Co-located monogenetic eruptions similar to 200 kyr apart driven by tapping vertically separated mantle source regions, Chagwido, Jeju Island, Republic of Korea

New eruptions in monogenetic volcanic fields conceptually occur independently of previous ones. In some instances, however, younger volcanic structures and vents may overlap with older edifices. The genetic links between such co-located eruptions remain unclear. We mapped and analysed the stratigraphic relationships between eruptive units on the 400 × 900-m island of Chagwido off the western coast of Jeju Island, a Pleistocene to Holocene intraplate volcanic field. Chagwido consists of an eastern, older tuff ring with a nested scoria cone and a western tuff, scoria and lava flow sequence. The two stratigraphic packages are separated by a prominent paleosol. The East-Chagwido tuff and scoria deposits were eroded and a period of intense weathering and soil development occurred, before a subsequent West-Chagwido tuff ring and scoria cone and lava complex was erupted. The two eruptions were fed by three chemically distinct magmas. The older eastern eruption consists of magma with composition transitional between high-Al alkalic basalt and low-Al alkalic basalt and has stratigraphic characteristics, composition and syn-eruptive trends akin to the neighbouring Dangsanbong tuff cone. This magma type is typical for the transitional stage from high-Al alkalic (pre 500 ka) to low-Al alkalic (post 250 ka) identified for the greater Jeju volcanic system. The East-Chagwido volcanic complex thus formed as the westernmost in a chain of three volcanoes along a fissure system, with a small volcanic remnant island Wado 1 km to the east and the large Dangsanbong tuff cone another 1 km eastward. A new Ar/Ar age of 446 ± 22 ka for Dangsanbong likely characterizes the age of the whole chain. The second, West-Chagwido eruption started with low-Al alkalic basalt forming a phreatomagmatic phase and ended with subalkalic basalt forming a scoria cone and lava flows. The occurrence of subalkalic lavas is known across Jeju to have started only at ~250 ka, and thus, the well-developed paleosol represents at least 200 kyr between the two co-located eruptions. The distinctive magma compositions show that each eruption tapped an independent region within the same underlying mantle source. These observations show that contrary to most assumptions of monogenetic volcanism, an already “tapped” source region may become fertile again through mantle convection/migration and eruptions can thus be expected from old vent sites in long-lived volcanic fields

Intraplate volcanism influenced by distal subduction tectonics at Jeju Island, Republic of Korea

The drivers behind the inception of, and the variable, pulsatory eruption rates at distributed intraplate volcanic fields are not well understood. Such broad areas of monogenetic volcanism cover vast areas of the world and are often heavily populated. Reliable models to unravel their behaviour require robust spatio-temporal frameworks within the fields, but an analysis of the potential proximal and distal regional volcano-tectonic processes is also needed. Jeju Island (Republic of Korea) is a volcanic field that has been extensively drilled and dated. It is also located near one of the world’s best-studied tectonic plate boundaries: the subduction zone in southwestern Japan, which generates the Ryukyu and SW Japan arcs. A new set o

Towed-camera investigations of shallow�intermediate water-depth submarine stratovolcanoes of the southern Kermadec arc, New Zealand

Southern Kermadec arc stratovolcanoes (of predominantly basaltic and andesitic composition) provide a depth transect of the transition between effusive and explosive submarine volcanism. Observations along 4.6 km of towed-camera track from the crests and upper flanks of the Clark and Rumble III volcanoes reveal a consistent pattern of substrate types that are interpreted to record effusive and explosive eruption processes. Below 700 m water depth, substrates are dominated by massive/blocky flows, pillow lavas, pillow and angular block talus, localized sheet flows, and minor granule-sand volcaniclastic detritus. The latter typically forms a substrate mode of 5�20%. Above 450 m, sand lapilli (that is in part winnowed and rippled), have a substrate mode of 50�100%, with minor components of massive flows, pillow lavas, and coarse talus. This difference in abundance of sand lapilli between 450 and 700 m is interpreted to record the transition between effusive and explosive (both phreatic and phreatomagmatic) eruptions. Between 600 and 700 m, a varied distribution of sand-lapilli abundance is interpreted as recording a mix of effusive and explosive eruptions, consistent with possible differences in the physical interaction of magma�water and known compositional magma heterogeneity.<br/