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Mt. Fuji: A Geologic History

Justin Luckner
ES 767 Global Tectonics
Dr. James Aber
Spring 2012

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Introduction

Mount Fuji is a volcano located on Japan's largest island of Honshu and is the tallest volcano in the country, rising 3,776 meters above sea level. In a country with hundreds of volcanoes, this volcano stands out. Considered sacred for hundreds of years by the Japanese people, today it is still a popular subject for the country's artists. While Mount Fuji may look like any other volcano, it has a very complex history created by its location.



Tectonics
Image of tectonic plates near Japan. Taken from http://volcano.oregonstate.edu/vwdocs/volc_images/north_asia/japan_tec.html

Japan resides on the eastern edge of the Eurasian Plate, with both the Pacific Plate and the Philippine Plate being subducted beneath , in a phenome non known as a triple junction. As these plates are subducted, they melt and create large amounts of magma which rise up to form the chains of mountains and volcanoes found on the island nation, of which Mount Fuji is the largest. The subducting plates are not the only source of magma found under Mount Fuji. Japanese scientists have also discovered a fracture in the Philippine Plate that behaves as a hot spot and helps to replenish and build up the cone. This unique situation is believed to be the reason for Mount Fuji's large size, as well as unusual composition.



Description

Mount Fuji is a stratovolcano, which means that its cone is made of alternating layers of lava and pyroclastic flows, which are comprised of fast moving rock and hot expanding gases.  Mount Fuji has the familiar cone-shaped silhouette like most other stratovolcanoes in the world, with gently sloped bottoms and becoming steeper at the summit. However, that is about the only thing Mount Fuji shares with other volcanoes of its kind. Where most stratovolcanoes are generally andesitic in composition, Mount Fuji is made up of mostly basaltic lava flows.








History

The history of Mount Fuji is not the history of a single series of eruptions from one volcano. In fact, it is the history of three different cones, beginning in the middle Pleistocene, with the formation of the Komitake volcano. The lava flows from this cone were andesitic in composition which is characteristic of other stratovolcanoes. Komitake was active until roughly 80,000 years ago, when another volcano rose up in the same area.

This second cone is known as Older Fuji, and began to eject the basaltic lava characteristic of the modern mountain. From 80,000 years ago until about 11,000 years ago, Older Fuji built up the alternating layers of lava and pyroclastics, as well as large mudflows. It is estimated that the volume ejected during this time totaled 250 km3.



The final stage in the creation of Mount Fuji began 11,000 years ago with the Younger Fuji cone. This is the modern view of the mountain, and its eruptions have been studied more in-depth. Eruptions during this period have been found to be quite explosive at times. The period from 11,000 years to 8,000 years ago was marked with mostly flank eruptions of massive lava flows from the north around the west side and to the southwest. From 8,000 to roughly 4,500 years ago, most of the lava flows were on the eastern side of the cone. Both of these periods were also characterized with small scale ejection of tephra, or solid blocks of rock in various sizes.

In the last 4,500 years, the cone has been built up with massive lava and pyroclastic flows from both the summit and flanks of the volcano. The lava was of a thick, blocky basaltic nature commonly called aa.  The last eruption from Mount Fuji happened in December of 1707. This eruption was very explosive, throwing ash and rock high into the air. Ash drifted to the northeast as far away as 280 km, and over the course of the 16 day eruption, had an estimated 0.68 km3 of material erupted.



Future Outlook

Due to the near proximity of Mount Fuji to Tokyo (less than 160 km), and with its history of explosive eruptions, Japan spends a lot of resources to monitor this volcano for any activity. Scientists record seismic activity for any signs the mountain may awake from its slumber and begin spitting smoke and fire over the land. The last seismic rumblings under the volcano were recorded in 2000-2001, but activity from the sleeping giant is currently very slow.




Sources

Mt Fuji Volcano. http://www.volcanodiscovery.com/fuji.html Accessed April 15, 2012.

Mt. Fuji-Geology. http://jove.geol.niu.edu/students/mdare/VirtualFieldTrip/Mt_Fuji/MountFujiGeology.html. Accessed April 15, 2012.

Geology of Fuji Volcano. http://www.eri.u-tokyo.ac.jp/VRC/vrc/others/fujigeol.html. Accessed April 16, 2012

How Volcanoes Work. http://www.geology.sdsu.edu/how_volcanoes_work/stratovolc_page.html. Accessed April 16, 2012.

Tectonics and Volcanoes of Japan. http://volcano.oregonstate.edu/vwdocs/volc_images/north_asia/japan_tec.html. Accessed April 15, 2012.