Bellringer Which ocean is getting smaller and which ocean is getting bigger due to subduction and sea floor spreading? Plate Tectonic Theory Notes How Plates Move • Earth’s crust is broken into many jagged pieces. The surface is like the shell of a hard-boiled egg that has been rolled. The pieces of Earth’s crust are called plates. Plates carry continents, oceans floors, or both. How Plates Move • The theory of plate tectonics says that Earth’s plates move because of convection currents in the mantle. Currents in the mantle carry plates on Earth’s surface, like currents in water carry boats on a river, or Cheerios in milk. How Plates Move • Plates can meet in three different ways. Plates may pull apart, push together, or slide past each other. Wherever plates meet, you usually get volcanoes, mountain ranges, or ocean trenches. Plate Boundaries • A plate boundary is where two plates meet. Faults form along plate boundaries. A fault is a break in Earth’s crust where blocks of rock have slipped past each other. Plate Boundaries • Where two plates move apart, the boundary is called a divergent boundary. • A divergent boundary between two oceanic plates will result in a mid-ocean ridge AND rift valley. (SEE: Sea-Floor Spreading) • A divergent boundary between two continental plates will result in only a rift valley. This is currently happening at the Great Rift Valley in east Africa. Eventually, the Indian Ocean will pour into the lowered valley and a new ocean will form. Plate Boundaries • Where two plates push together, the boundary is called a convergent boundary. • A convergent boundary between two oceanic plates will result in an ocean trench, when the more dense (older rock) subducts beneath the younger, less dense oceanic crust it collides into. It will also form an island arc as the crust melts and rises up as magma. (Japan) • A convergent boundary between two continental plates will result in a mountain range. (Himalayas) • A convergent boundary between one oceanic plate and one continental plate will result in a trench when the more dense oceanic crust subducts beneath the continental granite, as well as a volcanic mountain range as the oceanic crust melts and rises as magma. (Andes) Oceanic-Oceanic Convergent Plate Boundary Continental-Continental Convergent Plate Boundary Oceanic-Continental Convergent Plate Boundary Plate Boundaries • Where two plates slide past each other in opposite directions, with neither plate subducting or rising, this forms a transform boundary. Transform boundaries typically don’t create landforms, but they do create numbers earthquakes. (San Andreas Fault) .