The Toya – Usu post led me to the interesting volcanic history of Hokkaido, Japan. Found a multiple caldera complex in eastern Hokkaido with three calderas in close proximity to one another. Will write about these for the next few posts.
More than 70 large-scale eruptions have been recorded from these three calderas. Total tephra volume of the three in the last 1.7 Ma is over 1,000 km3. Discharge rate increased from 0.2 km3/yr to 2.0 km3/yr around 200,000 years ago. It is still high due to frequent activity from Mashu. Silicic magma systems of the three calderas formed independently.
The easternmost caldera is Mashu. It is a 6 km diameter caldera that truncates a stratovolcano. It was created some 7,000 years ago. There is a small andesitic stratovolcano, Kamuinupuri, that started forming some 4,000 years ago. A large, explosive eruption some 1,000 years ago created a 1.2 – 1.5 km crater on the top of the volcano. There is a small island in the center of the lake that is the tip of a dacitic dome. This caldera complex is on the eastern rim of the much larger neighboring Kutcharo (Kussharo) caldera.
The central caldera complex is Kutcharo, measuring 20 x 26 km in diameter. It was created 340,000 – 30,000 years ago. There is a post-caldera dacitic – rhyolite dome complex in the crescent-shaped lake that fills the western half of the caldera, the Atosanupuri stratovolcano – dome complex. Most of these domes were formed 10,000 – 1,000 years ago. There are no recent eruptions from this, though there is intense fumarolic activity around Atosanupuri and along the shores of the lake.
Westernmost caldera complex is Akan, a 13 x 24 km caldera. It hosts four post-caldera stratovolcanoes, three as the SW end of the caldera and one at the NE side. The only recent activity has been from the Meakandake group east of the caldera. It had a major eruption 13,500 years ago and multiple minor phreatic explosions in historic times.
Mashu is a 7,500-year-old caldera formed by the collapse of a stratovolcano following a climatic eruption. The caldera is steep-sided and occupied by a clear, picturesque lake. Visually, the crater lake is remindful of Mount Mazama / Crater Lake in Oregon, US. A small andesitic stratovolcano formed on the SE side of the lake beginning some 4,000 years ago. This volcano produced a large explosive eruption some 1,000 years ago. There is a small island in the middle of the lake that is the tip of a mostly submerged dacitic lava dome.
The entire region is part of the Akan – Mashu National Park, which encompasses all three calderas. There are less than 10,000 people living within 10 km of the volcano and lake, and just over 630,000 within 100 km.
Lake Mashu is famous for its clear water, some of the clearest in the world. It is also famous for the fog that covers its surface. The highest peak on the rim of the caldera is Mt. Mashdake (858 m). It is also known as Kamuinupuri (God’s Mountain).
The Akan – Mashu National park was designated in 1934. It is one of the longest established parks in Hokkaido. The region is forested with subarctic mixed coniferous forests. There are hot springs throughout the park. Bubbling mud pools are around Lake Akan and Mt. Ponpon. There is enough geothermal heat that there is a unique distribution of wildlife and plants usually not found this far north. There are crickets that are active even in the winter.
The park has two separate parts. The smaller western portion has Lake Akan and the Akan caldera. The larger eastern portion has Kutcharo and Mashu.
There are settlements of the Ainu People, Japanese indigenous people throughout Hokkaido. One of these is on the shore of Lake Akan with about 30 households. They are traditionally a hunting – gathering, fishing people. They venerate bears as the manifestation of high-ranking mountain god who descends to visit the earth from time to time dressed in meat and fur to benefit his people.
Lake Mashu is surrounded by steep crater walls 200 m high. It has no inlet or outlet. Lake transparency was measured in 1931 at 41.6 m, beating Lake Baikal’s 40.5 m at the time. Since then, red salmon and rainbow trout have been introduced into the lake, dropping its transparency down to 20 – 30 m. It is also one of the deepest lakes in Japan at 211 m at its deepest point. Water level on the lake does vary over time, with an average seepage of 1 – 5 mm/day with the seepage rate speeding up a bit at high water levels (above the boundary of lava and tuff).
There are no settlements on the lake. Access is restricted to hiking visitors from designated observation towers. These are referred to as observatories. You can find their locations online. The following link takes you to the Uramashu (Black Mashu) Observatory. Access to the lake itself is prohibited.
