During the 1950s, American Bruce Heezan put together some slow reeling acoustic profiling equipment and throwing dynamite from the stern of a ship, which caused explosions near the ocean surface, he recorded the results accordingly. He reinvestigated the topographic composition of the world's sea floors by recording the reflected sound from the sea floor on a microphone and then completely re-drew the bathymetric charts of the time. During this time, he discovered that a colossal deep-sea mountain range, known as the "Mid Oceanic Ridge System", encircles the earth. This is shown in Diagram 7.

　　He also found that a continuous rift exists in the central axis of the oceanic ridge.

　　He then claimed that the area around the central axis of the rift was where matter from the earth's mantle up welled from the sea floor, and interpreted that this was where the oceanic crust was newly formed and expanded in both directions. However, Heezan also interpreted the scale of this expansion remained within the "Mid Ocean Ridge" and that the diameter of the solid earth gradually got larger. Heezan's theory was eventually shown to be a mistake, because the diameter of the earth seems not to have got larger. However, his discovery of the "Mid Oceanic Ridge" should be regarded as one of the breaking developments in modern ocean geology.

　　Between 1961 and 1962, theories on the spreading of the ocean floor developed further, which would afterwards essentially change our image of the earth. American Robert Dietz and the founder of the Guyot, Harry Hess, advocated these theories. Dietz released his theory one year earlier than Hess, but in the interim, Hess had submitted his manuscript and was seeking criticism on his paper, so I put both their names forward as advocates.

　

　　Diagram 8 shows the east-west cross-section of the Northeast of Japan. Around 1930, while attempting to make a cross section of Northwest Japan, Kiyoo Wadati, discovered that a sloping surface of concentrated seismic activity existed to the depth of about 700km between the "Japan Trench" and the underside of the littoral province of Siberia.

　　After the Second World War, American H. Benioff ascertained that the surface that Wadati had discovered actually existed universally in all the world's trenches. From this time on, these concentrated surfaces of seismic focus were labeled the "Wadati-Benioff Zone".

　　Benioff interpreted these zones of friction as massive reverse faults that had been raised on the ocean floor by the continents.

　　Dietz and Hess, however, claimed that the surfaces of friction were caused by the endless subduction of the oceanic crust under the continental crust, as the upper surface of the oceanic crust and the lower surface of the continental crust maintained a particular distance, and the two crusts brushed against each other to form a very thin layer with much friction.

　　Dietz and Hess then claimed that the oceanic crust, which was generated in the central rift of the Mid Oceanic Ridge and spread at a speed of a few centimeters a year, would in most places that it reached, come in contact with the continental crust. Dietz and Hess adopted the theory that, while meeting with resistance of the continental crust, the oceanic crust would slide itself under it at an angle. They then determined that it was possible for the surface of the earth to move in this way without the radius changing. This is shown in Diagram 9.

　　Until this time, it was thought that the ocean floor was a quiet place that had always remained unmoved. This new discovery over turned this fundamental concept that had been followed for many centuries.

　　Occasionally, however, there are also cases when the crust of the ocean floor rises above the continental crust.

　　Typical representatives of these "Subduction Zones" are common in the Japan Trench, the Kuril Trench, the Izu/Ogasawara Trench, the Mariana Trench, the Nankai Trough, the Ryukyu Trench and other regions in the Pacific Ocean Rim. Recently, it has been suggested that this primary force that laterally moves the oceanic crust, is the heat convection that moves the mantle below it. The mantle is solid layer of solid state, but it has since been discovered that when it is put under pressure for a long period of time, that it transforms into a plastic-like form.

　　In this way, in line with the frictional surface of the subduction zone, when the continental crust that is swept over the upper surface of the oceanic crust by friction reaches a particular limit, it suddenly resists being destructed and returns to its former position. When this occurs, it creates a massive earthquake. Furthermore, the occurrence of accompanying localized earthquakes above and below this surface is also a common occurrence.

　　The reason why the "Subduction Zone" is actually an "Earthquake Zone" was at this point recognized.

　　When the continental crust strongly resists being destructed, thus creating a massive earthquake, it moves the seawater on its surface on a very large scale. This has been accepted as the reason for the outbreak of Tsunami's.

