Plate Tectonics Some History • The theory of plate tectonics had its beginnings in 1915 when Alfred Wegener proposed his theory of “continental drift” • He proposed that the continents fit together like a puzzle • He was not the first to notice this, but he was the first to realize that the continents were once joined together Fossils • Paleontologists had also found that there were fossils of similar species found on continents that are now separated by huge distances. • This seemed to show that the earth’s crust (lithosphere) was moving Controversy • Wegener’s ideas were very controversial because he didn’t have an explanation for why or how, just that they moved. • Although Wegener’s theory was later disproved, it was one of the first times that the idea of crustal movement had been introduced and it laid the groundwork for modern plate tectonics. Later Evidence • In the late 1950’s and early 1960’s, evidence emerged that suggested convection currents within the Earth’s asthenosphere could be the driving force behind the movement of the plates. • This theory led to equipment such as seismometers that showed activity along distinct belts around the world Tectonic Plates • These belts defined the edges of the tectonic plates • The lithospheric plates do not randomly meander about the Earth’s surface, but are driven by internal forces. • The mantle is believed to move in circular motions called convection currents Modern Theories • For many years, most scientists believed that mantle convection, seafloor spreading, and magma intrusion at mid-ocean ridges were the predominant mechanisms that drove plate motion. • However recently a new theory about subduction over mid-ocean ridges (sinking of a cold, dense, oceanic slab into a subduction zone) is considered the driving force Today • Although scientists know that forces within Earth drive plate motion, they may never know the exact details. • That plates move is indisputable, but how or why is the challenge • All of our landforms on earth such as mountain ranges, seafloor, trenches, ocean ridges, rift zones, and the volcanic ring of fire are all the result of plate movements