FAQ: A Look Inside Mount Merapi

The continuing eruptions and belches of steam and ash from Indonesia's Mount Merapi have sent locals fleeing, fearing larger eruptions and wondering when the spewing lava will stop.

Gunung Merapi, as it is known in Indonesia, is the most active of the nation's 129 volcanoes, and smoke can be seen emerging from the mountaintop at least 300 days a year. The name Merapi could be loosely translated as "Mountain of Fire" from the Javanese combined words "Meru," meaning "mountain," and "api," meaning "fire."

Here, OurAmazingPlanet looks at what scientists do and don't know about this very active volcano.

Where is Mount Merapi located?

Mount Merapi, which stands at about 9,551 feet (2,911 meters) tall, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta, on the island of Java. Merapi is the youngest and southernmost of a volcanic chain extending north and northwest, to the Mount Ungaran volcano.

Tectonically, Merapi is situated at the subduction zone where the Indo-Australian Plate is sliding beneath the Eurasian Plate. It is part of the Pacific Ring of Fire – a section of fault lines and volcanoes stretching from the western coast of South America, Alaska through Japan and Southeast Asia.

What type of volcano is Mount Merapi?

Mount Merapi is a stratovolcano – a tall, conical volcano composed of one layer of hardened lava, tephra (fragmented material produced by a volcanic eruption) and volcanic ash. These volcanoes are characterized by a steep profile and periodic, explosive eruptions.

Explosive volcanoes such as Merapi erupt suddenly and with terrifying force. These volcanoes form when cooled magma creates a lava plug blocking a crater. The plug traps hot gas and magma under the ground, where they build up until the pressure becomes too great, at which point hot gas and magma explodes out of the volcano in a shower of dust, ashes, cinders and volcanic bombs.

When did Mount Merapi last erupt?

In early May 2006, pyroclastic flows (a fast-moving current of extremely hot gas and rock, which travels away from the volcano at very high speeds) traveled up to 2 miles (4 km away) and the lava dome growth rate of Merapi was an estimated 3,531,460 cubic feet (100,000 cubic meters) a day, with an estimated total volume of 141,258,400 cubic feet (four million cubic meters). In addition, pyroclastic flows and rockfalls continued through the end of June 2006.

What was Mount Merapi's worst eruption?

The volcano's biggest and most devastating eruptions occurred in 1006 and 1930. The eruption of 1006 was so bad that many believe the existing Hindu kingdom in the area was destroyed, as it spread ash over all of central Java. During the 1930 eruption more than 1,300 people were killed.

Historically, pyroclastic flows and lahars (a mud or debris flow) accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the volcano's western-to-southern flanks and caused many fatalities.

More than 100 people have died so far in the current eruption.

When did Mount Merapi's eruptions start?

Merapi has been active for about 10,000 years, according to volcanologist John Seach in New South Wales, Australia. Eruptions have become more explosive over time, with viscous andesitic lavas that form long, thick flows often generating lava domes. (Andesite rocks are named for the Andes Mountains, where they are common).

Dome collapse has often generated pyroclastic flows and larger explosions, which have resulted in eruption columns, hot volcanic ash emitted during an explosive volcanic eruption. The ash forms a column rising many miles into the air.

Does Mount Merapi have any cultural significance?

The volcano is considered sacred by some local people who believe a supernatural kingdom exists atop Merapi, according to Indhanesia.com, an informational website about Indonesia. Every year a priest climbs to the top to make an offering.

Why are the current eruptions getting worse over time?

It is not unusual for eruptions to get worse over time. About half of volcanic eruptions follow this pattern, said Rick Wunderman, of the Smithsonian Institution's Global Volcanism Program. Wunderman told OurAmazingPlanet that many eruptions go critical over the first few hours, days and weeks of an eruption, but noted that others can take a long time.

What causes Merapi's eruptions to stop and start on a daily basis?

"You can compare a volcano to a can of spray paint in this case," Wunderman said. "When it gets close to the end of the container, it doesn’t come out uniformly anymore, and that can be for a lot of reasons: If the nozzle gets clogged; maybe the upper part of the volcano has cooled enough, or has become congested."

Whether the clog gets unstuck depends on several factors, including whether or not there is enough pressure to push the material out and the volume of material there is to push out, Wunderman said. There's also the distance that material has to travel.

"The material has to travel 30 miles [48 km] to get to the surface; there has to be enough propellant force to push them all that way and out," Wunderman said.

The fact that Merapi is a stratovolcano also determines its rate of eruption, he added.

"With many stratovolcanoes, there is a tendency to shove out stuff that isn't that energetic, called a lava dome. The lava dome doesn’t flow, it just sits as an unstable blob on top," Wunderman explained. But "if something such as an earthquake disturbs it, it can be shaken and rolled down and cause quite a bit of damage."

"Merapi is the poster child for unstable lava domes," Wunderman said. "The dome on Merapi rests on a steep, unstable environment, and it is easy for pieces to break off and do damage; for example, hot gases can be released and form a superheated, high speed cloud that rolls down the mountain."

Are there any signs that indicated the volcano will stop erupting any time soon?

"There are times when there are clear symptoms as to when a volcano will stop, but it is hard to know in this case because the volcano's behavior changes hour to hour and week to week," Wunderman said. "Some general signals that we go by are if the volcano is puffing and if earthquakes have become feeble or returned to their background levels (their usual state during times of inactivity for the volcano). In addition, with Merapi, the dome is there and eruptions can further build up that dome and resettlement of that dome can cause pieces to break off and fall. It is a difficult thing to estimate."

• The World's Five Most Active Volcanoes
• Indonesia's Explosive Geology Explained
• History's Most Destructive Volcanoes

This article was provided by OurAmazingPlanet, a sister site of LiveScience.