In the first half of the 1950's, through a survey made by Ewing from America, it was determined that in comparison to the continental crust, which has a thickness between 20km-60km, the thickness of the oceanic crust at 4km-10km is very much thinner.

　　In 1957, at the Scripps Institution of Oceanography of UCLA in America (I was sent by the Japanese Government to this institution between 1951-1955, and was where I obtained my Ph.D.), a distinguished Jewish physicist "Walter Munk", made a light-hearted claim at an academic association breakfast that if the attending members were willing to drill into shallow crust, which he had located some 4km under the deep-sea bed, they would be able to get their hands on some actual substances from the earth's mantle. Several geologists attending this breakfast, however, took his claim very seriously and soon began putting in the effort to realize Munk's beliefs. Amongst these geoscientists were Roger Revelle, the head of the Scripps Institution of Oceanography, Ewing the head of the Lamont Geological Observatory of Columbia University and Hess of Princeton University (the advocate of the "Ocean Floor Spreading Theory").

　　In the summer of 1959, at the first "International Oceanographic Congress" since the war (also attended by the yours truly), in a press release Revelle announced their plans for the worlds first deep sea drilling operation to be implemented in the Eastern Pacific Ocean. Eventually, it was pronounced that a specific commitment would be made to obtaining some substances from the earth's mantle by penetrating a hole through the Mohorovicic Discontinuity. The drilling project was named the "Mohole Project", taking the "Mo" from where the drilling was to be undertaken and adding it to the "hole" that was result. It was also announced that because the project was to be undertaken at a high risk of failure, that the initial bearing of responsibility would be taken by the United States, with a view to the internationalization of the project should success be achieved.

　　In 1961, CUSSI, a 3000-ton vessel was converted and equipped with the necessary drilling mechanisms and headed for the waters between Mexico and Hawaii. From the 3560-meter ocean floor, the drill penetrated through 171-meters of deep-sea sediment and 6-meters of basalt rock before the tip of the bit disintegrated. It appeared that it was going to take a long time to complete the "Mohole".

　　At this point, it was decided that for the time being it would be best to research the paleo-environment of the earth through paleontology and the implementation of drilling projects in the world's ocean floors, initially to immediately below the surface and then perhaps down to depths of several hundreds or thousands of meters.

　　The converted deep-sea drilling vessel was given the name "Glomar Challenger" (10,500 displacement tons), and was employed in the world's seas from 1968. When its ocean activities ended in 1983, it had clocked up some 96 research expeditions, with an outstanding success story in the collection of valuable research data.

　　During its third expedition, actual evidence of the spreading of the ocean floor was obtained and this proved to be one of the main supporting and powerful pieces of evidence in the "Plate Tectonic Theory".

　　From 1985, the Glomar Challenger's replacement vessel, the "JOIDES Resolution" continued the expeditions from leg 100, under the new title "Ocean Drilling Program (ODP)", and as of August 2000 had extended the completed research expeditions for the collection of data on the Paleo-environment to 190 and was midway through its 191^st research leg in the surrounding waters of Japan. The JOIDES Resolution is to be retired from service in 2003. The American deep-sea drilling vessels are pictured in Photograph 1.

　　From the 45^th expedition that was undertaken in 1975, the project was internationalized. Centered on the leadership of the United States, the new partnership consisted of the participation of six other countries, namely Japan, West Germany, United Kingdom, France and USSR (Russia). From this time the project became known as the "International Ocean Drilling Program". The participating countries in the project have since increased further, with the European Union, Canada, Australia and now also China being partners to the project. The present form of the project is scheduled to be wrapped up in 2003, but America already has plans drawn up for the replacement of the JOIDES Resolution and the continuation of the research project under a new means.

