Many of us learned the basics of plate tectonics in school. Yet only six decades ago, it was more theory than reality. Today, it helps us understand how our fundamental planetary geophysical system works.
Basically, the Earth’s surface is made up of eight major and several minor crustal plates that make up the oceans’ floors and the continental land masses. Continuing geophysical research is revealing more fine details.
These plates are in motion due to the injection of new crustal material (magma) at the seafloor. This activity occurs primarily along the mid-ocean ridge system. At 40,000 miles in length, it is the largest mountain chain on our planet. The system basically consists of two parallel ridges separated by a rift valley. It is in this valley that the new seafloor crust is being created. These places are called spreading centers. The fastest-growing plate segments can move as much as six inches per year—a very high speed in geological terms.
Since our planet is not increasing in size, there also has to be a compensating process that consumes crustal material. This occurs at certain edges of seafloor plates in a process called subduction, when one moving plate collides with another. The time from plate creation to destruction is about 200 million years.
Since continental crusts are less dense than those of the seafloor, they float above the convergence. As the more dense seafloor plate is forced downward, it is melted into magma. The collision between seafloor and continental plates creates intense frictional activity. This is why subduction zones are where most of our planet’s volcanic activity is found. In the Pacific, these energetic areas are referred to as “The Ring of Fire.” It is a zone stretching westward from the Aleutians to Kamchatka and southward past Japan to end off the Philippines.
The downward turning of the ocean plate creates extensive trench systems in all five oceans, though most are in the Pacific. With depths exceeding 20,000 feet, these are the deepest places in the World Ocean. Trenches are generally named for nearby continental features, such as the Kuril, Japan, Marianas, and Philippine trenches. The deepest places in trenches are usually named after the ships that discovered them. Thus the oceans’ deepest place, Challenger Deep, is named after the British survey ship HMS Challenger, which made the discovery in 1951.
However, these great depths to more than 36,000 feet represent less that 2 percent of the global seafloor area. For this reason they are little explored. There are just a handful of research platforms that can work at these depths. This is not the case with the rift valleys, where the average depth is on the order of 9,000 feet. In order to understand the fundamental processes of plate tectonics, both ends of this global-scale “conveyer belt” must be studied.
The most promising means to study both rifts and deeps is by the use of submersibles, manned and unmanned. Today there are nine that can reach the significant depth of about 20,000 feet. Diving this deep gives access to 98 percent of the seafloor. Access to the last 2 percent that makes up the trenches is currently limited to one manned and two unmanned submersibles.
The manned submersible is James Cameron’s one-person Deepsea Challenger that dove to the Challenger Deep in March 2012. The unmanned vehicles are the Japanese remotely operated vehicle Kaiko (two trips to the Challenger Deep) and the U.S. Woods Hole Oceanographic Institution’s Nereus (one trip). The Chinese have begun planning for a full ocean-depth manned submersible, but it will be many years before it is operational.
Many experts state that less than 10 percent of the World Ocean has been adequately studied. This suggests that it will take massive national investments to help mankind understand this single largest geographical feature on our planet. But it would be foolish to not devote more effort to the study of the trenches. If this is not done then we will really not understand the fundamental processes that govern the movements of the Earth’s outer skin.
As some have proposed, it is now time for an inner-space mission to Planet Earth.