To help explain how the lithosphere is floating on the asthenosphere, we need to examine the concept of isostasy. Isostasy refers to the way a solid will float on a fluid. The relationship between the crust and the mantle is illustrated in Figure 3. On the right is an example of a non-isostatic relationship between a raft and solid concrete. On the left, the relationship is an isostatic one between two different rafts and a swimming pool full of peanut butter.
With only one person on board, the raft floats high in the peanut butter, but with three people, it sinks dangerously low. Although it has about the same density as water, peanut butter is much more viscous stiff , and so although the three-person raft will sink into the peanut butter, it will do so quite slowly.
The raft with one person on it floats comfortably high. Even with three people on it the raft is less dense than the peanut butter, so it floats, but it floats uncomfortably low for those three people. The crust, with an average density of around 2. When more weight is added to the crust, through the process of mountain building, it slowly sinks deeper into the mantle and the mantle material that was there is pushed aside Figure 3.
When that weight is removed by erosion over tens of millions of years, the crust rebounds and the mantle rock flows back Figure 3. The crust and mantle respond in a similar way to glaciation. Thick accumulations of glacial ice add weight to the crust, and as the mantle beneath is squeezed to the sides, the crust subsides. When the ice eventually melts, the crust and mantle will slowly rebound, but full rebound will likely take more than 10, years. Large parts of Canada are still rebounding as a result of the loss of glacial ice over the past 12, years, and as shown in Figure 3.
The highest rate of uplift is in within a large area to the west of Hudson Bay, which is where the Laurentide Ice Sheet was the thickest over 3, m. Most subduction happens as an oceanic plate slips beneath a less-dense plate. Along with the rocks and minerals of the lithosphere, tons of water and carbon are also transported to the mantle. Hydroxide and water are returned to the upper mantle, crust, and even atmosphere through mantle convection, volcanic eruptions, and seafloor spreading.
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. The radius of the Earth is km. It has an average density of 5. The main layers are the crust, the mantle and the core.
The crust is the uppermost layer of the planet. It is between 5 and 80km thick. There are two types of crust, oceanic crust found beneath the oceans and continental crust. The oceanic crust is only km thick and made up mostly of basalt. The continental crust can be much thicker, up to 80km, and is made of less dense rocks such as silicate. The major element components of the crust are oxygen and silicon Si and thus the mantle is often referred to as the silicate mantle. The mantle lies beneath the crust to a depth of about km.
The mantle has many layers within the upper and lower mantle. The upper layer is the lithosphere below which is the asthenosphere. The transition zone is the layer between the upper and lower mantle distinguished by the km and km discontinuities, as revealed by seismic evidence.
The Core is made up of two layers, the inner core and outer core. Seismic evidence tells us that the inner core is solid while the outer core is liquid. The inner core has a radius of 1 km and the total radius of the core is km. It is through our study of the Earth that we can gain insight into the structure and composition of other planets.
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