The Formation of the Japanese Volcanic Island Arc

From GeoClasses

Contents 1 Abstract 2 Introduction to the Geography and Geology of Japan 3 Earthquakes and Volcanos in Japan: Life on the Ring of Fire 4 Formation of the Island Arc 4.1 Plate Tectonics: A Summary 4.2 Plate Tectonics and Japan 5 The Arcs and Trenches of Japan 5.1 Back-Arc Basin: The Sea of Japan 5.1.1 A Brief Digression on Japanese History as told by Rocks 6 The Cross-Section 7 The Ryuku's: Home of Okinawa 8 References

Abstract

Japan is a fascinating country. Not only is its culture rich, its country sides beautiful, and its cities break-taking - but Japan has an extremely interesting geological story to tell. When looking at the Japanese islands, the lower islands of Japan (the Ryukyu Islands) may seem to be more of what you would expect to see from an island arc caused by two oceanic plates. In contrast, the four main islands of Japan seem extremely large to be the product of one island arc. Indeed, the island nation of Japan owes the largest majority of its land to volcanic activity due to tectonics, however, research suggests that the main Japanese islands are the product of multiple arcs conjoining. Furthermore, research gives support to the idea that parts of modern Japan were once connected to mainland Asia via some sort of a land bridge. This project will investigate the current geological conditions in modern Japan, formation of the Japanese island arcs, and the geological forces that have acted on the arcs to give us the island nation we know today.

Introduction to the Geography and Geology of Japan

The Asian country of Japan is located in the northwestern portion of the Pacific Ocean and is South-Southeast of Korea and China. At its closest point, mainland Japan is approximately 200 miles from mainland South Korea. The Japanese mainland consist primarily of the four main islands: Honshu (the largest), Hokkaido, Kyushu, and Shikoku. While there are several thousand islands officially considered sovereign Japanese territory, the four main islands make up approximately 97% of the country's land mass. Japan is also highly mountainous with very few planes making agriculture, and indeed habitat, difficult in many parts of the country. Due to the large mountainous areas, the majority of the Japanese population is densely located along the coastal areas.

Earthquakes and Volcanos in Japan: Life on the Ring of Fire

Japan is part of a ring of highly active volcanic activity known as the Ring of Fire. The Ring of Fire is a horseshoe-shaped ring that extends around the Pacific Ocean. The ring is known for many oceanic trenches causes by plate subduction (see Plate Tectonics: A Summary) and many other areas of highly active and volatile plate tectonic activity. As a result, volcanos and earthquakes are extremely common along the ring. As one of the many countries along the ring, the same holds true for Japan. Moreover, according to the USGS (United States Geographic Society), more earthquakes occur in Japan than any other country in the world (relative to its size).

One of the most destructive earthquakes in recent history occured near the Japanese city of Kobe in January of 1995. Follow the link below to more information about that particular earthquake and Japanese experiences with earthquakes in general.

Earthquakes - A daily fact of life in Japan

Formation of the Island Arc

With a survey of the present day geography, topography, and life in this volcanic, actively seismic region, the vital question has presented itself: how did this geologically volatile island nation form? The formation of the Japanese island arc is a story of magnificent underground calamity. Unbeknownst to the early life forms in the eastern Asian waters, a mighty force in the interior of the earth was causing the crustal plates in the northwestern Pacific Ocean to collide. The force and pressure put on the oceanic crust during this process caused the earth in the Asian Pacific to become extremely unstable - giving way to escaping hot magma from the interior of the earth and digging huge trenches at the ocean bottom. This was the beginning of the island nation we now call Japan. The forces that acted on the crust to create the islands still act on the island nation today. These forces have become understood through the theory of plate tectonics.

Plate Tectonics: A Summary

The theory of plate tectonics is a redefinition and further explanation of the Continental drift theory - the idea that the continents move about on the Earth's surface. Plate tectonics stipulates that the earth's crust is divided into many plates (seven major, several more minor plates) that move, collide, and rift apart. It is these processes that help create many of the landform features we observe today such as volcanoes and large mountain ranges. While this theory helps explain observable landforms, it is our knowledge of the interior of the earth that provides the reason for the plate's movements. As the image shows, the needed "horsepower," if you will, for the movement of the tectonic plates is provided by the viscous silicate mantle which undergoes convection. Like a boiling liquid in a pot that is stirred into motion by the heat, the Earth's interior pressure and heat churn the mantle which in turn moves the lithosphere and thus the crust.

