The deep earth
Along the floor of the Ryukyu Trench, south of Japan, fields of mud volcanoes quietly erupt a mixture of sediment, methane gas, and water. These volcanoes are not driven by magma. Instead, they are conduits for fluids originating from deep within the Earth's crust. This geological feature is created by the Philippine Sea Plate subducting, or diving, beneath the Eurasian Plate at a rate of approximately 52 millimeters per year. The trench itself is a massive structure, stretching 1,398 kilometers and reaching a maximum depth of 7,460 meters.
As the oceanic plate descends, the immense pressure and increasing temperature force water out of the minerals within the crust. This process, known as dewatering, releases fluids that were trapped in the rock when it formed on the ocean floor millions of years ago. This deeply-sourced fluid then migrates upwards through faults in the overlying plate. As it rises, it mixes with gas and fine sediments, eventually erupting onto the cold seafloor to form large, conical mounds. Geochemical analysis of the erupted fluids confirms their origin, showing they have been altered by high temperatures (above 60°C) and interactions with deep-earth minerals.
Deep-sea exploration and discovery
The mud volcano fields in the Ryukyu Trench are extensive. One area off Tanegashima Island contains more than 30 such mounds. Another recently identified field at the trench's northern end in the Hyuga-nada area has at least 27 individual mud volcanoes. These formations are identified and studied using geophysical tools like 3D seismic reflection data and acoustic surveys, which can map the subsurface structures and detect the gas-rich fluids rising towards the seafloor.
Research vessels, including the scientific drilling vessel Chikyu, operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), have been essential in studying these remote features. By drilling into the seafloor and collecting core samples, scientists can analyze the physical and chemical properties of the erupted material. These studies provide direct evidence of the processes happening tens of kilometers below, in a region of the Earth that is otherwise inaccessible. The sediments erupted from some of these volcanoes are estimated to be from the middle to late Miocene epoch. These volcanoes are also an important pathway for materials like methane to travel from the deep subsurface into the ocean.