Sofia Martinez
The Earth's mantle is a solid layer that undergoes slow, continuous convective motion. It is composed of silicate rocky material with an average thickness of 2,886 kilometres (1,793 miles) and sits between the Earth's crust and its upper core. The mantle is divided into sections, including the upper mantle, transition zone, lower mantle, and core-mantle boundary. The upper mantle is further divided into the asthenosphere, which is composed of plastic flow, and the inner asthenosphere. While the Earth's mantle has not been explored in depth, studies of earthquake waves, heat flow, magnetism, gravity, and laboratory experiments on rocks and minerals have provided valuable information about its structure and composition.
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What You'll Learn
- The upper mantle is composed of plastic
- Olivine-rich rocks in the mantle deform due to 'disclinations'
- The mantle is divided into sections, including the upper mantle and lower mantle
- The mantle sits between the Earth's crust and its upper core
- The mantle is responsible for the Earth's volcanic and seismic activity
The upper mantle is composed of plastic
The Earth's mantle is a solid layer that undergoes slow, continuous convective motion. It is composed of silicate rocky material with an average thickness of 2,886 kilometres (1,793 mi). The mantle sits between the Earth's crust and its upper core. The upper mantle extends from about 7 to 35 km (4.3 to 21.7 mi) from the surface down to a depth of 410 km (250 mi). The inner part of the upper mantle is known as the inner asthenosphere, which is composed of plastic.
The Earth's mantle is plastic due to the presence of a low-viscosity layer. This layer changes the amount of 'coupling' between the outer crust and mantle convection. The crust is 'coupled' if mantle convection produces stresses in the crust large enough to deform it plastically. The low-viscosity layer is believed to exist at the base of the lithospheric plates, allowing plate tectonics to exhibit a variety of horizontal scales.
The upper mantle is composed of olivine, which makes up as much as 60% of this layer. Olivine is a mineral that does not exhibit enough defects in its crystal lattice to explain the deformations observed in nature. However, researchers have discovered that the crystal lattice of olivine exhibits highly specific defects known as 'disclinations'. These disclinations provide an explanation for the plasticity of olivine. When mechanical stress is applied, the disclinations enable deformation.
The plastic nature of the upper mantle is important for tectonic processes. Earthquake waves indicate that at a depth between 37 and 155 miles, the Earth's interior is less rigid than that above and below it. This plastic layer allows for the deformation of the Earth's mantle, facilitating the movement of tectonic plates and resulting in volcanic and seismic activity.
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Olivine-rich rocks in the mantle deform due to 'disclinations'
The Earth's mantle is a solid layer that undergoes slow, continuous convective motion. However, the mechanism by which olivine-rich rocks in the mantle deform has long been a mystery, as olivine does not exhibit enough defects in its crystal lattice to explain the observed deformations.
Olivine is the most abundant upper-mantle mineral and the weakest under a wide range of thermo-mechanical conditions. Researchers have now found that the crystal lattice of olivine exhibits highly specific defects known as 'disclinations', which had previously been neglected. These disclinations are located at the boundaries between the mineral grains that make up rocks. When mechanical stress is applied, disclinations enable grain boundaries to move, allowing olivine to deform in any direction. This motion of rotational defects is also known as disclination motion.
By observing these defects for the first time in samples of olivine using an electron microscope and specific image processing, scientists have provided an explanation for the plasticity of olivine-rich rocks in the mantle. This discovery has significant implications for understanding the dynamics of solids and materials science. Furthermore, it provides a powerful tool for studying the mechanics of solids and the multiscale modelling of the rheology of the upper mantle.
The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. This general description of the deformation process, including the motion of disclinations, resolves the olivine deformation paradox. High-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates are used to visualize these disclinations, which are found to decorate grain boundaries in olivine samples.
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The mantle is divided into sections, including the upper mantle and lower mantle
The Earth's mantle is a solid layer that undergoes slow, continuous convective motion. It is composed of silicate rocky material with an average thickness of 2,886 kilometres (1,793 miles). The mantle sits between the Earth's crust and its upper core. The upper part of the lithosphere is the Earth's crust, a thin layer that is about 5 to 75 kilometres (3.1 to 46.6 miles) thick. The mantle is separated from the crust by the Mohorovicic discontinuity, or "Moho", which is defined by a sharp increase in the speed of earthquake waves.
The mantle is divided into sections, including the upper mantle and the lower mantle. The upper mantle extends from about 7 to 35 kilometres (4.3 to 21.7 miles) from the surface down to a depth of 410 kilometres (250 miles). It is composed of olivine, which makes up as much as 60% of the upper mantle. The upper mantle is further divided into two main zones: the outer upper mantle and the inner asthenosphere. The inner asthenosphere is composed of plastic and is believed to be a low creep strength layer. The lower mantle reaches from 660 kilometres (410 miles) to a depth of 2,891 kilometres (1,796 miles).
The boundaries between the sections of the mantle are based on results from seismology. Earthquake waves indicate that at a depth between 37 and 155 miles, the Earth's interior is less rigid than that above and below it. This layer has an important bearing on tectonic processes. The mantle is also studied through heat flow, magnetic, and gravity studies, as well as laboratory experiments on rocks and minerals. While the Earth's mantle has yet to be explored at significant depths, indirect studies and human exploration of the Solar System continue to provide new insights into its structure and composition.