Plate Tectonics and Japan

The boundaries of the plates can behave in one of three ways. Where the plates are colliding is known as a convergent boundary. Where the plates are separating is a divergent boundary, and where they simply slide next to each other is known as a transform boundary. As shown in the image (The Tectonic Plates), there are at least four plates interacting in and around Japan: the Philippine, Pacific, North American, and Eurasian. The Philippine Plate is being subducted under the Eurasian plate, and the Pacific plate is being subducted under both the Philippine and North American Plate. Furthermore, the boundary between the North American plate and Eurasian plate is also a convergent boundary. Intuitively, this is a very geologically unstable region of the world. However, the story does not end at subduction.

When an Oceanic plate converges with a continental plate, the Oceanic plate will be subducted because it is more dense than the continental crust. When this happens, two important things will occur. First, a trench will form at the boundary. Second, the subducting plate scrapes off sediment from the continental plate as it is being pushed under the opposite plate. As the plate goes further under the crust, the water trapped in the crust lowers the melting temperature of the lithosphere creating magma which rises to the top eventually erupting and creating volcanic mountains. It is this process that helped create Japan at the boundary of the Eurasian and Pacific Plates.

It is important to realize that while Japan is an island nation today, it was not always completely isolated by water from the Eurasian mainland. During an ice age, when sea levels resend, the Japanese islands connect to both Korea and China to the South and Russia to the North. The Journal of Earth Systems Science Eduction from the Universities Space Science Research Association (Earth Science Division) has an interesting animation that shows the appearance of land bridges during ice ages that illustrates Japan's past as well. The Japanese Sea was not always as prominent of a feature either; however, this is discussed in detail in the Back-Arc Basin: The Sea of Japan section.

The Arcs and Trenches of Japan

Due to the fact that four main plates are interacting on each other in the northwest Pacific, Japan is actually the product of five different island arcs. The island arcs are, from northern Japan to southern: the Kuril Arc, the Northeastern Japan Arc, the Southwestern Japan Arc, the Izu-Bonin Arc, and the Ryukyu Arc.

As the image shows, the Kuril Arc is the result of the Pacific Plate subducting under the North American plate while the Northeastern Japan Arc is formed by where the Pacfic Plate is subducting further south along the North American Plate. Furthermore, the Pacific Plate is also responsible for the Izu-Bonin Arc where the plate is being subducted under the Philippine Plate. Due to the fact that all of these arcs are created by the subduction of the Pacific Plates, this arc system is also referred to simply as the Eastern Japan Arc system.

Similarly, both the Southwestern Japan Arc and the Ryuku Arcs are caused by the subduction of the Philippine Plate under the Eurasian plate and are thus called the Western Japan Arc. Thus, we have established that the many Japanese islands are the result of complex interactions between four tectonic plates as the subduction of the Pacific and Philippine Plates have played the largest roles. However, it is most interesting to note that the largest and most populated island in Japan, Honshu, is actually the product of two island arcs. One should take notice of the tectonic lines on the map of plates and notice that Honshu is indeed split nearly down the middle. The Itoigawa-Shizuoka Tectonic Line separates the island into two, distinct arcs. Geologist have long noted the immense amount of rapid geological change at this location. The area is not only unstable because of the general instability of the region, but this area has two different arcs with different "agendas," if you will, affecting and counter-affecting each other.

In addition to the arcs, ocean trenches can be identified with each arc system. Geologist are able to identify a trench with its specific arc due to similar processes occurring with each and thus have arrived at a list of trenches for each arc: The Kuril Trench, the Japan Trench (for the Northeastern Japan Arc), the Izu-Bonin Trench, and the Ryukyu Trench. A trench is created as the plate being subducted bends downward during the process. In addition to the trench that can be created, another phenonmenon associated with arcs is back-arc rifting. In other words, the crust beyond the arc thins and extends and creates a deep basin. This back-arc basin process is responsible for the two main basins near Japan: the Sea of Japan and the Kuril Basin. The next section discusses the back-arc basin creation process and the Sea of Japan.

Back-Arc Basin: The Sea of Japan

One of the unique features of a subduction zone is the creation of a back arc basin. A back arc basin is a product of rifting in the crust beyond the subduction zone opposite the trench. When looking at the photo of the trenches above, one can see the deeper blue just beyond the arcs opposite the subduction zones. The exact cause of these rifts has been somewhat debated because back arc rifting is not associated with every subduction zone in the world, thus one would be remiss to assert a general rule concerning their association with subduction zones. Nevertheless, in many cases, back arc rifting is observable in subduction zones. The reason behind them is thought to be the actions of the subducting plate in later stages. When the the subducting plate continues on under the overriding plate, many times the plate will bend back towards itself. This force can cause thinning and extension of the overriding crust. Such an event caused the spreading and opening of the Sea of Japan. The same phenomenon is also responsible for the Okinawa Trough and the Karil Basin. The back arc formation that caused the Sea of Japan is believed to have begun 15 to 22 million years ago (Ma). The back arc rifting has contributed significantly to the modern formation of the Japanese arc system, and the system is thought to have been fully complete approximately 2 Ma.