　　In terms of the way these research activities are undertaken, these unique deep-sea drilling vessels have only ever been equipped with a singular drilling pipe, and apart from the waste drawn up by the drilling process and taken as samples, the natural features of the seabed have been left as they were found. Furthermore, because only a singular drilling pipe has been used and blowout prevention devices have unable to be installed into the drilling equipment, the drilling vessel has steered clear of risky areas, such as those containing oil and natural gas layers that could be explosive and endanger the research activities of the vessel.

　　Under normal circumstances, double-pipes are used for the commercial drilling of oil and natural gas from the sea floor. The outer pipe is called the "Riser" and is thrust into the seabed from the deep-sea drilling vessel. An blowout prevention device is then attached to it on the seabed. The inner pipe is called the "Drill String". This is the drilling pipe, which at its tip contains the drilling bit. The heavy and muddy water like that of bentonite is pushed into the inner pipe, along with waste material from the drilling, rises up a gap on the outside of the inner pipe when below the ocean floor, and then above the sea floor is sent back to the vessel in a gap between the inner pipe and the riser.

　　The current deep-sea drilling vessel undertakes surveys using a drill string, with no riser.

　　In Japan, the huge contribution of the deep-sea drilling project to science is valued very highly. As a result, over ten years ago a goal was established to by Japan to make its own international contribution to science, leading to the development of a plan to construct a large-scale deep-sea drilling vessel that included a riser. This goal was blessed by Japan's diet when a 60 billion yen budget for the project was obtained. On the 25^th of April this year a groundbreaking ceremony was held and construction of the vessel began. The completion of construction is scheduled for 2004. The main body of the ship has been contracted to Mitsui Engineering and Shipbuilding Co. Ltd and the installation of the drilling equipment onto the vessel to Mitsubishi Heavy Industries. The administration of the construction and operation of the vessel has been handed over to Japan's JAMSTEC institution. The vessel is shown in Diagram 12.

　　The vessel has been named "Chikyu" (English translation: "Earth") and it has the following specifications.

　　Length of the Drill String: 10,000m (future plans are to extend this to 12,000m).

　　Length of the Riser: 2,500m (to be extended to 4,000m within he next few years).

　　Full Load Displacement: 60,000 tons.

　　There is much anticipation that this vessel will be able to penetrate the Mohorovicic Discontinuity and enable sample material to be gathered from the earth's mantle.

　

　　In the meantime, however, although the achievements of all the deep-sea drilling activities would be far too numerous to mention in their entirety, I would now like to give a brief account of a few of the main up comings of such research activities.

　　After the end of the war, the vessels that were particularly active in the research of deep-sea areas and especially the ocean floor of such deep-sea regions were "Submersible Research Vessels". I also feel that Japan may be the current leader in such submersible vehicles in the present day.

　　Auguste (father) and Jacques (son) Piccard, from Switzerland, are not only well known for their ascent record in a hot air balloon, but also for setting an underwater submersion record in the deep-sea.

　　In 1964, the French manned submersible research vessel "Bathyscaph", which was devised mainly by Jacques Piccard, achieved a dive to 4,050-meters off the coast of Dakar in Afrirca.

　　In 1958, Bathyscaph had already come to Japan and been submerged into the Japan Trench and other surrounding areas.

　　In the same year, the sister ship "Trieste" of the Alchimedes-type bathyscaph series was constructed in Italy. It was constructed as a submersible manned vessel that would be able to make dives to the deepest oceans of the world and was purchased by the U. S. Navy.

　　In 1960, Trieste reached the floor of the Challenger Deep in the Mariana Trench in one sweeping expedition. The vessel was manned by its designer Jacqes Piccard and Officer Don Walsh of the U. S. Navy. Prior to reaching the sea floor, they witnessed some shrimps from the view port of the vessel, but unfortunately when landing on the ocean floor a large amount of sedimentary mud floated up and prevented them from viewing anything else. They took photographs too, but nothing from within them could be identified. Amazingly, however, in the 40 years since this expedition, no other manned submersible vehicle has been able to break the record of this deepest voyage ever.

