This is the second of three posts on the East Hokkaido Caldera Field. This post is centered on the Kutcharo (Kussaro or Kuttyaro) volcanic complex.
Kutcharo is a central caldera complex, measuring 20 x 26 km in diameter. It was created 340,000 – 30,000 years ago via a series of major eruptions. Age of the entire system is around 400,000 years. The complex has four primary parts, the Kutcharo caldera itself, the Atosanupuri stratovolcano / caldera / dome complex filling the eastern portion of the caldera, the Nakajima stratovolcano forming an island in the middle of the lake, and the Mashu caldera complex on the SE caldera wall discussed in a previous post.
The Atosanupuri dacitic – rhyolite dome complex fills the eastern half of the caldera. It is a 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.
The Nakajima dome in the middle of the remaining lake is less than 10,000 years old. It is also a dacitic to rhyolitic dome.
The crater lake, Lake Kussharo is one of the largest and northernmost freshwater lakes near the Sea of Okhotsk. It is the largest crater lake in Japan and provides an important refuge for migrating birds and supports staging for multiple flocks of swan and duck species. Waters of the crater lake are acidic due to fumarole and hot spring activity. There are a few rainbow trout present which were introduced in 1951. These are located near inflowing streams that sufficiently dilute the acidic waters. There are several outdoor hot springs, a sand beach with naturally heated sand, and the small Wakoto Peninsula extending into the lake with active sulfurous vents on its tip. The lake is known at Japan’s Loch Ness, with rumors and reported sightings of a lake monster around 1973.
Unlike Mashu, there is great access to the lake with recreation activities including fishing, hiking, kayaking and cycling commercially available. Public baths warmed by hot springs are located on the lake shore. Cleanliness of the various public baths is highly variable and the subject to the occasional scathing online reviews. The lake is generally frozen over during the winter. The Kawayu hot spring resort is the main resort, located about 2 km NE of the lake. The Ainu Folklore Museum is located on the south bank of the lake. It is open May – October yearly.
There is a Lake Kutcharo in northern Hokkaido that is a stopping place for swans migrating from Siberia to southern Japan. Lake Kutcharo is not associated with the caldera or volcanic system. While the caldera and volcanic system goes by several names, the lake does not.
There are just over 8,300 people living within 10 km of the system and 680,000 within 100 km. Visitors generally ride train, bus or private / rented vehicles to access the region.
The Kutcharo caldera is the largest caldera in Japan at 26 x 20 km. The caldera forming eruption began with a large pyroclastic flow (Furume Welded Tuff) some 400,000 years ago. Subsequent caldera forming events deposited multiple pumice flows 210,000 – 35,000 years ago. The last of these was 35,000 years ago. Repose time between major eruptions was in the neighborhood of 20,000 – 40,000 years. The KpIV unit is the largest at 175 km3 dated around 120,000 – 115,000 years ago. Five eruption units are pumice fall only. The pyroclastic units are mainly dacitic to rhyolitic white pumice. The system started out as a basalt and basaltic andesitic system that shifted to dacites and rhyolites as the magma system evolved.
Kutcharo has a previously unrecognized resurgent dome 10 – 12 km in diameter, 360 m high produced by reinflation of the underlying magma chamber 28 – 23,000 years ago. It has an L-shaped graben, rhyolite lava, and 11 rhyolite domes around the resurgent dome within the graben. The architecture of Kutcharo closely resembles Valles and Long Valley calderas. Kutcharo shows signs of unrest (earthquakes and deformation) since 1938. Resurgence took place quickly after the caldera collapse, much quicker than other known resurgent dome formations.
The Nakajima Island dome was emplaced after the climatic eruption forming the Kutcharo caldera system. There is not a lot in the literature about Nakajima, though it is generally dated 17 – 10,000 years old, forming around the same time that the Atosanupuri system.
The Atosanupuri system is a younger volcanic complex that fills the eastern half of the Kutcharo caldera. There are 10 lava domes formed in this complex 15,000 years after the Atosanupuri caldera forming eruption. The most recent dome is Atosanupuri, also known as Mount Io (or Iwo) and was once mined for sulphur. The name Atosanupuri comes from the indigenous Ainu people and means naked mountain. It does host some alpine plant life and Siberian dwarf pine trees.
There is a relatively small Atosanupuri caldera measuring some 6 km in diameter located within the greater Kutcharo caldera rim. There are at least four large pyroclastic deposits associated with this system. They are Chanai-c, Nakashumbetsu Upper-a, -c and -e. Largest unit is Chanai-c at 6.9 km3 dated around 20,000 years ago. It is a white dacite pumice.
VOGRIPA carries four major eruptions out of Kutcharo 120,000 – 40,000 years ago. The eruptions 120,000 and 40,130 years ago were both VEI 7.2 and ejected over 170 km3 of material. The eruption 85,000 years ago was a VEI 6.4 and ejected over 25 km3 of material. The final eruption of this sequence was a VEI 6.1 that ejected 10 km3. Between 26,300 – 13,300 years ago, there are six eruptions listed, most between VEI 5.0 – 5.9. The smallest of these was the most recent, likely from Atosanupuri that was a VEI 4.6 ejecting less than a half a km3.
