Do plate tetonics cause problems

J. T. Wilson, in 1965, was the first to combine the concepts of continental drift and sea-floor spreading into a global picture of rigid moving plates. Support for this idea escalated rapidly in the late 1960s, when geophysicists presented further evidence for sea floor spreading and for the underthrusting, or subduction, of plates. This process, in which a lithospheric plate capped by oceanic crust thrusts below either a lithospheric plate capped by continental crust or another oceanic plate, explains why the Earth does not seem to have expanded as ocean floors behaved like paired conveyer belts symmetric around the mid-ocean ridges. It further accounts for both the distribution and compressional character of most major earthquakes.

Since 1968, there have been many demonstrations of sea floor spreading, continental drift, and hence plate tectonics. An early success, achieved at the outset of the international Deep Sea Drilling Project, was to date the ages of the magnetic anomalies over the ocean crust (obtained from MAGNETOMETERS towed behind Ships). This was done by drilling into the sediments resting on the oceanic basement. As expected, the further the drill site was from the mid-ocean ridge, the older was the basal sediment. Scientists are now trying to pinpoint the relative motions of plates through long baseline INTERFEROMETRY and lunar laser ranging.

One of the principal ways in which plate margins are mapped is by plotting earthquake activity and volcanoes, since the majority of these dynamic (and dangerous) processes occur where plates are in the process of being born and dying. Below: An earthquake in Anchorage, Alaska, in 1964 caused a permanent tilt of the land mass along the southern Alaskan coast line.