　　Through a telegraphic message from the U. S. Navy Headquarters, notice was given prohibiting further deep-sea voyages by the Trieste, when concerns were raised about a small leak that occurred when the vessel was in shallow waters. This resulted in the abortion of a second attempt by the submersible research vessel. Walsh later told me, in person, that he knew the increasing pressure at lower depths would prevent further water leakage, but he had no choice outside of obeying orders.

　　The floatation material used in Bathyscaph was gasoline. Compared to the pressure resistant cabin made from spheric steel, where people manned the vessel, the buoyancy material of the vessel was much heavier, which caused much resistance in both submerging to the bottom and rising to the surface, consuming much time.

　　Following this, the vessels began to use many small hollow glass balls packed into a container as buoyancy material. As a result, the weight and size of the vessel was reduced and its operational performance improved markedly.

　　In 1977, the American submersible research vessel "Alvin" discovered a thermal chimney in the central axis of the Galapagos Ridge in the Eastern Pacific. In the neighboring area an ecosystem containing a colony of living matter, not known to have existed at that time, was also discovered. It was determined that it survived off the nutrients in the sulfur discharged by the thermal chimney. This completely changed the concept held in relation to ecosystems. It was from this time that people began to challenge the reasoning behind the origin of life on earth and started to suggest that life on earth began not on the water's edge of the ancient earth, but on the ocean floor.

　

　　In 1979, the submersible vessel "Alvin" made another discovery, this time of the "Black-Smoker", a hydrothermal vent located in the central axis of the oceanic rise in the Eastern Pacific, which happened to be up welling at the time. It was found to contain an affluent amount of valuable copper, nickel and cobalt. From this discovery it the possibility that metal resources besides manganese nodules might also exist in the deep-sea became more apparent. This discovery was also the reason behind the innovative claim that black ore (Kuruko) and other mineral deposits found in the islands of Japan had become interfused with land by an accretionary wedge, which carried them up from their origin in the ocean floor.

　　Following the war Japan also constructed the submersible vessels "Kuroshio" and "Yomiuri", and they were put to extensive use. Their submersible depth, however, was only in the range of several hundred meters, and similar vessels existed in many other countries.

　　In 1981, JAMSTEC's manned submersible vessel "Shinkai 2000" (English translation: "Deep-Sea 2000") was completed and was commissioned into service. This was a groundbreaking achievement and ever since its introduction new discoveries have continued to be made through many different research activities. I also believe that making Shinkai 2000 available for use by all of Japan's researchers has also lead to its furthered success.

　　In 1984, the "KAIKO Project" was initiated between France and Japan. This was a joint project that aimed to target the Japan Trench and the Nankai Trough, which are regarded to be typical of the world's subductive zones. From France the manned submersible vessel "Jean Charcot" was brought to Japan and sound wave tests were carried out on the area. In the following year, taking the vessel "Nadir" as its mother ship, the manned submersible vessel "Nautile" (submersible depth: 6000m), also came to Japan and participated in the research activities.

　　The series of operations undertaken in the "KAIKO Project" are currently still running. On the Japanese side, for the first few years of the project a vessel owned by the Ocean Research Institute of the University of Tokyo took part in the research and in the following years JAMSTEC's vessels have participated to a large extent in the programs research activities.

　　In 1989, another of JAMSTEC's manned submersible vehicles "Shinkai 6500" was completed and put into service. As a manned submersible vehicle, this vessel was constructed with the ability to reach the worlds deepest waters and signaled the beginning of Japans leading role in such technology.

　　In 1991, Shinkai 6500 was submersed 6527 meters to the sea floor of an oblique plain on the Japan side of the Japan Trench, and established a new world record for the deepest manned voyage ever.

