Blasts from the past: RI’s volcanic history

Blasts from the past: RI’s volcanic history
Philip Rich, Contributor, Jakarta | Fri, 12/10/2010 9:55 AM

Troublemakers: Mt. Bromo (large crater) and Mt. Semeru belong to the Bromo Tengger Semeru National Park. Courtesy of Philip RichTroublemakers: Mt. Bromo (large crater) and Mt. Semeru belong to the Bromo Tengger Semeru National Park. Courtesy of Philip Rich Mount Merapi’s recent eruptions, tragically ending the lives of more than 300 people, are just the latest reminder that we live in the most volcanically active country on earth, with almost 130 active volcanoes in Indonesia.

Following the Merapi accounts in the news and perhaps reading one-off accounts of specific eruptions, such as Simon Winchester’s book Krakatoa: The day the World Exploded or watching the occasional Discovery or National Geographic feature on the Toba super volcano or the Tambora eruption, we may find ourselves struggling to put all the facts together.

Just how much bigger was Tambora compared to Krakatau? How do these volcano eruptions compare to other major events elsewhere like the Mt. Vesuvius eruption that buried Pompeii. What’s the timeline? Where does the recent Merapi activity fit in?

Mike Dobie, a Jakarta-based geotechnical engineer, develops solutions to civil engineering problems using reinforced soil techniques. His job is to develop firm foundations for his clients. The job speaks of establishing order and structure, built to last.

So when Dobie turned his attention to researching and understanding the history and relative scale of major volcanic activity in Indonesia and its place in the world picture, he wasn’t about to do a half job.

In a November presentation to Indonesian nature lovers group Go Wild!, Dobie explained some of the basic terminology used to describe different types of volcanoes.

He pointed out Indonesian volcanoes are mainly stratovolcanoes, that are steep, conical volcanoes built by eruption of (non-basalt) lava, tephra (or fragments of rock of all sizes) and pyroclastic flows (ground-hugging avalanches of hot ash, pumice, rock fragments and volcanic gas that rushes down the side of a volcano at speeds up to 100 kilometers an hour).

This type of volcano is a common feature in subduction zones, like in Java and Sumatra, where the Indo-Australian tectonic plate slips under the Eurasian plate.

As a result of the country’s location at this plate’s boundary, natural disasters are a fact of life here.

This same tectonic plate movement also accounts for the earthquake activity, and consequent tsunamis resulting from that, a double jeopardy effect.

Variations of the same effect are found all around the Pacific Rim, leading to the popular name “Ring of Fire” to describe the whole area.

Dobie captured the audience with a chart he has developed which makes it easy to see the relative sizes of different volcanic eruptions over time, both in Indonesia and overseas, measured based on cubic kilometers of ejected material — which defines the Volcanic Explosivity Index (VEI). One cubic kilometer of rock is a massive amount; it weighs in at 2.5 billion tons.

The chart places these major events on a timeline, with a logarithmic scale that places most emphasis on the activity over the past 100,000 years, the time within which human history begins.

Dobie then went through some of the biggest events on that scale.

The big one

Many visitors to peaceful Lake Toba or Samosir Island in North Sumatra may be unaware of the cataclysm that took place there.

Over 70,000 years ago, it is believed the earth experienced the largest volcanic eruption in human times. The Toba super volcano exploded. Up to 3,000 cubic kilometers of material were ejected, blanketing all of Southeast Asia in ash.

It’s conjectured that the resulting volcanic winter, caused by all this extra volcanic material in the atmosphere may have severely impacted human food supplies resulting in a dramatic population decline.

Furthermore, genetic evidence suggests that all humans alive today, despite apparent variety, are descended from a very small population, perhaps between 1,000 to 10,000 breeding pairs about 70,000 years ago. Some scientists believe Toba may have been the culprit.

Bromo and Batur

If you have been to Mt. Bromo in East Java, you will have noticed that Bromo itself sits in the remains of what was once a much larger volcano. It’s now called the Tengger caldera, but was once a 4,500-meters-high volcano, 800 meters higher than neighboring Mt. Semeru, the highest point on Java today.

Around 45,000 years ago, a massive explosion here finally left the sand sea caldera we see today.

In Bali, perhaps you have stopped for a snack at the town of Kintamani, to take in the panoramic view of the new Mt. Batur rising from the caldera floor. If so, you are perched on the rim of the old Mt. Batur caldera now partially filled with Lake Batur, created by a similar process to that in East Java.


