Mount Merapi

Indonesia is an exciting place for geoscientists, but the rest of us get a little confused by all the headlines about erupting volcanoes there: Tambora, Bromo, Sinabung, Agung, Merapi . . .

Let’s focus on that last one, because it has recently begun to stir again.

Perhaps you’ve seen this video from May 11, 2018:

Indonesian volcanologists later determined that was a steam-driven (“phreatic”) explosion that happened when magma came in contact with ground water circulating in the volcano’s walls.

Wonder why ordinary people would be relaxing and eating in such a desolate place?

It’s a popular trip among tourists and locals alike whenever the volcano is in repose, as it had been since 2014 until that phreatic blast–the first of three that day.

A bit lower down from Merapi’s double-peaked summit, away from the bare rock, is a thickly forested national park. Besides the excitement and challenge of walking or driving up an active volcano, people come here for views of the surrounding countryside as well as to see a variety of alpine and tropical mountain plants, bamboo groves, a rainforest on the southern flank, and the rare and endangered Merapi orchid Vanda tricolor. There are also Javanese eagles and leopards up there.

It’s accessible, too–just a few hours from downtown Yogyakarta, less than 20 miles (30 km) away. This city has been called the “soul of Indonesia”, and its metropolitan area is home to over three million people. Towns and farming villages dot the fertile land between the city and the volcano, and there are even settlements on the fire mountain’s flanks up to about 2-1/2 miles (4 km) from the summit crater in between the two peaks.

However, visiting Merapi is never routine or boring. There are very clear signs of geological violence in the recent past

Mount Merapi is explosive; it frequently erupts and has had many pyroclastic flows, more than 80 since 1768.

Merapi’s activity and its proximity to so many people have put it on a list of the planet’s most dangerous volcanoes.

Eruption history

There are lots of volcanoes in Java, Sumatra, Bali and Nusa Tenggara because of plate tectonics. The Indo-Australian crustal plate is diving under the Eurasian plate here. In any subduction zone like this, the descending plate takes lots of seawater down to where it’s heated and turns silica-rich molten rock into a sticky polymerized goop that squeezes up onto the surface sort of like toothpaste in an eruption, rather than flowing along as the present lava does at Hawaii’s Kilauea.

Here’s what one type of such lava (from a different subduction zone) looked like as it was erupted at Mount St. Helens between 2004 and 2008:

That’s not exactly Hawaiian-style fountaining activity. Though we can’t see them in operation during an eruption, chemistry and physics rule the day!

Water and seafloor material also contribute dissolved gases to magma at subduction-zone volcanoes like Mount Merapi. These gases come out of solution and pressurize the volcanic edifice or the dome of sticky lava that forms at the vent..

In plain English, the result of all this physical geochemistry is sometimes a huge explosion. Mount Merapi had very dramatic eruptions before 1800 AD because its vent system tended to be plugged. Events back then were often plinian in scale.

Pliny the Younger first described such a cloud, from the eruption that wiped out Pompeii, and may have killed his uncle: “A cloud . . . was ascending, the appearance of which I cannot give you a more exact description of than by likening it to that of a pine tree, for it shot up to a great height in the form of a very tall trunk, which spread itself out at the top into a sort of branches . . . ” (Image and text source)

After 1800, for some reason Mount Merapi was able to “breathe” better, but its eruptions since then have not been boring. Erupted lava piles up in a big dome that eventually collapses in a series of pyroclastic flows, some of which have run out over 12 miles (20 km) from the summit.

That was posted on YouTube by veteran volcano photojournalist Geoff Mackley. A little over a year later, Mount Merapi had a major eruption, VEI 4. After activity there picked up in September 2010, the first blast came on October 26. By November, the 3.5 million cubic meter lava dome had collapsed in a series of flows out as far as 10 miles (15 km) from the summit.

The eruption intensity slowed down in mid-November. By the beginning of December the worst was over, although ever since then rain at the peak has remobilized the 2010 ash into cement-like mud flows, known worldwide by their Indonesian name–lahars.

The 2010 eruption of Mount Merapi killed 347 people and forced over 410,000 people to evacuate.

Here is the full official version of this volcano’s 700,000-year-long history, already translated into English via Google Translate. I have used it quite a bit for the information up above.

Present activity at Mount Merapi

Ten days after the May 11th phreatic eruptions, ash again rose a little ways from Merapi’s summit. This time, said volcanologists (translated by Google), the material was new stuff from inside the volcano, not something old that blew out as a result of a steam explosion.

They do not expect an imminent repeat of the 2010 eruption, per news reports (Indonesian) from May 25.

As of the time that I write this (May 28th), Merapi remains at a Level II alert–a watch based a slight increase in seismic activity. Climbing activities are temporarily suspended and there is a three-kilometer (two-mile) no-go zone around the summit crater.

The next highest warning levels, if they should come, are III (increased seismic activity may soon progress to eruption or catastrophic situation) and IV (an eruption or other crisis that can cause disaster at any time).

If there are any changes in the next few weeks to months, I will update this post below.

Let’s hope it stays quiet.

The Merapi orchid “Vanda tricolor” (Juan Camilo Trujillo, CC BY 2.0)

May 31, 2018: So far, so good!


June 1, 2018: Posts about volcanoes tend to be open-ended–Merapi has had two blasts, per this source (translated via Google). Some people near the volcano fled, and residents are advised to use masks for ashfall and to take out their contact lenses.

Per the official report at this writing, which can be downloaded in PDF where it says “sini” (again, translated via Google), Mount Merapi remains at level II. Seismic evidence shows that magma is still 2 miles (3 km) or more beneath the summit; there has been no significant change in deformation; and the SO2 degassing hasn’t changed. Some hot material was blasted out, though, since the officials say some vegetation up there is burning.

How helpful it is to have instruments on volcanoes, so you can estimate (sometimes) how much time you have before it erupts! In this case, Indonesia probably has a bit more time to prepare before Mount Merapi once again begins to extrude lava and build a dome.

But it’s unlikely that anyone will be climbing up near the summit again any time soon.


Featured image: Sunrise over Borobudur Temple with Mount Merapi in 2016, Cmichel67, CC BY-SA 4.0.


BPPTKG (via Google Translate)

Global Volcanism Program: Mount Merapi. Last accessed May 28, 2018.

Mount Merapi activity report, May 18-24

Wikipedia (Indonesian). 2018. Gunung Merapi. Last accessed May 28, 2018, via Google Translate.
—. 2018. Taman Nasional Gunung Merapi (Mount Merapi National Park). Last accessed May 28, 2018, via Google Translate.

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