate at a rate of about 6 cm (2.4 inches) per year. As the Australia Plate grinds downward beneath the island of Java, the stresses generated by this massive earth-moving project trigger frequent earthquakes. Partial melting of the overlying crust provides fuel for Indonesian volcanoes.

There is no evidence to suggest a direct link between the May 27 earthquake and the ongoing eruption of Merapi. Here in Hawai`i, microearthquakes and volcanic activity are often so closely associated that it is obvious that the earthquakes are triggered by the movement of magma beneath the surface.

In the case of Java, however, both larger earthquakes and volcanic activity are concentrated above the subduction zone, and it is likely just bad luck they occurred simultaneously. Indonesian volcanologists did observe that the activity at Merapi seemed to escalate following the earthquake.

If the Sunda trench sounds familiar, it's because the magnitude-9.1 earthquake of December 2004 occurred near its northern end, offshore of Sumatra. That earthquake, the third largest since 1900, unleashed the tsunami that devastated coastlines from south Asia to east Africa.

If life in Hawai`i sometimes seems fraught with natural hazards, take time to reflect on how lucky we are not to be sitting over a subduction zone. We extend our heartfelt sympathy to the Indonesians who were affected by the recent earthquake and hope that Merapi will quiet down soon.

————————————————————————————————————————————————————————————————

Volcano Activity Update

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 frequent surface flows breaking out of the tube near the 2,300-ft elevation, and a persistent, but sluggish, flow, known as the "March 1 breakout," active on the coastal plain. In the last week, activity on the March 1 breakout was limited to a small area 1.8 km (1.1 miles) from the coast.

Lava is still 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 surface area of 17 ha (43 acres).

Access to the ocean entry remains closed, due to significant hazards. The National Park has reopened the surrounding area, however. 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.

Two earthquakes beneath Hawai`i Island were reported felt within the past week. The first was a magnitude-1.7 earthquake that occurred at 10:55 a.m. H.s.t. on Friday, June 2, and was located 3 km (2 miles) south-southeast of Ka`ena Point in Hawaii Volcanoes National Park at a depth of 42 km (26 miles). The second, with a magnitude of 2.5, occurred at 1:17 a.m. H.s.t. on Monday, June 5, and was located 5 km (3 miles) northeast of Kawaihae at a depth of 14 km (8 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit, with four short-period earthquakes located. Extension of lengths between locations spanning the summit, indicating inflation, continues at slow rates.