　　In 1995, JAMSTEC's unmanned submersible vessel, the "Kaiko" was completed and also commissioned into service. It achieved a successful operation on the Challenger Deep in the Mariana Trench, not only taking photographs of the seafloor, but also taking samples of many different organisms and bacteria with its robot arm. Amongst the samples taken were many unknown species. In the following year, taking advantage of the newfound ability to track the vessels position accurately through GPS, Kaiko made another voyage to the same piece of sea floor where further research and sample measures were taken. At the present time, Kaiko is one of the world's leading remotely controlled vehicles, capable of making voyages to the world's deepest waters.

　　Submersible research vehicles, like that of deep-sea drilling vessels, are a very powerful means of research, because they allow us to lay our hands on actual samples of material from the deep-sea regions out of our immediate reach.

　　On the turn of the Cretaceous Period of the Mesozoic Era to the Paleogene Period of the Cenozoic Era, 65 million years ago, dinosaurs ceased to exist and the earth became a flourishing environment for the evolution of mammals. Many other forms of living matter were also exchanged. The reason for this change was for a long time a mystery that puzzled geologists and paleontologists alike.

　　The place that corresponds to this turn of periods is called the "K/T Boundary". In the various places throughout the world to which the K/T Boundary corresponded, the existence of thin dark colored layers of earth had been recognized from early times. Within these layers a large amount of the element iridium existed.

　　In 1980, an American father/son combination, Luis (father, physicist and Nobel Prize Winner) and Walter (son and geologist) Alvares, from the fact that iridium, which by nature is found on earth in only very small quantities, was found to be contained in these areas, suggested that an explosion caused by the collision into the earth's surface of a small celestial mass containing such iridium could explain the extinction of the dinosaurs. They claimed that the dust created form the explosion on impact would have darkened the skies and prevented the suns rays from reaching earth. This would have resulted in much plant life perishing and the distinction of the herbivorous dinosaurs that fed on it. Soon after, the carnivorous dinosaurs that fed on the herbivorous ones also would have been unable to continue to survive. The Alvares suggested the only animals that may have been able to survive would have been rat-sized mammals.

　　This happening was assumed to have progressed over the entire globe. Many marine and land animals became extinct, and then some time later the world entered the Cenozoic Era, where the few animals that survived one by one began to prosper. Alvares theory sparked great controversy throughout the world.

　　Soon after, a theory arose that the place where the celestial mass fell was the crater located in the depression of the tip of the Yucatan Peninsula in Mexico. It was further inferred that the collision of the celestial body into the earth's surface would have created a massive Tsunami that would have reached an extensive proportion of the earth's surface, causing destruction to plant and animal life and contributing to the extinction of dinosaurs and other animals alike.

　　Between January 8^th and February 14^th in 1997, the deep-sea drilling research vessel "JOIDES Resolution" carried out a drilling in the Atlantic Ocean off the coast of Florida during its 171^st expedition, which aimed to explore the K/T Boundary.

　　From the sample measures obtained, strong evidence that verified that the event in all probability did occur was found in each of the several holes drilled. Substances of a glassy nature, which had obviously been melted by the intense heat created by impact of the celestial body, were determined from amongst the sample material and dust that had been thrown up by the collision and then gradually settled into the sediment of the sea floor was also discovered.

　　The results of this research were quickly reported in 1997. Most geologists and other researchers were convinced of the reality of Alvares theory by the credibility shown by this research.

　　This achievement, just four years ago, must be regarded as one of the big accomplishments of deep-sea drilling activities that have continued to be undertaken for all these years.

　　In recent times, thanks to high temperature / high pressure experiments, acoustic tomography, and the rapid progression of recent computer technology, it has become possible to re-create the conditions and processes that the earths solid state passed through, and by piecing together theory with the verification able to be made, rational explanations for phenomenon have begun to be formed.

　　From about 1983, research from this point of view began to make rapid progress. In this day and age, things have progressed so far that conditions occurring in the solid Earth can be re-produced.

　　In particular, research related to solid phase rock composition in the short term, and the plasticity shown by the mantle in its liquid state over the longer term, have developed remarkably.

