Indonesia’s Fabulous And Deadly Volcanoes
March 11, 2023. Indonesia’s most active volcano, Mt Merapi, erupted on Saturday spewing ash, shattered rock, lava and hot gases that blocked out the sun and carpeted the roads and shrouded the houses in eight villages near the base of the volcano with ash. Local mines on the slopes were closed and all tourist activities were halted as the eruptions continued through Saturday. At one point, after the eruption spewed clouds of hot ash hundreds of feet into the air, a pyroclastic cloud of ash and hot gases poured from the vent and surged 4 miles downslope to threaten villages and residences near the base of the volcano.
Not all of us know where Indonesia is…or that it is famous (or infamous) for its deadly volcanism.
Indonesia is an island-archipelago-nation lying close to the equator, north of Australia, between the Indian Ocean and the Pacific Oceans. Its southern boundary is a two-thousand mile long, east-west oriented, archipelago of volcanic islands. With a population of nearly 300 million, Indonesia is the fourth most populous in the world. However, with most of its 17,000 islands uninhabited and with its mix of mountainous islands, blue sea, and lush tropical forest, Indonesia ranks number one (1) out of fifty nations acclaimed for their natural beauty. Its lovely and attractive mountainous landscape is a major part of its allure, but obscures the fact that Indonesia is also the land of the most active, most dangerous and most well-known volcanoes. Note that Meropi is erupting as I write today.
Indonesia’s mountainous islands are almost all volcanic in origin. Of its total 128 volcanoes, 80 are actively erupting today, while five of these active ones are mostly unseen as submarine volcanoes. Of the active volcanoes, about 40 have caused almost all of the the nearly 200 deadly eruptions in Indonesia over the past nearly 2000 years of human records. During the historic period, a total of more than 57,000 people have died in this area as a result of earthquakes, lava flows, ash deposits, nuee ardent, pyroclastic flows, tsunamis, and other related disasters associated with these eruptions.
Southern Indonesia, which comprises a narrow almost two thousand mile long, east-west trending arc of volcanic islands, is the site of the most frequent, powerful and well known volcanic eruptions in the world. Mt Toba, located in western Indonesia’s North Sumatra, erupted violently some 73,000 years ago; infamous Krakatoa located about 700 miles east of Mt Toba, in the Banda Strait, famously erupted violently in 1883; and Mt. Tamboura, sited about 871 miles further to the east on the island of West Nousa erupted as a “super volcano” in 1815. This 1815 eruption is claimed to be the most violent ever recorded in world history. And finally in the category of presently active volcanoes, Mt Merapi in central Java, located only about 360 miles east of Krakatoa, is considered to be the most deadly active volcano in modern history,
In fact, two of the most infamous and destructive of all known volcanic eruptions occurred in Indonesia. The best known was the well known eruption of Krakatoa in 1883. But less well known but of greater violence and more damaging was the world’s most violent and destructive historic era eruption ever recorded, which occurred when Mr. Tambora erupted on April 10 to April 15, in 1815.
In terms of destruction of residences, financial damage, climatic impact, and human deaths the Tambora eruption is unparalleled. It is considered the most violent eruption in human history and resulted in the direct deaths of 11,000 people. However the long term climatic effects of the eruption had a world-wide impact causing deadly floods, crop failures which led to famine and more hundreds of thousands of death. The year 1816 is known as the “year with no summer”. In several temperate parts of the world snow fell in the month of July, and July temperatures dropped into the low 30s. Crops failed, and famine plagued large area of the world. The cooling effect on the celibates has been estimted to have caused
Volcanologists have established a “violent eruption index” ( or VEI) for these exceptionally violent sometimes called “super volcanoes”. The oils t eruption index or VEI has a 0 to (8) or 10 range based on how much material was ejected for how long the eruption lasts.
Several super volcanoes in earth history have produced massive eruptions that have had global implications. The eruption of the Yellowstone super volcano in Wyoming, USA, about 640,000 yers ago ranked an 8 on the VEI. Then 73,000 years ago, Mt Toba in Indonesia erupted with a possible VEI rank of 9. But in historic times it is Mt Tamboura that has been the most destructive and most violent. In 1815 Tamboura But it is Mt. Tamboura in Indonesia which erupted in April of that year with a rank of VEI 7 which remains the most violent volcanic eruption in human history.
Prior to April 10, 1815, Mt Tamboura on the island of Sumbawa in Ιndonesia was that nation’s tallest mountain, with its vent at an elevation 14,100 feet. After the violent eruption of that date Mt. Tamboura topped at only 9350 feet. (14,100-9,350 = 4,750 feet). The eruption blew the top 4,750 feet of Tamboura into dust, rock fragments and fine ash. That was followed by hot ash and gas flows (pyroclastic flows) streaming down the lower slopes toward the sea. The plume of debris was blown up into the atmosphere and estimated to rise to about 21 miles above the surface. The top nearly one mile high cone of the volcano was turned to fine ash, estimated to comprise about 35 cubic miles of fine dust and rock debris which was driven high into the upper levels of the atmosphere and remained lofted for long periods of time. The ash cloud was carried around the globe reflecting sunlight and darkening the sky . Global temperature dropped a full degree Fahrenheit the following year.
