I have for decades enjoyed scary movies, having grown up on the post-WWII science fiction B-movies of the late 1950s and early 1960s. Print media feeds into this with periodic scaremongering over Yellowstone, Campi Flegri, and other sites of significant volcanic activity in the past. The most recent episode of this was a group of stories about discovery of a growing rhyolite dome inside the Kikai caldera south of the southern Japanese island of Kyushu and the imminent eruption of a Japanese supervolcano.
One of the things that normally happens following a caldera-forming eruption is dome emplacement. So why is this one special? Why the current excitement? Perhaps it is time to take a look at the Kikai caldera.
Kikai caldera is a mostly submerged volcano some 45 km SW of Kyushu. There are three small islands above sea level. All are on the northern portion of the caldera rim. These islands are Satsuma-Iojima, Showa Iojima and Take-shima. Showa Iojima was formed during a 1934 submarine eruption.
There are settlements on Satsuma Iojima with perhaps 142 people living on the island. Hot springs with significant iron and sulfur output in the floor of the port turn the water yellow, giving the island the name Iojima or Iwo-jima (Sulphur Island).
There is a bi-weekly ferry service between Satsuma Iojima and Kagoshima. Trip time is just over 5 hours and 120 km.
The island hosted a Sulfur mine that closed in 1964 due to increased volcanic activity. Population of the island was as high as 604 before the mine closed. There was an attempt to create a resort complex in the early 1980s that went bankrupt. Current economy of the island is based on fishing, agriculture, and seasonal tourism.
Volcanic activity on Kyushu has generally moved south over time. It is classified as back arc volcanism and has been prevalent in Kyushu for the last 2 Ma. Activity has become significantly more explosive in the last 300 ka. It includes huge calderas, small calderas, lava dome complexes, and stratovolcanoes. Early eruptions were more basaltic and andesitic in nature with voluminous lava flows. Later eruptions are more rholitic with massive ignimbrite emplacement. There is significant mixing of eruption types throughout the arc over time. The volcanic arc extends south of Kyushu towards Taiwan.
There are five caldera systems active in Kyushu over the last 300 ka. From south to north, these include Kikai, Ata, Aira, Kirishima and Aso.
Kikai had the most recent eruption, the last of three some 7.3 ka. There is some dispute at the dating of the most recent caldera forming eruption with some sources dating it at 6.3 ka. The later number appears to be most accurate. Kikai’s initial volcanic activity started 700 ka. It is the northernmost of what is called the Ryukyu Volcanic Arc. As you move south along the arc away from Kikai, the volcanoes are primarily andesitic and pre-caldera. Volcanic activity has moved south along the arc in recent years with a number of pre-caldera stratovolcanoes above the waves.
Ata has its last major eruption some 100 ka. There was a smaller caldera formed 4 ka. It is partially submerged. The main Ata caldera is 25 x 12 km mostly submarine structure. The most recent activity created a 4.5 km caldera. Total volume from caldera-forming eruptions from Ata is estimated at over 300 km3 and created a rhyolitic and dacitic pyroclastic flow plateau.
Aira is home to Sakurajima. There were three episodes of pre-caldera activity at Aira, ending 25 ka. The majority of this activity was andesitic and rhyolitic lava and pyroclastic flows in the northern part of the caldera area. Erupted materials suggest that there was a large magma chamber under Aira in place since 40 ka. It had a pair of caldera-forming eruptions 22 ka and 16 ka. The 22 ka eruption ejected 400 km3. The most recent caldera forming eruption was 6 x 3 km. The larger caldera measures 23 x 17 km. Pre-Aira volcanism started some 900 ka.
Kirishima is a bit older and has two separated calderas that formed 500 – 400 and 300 ka. Both calderas produced voluminous dacitic and rhyolitic pyroclastic flow plateaus. The larger caldera measures 15 x 10 km. This complex formed stratovolcanoes since 100 ka with many relatively recent eruptions.
Final major caldera is Aso. It had four caldera-forming eruptions 300 ka – 90 ka. The most recent eruption ejected around 200 km3 of material. It measures 25 x 18 km. Total ejecta from eruptions at Aso are estimated at over 800 km3. Flank volcanoes formed along a fault running west from Aso. The first of these started forming 300 ka. All the caldera forming eruptions created widespread pyroclastic flow plateaus.
