Earth’s Active MantleEarth is the only planet in our solar system with a continually active mantle. Mercury and Mars have solid, unmoving interior structures. Venus has an active mantle, but the structure of its crust and atmosphere prevent it from changing the Venusian landscape very often.Explosive StudyExplosions, just like earthquakes, trigger seismic waves. Body waves from powerful nuclear explosions may have revealed clues about Earth’s interior—but such seismic study is prohibited as part of the Comprehensive Nuclear Test Ban Treaty.Mantling ConductivitySome mantle maps display electrical conductivity, not seismic waves. By mapping disturbances in electrical patterns, scientists have helped identify hidden “reservoirs” of water in the mantle.The mantle is the mostly-solid bulk of Earth’s interior. The mantle lies between Earth’s dense, super-heated core and its thin outer layer, the crust. The mantle is about 2,900 kilometers (1,802 miles) thick, and makes up a whopping 84% of Earth’s total volume.As Earth began to take shape about 4.5 billion years ago, iron and nickel quickly separated from other rocks and minerals to form the core of the new planet. The molten material that surrounded the core was the early mantle.Over millions of years, the mantle cooled. Water trapped inside minerals erupted with lava, a process called “outgassing.” As more water was outgassed, the mantle solidified.The rocks that make up Earth’s mantle are mostly silicates—a wide variety of compounds that share a silicon and oxygen structure. 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.The temperature of the mantle varies greatly, from 1000° Celsius (1832° Fahrenheit) near its boundary with the crust, to 3700° Celsius (6692° Fahrenheit) near its boundary with the core. In the mantle, heat and pressure generally increase with depth. The geothermal gradient is a measurement of this increase. In most places, the geothermal gradient is about 25° Celsius per kilometer of depth (1° Fahrenheit per 70 feet of depth).The viscosity of the mantle also varies greatly. It is mostly solid rock, but more 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 volcanoes, seafloor spreading, earthquakes, and orogeny (mountain-building).The mantle is divided into several layers: the upper mantle, the transition zone, the lower mantle, and D” (D double-prime), the strange region where the mantle meets the outer core.Upper MantleThe upper mantle extends from the crust to a depth of about 410 kilometers (255 miles). The upper mantle is mostly solid, but its more malleable regions contribute to tectonic activity.Two parts of the upper mantle are often recognized as distinct regions in Earth’s interior: the lithosphere and the asthenosphere.LithosphereThe lithosphere is the solid, outer part of the Earth, extending to a depth of about 100 kilometers (62 miles). The lithosphere includes both the crust and the brittle upper portion of the mantle. The lithosphere is both the coolest and the most rigid of Earth’s layers.The most well-known feature associated with Earth’s lithosphere is tectonic activity. Tectonic activity describes the interaction of the huge slabs of lithosphere called tectonic plates. The lithosphere is divided into 15 major tectonic plates: the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca.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. Isostasy describes the physical, chemical, and mechanical differences that allow the crust to “float” on the sometimes more malleable mantle. 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 characterized 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.AsthenosphereThe asthenosphere is the denser, weaker layer beneath the lithospheric mantle. It lies between about 100 kilometers (62 miles) and 410 kilometers (255 miles) beneath Earth’s surface. 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. Ductility measures a solid material’s ability to deform or stretch under stress. The asthenosphere is generally more viscous than the lithosphere, and the lithosphere-asthenosphere boundary (LAB) is the point where geologists and rheologists—scientists who study the flow of matter—mark the difference in ductility between the two layers of the upper mantle.The very slow motion of lithospheric plates “floating” on the asthenosphere is the cause of plate tectonics, a process associated with continental drift, earthquakes, the formation of mountains, and volcanoes. In fact, the lava that erupts from volcanic fissures 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.Transition ZoneFrom about 410 kilometers (255 miles) to 660 kilometers (410 miles) beneath Earth’s surface, rocks undergo radical transformations. This is the mantle’s transition zone.In the transition zone, rocks do not melt or disintegrate. Instead, their crystalline 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.WaterPerhaps the most important aspect of the mantle’s transition zone is its abundance of water. Crystals in the transition zone hold as much water as all the oceans on Earth’s surface.Water in the transition zone is not “water” as we know it. It is not liquid, vapor, solid, or even plasma. Instead, water exists as hydroxide. Hydroxide is an ion of hydrogen and oxygen with a negative charge. In the transition zone, hydroxide ions are trapped in the crystalline structure of rocks such as ringwoodite and wadsleyite. These minerals are formed from olivine at very high temperatures and pressure.Near the bottom of the transition zone, increasing temperature and pressure transform ringwoodite and wadsleyite. Their crystal structures are broken and hydroxide escapes as “melt.” Melt particles flow upwards, toward minerals that can hold water. This allows the transition zone to maintain a consistent reservoir of water.Geologists and rheologists think that water entered the mantle from Earth’s surface during subduction. Subduction is the process in which a dense tectonic plate slips or melts beneath a more buoyant one. 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.Lower MantleThe lower mantle extends from about 660 kilometers (410 miles) to about 2,700 kilometers (1,678 miles) beneath Earth’s surface. 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 corresponds 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.D Double-Prime (D’’)Beneath the lower mantle is a shallow region called D'', or “d double-prime.” In some areas, D’’ is a nearly razor-thin boundary with the outer core. In other areas, D’’ has thick accumulations of iron and silicates. In still other areas, geologists and seismologists have detected areas of huge melt.The unpredictable movement of materials in D’’ is influenced by the lower mantle and outer core. 