　　It is thought that heat convection in the mantle is continuous, but it is also estimated that the scale of the convection is very irregular. The dynamic state of the mantle or the phenomenon caused by such dynamics is known as "Plume Tectonics".

　　In 1983, T. Lay and D. Helmberger pointed out that a 200km thick thermally and quantitatively unstable layer of rock may exist in the lithic mantle, 2900km below the earth's surface in the section directly above the boundary between the mantle and the earth's core. They labeled this the "D Layer" (from D-Double Prime Layer).

　　In 1983, in a similar way, R. Jeanlog and A. R. Thompson discovered that a phase transition zone existed in the mantle about 670km from the earth's surface. Using this as a boundary, they then suggested dividing the mantle into the "upper mantle" and the "lower mantle".

　　These two claims set off some breaking developments in the research of the mantle's dynamics or in other words "Mantle Plume Tectonic Research".

　　Many Japanese scholars have been involved in the leading edge of much of this research.

　　One large transition was noticeable at the end of the Second World War, when Japanese researchers and scholars, who had tended to center their activities in the areas immediately surrounding Japan, gradually become to approach their research from a more global viewpoint.

　　The upper mantle is correspondent to the asthenosphere and is a lithic layer that contains Olivine, Garnett and Pyroxene composites. It is also plastic in nature.

　　The lower mantle is of a much softer nature. Through high temperature / high pressure experiments it has been determined that this layer is made up of a homogenous lithic substance known as perovskite, which is not only very soft, but also possesses high plasticity.

　　The "D Layer" is made up of more highly temperate, but slightly lighter perovskite layer.

　　The outer core of the earth is mainly comprised of a heavy iron that is highly temperate and of a liquid nature. In the zone containing the boundary between the mantle and the core the temperature is estimated to be in the range of 3900-5000℃.

　　The molten iron in the core is unable to ascend over the boundary into the mantle, because it is so heavy. However, the "D Layer" at the bottom of the lithic mantle, which is warmed by the outer core, is thought to become highly temperate and produce much magma. This is then believed to quickly ascend through gaps in the mantle, because it is much lighter than the mantle above it, or ascend more gradually as heat convection.

　　This highly temperate magma that ascends through the mantle, as part of heat convection, is known as "Hot Plume". Descending magma that has been cooled to a certain extent is called "Cold Plume". Large-scale plume is called "Super Plume", regardless of whether it is ascending or descending.

　　The magma discharged from the hot spots in the Mid Oceanic Ridge is hot plume that has erupted to the surface of the earth's solid state.

　　Oceanic plates that are cooled and subduct are called "Slabs". These slabs rub with the surrounding area in the upper mantle and occasionally destruct, which causes earthquakes. They then slowly descend to the bottom of the upper mantle, and after collecting for sometime, they burst through the bottom of the upper mantle into the lower, because they are cooled and become very heavy. The slabs continue to descend, and then enter the "D Layer" as cold plume. However, as the slabs descend through the lower mantle, they don't create as much friction and no earthquakes result.

　　This explained why the lower limits in regard to seismic focus of the Wadati-Benioff Zone seemed to remain below 700-meters.

　　Wilson, the advocate of the hot spots and the transform faults, suggested that each subductive zone continued to descend through the mantle and after building up for some time at the bottom of the upper mantle, became a cold super plume. At the same time, the descent of the cold super plume then brought the continental plates on the earth's surface together to form one huge continent. He went on to suggest that this descending super plume was then warmed to a highly temperate condition by the "D Layer", converting it into hot super plume, which after ascending to the earth's surface, put cracks in the massive continent earlier formed, and went on to spread across the ocean floor and form a new oceanic plate.

　　Wilson claimed this is how the continental plate of the earth was repeatedly divided and brought back together a number of times during the history of the earth. This is known today as the "Wilson Cycle".