The largest caldera forming eruption of Kutcharo (Kp IV) took place 120,000 – 115,000 years ago. It produced at least 175 km3 of material. The eruption started with an ash and pumice fall quickly followed by large pyroclastic flows as multiple vents opened up on either end of the forming caldera. Andesitic magmas were sequentially injected into the zoned magma chamber during the eruption. Both dacitic and rhyolitic magmas were produced during this eruption. There were at least three different andesitic magmas injected into the zoned chamber hours to weeks before the eruptions. These injections may have triggered the eruptions.
Kp IV is divided into four units. Unit 1 is a widespread ash fall deposit from a phreatoplinian eruption. Unit 2 is a subplinian eruption that produced a smaller ash fall deposit. Unit 3 is a voluminous pyroclastic flow deposit during the caldera forming phase. And Unit 4 is a small scoria-rich pyroclastic flow. There was also a small amount of scoria included in Unit 3. Units 1 – 3 are sequential, indicating the eruption ramped up to the climatic phase almost immediately. There is a short but well-defined break between Units 3 and 4. Later erupted materials are banded with and brown pumices and scoria. The pumices are chemically similar. There are some variations among the scorias, including banding between scoria and pumice in Unit 3. Unit 3 also appears to have been erupted from multiple vents. The multiple vent system developed early in the eruptive sequence on the north and south sides of the system allowing the eruption to rapidly ramp up to its climax. Three different mafic magmas were independently injected and discharged as components of the rhyolites from the northern vent system making up Units 3 and 4.
Eruptive history of post-caldera central volcanoes of Kutcharo (Atosanupuri, Nakajima and Mashu (described in the previous post)) is well documented by tephra layers above the youngest caldera forming ignimbrite Kutcharo layer (Kp I). There are more than 20 thin scoria layers visible. From these layers, there were 10 explosive eruptions from Atosanupuri / Nakajima volcanoes 25 – 13,000 years ago, producing at least 16 km3. In contrast, the Mashu system produced over 90 km3 between 35,000 – 1,000 years ago.
Atosanupuri domes are all dacites. Their creation is grouped into two periods. Seven of these were formed 10,000 – 5,500 years ago. An eighth was formed 5,500 years ago. This one also discharged a pyroclastic flow. The final two between 5,500 – 1,00 years ago. The Atosanupuri lava dome (also called Mount Iou or Io) is the newest dome and is located in the NE part of the complex. It is the youngest dome and has ongoing fumarolic activity on its surface. There is evidence of a phreatic eruption several hundred years ago, but there is no historic record of the eruption. That eruption created the Kumaotoshi crater. Atosanupuri is considered an active magmatic system.
There have been five phreatic eruptions from Atosanupuri between 1,500 – 1,000 years ago. The most recent eruption is dated at 300 – 400 years ago. Dates of all these are based on analysis of widespread tephra deposits.
Atosanupuri was mapped by satellite Synthetic Aperture Radar (SAR) measuring deformation 1993 – 2016. Unfortunately, coverage was not continuous and the interferometry results from three different satellites are not compatible. Coverage periods are 1993 – 1998, 2007 – 2010, and 2014 – 2016.
A large uplift of the complex took place 1993 – 1995 and turned into subsidence after 1995. The deformation field was about 10 km in diameter. Largest uplift was 20 cm over the period. There was an earthquake swarm in 1994 at a depth less than 10 km. Largest quake in this swarm was M 3.2.
Analysis of the uplift, subsidence and earthquakes associated with it suggest a low velocity structure exists between 6 – 14 km beneath the surface. The position of the structure remained static through the inflation episode and is referred to as an inflating sill in at least one paper. Subsidence was observed between 2007 – 2010. Due to a break in satellite coverage, no precise start date for beginning of this subsidence is known. The length of this episode is thought to extend for a good 10 years.
There is a second type of small constant subsidence of some lava domes over time also measured. This is thought to be mechanical relaxation of the dome structures.
There have been several earthquakes recorded near the volcano since 1950 greater than M 5.0. The largest was a M 6.5 in 1967. Another small swarm was observed in summer 1998. This coincided with another small earthquake swarm. Most earthquakes around Atosanupuri is thought to be caused by magma movements.
Deformation of volcanic domes that do not show obvious volcanic phenomena is troubling, leading for one set of authors to call for continuous monitoring of the entire Atosanupuri complex to assess the potential for future eruptions.
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.
Kutcharo is located at the intersection of two regional faults that accommodate the ongoing of the Kuril forearc sliver caused by the oblique subduction of the Pacific Plate. There was a M 6.0 earthquake within the caldera in 1938 thought to be caused by magmatic fluid intrusion along the caldera boundary.
Kutcharo is a large, robust volcanic system with active subsurface magma movements. Most recent eruptions involve dacites and rhyolites. It is located along an active weak lineament in the crust. Local earthquakes, inflation and subsidence are all thought to be due to magma movements underneath the caldera. Atosanupuri has active fumaroles and there is a healthy, active hydrothermal system in and around the crater lake. Fortunately, this large, active system is located relatively far from large cities.