　　Moreover, it has also been discovered that when the upper surface of the oceanic crust ranges between a depth of about 90km-150km and depending on the pressurizing and temperate conditions of the area, it is also an environment where rock is quite easily melted and magma formed. This magma, being lighter than the surrounding rock, identifies weak vertical surfaces and cracks, makes its way to the surface of the earth and erupts. This is how volcanoes and volcanic zones are formed.

　　Aside from volcanoes that stand independently of any other, the subductive zone, without fail, passively activated all other volcanic areas that exist on earth.

　　Furthermore, the great pressures that exist in the subductive zone also bend the continental crust to form the "orogenic zone".

　　In this way, the formation mechanisms, which had up to this point been a mystery, one by one became clearer and clearer, and many related phenomena began to be understood.

　　The subduction zone is concurrent with "Orogenic Zones", "Seismic Zones", "Volcanic Zones", ocean trenches (or troughs when less than 5000m in depth) and tsunamis, and therefore is known as the "Active Zone". In contrast to this, like both coasts of the Atlantic Ocean, the "Passive Zone" is where the oceanic crust hasn't submerged itself under the continental crust, but has just created a "Contact Zone" that pushes the continental crust laterally.

　　The next piece of text is about events that occurred after the "Tectonic Plates Theory". This discovery is shown in Diagram 10. In contrast to the claims of Wegener, around the time when Pangaea split into the northern and southern continents, and the Tehtys Sea was formed in between them, R. Dietz and J. Holden believed that the Indian subcontinent was part of the east side of Gondwana. They then hypothesized that the Indian subcontinent broke away from Gondwana, drifted north and collided with Eurasian Continent. This in turn, they claimed, decimated the Tehtys Sea and pushed up the orogenic zone of the Himalayas. This activity explains the reason why there is a prolific amount of fossils and seabed sediment from the Tehtys Sea found in the high altitudes of the Himalayan mountain ranges. Furthermore, Dietz and Holden also perceived that the Madagascar Island, which contains much continental rock dating back several billions of years, was a fragment that was left behind when the Indian subcontinent drifted north.

　　This account of happenings has been widely accepted as a theory that explains many facts.

　

　　In 1929, Motonori Matsuyama of Kyoto University found an opposite magnetization in a particular piece of basalt rock, and in the following years this marked the discovery that under normal circumstances the earth's magnetism reverts and inverts repetitively.

　　The movement of the heat convection in the outer core of the earth apparently changes direction from time to time. In response to this, the earth's magnetism seems also seems change between normal and reverse magnetism repeatedly. Each magnetic cycle is thought to last about one million years, with frequent changes from normal to reverse magnetism. Magnetism is lost in the period of transaction between them, but this correction period is thought to be very short.

　　After the Second World War, research into paleomagnetism progressed further and for example from 4.5 million years ago, the periods in the present cycle have been named the Brunhes Epoch, the Matsuyama Reverse Epoch, the Gauss Epoch and the Gilbert Reverse Epoch. Matsuyama's name has been officially adopted in this naming process. At the present time, the normal and reverse periods of magnetism have been determined as far back as the Jurassic Period in the Mesozoic Era.

　　The year after the theory of the spreading of the ocean floor was advocated, F. Vine and D. Matthews announced the "Tape Recorder Theory" on ocean magnetic anomalies, or in other words, when an absolute magnetometer was dropped off the stern of a ship and the plus/minus deviation recorded from the average value, it was discovered that centered around the Mid Oceanic Ridge there existed several bands of line symmetric anomalies. They translated this as a natural anomaly that occurred when the expanding oceanic crust reached Curie Point, cooled again and then magnetized material at the time that the direction of the earth's magnetism was in normal or reverse.

　　They publicly declared that the similar bands on both sides of the symmetric line, which coincided with the axis of the Mid Oceanic Ridge, were fossils of the magnetic anomalies of the earth.

　　In the process of their investigations, off the west coast of the border between U.S.A. and Canada, Vine and Matthews discovered a small-scale axis of spread and a band of magnetic anomalies on both sides of it. This proved to be the underlining breakthrough in the discovery of the San Andreas Fault that runs along the west coast of the Continental Divide.

　　In 1968, J. Heirtzler estimated the age of the entire ocean from the bands of magnetic anomaly. Through this estimation he revealed that the spread of the Atlantic Ocean was relatively slow, while on the other hand the spread of the Eastern Pacific Ocean was quite fast.

