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Plate Tectonics • Lithosphere- 60 miles thick (crust and upper part of the mantle.) It is the solid outer layer of earth made up of tectonic plates. • Plate tectonics is the theory that large pieces of the lithosphere move and change shape. Plate Tectonics • Asthenosphere- hot, plastic portion of the mantle • Hypothesis 1- the plates of the lithosphere float on top of the asthenosphere • Hypothesis 2- the plates are driven by the force of gravity acting on their own massive weight. Plate Boundaries • Volcanoes and earthquakes are more likely to occur where tectonic plates meet. • Mountains and rift valleys also occur along plate boundaries. Mid Oceanic Ridges • Mid oceanic ridges are mountain ranges in ocean crust. In the center of the ridge is a rift valley. • Divergent Boundary- two plates move apart and a gap is made. Hot rock then rises and oceanic crust is made. Subduction • Convergent Boundary- where older lithosphere is destroyed. • Subduction- When one tectonic plate moves beneath another. • The Andes Mountains were made this way. Miscellaneous • Subduction of ocean crust makes volcanoes. • Islands could form where two plates meet • Colliding tectonic plates create mountains