Songqiao “Shawn” Wei

How many poles are there on Earth? Most people would answer two: the North Pole and the South Pole. Mountaineers may say three, including Mount Everest as a third. Geophysicists and oceanographers, however, would answer four, adding the Mariana Trench —the deepest place on the earth.

There are more than 10 trenches around the Pacific Ocean, and some of them, such as the Mariana Trench and the Tonga Trench, lie more than 30,000 feet under the sea. In other words, if we put Mount Everest (with an elevation of 29,029 feet) into these trenches, the mountain would not even reach sea level. On March 26, 2012, James Cameron reached the bottom of the Mariana Trench in the Deepsea Challenger submersible, inspiring others to explore. However, because of the extreme high pressure, trenches still remain unreachable in most cases, and thus remain the most mysterious places on Earth.

According to the theory of plate tectonics, ocean trenches are produced by a process called subduction. The lithosphere, the rigid outer most rocky shell of the Earth, is divided into several plates, such as the North American plate and the Pacific plate. These plates move relative to one another, determining three types of plate boundary: convergent, divergent and transformative. The divergent boundaries normally appear as mid-ocean ridges, and rocks are generated by the magma of the mantle.

In contrast, at a convergent boundary, since one plate moves under another plate and sinks into the Earth’s mantle, these rocks return to the mantle. The convergent rate varies from tens to hundreds of millimeters per year. The area where two plates move towards one another and one slides underneath the other is called a subduction zone. The trench is the most observable sign of a subduction zone on the Earth’s surface. The Pacific Ocean is surrounded by a series of subduction zones, including the ones at the coasts of Japan, Alaska, Washington State, Mexico, Chili and Tonga.

There are many unknown but interesting problems in the subduction zones, such as the dynamic process of the motion, the water flow between the mantle and the surface, and so on. But the most important issues affecting our daily lives are earthquakes and volcanoes.

Scientists have noticed that more than 90 percent of earthquakes and 75 percent of all volcanoes are in a narrow band surrounding the Pacific Ocean, and so they have named this band the Pacific Ring of Fire.

Modern geophysical studies reveal that this “ring” is a production of the subduction zones around the Pacific Ocean.

As can be imagined, when one plate slides beneath another, the large friction on the interface between two plates prevents this relative motion. Even though the high temperature in the mantle and the water brought with the subducting plate can facilitate the sliding process, the stress on the interface still accumulates over a long period. This stress can be released frequently as small and harmless earthquakes. However, if the stress has been accumulated for a long time without any release, a catastrophic earthquake may occur. Unfortunately, so far no one can predict when the stress will be released or what the trigger of an earthquake is. Since the subduction zone usually extends thousands of miles along the surface, once the stress is released on one end of the subduction zone, the residual stress may focus on the other end, thus causing another earthquake.

The great Christchurch earthquake in February 22, 2011, is just such a case. New Zealand is on the same subduction zone with the Tonga Trench. In September 29, 2010, three earthquakes with a magnitude of more than 7.8 occurred near Tonga, Samoa and American Samoa, all located on the north end of the Tonga subduction zone. These earthquakes and the subsequent tsunami killed 192 people. After about five months, an earthquake with magnitude of 6.3 then occurred near Christchurch, the second largest city in New Zealand, killing 185 people and destroying the entire downtown area. Seismologists believe that these earthquakes were all caused by the movement of the Pacific plate westward beneath the Australian plate.

Another product of the subducting process is the volcanic activity surrounding the Pacific Ocean. When the oceanic plate subducts, it brings a large amount of water, which has been dissolved in the minerals. As the plate sinks deeper, the increasing pressure extracts the water from the subducting plate into the mantle. This supplementary water dramatically decreases the melting point of rock and thus produces plenty of magma. This magma with lower density then rises and finally erupts, reaching the surface and forming volcanoes. If the overlying plate is another oceanic plate, the volcanoes appear as an island chain or as submarine volcanoes, such as the Japan islands and the Tonga islands. In contrast, if the overlying plate is a continent, the volcanoes form a huge volcanic mountain range, such as the Andes.

Although more earthquakes have been reported to the public recently, the total number of all earthquakes and that of big earthquakes has not increased. In other words, our Earth is behaving normally, as it always has. The seismology group from Washington University in St. Louis has been operating several research projects in the Tonga-Fiji region and the Mariana-Guam region. We use various seismological techniques to study the natures of these subduction zones, in order to better understand the earthquakes and geological evolutions of the Pacific Ring of Fire.