Seafloor hydrothermal activities, in which hot water vigorously spouts out from chimney-like holes (chimneys) formed in the ocean bed, are one of the most active geological phenomena on the earth. Shifts within the earth are observed through the movements of heat and substances such as heavy metals caused by seabed volcanic activities followed by hot fluid circulation (see Photo). This hot fluid circulation supplies copper, lead, zinc, iron, gold, and silver as sulfides to the surface of the seabed. Also, an ecosystem peculiar to hydrothermal fields is formed there.

AIST is conducting basic surveys and research on seabed hydrothermal systems in active ocean areas including ones around Japan and ones in the central oceanic ridge. We are collecting data using manned submersibles, unmanned vehicle, and benthic multi coring system and are working to determine the mechanism of the movements and accumulations of substances and to produce a model of the deposit formation mechanism.

The Izu and Ogasawara Arc, equal to the Honshu Arc in size, is located in the area of sea south of Tokyo. In this seabed, there are many active seabed volcanoes on the west side of the Trench as well as islands in the southern and northern directions. The line of volcanoes along the trench is called a volcanic front. On the west side of this volcanic front, long and narrow depression, called a back-arc rift, lies from the north to the south. Many submarine calderas existing in the volcanic front and back-arc rift have concave topography where active hydrothermal activities are taking place. At submarine calderas with active hydrothermal activities, sea water sinks into the depth through cracks. Then the penetrated water is heated by the heat of the magma chamber and rises again to the surface of the seafloor as a hot fluid solution containing heavy metals and produces a potential sulfide deposit abundant in heavy metals, called a kuroko-type deposit (Figure 1). The same phenomenon as seen in the Izu and Ogasawara Arc is also taking place in the Ryukyu Arc.

In the exclusive economic zones around Japan, many potential metal deposits formed by the processes mentioned above (marked in Figure 2) containing an abundance of gold, silver, copper, lead, and zinc, exist in volcanic fronts and back-arc rifts.