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Developing the Theory of Plate Tectonics 8.9A describe the historical development of evidence that supports plate tectonic theory Pangaea: a supercontinent • Alfred Wegener (1912) proposed that Earth’s continents fit together like a puzzle at one time but has since broken apart. • This was known as continental drift. Mountain ranges aligned, which supported continental drift. What caused the continents to drift? • Wegener was unable to explain the cause of continents moving apart. • He proposed maybe Earth’s rotation “spun” the continents away. • The scientific community dismissed this explanation. • Continental drift was not universally accepted. Movement in Earth’s mantle • Geologist Arthur Holmes proposed that continents sit on top of Earth’s mantle which is fluid. • Convection currents move through the mantle. • The currents have enough force to move continents. Action of Convection Currents • Where currents rise, continents will break or push up. • Where currents fall, continents would crumple and form mountains. Big shift in idea • Where scientists once believed that the continents were too big to move, evidence showed that continents move constantly. • This is the theory of plate tectonics. – Plate = large, rigid slab of solid rock – Tectonics = from the Greek meaning “to build” The theory of plate tectonics suggests that the Earth’s surface is made of plates (rock layers). Evidence for Plate Tectonics Continental drift Evidence for Plate Tectonics • Related fossils found that match up with idea of continental drift Evidence for Plate Tectonics • Mid-Atlantic Ridge separating Evidence for Plate Tectonics • Mountain ranges forming where plates collide