Blasts from the past and present – Indonesian volcanoes

MANY will recall from our Geography lessons at school the cataclysmic eruption of Gunung Krakatoa in 1883. Alfred Russel Wallace in the 10th edition of his famous book, ‘The Malay Archipelago, 1890’, wrote: “One of the chief volcanic belts upon the globe passes through the Archipelago in a curving line.” This was written before Alfred Wegener’s 1912 concept of Continental Drift and much later in the early 1960s the theory of Plate tectonics was proposed.

In updating his book, Wallace added the footnote: “More recently, in 1883, the volcanic island of Krakatoa was blown up in a terrific eruption the sound of the explosions being heard in Ceylon (Sri Lanka), Manilla, and West Australia. The atmospheric disturbance was so great that airwaves passed three and a quarter-times around the globe, and the finer particles floating in the higher parts of the produced remarkable colours in the sky … in all parts of the world.”

He very briefly mentions Gunung Tombora (Tambora) in Sumbaya as a great eruption of 1815.

Indonesia sits on the ‘Pacific Ring of Fire’ and possesses the most volcanoes in the world with currently 130 active ones along the Sunda Arc where there is a 3,000km-long southern chain of volcanic islands. Here, the Indian Plate is subducted under the Asian Plate. I shall simply concentrate on the eruptions of Mount Tambora in 1815 and the more recent eruptions of Mount Merapi up to January 2024 and Mount Ruang as recently as April 2024.

Mount Tambora

Situated in the Lesser Sunda Island where the Australian Plate converges on the much smaller Sunda Plate at an average rate of 7.8cm, this stratovolcano, made up of alternate layers of ash and lava, became active again in 1812 with its inevitable eruption in April 1815.

This huge eruption has been accredited with an eruptive magnitude of 7 on the Volcanic Explosive Index (VEI) whereas Krakatoa is measured lower at 6 on the VEI.

Explosive eruptions from the central vent led to pyroclastic flows and lava together with tsunamis and the eventual caldera collapse as its huge magma chamber emptied.

After a series of rumblings on April 5, 1815, there followed thunder-like explosion sounds with ash falling on East Java. Sumatra even recorded the ‘sound of firing guns’ some 2,600km away on April 10-11.

In the early evening of April 10, chunks of pumice up to 20cm in diameter rained down to be followed an hour later by an ash fall. The continuous uplift of hot air and steam caused heavy rain to fall for the next week with the ash blown westwards to west Java and South Sulawesi.

Day skies were darkened into night up to 600km from the mountain. Upon reaching the sea, the pumice lava flows cooled to create ‘pumice rafts’, which floated up to 5km away with one even reaching Calcutta in the Indian Ocean in October 1815.

Before the eruption Mount Tambora’s height was 4,300m, higher than Mount Kinabalu today, and after it was reduced to 2,850m. This volcano continues to rumble after its massive explosion and a much smaller eruption occurred in 2011.

Tambora’s volcanic worldwide effects

Plumes of ash reached the stratosphere at height of 43km with this fine ash remaining at between 10km to 30km high for months and years following. Millions of tonnes of ash were ejected with global winds redistributing this sulphate laden ash worldwide. Owing to the density of the ash particulates in the northern hemisphere, 1816 was known as ‘The Year Without A Summer’ as the sunlight was dimmed to such an extent.

Britain did not escape the ash fall with long and brightly coloured sunsets of pink, purple, and red colours. The English painter John Constable, who died in 1837, left a legacy of fine art in his landscapes during this period, subsequent to Tambora’s eruption with ash laden clouds and strange sky colourings.

Global summer temperatures plummeted with June frosts in New England and snow falling in New York and Quebec City. Food crops were written off in the northern hemisphere because of cool temperatures and heavy rains leading to failed harvests.

Famine existed in Ireland and Wales!

China and India were also affected with massive flooding in the Yangtse River valley and the spread of cholera in India attributed to a very late monsoon.

Wallace noted that in 1815: “12,000 people were destroyed” by this volcanic eruption but recently it has been estimated that worldwide deaths amounted to between 71,000 to 117,000 people owing to volcanic ash inhalation, famine, and epidemics.

Mount Merapi

The name of this stratovolcano literally means ‘mountain of fire’. Situated in central Java, it is considered the most active volcano in Indonesia. Today, it stands at 2,910m high, and it has been erupting regularly since the 16th century. It is probably the most densely populated area of all Indonesian volcanoes with villages occurring on its flanks up to a height of 1,700m with thousands of inhabitants.

Several recent outbursts have occurred this century with large eruptions in November 1994 and October 2010. The latter explosion killed 353 people and 350,000 people were forced to leave their houses as pyroclastic flows combined with heavy rain caused lahars.

The April 2006 eruption resulted in a large lava flow and a gaseous cloud. It was the 2018 eruption that caused the evacuation of villagers within a 5km radius of the volcano and the 2021 outburst saw lava flows and clouds of ash later to fall out blanketing local residents.

Last year, in March 2023, a 7km lava flow with a column of hot air rising to 100m followed 10 months later, in January 2024, with a smaller lava flow of 2km and a much smaller hot cloud.

Who knows what and when a further eruption will occur on this mountain of fire?

Mount Ruang

This is a much smaller volcano than the two previously mentioned and it stands at 725m above sea level as an island, containing a partial lava dome and housing 843 people. Prior to its eruption this year, in April 2024, it first experienced a large eruption in 1871, which resulted in a tsunami affecting the nearby inhabited island of Tagulandang, killing 400 residents of a village.

In the mid-morning of April 16 this year, eruptions forced its inhabitants to leave for a nearby island of Tagulandang. Further eruptions followed the next day, and a tsunami alert was declared with the subsequent evacuation of 11,000 people from Tagulandang to mainland Sulawesi.

The eruptions died down for a fortnight but began again on April 30 with further evacuations of people.

The plume of sulphur dioxide emitted from the volcano’s vent extended over 1,000km and fear existed of reduced visibility, acid rain and air pollution downwind.

Both Malaysia Airlines and AirAsia together with smaller airlines cancelled all flights to Sarawak and Sabah on April 19 for fear of volcanic ash damage to jet engines and reduced visibility. The Kota Kinabalu Flight Information Region observed volcanic ash clouds.

‘Better safe than sorry’ was the order of the day!

This volcano is still rumbling like so many of those in this arc, and in the coming years there will be numerous occasions when Indonesians may be forced to flee their homes as volcanic clouds are distributed globally and weather patterns disrupted worldwide. Such are the forces of nature with the inevitable destruction of people, their homes, livelihoods, and even flights.

With seismic monitoring stations located on many volcanoes worldwide there is now a greater probability that alerts may be sent out to warn local people of imminent eruptions. Such a system exists in Italy. I visited a seismic station there on the volcanic island of Vulcano in the Aeolian Islands, north of Sicily, for two weeks in 2018 with that volcano emitting sulphurous fumes and the nearby island of Stromboli lighting up the night skies with its explosive eruptions.

Who knows where and when the next so-called dormant volcano will blow its top or a new one will emerge from the seabed?