During the Mesozoic era a large ocean, Tethys Ocean, floored by oceanic lithosphere existed in- between the supercontinents of Gondwana and Laurasia. This large oceanic plate was consumed at subduction zones. At the subduction trnches the sedimentary rock layers that were deposited within the prehistoric Tethys Ocean buckled, were folded, faulted and tectonically mixed with huge blocks of crystalline basement rocks of the oceanic lithosphere. The Eurasian margin is thought to have been geologically similar to the Western pacific region today. Volcanic arcs and back-arc basins formed and were emplaced onto Eurasia as ophiolites as they collidd with microcontinents. These microcontinents had been pulled away from the Gondwana continent further south. Therefore Turkey is made up of several different prehistorical microcontinents. During the Cenozoic folding, faulting, and uplifting, along with volcanic activity and intrusion of igneous rocks was related to major continental collision between the larger Arabian and Eurasian Plates.
Present-day earthquakes range from barely perceptible tremors to major movements measuring five or higher on the open-ended Richter scale. Turkey's most severe earthquake in the twentieth century happened in Erzincan on December 27, 1939. It damaged most of the city and caused about 160,000 deaths. Earthquakes of moderate intensity often continue with sporadic aftershocks over periods of several days or even weeks. The most earthquake-prone part of Turkey is an arc-shaped region stretching fom the general vicinity of Kocaeli to the area north of Lake Van on the border with Armenia and Georgia.
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