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Indonesia Volcanoes and Volcanics

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Map, Major Volcanoes of Indonesia with eruptions since 1900 A.D.

Indonesia consists of more than 13,000 islands, spread over an area approximating that of the conterminous United States. ... Although Chinese records show a Krakatau eruption in the 3rd century AD, and some 17 additional historical eruptions are reported from Kelut as well as Krakatau through the 15th century, uncertainty surrounds many of them. Europeans first began to document eruptions in 1512 (Sangeang Api and Gunungapi Wetar), about the time Portugal gained control of the Mollucan clove trade. ... The disastrous Krakatau eruption of 1883 was followed by several devastating eruptions on other islands and in 1920 a Volcano Survey was established by the government, leading to much improved volcano monitoring and reporting. ... The Volcanological Survey of Indonesia (VSI) now operates a network of 64 volcano observatories continuously monitoring 59 volcanoes.

The great sweep of the Sunda Arc, over 3,000 kilometers from NW Sumatra to the Banda Sea, results from the subduction of Indian Ocean crust beneath the Asian Plate. This arc includes 76% of the region's volcanoes, but those on either end are tectonically more complex. To the NNW, the basaltic volcanism of the Andaman Islands results from short spreading centers, and to the east the Banda Arc reflects Pacific Ocean crust subducted westward. North of this arc, tectonic complexity increases, with converging plate fragments forming multiple subduction zones, mainly oriented N-S, that in turn produce the Sulawesi-Sangihe volcanoes on the west and Haimahera on the east of the collision zone.

Indonesia leads the world in many volcano statistics. It has the largest number of historically active volcanoes (76), its total of 1,171 dated eruptions is only narrowly exceeded by Japan's 1,274, and these two regions have combined to produce 1/3 of the known explosive eruptions. Indonesia has suffered the highest numbers of eruptions producing fatalities, damage to arable land, mudflows, tsunamis, domes, and pyroclastic flows. ... Four-fifths of Indonesian volcanoes with dated eruptions have erupted in this century ...

The symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The NE and SW flanks of Agung extend to the coast, and the volcano rises above the caldera rim of Batur volcano on the NW. The summit of Agung contains a steep-walled, 500-meter-wide, 200-meter-deep crater. The flank cone Pawon is located low on the SE side of Gunung Agung. Only a few eruptions have been recorded in historical time.

Volcanic eruptions are thought to be responsible for the global cooling that has been observed for a few years after a major eruption. The amount and global extent of the cooling depend on the force of the eruption and, possibly, its latitude. When large masses of gases from the eruption reach the stratosphere, they can produce a large, widespread cooling effect. As a prime example, the effects of Mount Pinatubo, which erupted in June 1991, may have lasted a few years, serving to offset temporarily the predicted greenhouse effect.

Global cooling often has been linked with major volcanic eruptions. The year 1816 often has been referred to as "the year without a summer". It was a time of significant weather-related disruptions in New England and in Western Europe with killing summer frosts in the United States and Canada. These strange phenomena were attributed to a major eruption of the Tambora volcano in 1815 in Indonesia. The volcano threw sulfur dioxide gas into the stratosphere, and the aerosol layer that formed led to brilliant sunsets seen around the world for several years.

However, there is some confusion about the historical evidence that global cooling may be caused by volcanic emissions. Two recent volcanic eruptions have provided contradictory evidence on this point. Mount Agung in 1963 (Indonesia) apparently caused a considerable decrease in temperatures around much of the world, whereas El Chichon in 1982 (Mexico), seemed to have little effect, perhaps because of its different location or because of the El Nino that occurred the same year. El Nino is a Pacific Ocean phenomenon, but it causes worldwide weather variations that may have acted to cancel out the effect of the El Chichon eruption.

As observed after several eruptions, including Agung in 1963 (Indonesia) and El Chichon in 1982 (Mexico), stratospheric warming and lower tropospheric and surface cooling have been documented after the Pinatubo eruption. Labitzke and McCormick (1992) show that warming in the lower stratosphere (16 to 24 kilometers or 30 to 100 mbar) of up to 2 to 3 degrees C occurred within 4 to 5 months of the eruption between the equator and 20degreesNorth latitude, and it was also later noticed in middle northern latitudes (Angell, 1993). The warming distribution closely mirrored the dispersal pattern of the aerosol cloud; this mirroring strongly suggests that the warming was due to absorption of radiation by the aerosols. The warming was more intense in southern temperate-polar latitudes, perhaps due to the presence of aerosols from the Mount Hudson (Chile) eruption. Such temperature changes can influence stratospheric dynamics (Pitari, 1992). Since the peak of stratospheric warming in late 1991, temperatures in the 18- to 24-kilometer region have cooled considerably, passing the average in early 1993; temperatures in 1993 were the coldest ever recorded (Christy and Drouilhet, 1994; Monastersky, 1994) and may be related to ozone destruction in the lower stratosphere. Stratospheric temperatures also plummeted and stayed cooler than average for 7 years after the El Chichon eruption.