The volcano is an andesitic / basaltic andesitic volcano that has recently produced dacites. Smithsonian GVP also reports it as a basalt / pico-basaltic volcano. In addition to the Mashu caldera, the system includes the Kamuinupuri (Mashudake or Kamui) and Nishibetsu (Nisibetu) stratovolcano cones on the periphery of the caldera and the Kamuishu (Kamuishuto) dome in the middle of the lake. Nishibetsu probably predates the caldera.
The entire system is a parasitic volcano on the edge of the neighboring and much larger Kutcharo caldera system. It grew in three stages: a stratovolcano 17 – 12,000 years ago, caldera 11 – 7,000 years ago, and central cone 4 – 1,000 years ago. Its magmas are chemically distinct from neighboring Kutcharo. There are chemical variations in erupted magmas from felsic to mafic over a short period of inactivity. This is interpreted as a zoned magma chamber.
Although the volcano is famed for its crater lake, the real geological gem is the surrounding tephras and pyroclastic deposits. These have been studied since 1940. The history of its formation comes from study of the pile of lavas, pyroclastics and other volcanic debris exposed by the caldera collapse. The volcanic products of this pile are basalts, mafic and felsic andesites, and dacites.
The Mashu caldera is a deep oval-shaped depression, 7.5 x 5.5 km. It has a flat floor with a maximum depth of 211 m. The caldera wall is 150 – 350 m above the lake, 500 – 700 m above sea level. There are more than 10 dikes located in the caldera wall extending radially from the ancestral center of the cone.
The Nishibetsu-dake volcano is SE of the caldera, thought to be a satellite volcano formed during the stratovolcano building stage of Mashu. It is andesitic and old enough to be described as ruined.
Two post-caldera volcanoes erupted inside the caldera. Kamuishu is a small dacite lava dome in the center of the lake. Its base is over a kilometer in diameter at the bottom of the lake. Kamuinupuri is a steep sided andesitic stratovolcano on the SE shore of the lake. It partially fills the caldera. Its most recent activity dates back 1,000 years ago. There are no fumaroles present on it at this time.
Initial eruptions took place some 17,000 years ago with eruption of andesitie lavas, scoria and lapilli building a stratovolcano on the eastern wall of the Kutcharo caldera. These went on intermittently for a long time. There was a quiet period during this. Older somma lavas were various andesites and basalts. Younger somma lavas began erupting with felsic andesite pumice, mainly on the NW part of the caldera wall. These were welded, up to 20 thick, and eventually covered with additional andesitic lavas. The ancestral Mashu was thought to be over 1,000 m high.
Caldera building stage took place after an extended period of quiet. Eruption activity changed to highly explosive eruptions leading to caldera formation some 7,000 years ago. This was followed by ash and pumice fall eruptions followed by large volume pyroclastic flow. The violent activity took place in a short period of time, forming the caldera. Pyroclastic flows were distributed in all directions. Prevailing winds took most of the air fall to the west. Pumice fall at the eastern caldera wall is welded. These eruptions widely devastated local vegetation and formed sand dunes to the east of the caldera.
The final stage of central cone building (ongoing) started after another 2,500 – 3,000-year quiet period some 4,000 years ago. Kamuinupuri began erupting with felsic pumice and andesitic lavas. It formed a steep-sided cone. Explosive activity took place around 1,000 years ago creating a large crater (small caldera?) on the top of the volcano. Pumice fall deposits are thick near the crate and partly welded. During this time, the Kamuishu dacite dome began erupting at the center of the caldera floor.
Although Mashu does not erupt often, when it does go, it gets its money’s worth with a vigorous eruption. The Smithsonian GVP lists seven eruptive periods, the most recent a VEI 5 around 1080. This one formed a 1.2 x 1.5 diameter crater on top of Kamuinupuri. There are another six earlier eruptions listed with the largest a VEI 6 some 7,500 years ago (the caldera forming eruption). Intervening eruptions are 3 VEI 4, a single VEI3 and a single VEI2.
The caldera-forming eruption took place at the end of an eruption sequence starting some 12,000 years ago. The main eruption took place some 6,700 years ago and produced over 18.6 km3 of material. Kamuinupuri also produced a 4.6 km3 and 6.6 km3 following the caldera-forming eruption.