　　It can therefore be said that Pangaea, the last massive continent formed, is in the process of being scattered over the earth's surface. The Deccan Plateau, a basaltic plateau in India, was known to have been formed 65 million years ago by an eruption that caused a fission of incredibly hot magma, which spread very quickly over a wide area and solidified. However, the origin of the magma had proved a mystery since the start of civilization. "Wilson's Cycle" is very convincing when you consider that this magma was in actual fact hot plume, which ascended very rapidly through the earth's mantle from the "D" layer.

　　I believe, however, that the massive vibration caused by the celestial body when it crashed into the earth's surface in the vicinity of the Yucatan Peninsula, some 65 million years ago, may have been what stimulated the in the "D" layer and invited the ascent of the hot plume to a the earth's surface, which by coincidence erupted in the vicinity of the Deccan Plateau. I have also put forward such a theory in many presentations that I have made in recent times.

　　There are several places on the floor of the Pacific Ocean were oceanic rises can be found, aside from that of the Mid Oceanic Rise. In these locations, many guyots and atolls exist. These are interpreted to be the traces left by the several ascents of hot plume in the earth's history.

　　In 1964, Englishman W. B. Harland announced that while there was a period when the earth was widely covered by glacial formations near the end of the Precambrian Era, from the direction of the residual paleomagnetism it is apparent that all the continental fragments were scattered around the location of the equator at that time.

　　Russian K. Budkyo picked up on this theory, and suggested that because the suns brightness and heat would be strongly reflected by the fields of ice, the surface of the earth would have been unable to be warmed. On the contrary, he announced, at a certain point it would have been possible for the continents to develop if they were gathered around the shallow seas of the equator.

　　In the few years from this time (as of the year 2000), from a similar viewpoint, Americans P. F. Hoffman and D. P. Schrag, through their research findings, advocated that in between 750 million and 580 million years ago the earth repetitively moved in an out of glacial periods four times, and suggested that these were accompanied by warm periods before and after each ice age, during which animals prospered and evolved.

　　This "Snowball Earth Hypothesis" continues to arouse the interest of many researchers and scholars throughout the world today. So much so, in fact, that warnings about the next freezing of the earth are no longer an unusual part of research presentations and predictions for the near future.

　　Japan is an island arc located on the eastern tip of the Eurasian continent, facing the "Western Pacific Ocean".

　　Some 20 million years ago a secondary spreading of the ocean floor was caused (discovered by deep-sea drilling activities), as the Pacific Plate continuously subducted into the Eurasian plate. This created the marginal "Sea of Japan" and pushed the eastern edge of the Eurasian continent out into the Pacific to form the island arc of Japan.

　　The creation of the Philippine Sea Plate, which is the same marginal sea as the Sea of Japan, is estimated to have been formed much earlier in time, probably more than 40 million years ago.

　　Fully-fledged research into the geology of Japan began a few years after the Meiji Restoration of 1868.

　　Between 1875 and 1885, German goelogist Edmund Nauman revealed an outline of the geology of Honshu, Shikoku and Kyushu in the form of the "Figure of the Geologic Structure of Japan", which was later published in 1893. His revelations were very similar to the actual geology of Japan and so famous is his publication that it is still used in a variety of textbooks today.

　　Nauman pointed out the existence of the "Fossa Manga", a large trough that runs north to south across the central region of the island of Honshu and the "Median Tectonic Line", which runs east to west across Southwestern Japan, as if to bisect Japan into north and south.

　　Memories of Norman remain, for example, in "Nauman's Elephant" and his presence is still very much felt, even today.

　　The first geological research undertaken on Hokkaido was the three-year survey lead by American B. Lyman from 1873. Lymans research was not scientifically orientated, but was rather directed towards making a survey of the underground resources in the region. Furthermore, his "Geological Map of Ezo (Hokkaido)" was later extensively revised.

　　In 1880, a geology department was established in the Science Faculty of the Tokyo University and in 1882, the Geological Survey of Japan was established.

