With the invention of SONAR, submarine crews were astonished to find mountains under the ocean.
Ocean ridges are underwater mountain systems formed by the interactions of tectonic plates. Ocean ridges rise as much as 3,000 meters from the ocean floor, making them larger than even the Himalayas, but entirely underwater. These mountain ranges are bisected by a deep trench down the middle; this trench is as much as 2,000 meters deep. The trench and ridge pattern results from the separation of the sea floor and the formation of new rock along the fault line.
The Earth’s crust is composed of a series of large plates, known as the lithosphere, which float on an underlying layer of liquid magma known as the asthenosphere. The asthenosphere is heated by the radioactive decay of elements, such as uranium, thorium and potassium, which keeps the magma in its liquid state. Variations in temperature and pressure within the asthenosphere create convection currents within the liquid that propel the tectonic plates in different directions around the surface of the earth.
Tectonic plates can either move towards, slide past or move away from one another. Ocean ridges are formed by tectonic plates moving slowly away from each other in a type of motion known as a divergent boundary. As the two or more plates diverge, this puts stress on the rock along the boundary line.
When enough stress has built up along the divergent boundary, the rock cracks and liquid magma wells up through the fracture. The magma, which was previously under high pressure in the asthenosphere, decompresses and expands when it reaches the comparatively lower pressures of the lithosphere. The magma cools and forms new rock, which becomes part of an ocean ridge system. However, the rock is much hotter than that of the rest of the ocean floor and remains less dense and rises far above the sea floor for at least 60 million years, and during this time the rock slowly cools and contracts to the same density as the rest of the sea floor.
Symmetrical Rock Formation
As new rock is formed, the rock is deposited equally on either side of the fault line. As magma cools into solid rock, particles within the magma align themselves to the magnetic poles of the Earth, which change over time. Stripes of rock are found on either side of an oceanic ridge in a mirror-image pattern. The magnetic alignment of particles within a stripe will match that of its corresponding mirror image.
Ocean ridges are found in the center of the ocean floor. The geological activity at these ridges is the cause of sea-floor spreading. However, oceanic tectonic plates are generally thinner than those of continental plates and, unless the oceanic plate and continental plate are moving in the same direction, the oceanic plate will not grow indefinitely. At the edges of the oceanic plate, deep-sea trenches form where rock is pushed under the continental plate. This rock undergoes partial melting and some of the oceanic plate is recycled back into magma.