The VOGRIPA database lists another 17 VEI 4 – 6 eruptions 33,700 – 11,800 years ago. Bulk volume ejected in these is between 0.1 – 20 km3.
The 1080 VEI 5 ejected 4.6 km3 bulk volume. The caldera-forming event 7,500 years ago ejected at east 18.6 km3.
Total discharge rate of this region, the East Hokkaido Caldera Cluster (EHCC) measures close to that of the Altiplano – Puna Volcanic Complex, or about half to a third of Taupo / Yellowstone discharge rate of 5 – 7 km3/yr. The EHCC discharge rate has not decreased over the last 35,000 years. Most of this has come from Atosanupuri at Akan and Mashu. Mashu’s activity is more explosive and voluminous.
There is a general evolution of eruptive products through the system’s history from mafic to felsic. Early stages of activity erupted mafic lavas. During the latter stages, eruptions were separated by long periods of quiet and became more explosive producing felsic materials. Within short periods of time, this trend reverses indicating a zoned magma chamber.
The caldera forming eruption took place some 7,700 – 7,600 years ago beginning with phreatomagmatic eruptions and progressing to a magmatic eruption producing widespread ash fall and pyroclastic flows. Total volume of the excavated caldera is estimated at 9 km3, much smaller than the estimated volume of the ancestral Mashu. The caldera is referred to as a Krakatoan type.
The Kamuishu and Kamuinupuri structures started building some 5,400 years ago, with most initial activity for the next 3,000 years. There have been five significant eruptions from Kamuinupuri since it started building. The first was 5,400 years ago starting with a phreatomagmatic eruption that discharged air fall and pyroclastics. This began a series of eruptions that lasted from 5,400 – 4,000 years ago producing air falls and pyroclastics. This sequence ended some 4,000 with a VEI 4 Plinian eruption that produced pyroclastics. After another period of quiet, another series of four main eruptions took place 2,500 – 1,600 years ago.
The final eruption from Kamuinupuri took place 1,000 years ago, a VEI5 producing 1.8 km3 dense rock equivalent. The most recent eruption from this vent was the largest one.
The only historic activity from Mashu took place in 2003 with a series of earthquakes between Feb – June. The largest was a M3.6.
As we previously discussed with Toya and Usu, major tectonic activity on NE Hokkaido is driven by subduction of the Pacific Plate under the Okhotsk Plate. The Kuril Trench has been active for at least 30 – 36 Ma. Extension in the Sea of Japan and the Sea of Okhotsk to the west of Hokkaido is recent at a few million years.
Crustal evolution of central Hokkaido has been dominated by a series of collision and accretion events. This created three main tectonic belts situated mainly north-south. The western portion of these belts was severely deformed by collision between the Okhotsk and Eurasian Plates, subsequent clockwise rotation of the Okhotsk Plate, SW movement of a forearc sliver between the Kuril Islands and the Kuril Trench. Everything comes together in western Hokkaido. The East Hokkaido Caldera Field is located along the movement line between the Okhotsk Trench and the Kuril sliver (block?, microplate?). Crustal thickness in eastern Hokkaido is around 25 km and gets significantly thicker to the west. The subducting Pacific Plate underlies the entire region.
Hokkaido is located at the intersection of two active island arc-trench systems: NE Japan arc – Japan Trench and the Kuril arc – Kuril Trench systems. The ancient Kuril arc along the southern margin of the Okhotsk Plate collided with the Eurasian Plate in the late Eocene. After this collision, subduction along Hokkaido, Sakhalin and the southern margin of Siberia stopped, with only the Kuril trench surviving. Since then, the Kuril forward sliver (block?) has been migrating SW and colliding with the NE Japan arc. The easternmost part of Hokkaido is geologically considered to be the southern margin of the Kuril arc.
Mashu is among the most recently active of the Akan – Mashu caldera field. The Kamuinupuri volcano most recently cooked off a violent, ashy VEI 5 eruption, that left a small caldera / large crater in the top of the volcano. Eruptions from this satellite vent appear to be getting more violent over time which is concerning. On the other hand, there is a complete lack of active fumaroles or an active hydrothermal system associated with it. There are still periodic volcanic earthquake swarms, the last being 2003. This is an active and dangerous system with a highly evolved magma chamber beneath it.