The historically active Batur volcano is located at the center of two concentric calderas NW of Agung volcano. The SE side of the larger 10 x 13 km caldera contains a caldera lake. The inner 7.5-kilometer-wide caldera, which was formed during emplacement of the Bali (or Ubud) ignimbrite, has been dated at about 23,670 and 28,500 years ago (Wheller 1986, Sutawidjaja et al. 1992). The SE wall of the inner caldera lies beneath Lake Batur; Batur cone has been constructed within the inner caldera to a height above the outer caldera rim. The Batur stratovolcano has produced vents over much of the inner caldera, but a NE-SW fissure system has localized the Batur I, II, and III craters along the summit ridge. Historical eruptions have been characterized by mild-to-moderate explosive activity sometimes accompanied by lava emission. Basaltic lava flows from both summit and flank vents have reached the caldera floor and the shores of Lake Batur in historical time. The caldera contains an active, 700-meter-tall stratovolcano rising above the surface of Lake Batur. The first historically documented eruption of Batur was in 1804, and it has been frequently active since then.

The forested slopes of Galunggung volcano in western Java are cut by a horseshoe-shaped caldera breached to the ESE that has served to channel the products of recent eruptions to the SE. The "Ten Thousand Hills of Tasikmalaya" dotting the plain below the volcano are debris-avalanche hummocks from the collapse that formed the breached caldera about 4,200 years ago. Although historical eruptions, restricted to the central vent near the caldera headwall, have been infrequent, they have caused much devastation.

On the southeast slope of Galunggung Volcano on the densely populated island of Java, a hummocky deposit called the "Ten Thousand Hills of Tasik Malaja" drew the attention of European geologists in the early part of the 20th century. Dutch geologist B.G. Escher hypothesized that a breakout of a crater lake resulted in a watery landslide that formed the deposit. The hummocks were likely material left behind as the more watery parts of the slide flowed away. Austrian geologist F.X. Schaffer suggested that the hummocks might be manmade; as the local people cleared the land to make ricefields, they made dumps of the boulders and cobbles that they found. The dumps became hummocks, and were used as sites for homes and fruit trees, as they offered protections from hostile people as well as from the mosquitoes and rats of the rice fields. Schaffer noted that the volume of material might seem large for "occidentals but it is not beyond the powers of the numerous and industrious Malays."

The horseshoe shape of Galunggung's crater and the nature of the hummocks, however, suggest a different cause for the formation of the Ten Thousand Hills. Since 1980, geologists from the Volcanological Survey of Indonesia and the U.S. Geological Survey have reinterpreted the deposit as a debris-avalanche deposit. Quarry exposures show pieces of the old volcano -- the block facies -- shattered but intact, that are similar to the deposits at Mount St. Helens and Mount Shasta. Radiocarbon dates of a lava flow within the deposit show that the debris avalanche is less than 23,000 years old.

The Ijen volcano complex consists of a group of small stratovolcanoes constructed within the large 20-kilometer-wide Ijen (Kendeng) caldera. The north caldera wall forms a prominent arcuate ridge, but elsewhere the caldera rim is buried by post-caldera volcanoes, including Gunung Merapi stratovolcano, which forms the 2799 meter high point of the Ijen complex. Immediately west of Gunung Merapi is the renowned historically active Kawah Ijen volcano, which contains a nearly one-kilometer-wide, turquoise-colored, acid crater lake. The picturesque lake is the site of a labor-intensive sulfur mining operation, in which sulfur-laden baskets are hand-carried from the crater floor. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of post-caldera cones forms an E-W-trending zone across the southern side of the caldera. The active crater at Kawah Ijen has an equivalent radius of 361 meters, a surface of 41 x 106 square meters, it is 200 meters deep and has a volume of 36 x 106 cubic meters (Pasternack and Varekamp, 1997).

Kaba, a twin volcano with Mount Hitam, has an elongated summit crater complex dominated by three large historically active craters trending ENE from the summit to the upper NE flank. The SW-most crater of Gunung Kaba, Kawah Lama, is the largest. Most historical eruptions have affected only the summit region of the volcano. They mostly originated from the central summit craters, although the upper-NE flank crater Kawah Vogelsang also produced explosions during the 19th and 20th centuries. In 1833 an eruption ejected water from the crater lake, forming lahars that produced damage and fatalities at Talang, Klingi, and Bliti villages.

Karangetang (Api Siau) volcano at the northern end of the island of Siau, north of Sulawesi, contains five summit craters along a N-S line. Karangetang is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts has also produced pyroclastic flows.

