What do the upper mantle and the crust form

The mantle is also chemically distinct from the crust, in addition to being different in terms of rock types and seismic characteristics. This is due in large part to the fact that the crust is made up of solidified products derived from the mantle, where the mantle material is partially melted and viscous. This causes incompatible elements to separate from the mantle, with less dense material floating upward and solidifying at the surface. The crystallized melt products near the surface, upon which we live, are typically known to have a lower magnesium to iron ratio and a higher proportion of silicon and aluminum.

These changes in mineralogy may influence mantle convection, as they result in density changes and as they may absorb or release latent heat as well. Between the upper and lower mantle, there is also what is known as the transition zone, which ranges in depth from km miles.

The lower mantle lies between , km , miles in depth. However, due to the enormous pressure exerted on the mantle, viscosity and melting are very limited compared to the upper mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous. Denser elements, like lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core.

Because of its high temperature, the outer core exists in a low viscosity fluid-state that undergoes turbulent convection and rotates faster than the rest of the planet. This causes eddy currents to form in the fluid core, which in turn creates a dynamo effect that is believed to influence Earth's magnetic field. The average magnetic field strength in Earth's outer core is estimated to be 25 Gauss 2.

The only reason why iron and other heavy metals can be solid at such high temperatures is because their melting temperatures dramatically increase at the pressures present there, which ranges from about to gigapascals. Because the inner core is not rigidly connected to the Earth's solid mantle, the possibility that it rotates slightly faster or slower than the rest of Earth has long been considered.

By observing changes in seismic waves as they passed through the core over the course of many decades, scientists estimate that the inner core rotates at a rate of one degree faster than the surface. More recent geophysical estimates place the rate of rotation between 0. Recent discoveries also suggest that the solid inner core itself is composed of layers, separated by a transition zone about to km thick.

This new view of the inner core, which contains an inner-inner core, posits that the innermost layer of the core measures 1, km miles in diameter, making it less than half the size of the inner core. It has been further speculated that while the core is composed of iron, it may be in a different crystalline structure that the rest of the inner core. What's more, recent studies have led geologists to conjecture that the dynamics of deep interior is driving the Earth's inner core to expand at the rate of about 1 millimeter a year.

This occurs mostly because the inner core cannot dissolve the same amount of light elements as the outer core. The freezing of liquid iron into crystalline form at the inner core boundary produces residual liquid that contains more light elements than the overlying liquid. This in turn is believed to cause the liquid elements to become buoyant, helping to drive convection in the outer core. This growth is therefore likely to play an important role in the generation of Earth's magnetic field by dynamo action in the liquid outer core.

It also means that the Earth's inner core, and the processes that drive it, are far more complex than previously thought! Yes indeed, the Earth is a strange and mysteries place, titanic in scale as well as the amount of heat and energy that went into making it many billions of years ago.

And like all bodies in our universe, the Earth is not a finished product, but a dynamic entity that is subject to constant change. And what we know about our world is still subject to theory and guesswork, given that we can't examine its interior up close.

As the Earth's tectonic plates continue to drift and collide, its interior continues to undergo convection, and its core continues to grow, who knows what it will look like eons from now? After all, the Earth was here long before we were, and will likely continue to be long after we are gone. Explore further. More from Earth Sciences.

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This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The higher mantle temperature caused a greater amount of melting in the upper mantle, resulting in more magma released to the surface to form thicker crusts [ 3 ]. The formation speed of the early oceanic crust was also likely to be faster than current speeds, due to the higher recycling rates caused by higher upper mantle temperatures [ 1 ].

The early oceanic crust is likely to be basalts in composition, and this could have resulted in the first plate tectonic activity.

The basalt crust is denser than the molten mantle, so the basalt crust could have subsided into the upper mantle, leading to the recycling of crusts [ 3 ]. The oldest continental crust appeared about 4 billion years ago; however, granite continental crust only appeared about 3 billion years ago.

There is no other planet in the solar system that has a continental crust except our Earth, mainly because it requires the presence of water on a planet and the subduction of crusts [ 4 ]. The seawater cools the hot mantle at the subduction zones, and it allows fractional crystallisation to take place to produce a granite crust [ 1 ].

It is the uppermost top component of the lithosphere and floats on top of the upper mantle [ 6 ]. The crust plus the upper mantle is separated by the Mohorovicic discontinuity—a seismic and compositional boundary [ 6 ].

