Mount Fujiyama (Japan): Volcanoes at Subduction Zones
1. Zoom from Greece to Japan.
2. Show aerial/satellite pictures of Mt. Fuji
a. Point out nearly perfect conical shape
b. Show nearly perfect triangular shadow.
3. Show views of Mt. Fuji
a. Show pilgrimages
i. Mt. Fuji is the most sacred mountain in the Shinto religion.
b. Show artwork by Hiroshige/Hokusai – deeply important part of
national psyche
4. Q: why are there volcanoes like Mt. Fuji in Japan, and how did it get so
perfectly shaped?
5. Show map of global volcano distribution and describe.
6. Mention briefly the 2011 Japan earthquake.
7. Show map of global earthquake distribution and describe.
a. Point out the similarities between both maps.
b. Pose the question, what could be causing this.
c. As aside, discuss the correlation/causation conundrum.
8. Show global plate tectonic map, discussing similarities with EQ and
volcanism maps.
a. The plates (and continents within them) are always moving: Show
animation of historical plate motions.
b. Point out unique situation for Japan…at the juncture of 4 plates
c. (only one other place like that – in Indonesia – another very
tectonically active place, as we will see)
d. Show diagram of a subduction zone.
e. Show Japan subduction zone from bathymetry and seismic
tomography
f. Deep trench (deepest is just to the south: Marianas – 11 km deep)
g. Japan’s basement rocks were once at the edge of the Asian continent
h. Q: How can this be? How can Japan be pulled away at a collision zone?
i. A: seafloor goes DOWN, creates a suction force that pulls japan
toward the trench, opening the Japan Sea
9. Back to the question of why there are volcanoes in Japan. Now it is really
confusing, because we take an island (Japan), stuff a lot of cold rock under it
(old, cold Pacific Ocean floor), and rock melts! Should be the opposite
a. Earth gets increasingly hot with depth – almost 6000C at the center.
b. Only the intense pressure, squeezing rock, keeps it from melting
(most of mantle is solid)
c. A: WATER. The ocean water allows the rock to melt and become
magma, rising to the surface to form Mt. Fuji. The magma is not
unusually hot, it just melts more easily with the added water.
10. Now, back to the question about the conical shape of Fuji.
a. Problem – just lava gives you Hawaiian-type shield volcanoes
b. Just ash doesn’t last long – gets washed/eroded away.
c. Fuji is a Composite Volcano (Stratovolcano)
i. Strato = layers
d. Made from alternating layers of ash and lava
i. Ash falls most around the vent, causing vertical build-up of
cone
ii. Lava holds it in place
e. Fuji gets different kinds of eruptions, depending upon things like the
amount of water, and the temperature
i. Too much water – explosive, like Santorini
ii. Too cold (or too little water) – magma is sluggish, can build up
to a large explosion
iii. Warmer, or right amount of water – lava flows out, covering
and locking in the ash layers.
f. The volcanic cones can grow very large, though they can also blow up,
like Santorini or Krakatoa, and have to start building again.
i. We have seen from Mt. St. Helens (the north face blew off in
1980), that there are repeating cycles (and cycles within
cycles) as a stratovolcano grows and eventually explodes.
11. Now we can understand why these big earthquakes keep hitting Japan
(explain tectonics).
a. (I didn’t include earthquakes as any of the Geological Wonders
because there isn’t any place you can go to look at one, unless by great
misfortune you happen to be in the midst of a big one. But they are an
important geological process, and will play a role in the formation of
several of Geological Wonders I WILL talk about)
b. Show map of 2011 Japan earthquake and its aftershocks.
c. One of the largest ever (Larger: 1960 Chile; 1964 Alaska; 2004
Sumatra)
d. Show photo from 1923 Kanto EQ
e. Show Tokai Gap map
f. Show photo of the tsunami and explain.
g. Show NOAA animation of tsunami propagation.
12. Before I show my Top 5 list, I must say that there are a lot of places to go see
subduction zone volcanoes (stratovolcanoes).
a. Some in the Mediterranean (as we already saw last lecture), but most
are in the Ring of Fire. (show map)
b. There are 1500 active or potentially active above-ground volcanoes
around the world, and about 500 of these have erupted in historic
times. 700 of these are stratovolcanoes, and most of these are around
the Ring of Fire.
c. In the US, most are in Alaska, but there are also man in the Pacific
Northwest, including Mt. St. Helens, Crater Lake, Mt. Hood, Mt. Shasta,
Mt. Baker, and Mt. Rainier.
13. So, here are my Top 5:
a. Mt. Fujiyama
b. Mount Rainier (Washington, USA)
i. Tacoma and Seattle built upon the mudflows from its last
eruption
c. Cotopaxi (Ecuador)
i. One of my favorite paintings (Frederic Church)
d. Popocatapetl (Mexico)
e. Chimborazo (Ecuador)
i. Top is the farthest point on land from earth’s center (because it
is at the Equator)
14. Next lecture: Galapagos Rift
Questions:
1. What is the connection between the geologic setting of
this lecture and the setting of the previous lecture?
2. If lava keeps coming out at volcanos like Mt. Fuji, why
don’t they eventually become incredibly tall, like Mt.
Everest?