Mount Fuji -- Geology

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Background

Ancient Japanese people revered the nearly perfectly symmetrical Mount Fuji as sacred, and it is a shrine to the followers of the Shinto and Buddhist faiths.  For centuries, only the holy men and male followers were permitted to climb the mountain for the associated spiritual experience.  However, after the Shogun were defeated, the mountain was opened to all who wished to climb it.  Since then several quotes regarding the mountain have surfaced, such as "If you never climb Mount Fuji, you are a fool, but if you climb it more than once you are also a fool" and "Fuji-san is a mountain for looking at, not climbing".  The mountain has become a cultural icon to the people of Japan, and images of it are held in high regard.  Thousands of people reach the summit each year during the official climbing season, July to August.  
Location

Topographic map of Mount Fuji and surrounding area. Mount Fuji is a snow capped prominent peak in the center of the map with various sharp peaked hills surrounding it

Mount Fuji is the highest mountain in Japan with an elevation of 3,776 meters.  It resides on Honshu, the largest of the Japanese islands, and it is located on the island due west of Tokyo.  On a clear day, particularly in the winter, it can be seen from Tokyo.  The volcano lies between the prefectures Shizuoka and Yamanashi. (Dahlby, 2002)

Mount Fuji's location and activity are of particular interest to the people of Japan.  It is less than 100 miles from the capital, Tokyo, and this is easily Japan's most populated city.  Tokyo has a population of approximately 12  million people, and the country has 128 million people.  Therefore, nearly ten percent of the entire population is in this one city.
Human Impact

Since volcanic eruptions are a threat to human civilizations, Japan devotes significant resources through the Volcanological Society of Japan and the University of Tokyo's Volcano Research Center to monitor the activity of this nearby volcano.  

When earthquakes occur, seismologists record the activity on seismometers located near the volcano.  The scientists then look at the data collected to determine whether the volcano is likely to erupt.  If the earthquakes are few and of low magnitude, it is unlikely that any volcanic activity will occur.  However, if the earthquakes increase in magnitude and frequency, careful observations must be made to determine the possibility of an eruption.

In addition to seismic data, physical assessments are made by GIS (Geographical Information Systems) technology.  This tool takes precise measurements of the earth's surface via satellite.  When differences are measured over time, patterns can be established.  For example, in Yellowstone National Park, that caldera habitually rises and falls.  So, if a similar pattern is established at Mount Fuji, any significant changes may indicate volcanic activity.

This type of information can allow the people in the area to prepare for a pending disaster.
Geologic History


Photo of tectonic plates converging near Japan. The Eurasian plate is westmost, then the Phillippine Sea plate is central, and the Pacific plate is east. Mount Fuji is a composite volcano (stratovolcano) that was created through a series of volcanic eruptions. The volcano is above a subduction zone where the Philippine Sea plate is sinking beneath Japan.  However, Mount Fuji is not solely generated by the subduction of plates.  Scientists at Kyoto University have discovered a tear in the Philippine Sea Plate directly beneath Mount Fuji that permits enormous volumes of mantle material to replenish Mount Fuji's magma chamber.  It is speculated that the rip ("hot spot") was created around two million years ago during a nearby continental collision, and this hot spot is the primary source feeding Mount Fuji. (New Scientist, 2004)
(http://www.huttoncommentaries.com/ECNews/AGUreports/Dec2002/japan_trenchs.jpg)

During hot spot activity, rock is melted, and it creates several small pockets of magma.  Generally, volcanoes in these types of areas are much smaller than Fuji, but the special circumstances (hot spot activity plus subduction zone location) cause its immense size and frequent activity.  The volcano produces roughly 10,000 km3 per every 100,000 years, and another volcano in a similar situation would be expected to produce a magnitude lower than that.  (New Scientist, 2004)

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km3 in perspective

An average football field is roughly 300 m (160 ft) wide by 90 m (300 ft) long.  An average trailer from a semi-truck is 15 m (50 ft) long by 2.5 m (8.5 ft) wide by 3 m (10 ft) -- the overall area of the trailer is 112.5 m3 (1.125 x 10-9 km3).  If we pack semi-trailers filled with volcanic material, 10 wide by 30 long in one football stadium, and then stack them on top of one another 100 high (total of 30,000 trailers), we will have about 1/3000th of what we need to fill 1 km3 (3000 football fields).  So, if we are talking about 100,000 km3, we would need roughly 3 million football fields full of our trailers to equate the volume of material produced by Mount Fuji every 100,000 years.

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As indicated in the table below, the dominant rock type is basalt, and this is unexpected in this situation.  Because of the mantle material punching through the rip in the Philippine Sea Plate, basalt is produced instead of andesite.  Basalt is expected at mid-ocean ridges and other isolated "hot spot" volcanoes such as the Hawaiian volcanoes.  The earliest eruption was comprised of andesite, and that suggests that the rip was not present.  Andesitic rocks are associated with subduction zone volcanoes.

Initially the volcano was thought to have been created by three eruptions.  However, a three-year drilling project that ended in February 2004 revealed evidence of an older eruption.  Each of these eruptions is given a name, and from oldest to youngest they are:
 
Cone name Time period Rock Type  
Sen-komitake Several hundred thousand years ago (before Komitake)
 Andesite      
Komitake    
Several hundred thousand years ago Basalt      
Old Fuji
~100,000 years ago Basalt      
New Fuji ~10,000 years ago Basalt    

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 References

Dahlby, T.  "Fuji, Japan's Sacred Summit".  National Geographic.   August 2002.  pp. 26 - 45.

Hutton, W.  "The Hutton Commentaries".  <http://www.huttoncommentaries.com/ECNews/AGUreports/Dec2002/AGU_Dec2002MeetingReport.htm>

"Why Mount Fuji Rises So High."  New Scientist, May 29, 2004. p. 17 (Journal reference: Geophysical Research Letters (DOI: 10.1029/2004GL019477)