The crust is thickest under mountain ranges and thinnest under mid-ocean ridges [ 6 ]. There are two main types of crust, the continental crust underlie continents and the oceanic crust underlie ocean basins , the latter being denser and thinner but both being less dense than the mantle [ 6 ].

Continents are generally antipodic to oceans [ 9 ]. Conrad discontinuity, which lies at a depth of 5—20 km, separates the continental crust and oceanic crust [ 1 ].

Unlike continental crust, the oceanic crust has no granitic zone, but the mantle beneath the oceanic crust is possibly richer in radioactive elements than the mantle below the continental crust.

The different locations of heat sources and thermal conductivity of the crust give rise to temperature variations in the different crust types. Within the continental crust, there are four layers—the upper, middle, lower and the lowest layers [ 10 ].

The first layer is mainly made of sedimentary rocks and volcanic rocks, and the P-wave velocities in this layer are less than 5. The second layer is mainly made of granitic plutons and metamorphic rocks low-grade , and the P-wave velocities in this layer are between 5. The third layer is mainly made of gabbroic cumulate, and the P-wave velocities in this layer range from 6.

The fourth layer is usually thin or missing [ 11 ], and the P-wave velocities in this layer are between 7. The crust is carried as plates, which are slabs of lithosphere that carry oceanic crust, continental crust or both. They are carried by convection currents in the mantle, a process known as plate tectonics, which is driven by internal heat [ 1 ]. The plates meet at plate boundaries, which can be convergent boundaries, divergent boundaries or transform faults [ 1 ].

Interactions at plate boundaries, such as between crusts or between the crust and the mantle, can give rise to tectonic features such as oceanic ridges and volcanic arcs [ 6 ]. For example, new oceanic crust develops at the opening rifts of divergent boundaries, forming mid-oceanic ridges through ridge push [ 12 ]. This is seen in the Aleutian Islands. The colliding continental crust would result in the crust deforming into fold mountains [ 12 ], and an example of this is the Himalayan mountain range.

Continental margins are long narrow belts [ 13 ] that form at the outer edges of major landmasses [ 14 ] that include continental and submarine mountain chains.

These margins could be passive, active or transform. Passive margins are found between continental and oceanic crust and are tectonically inactive, thus having a smooth relief [ 14 ]. Active margins have more tectonic and seismic activity and have features such as volcanoes and high sediment availability [ 14 ]. These are the fragments of oceanic crust and of the upper mantle that have undergone tectonic emplacement onto the continental crust [ 15 ].

They can be incorporated into both passive and active margins [ 16 ] and could be evidence of features of ancient oceanic crust that have since been consumed by subduction [ 15 ]. Minerals have definite chemical composition, whereas rocks are made up of minerals and have no specific chemical composition. The three main kinds of rocks: igneous, sedimentary and metamorphic.

Igneous rocks are formed by crystallisation of magma or lava. Sedimentary rocks are formed from lithification. Metamorphic rocks are formed from igneous and sedimentary rocks that undergone high temperatures and pressures, stress and fluid activity. There are more than known minerals.

They are all silicates and are also called rock-forming minerals. Amongst the silicates, feldspars are the most abundant with plagioclase being the largest portion [ 18 ]. Minerals are formed by crystallisation through cooling of magma or lava and liquids.

Another process is the evaporation of the liquid containing minerals, which result in the precipitation of material in the form of mineral veins. Elements are the building blocks of minerals. The lower mantle is hotter and denser than the upper mantle and transition zone. The lower mantle is much less ductile than the upper mantle and transition zone. Although heat usually correspond s to softening rocks, intense pressure keeps the lower mantle solid. Geologists do not agree about the structure of the lower mantle.

Some geologists think that subducted slabs of lithosphere have settled there. Other geologists think that the lower mantle is entirely unmoving and does not even transfer heat by convection. In still other areas, geologists and seismologist s have detected areas of huge melt. The iron of the outer core influences the formation of a diapir , a dome -shaped geologic feature igneous intrusion where more fluid material is forced into brittle overlying rock.

The iron diapir emits heat and may release a huge, bulging pulse of either material or energy—just like a Lava Lamp. This energy blooms upward, transferring heat to the lower mantle and transition zone, and maybe even erupting as a mantle plume. At the base of the mantle, about 2, kilometers 1, miles below the surface, is the core-mantle boundary, or CMB.

