THE GREAT TIANCHI ERUPTION AND ITS PRE-ERUPTION HISTORY AND CONDITIONS

Huge explosive volcanic eruptions probably present the ultimate natural hazard to mankind, yet our understanding of such catastrophic eruptions is highly incomplete. The great 10^th century Tianchi (or Baitoushan in Korean) eruption represents one of the two largest explosive eruptions (along with the 1815 Tambora eruption) in the past two thousand years. The Tianchi volcano from the Changbaishan Volcanic Field spreads over the border of northeast China and North Korean (E 128°, N 42°). The huge eruption ejected about 100 km³ of tephra and resulted in the formation of a 5 km diameter caldera: the Tianchi (Heaven Lake) caldera.

In spite of recent geochemical and geophysical investigations of the Tianchi volcano, little is known about its pre-eruption history and conditions or the trigger of eruption. Here we report results of high-quality uranium-thorium dating of zircons from the Tianchi volcanic rocks. Fourteen zircon grains yield a zircon isochron age of 8.6±1.3 ka (thousand years), with MSWD of 1.3. Our data indicate that the eruptible rhyolitic magmas were stored in the crust for only 7.6±1.3 ka before eruption. The good quality of the U-Th zircon isochron age, the unimodal age distribution, the simplicity of the zircon crystals, and the accuracy of the young eruption ages help provide robust estimate of zircon residence time in the eruptible rhyolitic magmas from Tianchi volcano.

Based on titanium-in-zircon geothermometer, the Tianchi rhyolitic magmas were formed at very low temperature (648 ºc) and were highly enriched in water (>4%) before eruptions. Intrusions of hotter mafic magmas into a pre-existing low-temperature, high water content, young rhyolitic magma chamber triggered the explosive eruption in the 10^th century.