Download Plates are large rigid slabs on Earth`s surface. Interact at boundaries

Plates are large rigid slabs on Earth’s surface. Interact at boundaries such as the San Andreas Fault. Boundaries are not often at edge of continent; often in the ocean. Plate boundary is exposed at Iceland, on the Mid‐Atlantic Ridge. This is an area of rifts and earthquakes, geysers and hot springs, basaltic volcanic activity, which fills the rift as the plates diverge. It is a divergent boundary. Where plates move apart, rifting allows magma from mantle to fill in the gap. Described as sea floor spreading, this occurs at the center of the oceanic ridge system: the largest mountain range in the world. Plates grow where this sea floor spreading occurs. Plates can grow if other areas have plates shrinking. This occurs at subduction zones, where there are convergent boundaries, characterized by a dramatic topographic feature called a deep sea trench. There are three types of convergent boundaries: ocean‐ocean like the Mariana Trench where the magma is more iron‐rich; ocean‐continent like the Andes with high silica magma; and continent‐
continent like the Himalayas that have little volcanism because there is no subduction. Convergent margins that involve at least one oceanic plate have subduction, the trenches, and a chain of active volcanoes created by the consumed oceanic lithosphere. The downgoing slab is pulled by gravity and currents in the mantle. Earthquakes trace the descent of the subducting plate into the mantle. Friction helps melt mantle rock where the slab is subducting. This melted rock reacts with overlying rock to make more magma. The magma creates a chain of volcanoes, which may appear as an arc of islands like the Aleutians, or mountains on the edge of a continent like the Andes. The eruptions in continental areas are violent, due to the high‐silica magma created in this environment. The largest mountains in the world form when two continental plates collide. There is little volcanism, because there is no subduction. A third type of boundary, that also has little volcanism, is where plates slide past one another. This is a transform boundary. The San Andreas Fault is one of these, connecting the spreading center of the Gulf of California to the Juan de Fuca Ridge, another spreading center off the Oregon coast. The crust plus the uppermost mantle are bonded together. The unit is rigid, and about 100 km thick. It is called the lithosphere. The lithosphere moves around on mobile zone of upper mantle below it. The mobile zone is called the asthenosphere. Plates slide on the mobile asthenosphere. The motion seems to be a convection cell, supported by high heat flow at the oceanic ridges. Primary heat of formation of Earth and radioactive decay of uranium and thorium provide heat to drive these convection cells. The mantle rocks flow over time, allowing the lithosphere to move. Plates may be pushed off the ridges by upwelling material. They may slide off of the ridges because of the slope and the mobility of the asthenosphere, to be forced down into mantle elsewhere. Pull of the downgoing slab by gravity may be part of what drives the motion. Slabs with long lengths of trench at one boundary are moving the most rapidly, so this is likely the most important mechanism of movement, but the others likely contribute. Most data supports the idea that there are two sets of convection motion, one in the upper mantle, another in the lower mantle. Earthquakes occur to a depth of about 700 km. Definition of slabs does not appear below this depth either. The lower mantle convection heats the upper mantle, but there doesn’t seem to be mixing between the two zones. There is a sharp seismic boundary between the two. The core is of different composition than the mantle. The core is metallic, and the outer core is liquid, with active, more‐vigorous convection than the mantle. Convection changes the shape of Earth. Removing the topography of the continents and oceans, there is a remaining irregular surface: bumps and swells at hot limbs of convection cells, depressions and down warps at the cold limbs. In addition, there are mantle plumes: topped by active volcanoes that may form islands. Hawaii is one. The mantle has a stationary hot‐spot, and the lithosphere moves over it, and the volcanoes get dragged away from it. The result is an ever‐older trail of islands/seamounts created by the hot‐spot. After the volcano is dragged away from the hot‐spot, the island is eroded away. Some may support coral reefs that grow at sea level, leaving a coral atoll, but the volcanic part is all below sea level. There is a new volcano forming southeast of Hawaii: Lo'ihi, which may be an island after 20,000 to 100,000 years from now. Some hot‐spots may be under continents too: Yellowstone is one of these places. The plate tectonics model does have some unanswered questions. There is additional testing and investigation of predictions to fine‐tune this theory.