Lahar deposits generated after the Millennium eruption of the Changbaishan Tianchi volcano in the Erdaobaihe River system, China

Abstract

After the Plinian Millennium Eruption (including the ME-I and ME-II eruptive stages) of Changbaishan Tianchi volcano in 969 ± 20 CE, two large-volume lahars were deposited mainly on the northern flank of the Erdaobaihe River, which flows through the only outlet from Tianchi Lake: (1) a lahar deposit containing gray and black pumice (Lahar (G&B) P) and (2) a lahar deposit containing lithic particles (Lahar L). Based on the sedimentary characteristics of lahar outcrops and grain-size and compositional analyses performed using standard dry sieving techniques, examination of thin sections and geochemical analyses, the origins, transport and deposit processes and fluid phase variations of the two lahar deposits are studied. These two lahar deposits are fan-like in the flat medial area (23–60 km from the crater) and fill channels in the distal mountainous area (>60 km from the crater). Lahar L in the medial area represents a complete composite lahar flow deposit with a frontal debris head (very coarse gravel texture, extremely poor sorting, massive and matrix-supported structure) and a hyperconcentrated fluid phase body and tail (coarse sandy texture, very poor sorting, faint horizontal stratification with thin lenses of gravel, clast-supported structure). Lahar L in the distal area represents the material that passed the debris flow head, and the fluid phase changes from debris flow to hyperconcentrated (poor sorting, clear thin horizontal stratification) downcurrent. The fluid phase of Lahar (G&B) P changes from debris flow in the medial area (medium gravel to fine sand, very poor sorting, thick horizontal stratification with floating pumice on each unit surface) to hyperconcentrated flow (poor sorting, thin horizontal stratification, clear normal graded bedding) and to stream flow (moderate sorting, cross-bedding) in the distal area. Lahar (G&B) P was likely formed by the mixing of water from Tianchi Lake and pyroclastic flows of ME-II. Lahar L was likely triggered by a sudden release (dam break) from Tianchi Lake. An assessment of the risk posed to the stability of Tianchi Lake is as important as an assessment of the eruption risk.