Mantle Convection and Plate Tectonics (article) | Khan Academy
The Lithosphere-Asthenosphere boundary (LAB) represents a mechanical difference between Earth's inner structure can be described both chemically ( crust, mantle, core) and mechanically. The Lithosphere-Asthenosphere boundary . The mantle's convective motions break the lithosphere into plates and move them around the surface of the planet. These plates may move away from, move by. The lithosphere includes the brittle upper portion of the mantle and the The asthenosphere is viscous, and the lithosphere-asthenosphere.
Halfway through their journey, the waves can reflect off of the surface or a melt layer. The longer path goes all the way to the surface before the waves are reflected. A shorter path is possible if the waves get reflected off of a melt layer at the lithosphere-asthenosphere boundary represented by the yellow-orange area beneath the hotspot.
- Mantle Convection and Plate Tectonics
- New understanding of Earth's lithosphere-asthenosphere boundary beneath the Pacific Ocean
- Lithosphere-Asthenosphere boundary
Waves taking this shorter path will arrive several tens of seconds before waves that miss the melt layer and have to travel to the surface. Nicholas Schmerr Older techniques made imaging seismic discontinuities shallower than kilometers quite difficult, and regions beneath the oceans could only be accessed where seismic stations were installed on ocean islands or by deploying ocean bottom seismometers, giving an incomplete picture of where the Gutenberg occurs beneath the Pacific Ocean.
He discovered that the seismic discontinuity is not a Pacific-wide phenomenon, but rather only detectable beneath regions with recent surface volcanism.
Earth's Crust, Lithosphere and Asthenosphere - Windows to the Universe
He also found the Gutenberg appears to become deeper beneath older crust, confirming the discontinuity is, indeed, related to the LAB. Schmerr proposes that the Gutenberg is formed by partially molten rock produced in the asthenosphere that collects and ponds at the base of the lithosphere. Decompression of hot rock at small-scale upwellings or hot mantle plumes is responsible for generating the melt.
Plumes will thermally reheat the lithosphere, making it shallower than would be expected underneath older crust.The Lithosphere
This means plate tectonics are enabled on Earth because of mantle composition and or grain size, not necessarily the presence of melt. These plates may move away from, move by, or collide with each other. This process forms ocean basins, shifts continents, and pushes up mountains.
Tectonic plates break apart and diverge where the mantle beneath is upwelling. In such regions mid-ocean ridges develop, and new lithosphere and crust form to replace the material that is moving away.
Where plates converge, usually where the mantle is downwelling, one plate is forced beneath another. When this involves plates with embedded continental crust, mountain belts such as the Alps and Himalayas form.
If the collision involves plates with oceanic crust, subduction zones form where one plate descends into the mantle beneath the other plate.
Above these subduction zone chains of volcanoes and island arcs like the Aleutians, develop. Below the Aleutian island arc, at depths of to kilometers, water is forced out of the subducted Pacific plate. This water lowers the melting point of the overlying mantle, causing it to melt. The melting forms magma, which rises to feed the 55 currently active volcanoes that make up the island arc.
California's San Andreas fault system is an example.
Lithosphere-Asthenosphere boundary - Wikipedia
So mantle convection not only accounts for ocean basins, continents, and mountains, it is also the ultimate reason for nearly all earthquakes and volcanoes. The San Andreas is probably the best-known transform fault boundary.
In fact, the connected mid-ocean ridge system is in essence a 80, km long volcano. Over the course of several million years, all of the water in the world's oceans cycles through the oceanic crust at mid-ocean ridges.
This is how the hydrothermal circulation influences ocean composition. How about atmospheric and oceanic circulation? Atmospheric circulation involves the movement of large air masses, and mountains act as barriers, redirecting circulation in the atmosphere.
Also, as tectonic plates move, continents and ocean basins open and close.