The listed calderas are not the only volcanoes in this region. There are many other including Unzen.
Kikai caldera is on the volcanic front formed by the subduction of the Philippine Sea plate at the Nankai trough and the Ryukyu trench. There are three islands, Take-shima (Take-sime), Satsuma Iojima (Iwo-jima, Tokara Iwo-zima), and Showa Iojima (Syowa Iwo-zima), on the northern rim of the caldera. The third and smallest island was formed during a submarine eruption in 1934. It is described as either a volcanic dome or a small shield volcano measuring 500 x 300 m.
Satsuma Iojima consists of two volcanic structures, a rhyolite dome named Iodake, and a scoria cone called Inamuradake. Iodake on the eastern end of the island is the most recently active of all the volcanic structures above the surface. The island itself measures 5.5 km long. There have been mild to moderate explosive eruptions from Iodake during the past several decades.
There are active fumaroles, hot springs, both on the island and offshore in the harbor and other protected inlets. Some of these are sufficiently active so as to turn the ocean water orange – brown and yellow.
The newest island, Showa Iojima, formed in 1934’s eruption pokes only 24 m above the surface of the ocean. It is 2 km to the east of Satsuma Iojima.
The third island, Take-shima is not recently active, and is located further along the caldera rim to the east of Satsuma Iwo-jima. It measures 4.5 km.
Magmas of Satsuma Iwo-dake vary, suggesting a shallow, layered magma chamber. The rhyolites of Iodake are similar to those of the caldera-forming eruption, with a proposed depth of 3 km. The basalts that built Inamura-dake came from a deeper and hotter magma chamber, 3 – 5 km. None of these eruptions ejected juvenile material.
The Kikai caldera shows a double-caldera structure. The inner structure is 17 x 15 km. The outer one is 24 x 19 km. The northern rim of both calderas overlap. This is also where the aerial portions of the caldera rim (islands) are located.
The large resurgent dome is located in the middle of the smaller caldera. There are also areas of uplift marking the caldera rims. Active fumaroles and volcanic hot vents have been identified on the dome, though they have not been individually located yet.
Satsuma Iodake has well-documented and monitored hot springs and other thermal features. These did not reflect any changes during the June 2013 eruptions.
The most recent Kikai caldera is completely submerged and formed during a submarine eruption some 7,300 years ago. That eruption ejected over 500 km3 of debris. Recent research surveys and mapping analyzed a rhyolite dome 32 km3 in volume that formed after the caldera collapse. The dome is still active. The magma system appears to have evolved chemically and physically since the caldera-forming event.
There are at least three caldera-forming eruptions known from this volcano. The oldest, Koabi, took place 140 ka, ejecting 130 km3 of material. The next one, Tozurahama, took place 95 ka, ejecting 130 km3. The most recent was the Akahoya 7.3 ka, ejecting 500 km3.
The Akahoya eruption included plinian tephras and pyroclastic flows. The climactic eruption put a pyroclastic flow across 45 km of ocean and onto southern Kyushu Island 100 km from the caldera, almost all the way to Kagoshima. Ash made it as far as 1,000 km from the caldera. The caldera collapse looks to have taken place during the climactic eruption. This collapse may have been relatively slow, as there is not convincing evidence of tsunamis on the islands surrounding Kikai.
The eruption devastated life on southern Kyushu, destroying a forest and bamboo grass bush replacing them with a new grassland. Forests took at least 600 years to start recovering. Settlements came back slowly also, with settlement sizes being smaller than those before the eruption as long as 2,000 years after the eruption. There were at least two massive earthquakes during the eruption. It is unknown how many people these earthquakes killed before the arrival of the pyroclastic flows.
As the entire caldera was mostly subaerial, there was a significant amount of water involved in the eruption. Satsumo Iodake and Taka-shuima represent portions of the original northern rim of the caldera. We do not know how much of the volcano was above the water at the time of the caldera-forming eruptions. My suspicion is not much.
Post caldera volcanic activity started building the resurgent rhyolite dome 6 ka. This activity was followed with a rhyolitic dome on Satsuma Iojima, Iodake with a volume around 1.1 km3. A second basaltic pyroclastic cone, Inamura-dake started building at 3.9 ka. An estimated 45 km3 has erupted since the 7.3 ka eruption, indicating a probable large magma chamber under the volcano.