The relatively small Kelut stratovolcano contains a summit crater lake that has been the source of some of Indonesia's most deadly eruptions. A cluster of summit lava domes cut by numerous craters has given the summit a very irregular profile. Satellitic cones and lava domes are also located low on the eastern, western, and SSW flanks. Eruptive activity has in general migrated in a clockwise direction around the summit vent complex. More than 30 eruptions have been recorded from Gunung Kelut since 1000 AD. The ejection of water from the crater lake during Kelut's typically short, but violent eruptions has created pyroclastic flows and lahars that have caused widespread fatalities and destruction. The construction of drainage tunnels beginning in 1926 to lower the lake level has greatly reduced the human impact of recent eruptions.

The renowned Krakatau volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 AD, destroyed the ancestral Krakatau edifice, forming a 7-km-wide caldera. Remnants of this volcano formed Verlaten and Lang Islands; subsequently Rakata, Danan and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan volcanoes, and left only a remnant of Rakata volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau), constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan, has been the site of frequent eruptions since 1927.

Krakatau 1883 Eruption

The twin volcanoes Lokon and Empung, rising about 800 meters above the plain of Tondano, are among the most active volcanoes of Sulawesi. Lokon, the higher of the two peaks, whose summits are only 2.2 kilometers apart, has a flat, craterless top. The morphologically younger Empung volcano has a 400-meter-wide, 150-meter-deep crater that erupted last in the 18th century, but all subsequent eruptions have originated from Tompaluan, a 150 x 250 meter-wide double crater situated in the saddle between the two peaks.

Merapi, one of Indonesia's most active volcanoes, dominates the landscape immediately north of the city of Yogyakarta in one of the world's most densely populated areas. Merapi is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. The steep-sided modern Merapi edifice, its upper part unvegetated due to frequent eruptive activity, was constructed to the SW of an arcuate scarp cutting the eroded older Batulawang volcano. Pyroclastic flows and lahars 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 during historical time. Since 1953, activity has been characterized by extrusion of lava into the summit crater, with periodic lava dome collapse and nuée ardente formation. Summit lava dome growth has continued since the 1969 gas explosion. It is monitored from the Merapi Volcano Observatory (MVO) in Yogyakarta.

The name Peuet Sague (summit elevation - 2,780 meters), which has numerous spelling variants (e.g. Peut Sagoe, Peuet Sagu, Puet Sagu, Peuet Sagoee), means "square." Accordingly, Peuet Sague, a stratovolcano, contains four summit peaks. It is located in Sigli Regency, Aceh Province near Seulawah Agam and Burni Telong volcanoes. The crater believed to be active resides SE of one of the peaks of the lava dome (Mount Tutung). This narrow crater has a diameter of about 70 meters and a depth of 80 meters.

Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises abruptly to 3,676 m above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S line cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE.

The small conical volcano of Soputan on the southern rim of the Quaternary Tondano caldera is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated Soputan volcano is located SW of Sempu volcano and was constructed at the southern end of a SSW-NNE trending line of vents. During historical time the locus of eruptions has included both the summit crater and Aeseput, a prominent NE flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

Global cooling often has been linked with major volcanic eruptions. The year 1816 often has been referred to as "the year without a summer". It was a time of significant weather-related disruptions in New England and in Western Europe with killing summer frosts in the United States and Canada. These strange phenomena were attributed to a major eruption of the Tambora volcano in 1815 in Indonesia. The volcano threw sulfur dioxide gas into the stratosphere, and the aerosol layer that formed led to brilliant sunsets seen around the world for several years.

The massive Tambora stratovolcano forms the entire 60-kilometer-wide Sanggar Peninsula on northern Sumbawa Island. Tambora grew to about 4000 meters elevation before forming a caldera more than 43,000 years ago. Late-Pleistocene lava flows largely filled the early caldera, after which activity changed to dominantly explosive eruptions during the early Holocene. Tambora was the source of history's largest explosive eruption, in April 1815. Pyroclastic flows reached the sea on all sides of the peninsula and heavy tephra fall devastated croplands, causing an estimated 60,000 fatalities. The eruption of more than 150 cubic kilometers of tephra formed a 6-kilometer-wide, 1250-meter-deep caldera and produced global climatic effects. Minor lava domes and flows have been extruded on the caldera floor during the 19th and 20th centuries.

Tambora 1815 Eruption

The 16-kilometer-wide Tengger caldera is located at the northern end of a volcanic massif extending from Semeru Volcano. The massive Tengger volcanic complex dates back to the early Pleistocene and consists of five overlapping stratovolcanoes, each truncated by a caldera. Lava domes, pyroclastic cones, and a maar occupy the flanks of the massif. The most recent of the Tengger calderas is the 9 x 10 kilometer-wide Sandsea caldera, which formed incrementally during the late Pleistocene and early Holocene. An overlapping cluster of post-caldera cones was constructed on the floor of the Sandsea caldera within the past several thousand years. The youngest of these is Bromo, on of Java's most active and most frequently visited volcanoes. Bromo is a centrally located cone, with a 700-meter-diameter crater.

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