Mid-Ocean Ridges

Abstract

Two thirds of our planet are covered by ocean, beneath which stretches basaltic oceanic crust. The eruptions that produce this crust are hidden from direct observation. Remote sensing instruments with which to record the countless volcanic eruptions that occur under the ocean waves each year are few and far between. Even the fact that mid-ocean ridges (MOR) are also belts of frequent, albeit low magnitude, earthquakes cannot be explained by magmatic activity alone. If we were able to remove the water layer of more than 3 km, we could stand in the center of the 20–30 km wide rift of the Mid-Atlantic Ridge (MAR), with its walls towering as high as 3 km above the valley floor at 4–4.5 km depth. In the center of this up to 8 km wide youngest and volcanically most active zone, we might never see a volcanic eruption in our lifetime. Moreover, our view along the axis of the ridge would be obstructed at less than 50 km distance by an offset of the ridge itself. Nevertheless, we can be certain that, if there were a sharply defined seam in the middle of the rift, a fracture about 3 m wide would open during a human lifetime of 70 years. This cleft would form between the Eurasian and African plates, moving eastward, and the American plate moving west. Lava would fill the fracture sooner or later. The fact that the most important crust-forming processes on Earth occur along the mid-ocean ridges and that these are magmatic in nature and associated with frequent volcanic eruptions, is based on both direct sampling and indirect geophysical measurement. Geologically older, uplifted ocean crust exposed on land, ophiolite complexes, provide another fundamental tool for our understanding of the structure and origin of present- day igneous ocean crust.