　　Research in this area progressed and a short time later it was found that the oldest piece of oceanic plate existed in the Western Pacific to the south of the islands of Japan. However, this was estimated to be only in the vicinity of 2-2.5 billion years old. This means, at this time, the older sea floor had already been absorbed into the earths inner through the subduction zone.

　　In 1953, Dietz came to Japan and spent a year studying at the Japanese Hydrographic Department. After putting right Japan's old hydrographic charts, he discovered that a mountainous undersea range, including the "Guyot", existed from the central Northern Pacific to the intersection between the Kamchatka Peninsula and the Aleutian Islands. In 1954 he publicly announced its existence and named it "Emperor's Seamount Chain".

　　In 1963, Dietz made a prediction that the mystery composite rock "ophiolite" that lies exposed near San Francisco was originally a substance from the ocean floor. Soon after, the truth of this prediction was proved.

　　In 1963, Canadian J. Tuzo Wilson advocated the "Hot Spot Theory". He believed that if the Mid Oceanic Ridge was the outlet line for material from the earth's mantle, there ought to also be output points. He claimed that Hawaii was one of those "Hot Spots". Hawaii is now the only volcano that actively exists on top of a hot spot. Wilson stated that the origins of the hot spots remain below the crust of the earth. Furthermore, he added that under the influence of the movement of the ocean floor, the upper section of the earth's crust was moving westward, so even though Oahu Island, Kauai Island and the like were actual volcanoes, they had already been made non-active, or in other words they had a rootless existence. This too, was later proved to be true.

　　Through further research it was then discovered that traces of activity from the Hawaii "Hot Spot" that spurted in fits and starts were identified on the floor of the Pacific Ocean, out past the Midway Islands from the Hawaii Islands, in the Kammu Seamount (of about 40 million years ago) and in the northern point of the Meiji Seamount (about 70 million years ago), which are both contained in the Emperor's Seamount Chain. I was lucky enough to be part of these research activities.

　　In 1965, J. T. Wilson advocated the idea of "Transform Faults". Many places along the central axis of the Mid Oceanic Ridge are broken up by an almost right angular fault of relative length and as a result the central axis has deviated. Wilson interpreted that, even after the central axis of the ridge moved, on a general scale magma from the earth's inner continued to spew out and spread on onto both sides of the ridge to form the positioning that exists today.

　　At some stage, when the central axis is realigned by the fault to a distance that puts it almost at right angles, because the discharge of the magma and the following lateral deviation continue to occur, both sides of the earth's crust on the fault begin to move in the opposite direction. Wilson named this unique kind of fault a "Transform Fault".

　　In contrast to this, he then labeled the normal faults, in which the whole block moves in a straightforward fashion in the opposite direction, as "Transcurrent Faults".

　　Next, Wilson reached a conclusion on this continuing movement. He revealed the phenomenon that shallow earthquakes occur frequently only in the central axis of the Mid Oceanic Ridge and the deviating part of the fault that is almost at right angles to it.

　　The northern extension of the central axis of the East Pacific Rise, which is the Eastern Pacific section of the Mid Oceanic Ridge, looks as if it extends into the Gulf of California. Researchers construed that this extending line is more than likely to be the San Andreas Fault that extends north from this region, which has been driven up into land.

　　Extending from the concept of transform faults put forward by Wilson, it was determined that the San Andreas Fault was not transcurrent, but actually a massive transform fault.

　　From this it was understood that the older the earth's formation, the further and longer were the distances between the two points separated by the fault. Moreover, it was also understood that the large earthquakes that occur in the subduction zone of Japan start with a vertical movement, whereas the earthquakes that occur along the San Andreas Fault start with a lateral movement.

　　As I have previously explained, the solid part of the earth's surface layer is not only he earth's crust, but also includes the upper layer of the mantle. It has a thickness in the range of 60km-200km and the average thickness is estimated to be approximately 100kms.

　　Between 1967 and 1968, in no particular sequence, Englishman D. P. McKenzie, Americans R. L. Parker and W. J. Morgan and Frenchman Xavier Le Picon all announced reports in this field.

　　The solidified layer on the surface of the earth's solid-state came be known as a "plate". The cross-sectional movement of this plate can be seen in Diagram 11. This is the same diagram used in Diagram 9, with the earth's crust being replaced by the earth's plate.

