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Encyclopedic Entry Vocabulary. As Earth began to take shape about 4. The molten material that surrounded the core was the early mantle.
Over millions of years, the mantle cooled. Common silicates found in the mantle include olivine, garnet, and pyroxene. The other major type of rock found in the mantle is magnesium oxide. Other mantle elements include iron, aluminum, calcium, sodium, and potassium. In the mantle, heat and pressure generally increase with depth. The geothermal gradient is a measurement of this increase. The viscosity of the mantle also varies greatly.
It is mostly solid rock, but less viscous at tectonic plate boundaries and mantle plumes. Mantle rocks there are soft and able to move plastically over the course of millions of years at great depth and pressure. The transfer of heat and material in the mantle helps determine the landscape of Earth. Activity in the mantle drives plate tectonics , contributing to volcano es, seafloor spreading , earthquake s, and orogeny mountain-building.
The upper mantle extends from the crust to a depth of about kilometers miles. The upper mantle is mostly solid, but its more malleable regions contribute to tectonic activity. The lithosphere is the solid, outer part of the Earth, extending to a depth of about kilometers 62 miles. The lithosphere includes both the crust and the brittle upper portion of the mantle. Tectonic activity describes the interaction of the huge slab s of lithosphere called tectonic plate s. The division in the lithosphere between the crust and the mantle is called the Mohorovicic discontinuity , or simply the Moho.
The Moho does not exist at a uniform depth, because not all regions of Earth are equally balanced in isostatic equilibrium. The Moho is found at about 8 kilometers 5 miles beneath the ocean and about 32 kilometers 20 miles beneath continents.
Different types of rocks distinguish lithospheric crust and mantle. Lithospheric crust is characterize d by gneiss continental crust and gabbro oceanic crust. Below the Moho, the mantle is characterized by peridotite, a rock mostly made up of the minerals olivine and pyroxene. The asthenosphere is the denser, weaker layer beneath the lithospheric mantle.
The temperature and pressure of the asthenosphere are so high that rocks soften and partly melt, becoming semi-molten. The asthenosphere is much more ductile than either the lithosphere or lower mantle. The asthenosphere is generally more viscous than the lithosphere, and the lithosphere-asthenosphere boundary LAB is the point where geologist s and rheologist s—scientists who study the flow of matter—mark the difference in ductility between the two layers of the upper mantle.
In fact, the lava that erupts from volcanic fissure s is actually the asthenosphere itself, melted into magma. Of course, tectonic plates are not really floating, because the asthenosphere is not liquid.
Tectonic plates are only unstable at their boundaries and hot spots. In the transition zone, rocks do not melt or disintegrate. Instead, their crystal line structure changes in important ways. Rocks become much, much more dense. The transition zone prevents large exchanges of material between the upper and lower mantle.
Some geologists think that the increased density of rocks in the transition zone prevents subducted slabs from the lithosphere from falling further into the mantle. These huge pieces of tectonic plates stall in the transition zone for millions of years before mixing with other mantle rock and eventually returning to the upper mantle as part of the asthenosphere, erupting as lava, becoming part of the lithosphere, or emerging as new oceanic crust at sites of seafloor spreading.
Some geologists and rheologists, however, think subducted slabs can slip beneath the transition zone to the lower mantle. Other evidence suggests that the transition layer is permeable , and the upper and lower mantle exchange some amount of material. It is not liquid, vapor , solid, or even plasma. Instead, water exists as hydroxide. Oceanic lithosphere is therefore pulled apart in several directions: that process creates the mid-ocean ridges where new, hot and light oceanic crust is created.
However, convection also drives plate tectonics. Picture this scenario: when you cook noodles in a pot of water, you create convection cells.
The noodles move upward in the middle of the pot where the temperature is higher, and then downward on the edges of the pan where the temperature is lower. Such convection cells exist inside the Earth's mantle. One difference is that the mantle is not liquid; rather, the solid rocks are so hot that they can slowly flow. Hot, less dense rock material goes toward the crust whereas relatively denser, less hot material goes toward the core.
At certain times and places, hot, upflowing rock material in these convection cells weakens continental crust to create rifts and eventually new ocean basins.
The East-African Rift, for instance, is the result of such a convection cell breaking up the African plate. Convection cells were responsible for the breaking up of supercontinents many times in Earth's history. Skip to main content. Climate Sea Levels Why will sea level rise not be the same everywhere? How can we date corals?
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What are the different types of rocks? Convection currents within the mantle provide one potential driving force for plate movement. The plastic movement of the mantle material moves like the flow of mountain glaciers, carrying the lithospheric plates along as the convection movement in the mantle moves the asthenosphere.
Slab pull, slab trench suction and ridge push may also contribute to plate movement. Slab pull and slab suction mean that the mass of the descending plate pulls the trailing lithospheric slab across the asthenosphere and into the subduction zone.
Ridge push says that as the less dense new magma rising into the center of oceanic ridges cools, the density of the material increases. The increased density accelerates the lithospheric plate toward the subduction zone. Heat transfer also occurs in the atmosphere and hydrosphere, to name two layers of earth in which convection currents take place. Radiant heating from the Sun warms the surface of the Earth.
That warmth transfers to the adjacent air mass via conduction. The warmed air rises and is replaced by cooler air, creating convection currents in the atmosphere. Similarly, water warmed by the sun transfers heat to lower water molecules through conduction. As air temperatures fall, however, the warmer water below moves back toward the surface and the colder surface water sinks, creating seasonal convection currents in the hydrosphere.
This has remained an open question ever since the advent of plate tectonic theory 50 years ago. Do the cold edges of plates slowly sinking into the Earth's mantle at subduction zones cause the motion observed at the Earth's surface? Or alternatively, does the mantle, with its convection currents, drive the plates?
For geologists, this is rather like the problem of the chicken and the egg: the mantle apparently causes the plates to move, while they in turn drive the mantle The scientists first had to find the appropriate parameters, and then spend some nine months solving a set of equations with a supercomputer, reconstructing the evolution of the planet over a period of 1.
Using this model, the team showed that two thirds of the Earth's surface moves faster than the underlying mantle, in other words it is the surface that drags the interior, while the roles are reversed for the remaining third. This balance of forces changes over geological time, especially for the continents. The latter are mainly dragged by deep motion within the mantle during the construction phases of a supercontinent, as in the ongoing collision between India and Asia: in such cases, the motion observed at the surface can provide information about the dynamics of the deep mantle.
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