Did you know that the Earth’s longest mountain range is underwater? The Mid-Ocean Ridge system, shown above snaking its way between the continents, is more than 56,000 kilometers (35,000 mi) long. This series of mountains and valleys marks where the Earth’s crustal plates are moving apart.

The driving force behind the process of plate tectonics is heat generated deep inside the earth’s core by radioactive decay. This heat reaches the surface primarily along the Mid-Ocean Ridge. One of the earth’s most dramatic topographical features, the Mid-Ocean Ridge is a continuous range of undersea mountains more than 12,000 feet high and 1,200 miles wide winding through 40,000 miles of the world’s oceans. It is here, at Mid-Ocean Ridges, that new sea-floor crust is produced and much of the earth’s internal heat is released.

At Mid-Ocean Ridges, two plates are pulling apart from each other as hot magma (liquid rock) emerges from the mantle and oozes forth as lava to fill the crack continuously created by plate separation. The lava cools and attaches itself to the trailing edge of each plate, forming new ocean floor crust in a process commonly known as sea-floor spreading.

All plates have a so-called ‘leading edge’ and ‘trailing edge’. The leading edge is simply the front of the plate, that edge which ‘leads’ the plate in the direction that it is moving. The trailing edge is the back end of the plate. At Mid-Ocean Ridges new crust is added to the trailing edge of each of the two separating, or diverging, plates. Hence, the further sea-floor crust is from the mid-ocean ridge, the older it is.
   
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