Mantle convection describes the movement of the mantle as it transfers heat from the white-hot core to the brittle lithosphere. The mantle is heated from below, cooled from above, and its overall temperature decreases over long periods of time. All these elements contribute to mantle convection.

Convection currents transfer hot, buoyant magma to the lithosphere at plate boundaries and hot spots. Earth's heat budget , which measures the flow of thermal energy from the core to the atmosphere, is dominate d by mantle convection. In this model, the mantle convects in a single process. A subducted slab of lithosphere may slowly slip into the upper mantle and fall to the transition zone due to its relative density and coolness.

Over millions of years, it may sink further into the lower mantle. Some of that material may even emerge as lithosphere again, as it is spilled onto the crust through volcanic eruptions or seafloor spreading. Layered-mantle convection describes two processes. Plumes of superheated mantle material may bubble up from the lower mantle and heat a region in the transition zone before falling back. Above the transition zone, convection may be influenced by heat transferred from the lower mantle as well as discrete convection currents in the upper mantle driven by subduction and seafloor spreading.

Mantle plumes emanating from the upper mantle may gush up through the lithosphere as hot spots. A mantle plume is an upwell ing of superheated rock from the mantle. As a mantle plume reaches the upper mantle, it melts into a diapir. This molten material heats the asthenosphere and lithosphere, triggering volcanic eruptions.

The Hawaiian hot spot, in the middle of the North Pacific Ocean, sits above a likely mantle plume. As the Pacific plate moves in a generally northwestern motion, the Hawaiian hot spot remains relatively fixed. Loihi, a mere , years old, will eventually become the newest Hawaiian island.

Geologists think mantle plumes may be influenced by many different factors. Some may pulse, while others may be heated continually.

Some geologists have identified more than a thousand mantle plumes. Until tools and technology allow geologists to more thoroughly explore the mantle, the debate will continue. The mantle has never been directly explored. Even the most sophisticated drilling equipment has not reached beyond the crust.

Drilling all the way down to the Moho the division between the Earth's crust and mantle is an important scientific milestone, but despite decades of effort, nobody has yet succeeded. In , scientists with the Integrated Ocean Drilling Project drilled 1, meters 4, feet below the North Atlantic seafloor and claimed to have come within just meters 1, feet of the Moho. Many geologists study the mantle by analyzing xenoliths. Xenolith s are a type of intrusion—a rock trapped inside another rock.

The xenoliths that provide the most information about the mantle are diamonds. Diamonds form under very unique conditions: in the upper mantle, at least kilometers 93 miles beneath the surface. Above depth and pressure, the carbon crystallizes as graphite , not diamond.

The diamonds themselves are of less interest to geologists than the xenoliths some contain. These intrusions are minerals from the mantle, trapped inside the rock-hard diamond. Xenolith studies have revealed that rocks in the deep mantle are most likely 3-billion-year old slabs of subducted seafloor.

The diamond intrusions include water, ocean sediment s, and even carbon. Most mantle studies are conducted by measuring the spread of shock wave s from earthquakes, called seismic wave s. The seismic waves measured in mantle studies are called body wave s, because these waves travel through the body of the Earth.

The velocity of body waves differs with density, temperature, and type of rock. There are two types of body waves: primary waves, or P-waves, and secondary waves, or S-waves. P-wave s, also called pressure waves, are formed by compression s. Sound waves are P-waves—seismic P-waves are just far too low a frequency for people to hear.

S-wave s, also called shear waves, measure motion perpendicular to the energy transfer. S-waves are unable to transmit through fluids or gases. P-waves primary waves usually arrive first, while s-waves arrive soon after.

Seismic reflections, for instance, are used to identify hidden oil deposits deep below the surface. The Gutenberg discontinuity is more popularly known as the core-mantle boundary CMB. This alerts seismologists that the solid and molten structure of the mantle has given way to the fiery liquid of the outer core. Cutting-edge technology has allowed modern geologists and seismologists to produce mantle maps. Geoscientists hope that sophisticated mantle maps can plot the body waves of as many as 6, earthquakes with magnitude s of at least 5.

These mantle maps may be able to identify ancient slabs of subducted material and the precise position and movement of tectonic plates. Many geologists think mantle maps may even provide evidence for mantle plumes and their structure.