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Billy Mitchell volcanoBilly Mitchell volcano is a small shield volcano, which is composed mainly of loose deposits from explosive eruptions (pyroclastic shield). The volcano is located immediately NE of Bagana volcano in central Bougainville Island.
Billy Mitchell's typical eruption style are explosive eruptions from the central vent, and the volcano is remarkable for having produced some of the largest explosive eruptions in Papua New Guinea in the past 10,000 years.
Background:The andesitic-dacitic Billy Mitchell volcano has a shallow, but steep-walled and 2.4 km wide caldera filled with an up to 90 m deep crater lake. A small lava dome forms an island near its southern shore. The lake drains through a gap in the northern caldera rim into the Tekan River.
Outside, Billy Mitchell volcano has a smooth eastern flank, which extend as far as 22 km on the eastern side. The other sides are rough and more eroded.
2 major explosive eruptions are known from the volcano, one about 900 years ago and the other about 370 years ago. They produced wide-spread dacitic pyroclastic-fall deposits that cover most of the northern half of Bougainville Island. The younger eruption could have produced the present-day summit caldera. Pyroclastic-flow and -surge deposits from Billy Mitchell extend 25 km to the eastern coast of Bougainville Island.
1580 plinian eruption and global climate effects
The explosive eruption of Billy Mitchell volcano around 1580 (uncorrected radiocarbon age, i.e. between 1480–1640) ranks as one of the largest in the world during the past 1000 years. It erupted about 14 cubic km of tephra (ca. 6 km3 of magma) and produced an ash layer over 300 km2. Ignimbrite from the eruption extends in a broad fan 20 km eastwards towards the Pacific Ocean. The eruption probably caused the formation of the present-day caldera.
The eruption has very likely caused a world-wide short-term cooling such as other large eruptions (e.g. Tambora in 1815, Krakatau in 1883, Katmai in 1912, Pinatubo in 1991).
Corresponding signals in tree rings and ice-cores from Antarctica and Greenland have been identified, one around 1495 and one around 1587, the latter of which is considered the more likely candidate to have recorded the Billy Mitchell eruption. A signal at this time appears in both Greenland and Antartica, which would only happen with eruptions at near-equator latitude.
Based on comparative studies, it has been estimated that the eruption caused a temporary temperature anomaly (mean temperature drop) on the Northern Hemisphere of ca. 0.2-0.3 degrees C for 1-2 years following the eruption (for comparison: the largest known explosive eruption in 1815 from Tambora volcano caused an anomaly of ca. 0.5 degrees and decreased over 3 years).
- Smithsonian / GVP volcano information
- McKee C O, Johnson R W, Rogerson R, 1990. Explosive volcanism on Bougainville Island: ignimbrites, calderas, and volcanic hazards. Proc Pacific Rim Cong 1990, 2: 237-245
- Briffa et al (1998) "Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years", Nature, v. 393, pp. 450-455
1030 plinian eruption
An earlier large explosive eruption (VEI5, ca. 2 km3 magma erupted), about half the size of the 1580 eruption, occurred around 1030 (radiocarbon dated). Ice core data indicate a peak in acidity caused by a large eruption at that time, but the large eruption of Baitoushan volcano in NE China 969 ±20 AD competes with the Billy Mitchell eruption for the ice core data. Unlike for the 1580 eruption, conclusive records on its effects on climate cannot be easily obtained.
Susanne Horn and Hans-Ulrich Schmincke (2000) "Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD", Bull. Volc., v. 61 (6), pp. 537-555
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