A Brief Digression on Japanese History as told by Rocks

Japan is also a complex mixture of many different kinds of rocks. Geologists have determined that the oldest rocks in the Japanese bedrock are ophiolites, granitic rock, and deep water sedimentary rocks. It is believed that these old rocks have their origin from the supercontinent of the Proterzoic era. However, the largest majority of rocks in Japan are those from accretionary prisms that were created before the rifting that caused the Sea of Japan. The second largest group of rocks, as one might expect, are igneous rocks followed by a large portion of sedimentary rocks.

Intuitively, we can understand the ratio of rock types to be in line with the way Japan formed: an island arc on continental crust. While there is a fair amount of pre-existing Eurasian rock (in the form of ophiolites thrust onto the continent before Japan's creation), the dominate types of rocks are accecionary from the subudcting plate, igneous rocks from volcanos, and sedimentary from weathered, mass wasted, and eroded volcanic rock.

The Cross-Section

Though the creation of Japan deals with at least four tectonic plates and involves serveral different arcs, trenches, and basins, I have attempted to provide a simple cross section of the forces at work that give us the east Asian landforms we know today. The first drawing, to your left, is from the perspective of southern Japan looking in the northeastern direction. The second drawing is from the opposite perspective.

The Ryuku's: Home of Okinawa

I spent nearly a year on the Japanese island of Okinawa recently. From April of 2006 to January of 2007, I was stationed on the island prefecture, and I had the opportunity to see many of its most beautiful landforms. While Okinawa is part of a volcanic arc due to the subducting Philippine plate under the Eurasian plate, the island of Okinawa is unique because it is not volcanic. The thought among geologists is that Okianwa had its beginnings as accretionary prisms and broke off at a point in geologic history. The rifting that has created the Okinawa trough is believed to to be the force that has moved Okinawa into its current location. Okinawa is largely composed of limestone and other sedimentary rocks. Due to the large limestone composition, the island is beautifully sprinkled with caves and cliffs to form a wonderful subtropic area to be stationed. Of course, be sure not to stay too long, the island is only 70 miles long and 10 miles wide and island fever can set in very quickly!

References

Asahiko, Taira. “Tectonic Evolution of the Japanese Island Arc System.” Annual Review, Earth Planet. 2001: 29:109-34.

Brian Taylor, Ed. Backarc Basins: Tectonics and Magmatism. Excertps available on Google Books. Retrieved June 27, 2008. http://books.google.com/books?id=I8AFhvNeimgC&pg=PA415&lpg=PA415&dq=japan+basin&source=web&ots=vWVPSwn_vz&sig=BMTI43PGaCWs3rsxramOqFo_s2w&hl=en&sa=X&oi=book_result&resnum=3&ct=result#PPP1,M1

Nishiwaki, H. “Introduction to the Landforms and Geology of Japan.” Retrieved June 27, 2008. http://glgarcs.lad.jp/index.html

Journal of Earth Science Eduction. “Ice Age Module.” Animated movie. Online article database. Retrieved June 28, 2008. http://jesse.usra.edu/articles/iceagemodule/resources/html/migrationvislarge.html

Publications International, Ltd. “12 of the Most Destructing Earthquakes.” Retreived June 27, 2008. http://science.howstuffworks.com/12-of-the-most-destructive-earthquakes1.htm

Wikipeida. The Free Encyclopedia. Retrieved June 27 and 28, 2005. Multiple pages: http://en.wikipedia.org/wiki/Neogene, http://en.wikipedia.org/wiki/Plate_tectonics, http://en.wikipedia.org/wiki/Back_arc, http://en.wikipedia.org/wiki/Japan_Trench, http://en.wikipedia.org/wiki/Philippine_Plate, http://en.wikipedia.org/wiki/Pacific_Plate, http://en.wikipedia.org/wiki/Pacific_Ring_of_Fire, http://en.wikipedia.org/wiki/Geology_of_japan, http://en.wikipedia.org/wiki/Japan

United States Central Intelligence Agency. The World Fact Book. “Japan.” Retrieved June 24, 2008. https://www.cia.gov/library/publications/the-world-factbook/geos/ja.html

United States Geological Society. Earthquake Center, Online. Retrieved on June 27, 2008. http://earthquake.usgs.gov/