Lahar deposits along the Nigorikawa River in the eastern foot of Zao Volcano, Japan, during the last 8000 years

Lahar phenomena and the accompanying eruptions at Zao Volcano, NE Japan, have been recorded historically from the 13th to 20th centuries. However, no studies have been conducted on lahar deposits. This study focused on the lahar deposits of the last ca. 8000 years from fluvial terraces (terraces I, II, and III in descending order of elevation) distributed along the Nigorikawa River at the eastern foot of the volcano. The lithologic, granulometric, and component features of the lahar deposits revealed gravelly non-cohesive debris flow, sandy hyperconcentrated flow, and muddy cohesive debris flow lahar deposits. The sandy and muddy matrix of the lahar deposits mostly originated in the scoriaceous magmatic and phreatic eruption deposits and/or phreatic-eruption-related products in the Zao youngest stage. The clasts of older lavas and basement rocks of all the lahar deposits were entrained during transportation through the river. Radiocarbon (14C) dating indicates the depositional ages of the lahar deposits in terraces I, II, and III to be ca. 8–6 ka, ca. 3.5–2.5 ka ca. and 1700–1900 CE, respectively. The two gravelly and three muddy lahar units of terrace I, two muddy units of terrace II, and at least two gravelly and three muddy units of terrace III correlate with the reported magmatic and phreatic eruptions. Based on chronostratigraphic positions and lithology, the 1895 CE phreatic eruptions may have triggered the two uppermost muddy units in terrace III. However, the gravelly and sandy units of terrace II revealed the thickest deposits, and are widely distributed among the sections of the study area, with no corresponding magmatic eruption deposits in the proximal area. Unknown large-scale magmatic eruptions or crater lake outbursts could have triggered the lahars that formed the sequence. This study revealed that lahar events occurred repeatedly during the past 8000 years and flowed down to distal area. These results indicate the likely occurrence of lahars, especially during and after the eruption at Zao Volcano.