Volcanic activity on Satsuma Iojima progressed in several stages following the 7.3 ka eruption. First started at Iodake with phreatomagmatic eruptions and pumice fallout. Rhyolitic lava followed. This was mostly effusive with continuing ejection of fragments, which built up an old volcanic edifice.
It was followed perhaps 3,000 years ago by Inamura-dake, which was effusive basaltic and scoria cone building. The eruption shifted to the western foot of the cone, with an explosive eruption followed by multiple rhyolitic lava flows from the crater and the formation of hyaloclastite in the shallow sea just offshore building up a marine terrace. Over the last 1,000 years, volcanic activity shifted to intermittent with pumice and bomb fallout.
The most recent island-building activity took place 1934 – 1935 2 km to the east of Satsuma Iojima 500 m beneath the ocean waves. Rhyolite formed Showa Iwo-jima Island.
The most recent eruption was June 2013 from the Iwo-dake rhyolitic dome on Satsuma Iwo-jima, which put vapor plumes some 300 – 900 m above the summit. There was seismic unrest in May but dropped back to normal levels afterwards. Weak tremor was detected for this eruption in June. The plume was difficult to observe through low lying clouds. There was ashfall on the village of Mishima some 3 km from the summit. The plumes also included SO2 emissions. Plumes continued through 2014.
Plumes were observed 1998 – 2000 and 2003 – 2013. They were accompanied by earthquakes 1998 – 2002 and 2009 – 2011.
Basic tectonics of southern Japan and Kyushu are driven by the subduction of the Philippine Sea Plate under the Amur Plate. General direction is NW.
Initial subduction under the Amur Plate was by the Pacific Plate. Around 45 Ma, the Philippine Sea Plate was created and started subduction under Kyushu. Its current rate of subduction is 7 cm/yr. Historic movement of the Philippine Sea Plate is not well known. The subduction zone SE Kyushu is called the Nankai Trough. It has formed the Ryukyu Volcanic Arc.
The triple junction now located just south of Tokyo formed between Kyushu and Taiwan and has migrated north ever since. It reached the southern tip of Kyushu some 22 Ma. Japan started to rift from Asia 32 – 23 Ma. Seafloor spreading in the Sea of Japan took place 23 – 12 Ma.
Minor forearc volcanism started NW of Kyushu 17 – 12 Ma. The Kyushu Palau Ridge collided with the Nankai trough south of Kyushu 15 – 12 Ma.
There is a suspected pause in subduction 10 – 6 Ma. This may have lasted between 14 – 8 Ma. What actually happened during this time period has not yet been well determined.
Modern day subduction resumed under Kyushu some 6 Ma. At the same time the Okinawa Trough started opening between the Ryukyu Volcanic Arc and Korea. Southern Kyushu rotated some 30 degrees counterclockwise, opening a graben / trough that was exploited by subsequent volcanism. There is evidence for possible slab rollback under Kyushu starting 5 Ma.
A volcanic – tectonic depression began forming in NE Kyushu some 6 Ma. There was also extensional faulting.
Over the last 2 Ma, there was another change in subduction direction of the Philippine Sea Plate more toward the NW. This changed the extensional regime to more compressional and in turn slowed down magma production a bit.
Today the subduction continues with numerous offshore thrust earthquakes. The Geology and Tectonics of Kyushu describes this with the following terms:
“A large variety of earthquakes representing virtually every sense of faulting occur in Kyushu, reflecting the complex and dynamic active tectonic setting there.”
Kyushu is a tough, tough region volcanically. It has a significant magma supply and has frequent caldera-forming eruptions. While the recent caldera outbreak is at present quiet, this quiet time does not necessarily mean the caldera outbreak is finished.
Try as I might, I was unable to find anything that I was able to interpret as significant concern about the recently imaged resurgent dome in the middle of the Kikai caldera. While it is active, may be growing, this does not necessarily mean that it is on the verge of erupting. Rather this means that the magma supply remains, and is reaching the surface.
Kikai is a dangerous active volcanic structure. It should be treated with a great deal of respect along with the other four caldera regions in Kyushu.