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The mantle sits between the Earth's crust and its upper core
The Earth is composed of four layers: the inner core, the outer core, the mantle, and the crust. The mantle sits between the Earth's crust and its outer core. The crust is the layer we live on and is the most widely studied and understood. It is extremely thin compared to the mantle, which is about 2,900 kilometers (1,802 miles) thick and makes up 84% of the Earth's total volume. The crust is composed of two basic rock types: granite and basalt. The continental crust is composed mostly of granite, while the oceanic crust consists of a volcanic lava rock called basalt.
The mantle is a solid layer that undergoes slow, continuous convective motion. It is much hotter and denser than the crust and has the ability to flow due to convection currents. Convection currents are caused by the very hot material at the deepest part of the mantle rising, then cooling, sinking, heating again, rising, and repeating the cycle. This movement of the mantle drives plate tectonics, contributing to volcanoes, seafloor spreading, earthquakes, and orogeny (mountain-building).
The upper edges of the mantle, between 100 and 200 kilometers (62 to 124 miles) underground, form a layer of partially melted rock known as the asthenosphere. This weak, hot, and slippery part of the mantle is believed to be what the Earth's tectonic plates ride upon and slide across. The outermost zone of the mantle is relatively cool and rigid, behaving more like the crust above it. Together, this uppermost part of the mantle and the crust are known as the lithosphere.
The transition zone of the mantle, located between 410 kilometers (255 miles) and 660 kilometers (410 miles) beneath the Earth's surface, is where rocks undergo radical transformations. In this zone, the crystalline structure of rocks changes, and they become much denser. This increased density may prevent subducted slabs from the lithosphere from falling further into the mantle, stalling them for millions of years before they mix with other mantle rock.
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The mantle is responsible for the Earth's volcanic and seismic activity
The Earth's mantle is a significant layer situated between the crust and the core, making up about 84% of the Earth's total volume. It is about 2,900 kilometres (1,802 miles) thick. The mantle is mostly solid, but its more malleable regions contribute to tectonic activity.
The mantle is divided into several layers: the upper mantle, the transition zone, the lower mantle, and the strange region where the mantle meets the outer core, known as D” (D double-prime). The upper mantle extends from the crust to a depth of about 410 kilometres (255 miles) and includes approximately the bottom 65 kilometres (40 miles) of the rigid, rocky lithosphere. The lithosphere is broken up into tectonic plates that float on the asthenosphere, a viscous area that spans the region from 100 to 200 kilometres (62-125 miles) below the Earth's surface. The very slow motion of these lithospheric plates causes plate tectonics, which is associated with earthquakes, volcanic activity, and mountain formation.
Volcanoes are most commonly found at the boundaries of tectonic plates. These plates are constantly moving, albeit very slowly, and they interact in three major ways: convergent boundaries, divergent boundaries, and transform faults. At convergent boundaries, plates collide or slide under one another, causing friction, high pressure, and melting, which can lead to volcanic eruptions. Divergent boundaries are where plates move apart, allowing magma to rise from the mantle and create new plate material. This movement of magma contributes to volcanic activity and is observed in the Mid-Atlantic Ridge.
Mantle plumes are another cause of volcanic activity. These are upwellings of superheated rock from the mantle that create "hot spots," volcanic regions not associated with plate tectonics. As a mantle plume reaches the upper mantle, it melts and triggers volcanic eruptions. The Hawaiian hot spot is thought to be a mantle plume, creating a series of volcanoes as the Pacific plate moves north-westward.
Seismic activity is closely linked to the movement of tectonic plates. Seismologists study the behaviour of the mantle by measuring the spread of shock waves from earthquakes, known as seismic waves or body waves. These waves reflect off different types of rocks, allowing scientists to identify the rocks present in the Earth's crust and mantle. By mapping these waves, seismologists can gain insights into the structure and dynamics of the mantle, contributing to our understanding of the Earth's volcanic and seismic activity.
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Frequently asked questions
The Earth's mantle is a solid layer that undergoes slow, continuous convective motion. It is made up of silicate rock and minerals, with the upper mantle extending from about 7 to 35 km (4.3 to 21.7 mi) from the surface down to a depth of 410 km (250 mi). The mantle is not literally made of plastic, but it does exhibit plasticity due to the presence of a low-viscosity layer and the deformation of olivine-rich rocks.
The plasticity of the Earth's mantle is supported by various sources of evidence, including earthquake waves, heat flow, magnetic and gravity studies, and laboratory experiments on rocks and minerals. Earthquake waves indicate that at a depth between 37 and 155 miles, the Earth's interior is less rigid, exhibiting plastic-like behaviour.
The notion of a deep plastic layer, also known as the "asthenosphere", arises from the observation that the Earth's surface morphology differs significantly from that of the Moon or Mars. This difference is attributed to the presence of a low-viscosity or low creep strength layer in the mantle, which affects the "coupling" between the outer crust and mantle convection.