　　Based on the assumption that the "Fossa Manga" was a collapsed rift valley, Hisakatsu Yabe called the western edge of this fault line the "Itoigawa-Shizuoka Tectonic Line" and considered that the eastern edge of the fault was covered by volcanoes like Mt. Asama and Mt. Fuji.

　　In the present day, however, the "Itoigawa-Shizuoka Tectonic Line" is strongly believed to be the plate boundary, and moreover there is a strong possibility that the eastern edge of the fault never existed.

　　In the metamorphic zones of Japan, high pressure/low temperature and low pressure/high temperature formation characteristics are always present. These are the special features characteristic of a subduction zone.

　　Diagram 13 shows the latest classification of the geological structure of Japan, which has been determined by taking into account various approaches to plate tectonics.

　　Southwestern Japan is broadly divided into the "Southwest Japan Province" in the north, and the "Shimanto Province" in the south and the tectonic line that divides them is called the Butsuzo Tectonic Line, south of the Median Tectonic Line. This latest classification translates the Median Tectonic Line as being a large-scale/low angle reverse fault, where the northern side of the crustal block has risen up on the southern block.

　　Formerly, Japan's volcanic zones where classified into seven different zones. However, the fact that volcanic zones can only be formed in the subduction zone, became clear after the theory on the spreading of the ocean floor was realized.

　　In 1970, Arata Sugimura classified the volcanoes in Japan into two zones; the "Eastern Japan Volcanic Zone" that corresponds to the subduction of the Pacific Plate and the "Western Japan Volcanic Zone" that corresponds to the subduction of the Phillipine Sea Plate. Sugimura's classification has been adopted ever since.

　　The marginal line near the oceanic trench axis or trough axis of a volcanic zone is a very distinct line and any further over the line into the trench or trough no eruptions occur. This marginal line is called the Volcanic Front. This is how it became known that while earthquakes can strike Tokyo, it's not possible for any volcanic eruptions to occur there.

　　In 1972, Arata Sugimura then announced that accompanying its movement up to the north, a small continental fragments that lay on top of the Philippine Sea Plate had one after another clashed with the island of Honshu and as a result had formed the Izu Peninsula. Through the inspection of the direction of the residual paleomagnetism in stratum of the Izu Peninsula and other research, it was determined that this was actually true.

　　However, at the present time in this region, the Philippine Sea Plate is cutting in under the Izu Peninsula and is in the process of subducting into the side of Honshu. The Suruga and Sagami Troughs have become the subducting axis in this process.

　　In Hokkaido, small continental fragments that rested on the top of two plates moved towards each other from the east and the west and collided. Several of the fragments from the western side subducted under those from the eastern side and solidified. Soon after, the Hidaka Volcanic Belt, which is part of the Eastern Japan Volcanic Zone, was formed as a result of the Pacific Plate subducting into it.

　　From historical records, those volcanoes that are thought to be capable of erupting are known simply as "Active Volcanoes", where as those without are labeled "Extinct Volcanoes".

　　In the locations where the Pacific Plate subducted under the Philippine Sea Plate (the same oceanic plate as itself), the Izu-Ogasawara Trench, the Mariana Trench, the Yap Trench and the Palau Trench were formed.

　　In the subductive zone, because the orogen becomes bent, on the oceanic trench side of the ridge a forearc basin is formed and on the opposite side of the ridge a backarc basin is created.

　　When sediment collects in the depressed areas of the forearc basin, their top surface becomes flat and they form deep-sea terraces. However, the top surfaces of a backarc basin rarely become flat.

　　The Mariana Trough and the Okinawa Trough are backarced basins. In the Philippine Sea there is the characteristic ocean floor configuration of the Kyushu-Palao Ridge that extends from north to south.

　　Until a little while ago, it was thought that the dispersion of the plates in the vicinity of Japan was very simple and was only made up of the Eurasia, Pacific and Philippine Plates that incorporate the islands of Japan.

　　It was Eiichi Honza who identified that extending above the Itoigawa-Shizuoka Tectonic Line there existed another tectonic line runs close to the Japan Sea side of Northeastern Japan.

