1. Iwojima (Introduction to Japanese Geology)

Why begin at Iwojima, which is, after all, a small, unremarkable island only one third the size of Manhattan?

The flag raising at Iwo-Jima, photographed by Joe Rosenthal  

First, Iwojima was the site of one of the bloodiest battles in modern times. Strategically, Iwojima was midway between Tokyo and American bomber bases in the Marianas; whereas the B-29s that were bombing the Japanese homeland could fly from the Marianas without refueling, their fighter escorts could not. For 36 days, 22,000 Japanese soldiers defended the island against 110,000 marines. On the American side, 6,825 marines died here; an additional 19,000 marines were wounded in the fighting. On the Japanese side, the survival rate was nearly zero: their strategy in coming to Iwojima was not to survive, but to dig in and slow the American advance toward Japan. See http://www.iwojima.com for capsule history and photos.

The second reason for beginning with Iwojima is geographical. Iwojima is separated from Japan by 650 statute miles of water. However, if we strip away the water and look at the earth's crust, it becomes clear that Japan and Iwojima are connected by a long ridge. This is not an accident: the same forces that created Iwojima also created Japan.

Sea floor surrounding Japan

This map was generated by satellites that trace the height of the earth's crust by measuring the gravitational pull of the earth's surface as the satellites pass over. Crudely speaking, high gravity = high altitude and low gravity = low altitude. This is especially useful for measuring parts of the earth's crust that are covered by water, like seas and oceans.

The first thing that we notice about Japan is that it is an island. But the water to the west of the island, known as the Sea of Japan, is much shallower than the water to the east of the island. The reason for this is simple: Japan is really the eastern edge of the continent of Asia, and the Sea of Japan is merely a water-filled depression in that continent.

  Major volcanoes of Japan (USGS map)

The real break, geologically speaking, occurs off of Japan's eastern coast, where there is a deep chasm in the crust of the earth, known as the Japan Trench. This trench was formed by the collision of two massive plates or pieces of the earth's crust: the Eurasian plate and the Pacific plate. Where the plates collide, the Pacific plate goes under the Eurasian plate in a process called subduction. (For an explanation, see Carolina Lithgow-Bertelloni's lecture on subduction zones. Also good is the PBS exhibit "Savage Earth: Hell's Crust.") This is what creates the deep trough or trench. The process, though, does not stop there. First, the area behind the edge of the top plate has a tendency to sink, forming what is called a marginal basin; the Sea of Japan, which we mentioned earlier, is an example of this. Second, while the area immediately behind the plate edge is sinking, the edge itself is behind pushed up by volcanoes. As the Pacific plate buries itself under Eurasia, it melts, creating a layer of molten rock. This molten rock pushes up the crust of the earth and returns to the surface, just behind the edge of the top plate, in the form of volcanoes. Did I mention that Japan has a lot of volcanoes?

This situation is not unique to Japan. The Pacific ocean is rimmed on all sides with volcanoes in a line that stretches, in the west, from New Zealand to Kamchatka, across the Bering Strait to Alaska, and down the coast of North and South America. This line of volcanoes is known as the Pacific Ring of Fire, and it marks the places where the Pacific plate collides with its various neighbors.

Pacific Ocean and Continents   Pacific Ring of Fire (in red)

Plate boundaries in the Ring of Fire