　　Under the plate in the mantle section, it was discovered that the sound wave velocity of the seismic waves was low and that the section was quite soft. As a result, it was interpreted that this is the section that moves in a lateral direction after convection has risen up into the mantle. Above this, the solid plate, which rests on this soft section, moves as a plate in conjunction with this lateral movement. This is what led to the discovery of the plate's existence.

　　The plate is also known as the lithosphere, and the soft area below the plate, where the flow of convection is in a lateral direction, was named the asthenosphere. Up until this time the earth's crust was regarded as being the equivalent of the lithosphere, but this definition was then amended, and the plate was made the equivalent of the lithosphere. The dynamics of the plate are known as "plate tectonics".

　　With the implementation of the plate concept, the dynamical explanation for the movement of the surface layer of the earth's solid-state could be made a lot more rationally, compared with the theory that only the earth's crust was solid.

　　The "Hot Spot Theory" and the "Transform Fault Theory" continued to be accepted in the same way.

　　The oceanic plate which was generated in the central axis of the Mid Oceanic Ridge and spread on both sides of the ridge across the ocean floor, became heavier as it cooled and by the time it began meeting with the continental plate, because it naturally became subductive, the theory arose that the lateral flow of convection in the mantle wasn't always necessary. The real truth behind this theory, however, is something that is still yet to be clarified. I believe that a more amiable explanation may be that there is a coexistence in the subductive movement of the oceanic plate, between the laterally flowing mantle convection and the oceanic plate itself, which becomes thicker and heavier as it cools.

　　The surface plate of the earth's solid state can be classified broadly into the continental plate and the oceanic plate. The activity in which the orogenic zone forms in the subduction zone is known as the Cordilleran-type orogenesis. The formation of mountains like the Himalayan mountain ranges, which are the result of a clash between two continental plates, are called collision-type orogenesis.

　　In reaction to the subduction of the oceanic plate, secondary mantle convection occurred close to the under edge of the continental plate and caused a small-scale spread of the ocean floor. The edge of the continent separated to form island arcs and in between the arcs and the continents themselves marginal seas were created. The Sea of Japan, the Okhotsk Sea, the Bering Sea and the Philippine Sea are all examples of such marginal seas.

　　The Philippine Sea is a large-scale marginal sea. After its formation, an ocean floor-spreading axis developed on its seabed, and formed the subductive zone of the surrounding region. The Nankai Trough, the Ryukyu Trench and the Philippine Trench are all examples of this subduction zone.

　　In 1977, American D. Seely advocated the concept of the accretionary wedge. This proved to be a very important claim for understanding the phenomenon that are related to the subductive zone. Seely claimed that upper surface of the subductive oceanic plate, including the deep-sea sediment and the ocean floor basalt rock beneath it, due to the resulting friction, gradually became stuck fast under the surface of the upper continental plate and affixed itself without completely sinking, therefore forming an accretionary wedge.

　　Then, Seely explained that when atolls and the like from within the ocean subducted and were dragged into the accretionary wedge, without returning to the mantle, they became set in the upper surface of the earth, and through erosion over time became exposed on the earth's surface.

　　This explained the reason why the mystery composite rock "ophiolite", which seemed to have deep-sea sediment and basalt rock-like origins, had become exposed on the earth's surface.

　　It was also then discovered that the limestone rock in Akiyoshidai of Yamaguchi Prefecture was actually a limestone mass that contained a prolific amount of coral fossil that had grown on a deep-sea solitary volcano, which had formerly risen from the ocean floor. The islands of Japan are abundant in limestone rock. It was then further discovered that there are two kinds of coral found interfused in this rock, which arrived there through the accretionary wedge. One of these corals was determined as being from the former ocean floor and the other to originate from a shallow sea are that had developed on the coast, during a warm period.

　　The actual measurement of the speed in which the ocean floor spread has already begun.

　　Using the VLBI method, the change in the interval between Hawaii and Kashima, Japan has been measured. It was found from Hawaii the ocean floor moved towards Japan at a speed of about 4cm per year. This may seem like a very minute movement per year, but given a million years, the sea floor would have reached Japan.

　　Recently, GPS's are being extensively used to obtain accurate positions on earth through simply sending and receiving signals to and from multiple satellites. It is becoming more and more common to repeat measurement positions by GPS and read off the amount of change between those measurements and calculate the amount of movement.