The Millennium Eruption
In late 946 CE, the volcano beneath Changbai Mountain produced one of the most powerful eruptions of the last 5,000 years. Modern analysis of preserved trees and Greenland ice cores confirms the date of this cataclysmic event, known as the Millennium Eruption. It registered as a 6 or 7 on the Volcanic Explosivity Index (VEI), placing it in the same category as the 1815 eruption of Tambora.
The eruption column shot gas and ash more than 30 kilometers into the stratosphere. It ejected an estimated 23 cubic kilometers of dense rock equivalent material. Pyroclastic flows—fast-moving currents of hot gas and volcanic matter incinerated the landscape, filling valleys with deposits over 100 meters thick. The ashfall was immense, blanketing an area of 1.5 million square kilometers across the Sea of Japan and parts of northern Japan with a layer of fine particles. Historical chronicles from a temple in Nara, Japan, describe "white ash falling like snow" on a November evening in 946. When the eruption subsided, the mountain's summit collapsed, leaving behind a massive five-kilometer-wide caldera.
The Caldera Lake Today
The caldera created by the 946 CE eruption is Tianchi, or Heaven Lake. Its surface sits at an altitude of 2,189 meters. The lake has a surface area of 9.82 square kilometers and holds approximately 2.04 billion cubic meters of water, supplied almost entirely by rain and snowmelt. With a maximum depth of 373 meters, it is the deepest alpine lake in China. The water is intensely blue and remains cold year-round.
The mountain is an active stratovolcano, and the lake is on the international border between China and North Korea. It is the source of three major regional rivers: the Songhua, Tumen, and Yalu. The volcano is not extinct. Geologists classify it as dormant, and it is actively monitored for seismic activity. Smaller eruptions occurred after the Millennium event, with the last confirmed activity taking place in 1903. Between 2002 and 2005, instruments detected ground swelling and seismic tremors, indicating a period of magma chamber recharge deep beneath the lake.