Why is all of this violent volcanic activity located in this linear island archipelago of southern Indonesia? As we will see below it may well be the result of the rapid rate at which the Australian plate is undergoing subduction below the Eurasian Sunda plate…and the types of rock materials involved in the process.
The earth’s crust is divided up into seven (7) major crustal plates that fit together like a jigsaw puzzle and are constantly moving over the earth’s surface, jostling each other, crumpling up their margins to produce mountain ranges, or colliding then driving down into the Earth mantle to produce earthquakes and in some places like the margins of the Pacific Ocean and Indonesia violently erupting volcanoes.
Much of the cause of Indonesia’s intensive vulcanism lies with neighboring Australia.
The continent of Australia is unique in that it occupies its own “Australian” crustal plate. More than 80 million years ago the Australian plate and Antartica (another crustal plate) were part of a former larger “supercontinent”—-known as Gondwana. About 47 million years ago, for some unknown as yet reason, the Australian plate broke away from Gondwana and began moving northward at a rate of about three inches per year (7.8 cm). The world wide average rate of growth (or movement) for crustal plates is only 0.6 inches per year (1.5 cm) or the rate of fingernail growth. Thus the Australian plate is moving five times faster than the average.
It may have taken about 40 million years for it to eventually collide with the Sunda section of the Eurasian plate. As the result of this collision, parts of the Australian plate has been driven under the Eurasian plate. The thrusting continued to press the slab downward into the hot mantle, undergoing a process termed subduction. As this part of the Australian plate descended, its slope and frictional drag tended to pull down parts of the surface of the Earth’s ocean crust to form a deep ocean trench, today known as the Java or Sunda Trench. This ocean-bottom feature is more than 2,000 miles long and in places over 4.5 miles deep. The deep, long curving underwater feature, located about 150 miles off the Indonesian coast, parallels the shoreline of western and southern Indonesia. See a world map.
But that is not the end of the story. As parts of the Australian plate descended into the hot mantle, the rock slab of the Australian crustal plate began to melt. This now molten rock material, forming a mass less dense than its surroundings, created “blobs” of molten rock or magma. The hot molten mass tends to rise toward the surface. The process is similar to what one can observe in a paraffin “Lava Lamp” as bubbles of melted colored paraffin rise toward the top of the lamp.
Under Indonesia the hot magma derived from the Australian plate actually melts its way upward, slowly rising though the earth’s crust as it approaches the surface. The massive blob or magma chamber, sometimes tens of miles in diameter or more, may incorporate other rock types into its mass as it rises. Others may come to rest lodged close to the surface and may remain in place for millennia. Often these buried masses of magma tend to under go chemical and physical alterations to their composition as they age, sometimes the end product of these alterations produce magma or molten rock of very viscous (rich in silica) and charged with superheated gases such as water vapor and carbon dioxide gas.
If continuously fed by more moleten material from below, the magma chamber may grow larger. The expansion cause the sloping rock surface of the overlying volcano to swell and become unstable. Rock slides and down slope movement (slumping) of the slopes of the volcano may reduce or release the confining pressure on the magma and gases trapped in the magma chamber. The instantaneous release of pressure, then may result in a catastrophic violent explosion of the magma and gases into the atmosphere and the eventual emptying of the magma chamber. This process produces the violent eruptions and the geologic features we call volcanoes.
I repeat myself here… but perhaps to good effect. During an eruption the molten, gas-infused magma may rise from depth to create or fill a magma chamber within the core of the volcano. These swelling movements on the slope of the volcano may alter the stability of the sloping surface producing mass wasting or landslides which rapidly expose the buried magma chamber to the low pressure at the surface. At this point the gases within the magma expand instantaneously, mix with the viscous molten magma to form towering clouds of dust and gas, incorporating lava bombs, coarse particles called tephra, and fine ash..as well as a frothy volcanic rock known as pumice which rise to gret hight above the volcano. In cases of very gaseous magma, eruptions produce almost lol ash and these materials fall back to earth, where they accumulate in cone like geological features we know as cinder volcanoes. Volcanoes make up the bulk of the landforms all along the Indonesian coast. All are the result of the process of subduction or driving of a h of Australian crustal plate deep into the mantle where it melts.
The magma which flows into the vent of Mt Tambora has intermediate levels of silica which make it more viscous. (Lavas can be basaltic (silica poor)…andesitic (intermediate silica)….rhyolitic (rich in silica) ). Tambora, like other steep sided intermediate or composite volcanoes has periods when it erupts less gaseous magma to form flows of less viscous low gas (basaltic) content lava. These eruptions form flows more quietly. These steep sided volcanoes have alternate eruptions, gaseous explosive (andesitic and rhyolitic)ones which produce violent explosions and huge clouds of ash and fine particulates alternating with quiet lava (basaltic) flows. The result of this process is a volcano with steep sides.
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