The seafloor mid-ocean ridge system encircles our planet for more than 40,000 miles. Yet this remarkable planetary feature is almost totally unseen above the oceans' surface. An exception is Iceland, where the ridge passes right through that nation.
"Ridge" is used to describe this mega-feature that in places rises several thousand feet above the seafloor. The system is actually two parallel ridges with a rift valley running between them that can be as much as 180 miles wide. It is here that the Earth's crust is being perpetually created anew.
Parts of this great mountain range were first discovered on the Atlantic seafloor in the 1950s. At first, it was thought to be unique to that ocean. Then, as more surveys were done, it was eventually determined that this was one continuous global feature linked to geological processes constantly changing the face of our planet.
Since the 1950s we have learned that the Earth's outer skin consists of seven or eight major crustal plates, plus several minor ones. The ever-moving plates are in a continual state of birth and destruction along their boundaries. Plate boundaries at a mid-ocean ridge comprise a place of creation known as a divergent plate boundary.
Creation occurs in the rift valleys of the mid-ocean ridge system at a water depth of about 9,000 feet. New crust is formed there when magma is forced up through the seafloor. The speed of spreading varies from place to place along the mid-ocean ridge system. In the mid-Atlantic, the rate is about 1 inch per year, while along the fast-moving Pacific Rise it can be as high as 7 inches per year. (The latter is about twice the rate at which a human fingernail grows.)
Since our planet is not increasing in size, there has to be a place where seafloor is consumed. This process is called subduction, when plates collide and one passes under another. The plate forced into the earth's interior is melted and essentially recycled. Deep-trench systems in the World Ocean are all subduction regions. The enormous forces of friction between two plates result in zones of extremely active volcanic and earthquake activity.
An example is the island archipelago of Japan that flanks the Japan Trench. Here, the Pacific Plate is being shoved under the Asian Plate. The process folds up the edge of the Asian Plate to create the mountainous topography of that volcanically active nation.
The average time between the creation of new seafloor at a spreading center and its destruction at a subduction zone is roughly 200 million years. Compared to the 4.6-billion year age of our planet, this is indeed a very rapid geological process.
Rift valleys—fields of hydrothermal vents created when water percolates down into the seafloor—are features that were first found and investigated in 1977. Beneath the seafloor the water comes into contact with magma, becomes superheated, and starts to rise rapidly through subsurface rock structures. The water's intense heat leaches metallic compounds out of the rocks and jets out of the seabed at temperatures as high as 750 degrees Fahrenheit. These jets are called "smokers" and can be clear, black, or white. Since the ambient temperature of seawater at this depth is in the low 30s, the entrained minerals precipitate out very quickly. The result is the formation of areas of mineral-concretion chimneys and pavements that can be of ore quality.
Remarkably, these hydrothermal vent fields also support an entire life system that has no relationship to photosynthetically created life on Earth. Instead of being powered by sunlight, this deep-ocean alternative is fueled by thermal heat and chemical compounds from the Earth's interior. This other life process on our planet was only discovered 33 years ago.
Over the past two decades, intensive studies of the global mid-ocean ridge system have shown that hydrothermal vent fields are found all along its length. But these features are transient, with "lifetimes" ranging from months to years. The ore-rich pavements created by the jets have attracted the interest of mining companies. By 2012, the Canadian company Nautilus Minerals plans to begin offshore mining operations in 5,300 feet of water at Papua New Guinea. For some time now, Japan, Korea, and China have been conducting research on the commercial possibilities of exploiting these seafloor resources. The big question is whether or not these extraction activities can profitably compete with land-based resource operations.
In the words of the Old West, it may be true that "Thar's gold in them thar hills."