　　In 1983, Kazuaki Nakamura declared that this tectonic line was actually a plate boundary and put forward the bold proposal that Northeastern Japan lies on the western edge of the North American Plate. His proposal was supported by the fact that there is a frequent occurrence of earthquakes along this tectonic line in the Sea of Japan.

　　Soon after, within Japan various theories were advocated to some degree try and separate the micro-plate of Northeastern Japan from the main body of the North American Plate. However, internationally most researchers adopt Nakamura's interpretation as being correct.

　　The surface beneath the Kanto region is believed to consist of three separate layers. After passing through the Sagami Trough there is supposedly the subducting Philippine Sea Plate and then exists the subducting Pacific Plate slab.

　　The Philippine Sea Plate slab underneath the Sagami Trough is thought to have descended gradually, pulling down with it the Miura and Boso Peninsulas that are on the front edge of the Kanto region.

　　Afterwards, when the Continental Plate in the Kanto region reached the point of destruction, at the same time creating a wide-ranging backlash against the subducting Philippine Sea Plate, the ground suddenly rose and this caused the Great Kanto Earthquake of 1923. However, irrespective of the repulsion displayed by the Continental Plate, the Philippine Sea Plate began descending again soon after and has continued to subduct in the same manner since.

　　The Great Kanto Earthquake was a wide spreading earthquake caused by pate repulsion.

　　In 1977, on the 57^th expedition of the "Ocean Drilling Program" mentioned earlier in this text, I was involved in the implementation of a number of deep-sea drillings on the landside of the inclined plane into the Japan Trench, off shore of Hachinohe, Kamaishi and the Northeast of Japan. There were 13 researchers aboard the ship at the time. The senior researchers were American Roland von Huene and myself, Jean-Paul Cadet, who is presently a professor at Paris University, came from France and all other participating researchers were from U.S.A. The biggest accomplishment made on this voyage was the determination of the start of the subduction of the Japan Trench. The beginning of the subduction period was found to be approximately around 23 million years ago.

　　In 1953, when entering the Tsugaru Strait and heading west over the Japan Trench on the Scripp's research vessel "Spencer F. Baird", I nearly choked in excitement when I became aware of a deep-sea terrace at a depth range of approximately 1000m-3000m. The discovery of such an existence had been an ambition of mine for many years.

　　Relying on the preliminary result of the multi-channel acoustic surveys on 438,439 drilling sites, indicated in diagram 14, I established a drilling near the eastern edge of the deep-sea terrace, with the agreement of my colleagues.

　　We drilled 1157.5m under the ocean floor at a depth of 1564.5m. The last 12 meters of the drilling was into stratum from the Upper Cretaceous Period and traces of land erosion were embedded on upper surface of it. Above this was a 45m thick layer of andesite-breccia. The surrounding area also showed evidence of continental and island arc volcanic activity. Above this was approximately 100m of sand that contained fossilized shallow-water shell fragments and the remaining 1100m was deep-sea argillaceous sediment.

　　From the history of these sedimentary layers, although we were not immediately able to make any discovery, we were able to ascertain that over a considerably long period of time this surrounding area had made its way on to land and then as a consequence of the descent of the axis of the Japan Trench, which began about 23 million years ago, it has continued to sink. This former surface of the earth is now found 2600m below the present surface of the earth. Taking the name of the "Oyashio Current" that flows over this area, we named this ancient land the "Oyashio Ancient Landmass". Diagram 15 shows the background behind how the "Oyashio Ancient Landmass" sunk to its present depth below the seafloor.

　　If you put this together with the result of this deep-sea drilling research that tells us that the formation of the marginal Japan Sea began some 20 million years ago, the timing of the continuous descent in the Japan Trench is also periodically consistent.

　　The deep-sea terrace was the upper surface of this ancient landmass, which had been leveled by off by sediment that had piled up in the forearc basin.