A pipeline of spouting mud

　The image of volcanoes is one of erupting thick streams of molten rock, but it is not only lava that erupts from underground. Sometimes it is mud that spouts out. Those volcanoes are called as mud volcanoes. In the same way that volcanoes are formed by accumulating lava, erupting mud piles up and forms a mountain. There are small mud volcanoes of a meter high, and there are also mud volcanoes which reach diameters of several kilometres and heights of several hundreds of meters. There are a few over 2,000 confirmed mud volcanoes worldwide, on land and on the sea floor, and they are also found in Japan. The mud volcano which was surveyed on this occasion is off the Kii Peninsula coast, in the Nankai Trough in the Sea of Kumano. The geography of sea of Kumano is a wide, flat formation in the ocean at around 2,000 meters depth, where about ten mud volcanoes can be found. Out of these it is the Fifth knoll where a drilling survey will be performed.

　What is the significance of investigating these mud volcanoes? Koichi Iijima of JAMSTEC who took part in the survey as a sedimentologist explains: “Mud volcanoes are carrying methane up from underground. This methane and methane hydrate are extremely interesting compounds.” The spouting of methane is also a characteristic of mud volcanoes. Deposits piled up on the seafloor contain organic matter such as the remains of living creatures. With new clay and sand depositing on top, this gets buried deep in the earth, the organic matter decomposes and methane is generated by methane-producing bacteria or through terrestrial heat. Most methane is formed deep in the earth. Methane hydrate (an ice like substance formed by methane and water under low temperatures and high water pressure), which has also attracted attention as a resource, exists buried in the cracks of the netlike structure inside mud volcanoes. Mud volcanoes are a very interesting subject of research, both as natural pipelines carrying methane from deep places and as methane hydrate icebergs.

　“When you look at the lithium isotopes in the erupted material from Fifth knoll, it seems that this lithium comes up from near the asperity zone several kilometers beneath the seafloor.” According to Iijima mud volcanoes are also so-called information pipelines. The asperity zone (slip plane) is where the earth’s crust that forms the Japanese Archipelago and the oceanic crust sinking deep into the earth, rub together. This is a place where earthquakes occur, and by investigating matter erupted from mud volcanoes, we may be able to learn about what occurs in these asperity zones deep down.

　What is the significance of investigating these mud volcanoes? Engineer Koichi Iijima of JAMSTEC who took part in the survey as a sedimentology expert explains: “Mud volcanoes are carrying methane up from underground. This methane and methane hydrate are extremely interesting compounds.” The spouting of methane is also a characteristic of mud volcanoes. Deposits piled up on the seafloor contain organic matter such as the remains of living creatures. With new soil and sand collecting on top, this gets buried deep in the earth, the organic matter decomposes and methane is generated by methane-producing bacteria or through terrestrial heat. Most methane is formed deep in the earth. Methane hydrate (an ice like substance formed by methane molecules and water molecules under low temperatures and high water pressure), which has also attracted attention as a resource, exists buried in the cracks of the netlike structure inside mud volcanoes. Mud volcanoes are a very interesting subject of research, both as natural pipelines carrying methane from deep places and as methane hydrate icebergs.

　“When you look at the lithium isotopes in the erupted material from Fifth Sea Hill, it seems that this lithium comes up from near the asperity zone several kilometres beneath the surface.” According to Iijima mud volcanoes are also so-called information pipelines. The asperity zone (slip plane) is where the earth’s crust that forms the Japanese Archipelago and the oceanic crust sinking deep into the earth, rub together. This is a place where earthquakes occur, and by investigating matter erupted from mud volcanoes, we may be able to learn about what occurs in these asperity zones deep down.