Between Java and Sumatra, in today’s Sunda Strait lie the remains of a volcanic caldera 40 to 60 kilometers across. It seems likely there was a series of major volcanic explosions in that area, as recently as 535 AD.

This is now called now called the “proto-Krakatau” explosion. It seems to be responsible for the formation of the Sunda Strait itself, and writer David Keys in his controversial book Catastrophe: An Investigation into the Origins of the Modern World suggests that the climatic effects of this eruption were felt around the world leading to famines and even the collapse of civilizations in locations as far apart as Mexico and the Middle East.

On a smaller but still devastating scale Krakatau of course famously erupted again on Aug. 27, 1883, with the explosion and resultant tsunami killing 36,000 people in coastal areas of Java and Sumatra.

Largest volcanic eruption: A section of Mt. Tambora’s crater can be seen. Mt. Tambora is an active stratovolcano, is located on the island of Sumbawa. Courtesy of Philip RichLargest volcanic eruption: A section of Mt. Tambora’s crater can be seen. Mt. Tambora is an active stratovolcano, is located on the island of Sumbawa. Courtesy of Philip Rich Tambora creating Frankenstein

Famous though it may be, the Krakatau eruption is not the largest volcanic event in Indonesia in relatively recent historical time.

That dubious honor belongs to Tambora, in the island of Sumbawa. Mt. Tambora exploded on April 10-11, 1815. Six times larger than the later Krakatau eruption, it remains the largest eruption in the world on a scale with that of Taupo in New Zealand 1,800 years ago.

No humans were in the vicinity for that one. Around 160 cubic kilometers of material were ejected when Tambora erupted.

For scale, the Merapi eruptions in November 2010 are estimated at 0.15 cubic meters. The famous Vesuvius eruption that buried Pompeii in 79 CE comes in at 4 cubic kilometers.

Tambora’s effects were felt across the world, as the material and gases ejected into the upper atmosphere created a global cold snap and the “year without summer” in 1816.

In July 1816, “incessant rainfall” during that “wet, ungenial summer” forced Mary Shelley and friends to stay indoors for much of their Swiss holiday. They decided to have a contest, seeing who could write the scariest story, leading Shelley to write Frankenstein.

Tambora also holds the dark distinction of being the eruption responsible for most deaths in human times. This time most deaths were not attributed to the explosion itself nor any subsequent tsunami, but to the devastating effect on the plant and animal life in Sumbawa, Lombok and surrounding areas resulting in a total estimated death toll of 92,000, over half of which resulted from famine.

Make the challenging trip to the caldera rim of Tambora today at 2,850 meters and gaze at the 7-kilometer wide depression up to 1-kilometer deep, remains of a peak that once reached 4,200 meters.

Recent cases

To close his talk, Dobie revisited the events around the June 1991 eruption of Mt Pinatubo in the Philippines. This is the second largest eruption on earth since Krakatau in 1883, (after Novarupta in remote Alaska, 1912). Dobie pointed out that the experience here was instructive as an example of the effects of a significant blast today, especially from one of the many active volcanoes in Indonesia.

No eruption had been anticipated from Mt. Pinatubo, but with earthquake activity increasing in the area during May 1991, monitoring activities were increased, and evacuation of people within a 16-kilometer radius was commenced.

Filipino volcanologists assisted by the US Geological survey, made a very accurate assessment of the volcanic risk, evacuating thousands of those most at risk only days before the major series of eruptions began.

Nonetheless, there were several hundred deaths, mainly caused by roofs collapsing under the weight of volcanic materials, turned into wet ash sludge by a passing tropical storm Yunya.

Millions of tons of sulphur dioxide were discharged into the upper atmosphere, causing an estimated global temperature drop of 0.5 degrees Celsius.

After all this, the recent activity at Mt. Merapi may seem small by comparison.

However Mt. Merapi is a member of a highly select club of only 16 “decade” volcanoes, worldwide. They are named decade volcanoes because the project was initiated as part of the United Nations-sponsored International Decade for Natural Disaster Reduction.

These volcanoes are deemed worthy of particular study in light of their history of large, destructive eruptions and proximity to populated areas.

“Indonesia’s location squeezed between two large tectonic plates means volcanic and earthquake activity will remain a fact of life here. Studying the phases of similar eruptions elsewhere and the responses to them, like at Mt. Pinatubo, can help Indonesia be better prepared in future,” said Mike.