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 2,900 kilometers (1,802 miles) below the surface, is the core-mantle boundary, or CMB. This point, called the Gutenberg discontinuity, marks the end of the mantle and the beginning of Earth’s liquid outer core.Mantle ConvectionMantle 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. Convection currents also transfer denser, cooler material from the crust to Earth’s interior through the process of subduction.Earth's heat budget, which measures the flow of thermal energy from the core to the atmosphere, is dominated by mantle convection. Earth’s heat budget drives most geologic processes on Earth, although its energy output is dwarfed by solar radiation at the surface.Geologists debate whether mantle convection is “whole” or “layered.” Whole-mantle convection describes a long, long recycling process involving the upper mantle, transition zone, lower mantle, and even D’’. 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. Convection currents may then transport the hot, buoyant material in D’’ back through the other layers of the 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.Mantle PlumesA mantle plume is an upwelling of superheated rock from the mantle. Mantle plumes are the likely cause of “hot spots,” volcanic regions not created by plate tectonics. As a mantle plume reaches the upper mantle, it melts into a diapir. This molten material heats the asthenosphere and lithosphere, triggering volcanic eruptions. These volcanic eruptions make a minor contribution to heat loss from Earth’s interior, although tectonic activity at plate boundaries is the leading cause of such heat loss.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. Geologists think this has allowed the Hawaiian hot spot to create a series of volcanoes, from the 85-million-year-old Meiji Seamount near Russia’s Kamchatka Peninsula, to the Loihi Seamount, a submarine volcano southeast of the “Big Island” of Hawaii. Loihi, a mere 400,000 years old, will eventually become the newest Hawaiian island.Geologists have identified two so-called “superplumes.” These superplumes, or large low shear velocity provinces (LLSVPs), have their origins in the melt material of D’’. The Pacific LLSVP influences geology throughout most of the southern Pacific Ocean (including the Hawaiian hot spot). The African LLSVP influences the geology throughout most of southern and western Africa.Geologists think mantle plumes may be influenced by many different factors. Some may pulse, while others may be heated continually. Some may have a single diapir, while others may have multiple “stems.” Some mantle plumes may arise in the middle of a tectonic plate, while others may be “captured” by seafloor spreading zones.Some geologists have identified more than a thousand mantle plumes. Some geologists think mantle plumes don’t exist at all. Until tools and technology allow geologists to more thoroughly explore the mantle, the debate will continue.Exploring the MantleThe 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 2005, scientists with the Integrated Ocean Drilling Project drilled 1,416 meters (4,644 feet) below the North Atlantic seafloor and claimed to have come within just 305 meters (1,000 feet) of the Moho.XenolithsMany geologists study the mantle by analyzing xenoliths. Xenoliths 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 150 kilometers (93 miles) beneath the surface. Above depth and pressure, the carbon crystallizes as graphite, not diamond. Diamonds are brought to the surface in explosive volcanic eruptions, forming “diamond pipes” of rocks called kimberlites and lamprolites.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. Diamond intrusions have allowed scientists to glimpse as far as 700 kilometers (435 miles) beneath Earth’s surface—the lower mantle.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 sediments, and even carbon.Seismic WavesMost mantle studies are conducted by measuring the spread of shock waves from earthquakes, called seismic waves. The seismic waves measured in mantle studies are called body waves, 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-waves, also called pressure waves, are formed by compressions. Sound waves are P-waves—seismic P-waves are just far too low a frequency for people to hear. S-waves, also called shear waves, measure motion perpendicular to the energy transfer. S-waves are unable to transmit through fluids or gases.Instruments placed around the world measure these waves as they arrive at different points on the Earth’s surface after an earthquake. P-waves (primary waves) usually arrive first, while s-waves arrive soon after. Both body waves “reflect” off different types of rocks in different ways. This allows seismologists to identify different rocks present in Earth’s crust and mantle far beneath the surface. Seismic reflections, for instance, are used to identify hidden oil deposits deep below the surface.Sudden, predictable changes in the velocities of body waves are called “seismic discontinuities.” The Moho is a discontinuity marking the boundary of the crust and upper mantle. The so-called “410-kilometer discontinuity” marks the boundary of the transition zone.The Gutenberg discontinuity is more popularly known as the core-mantle boundary (CMB). At the CMB, S-waves, which can’t continue in liquid, suddenly disappear, and P-waves are strongly refracted, or bent. This alerts seismologists that the solid and molten structure of the mantle has given way to the fiery liquid of the outer core.Mantle MapsCutting-edge technology has allowed modern geologists and seismologists to produce mantle maps. Most mantle maps display seismic velocities, revealing patterns deep below Earth’s surface.Geoscientists hope that sophisticated mantle maps can plot the body waves of as many as 6,000 earthquakes with magnitudes of at least 5.5. 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.
Term Part of Speech Definition Encyclopedic Entry abundance Noun
a buildup of something.
view or interpretation.
layer in Earth's mantle between the lithosphere (above) and the upper mantle (below).
layers of gases surrounding a planet or other celestial body.
Encyclopedic Entry: atmosphere birth name Noun
name given to someone by their parents or leaders at their birth.
body wave Noun
seismic wave that travels through the interior of the Earth.
line separating geographical areas.
Encyclopedic Entry: boundary boundary Noun
line separating geographical areas.
Encyclopedic Entry: boundary brittle Adjective
fragile or easily broken.
capable of floating.
to describe the characteristics of something.
substance having at least two chemical elements held together with chemical bonds.
instance of being pressed together or forced into less space.
maintaining a steady, reliable quality.
one of the seven main land masses on Earth.
Encyclopedic Entry: continent continental crust Noun
thick layer of Earth that sits beneath continents.
continental drift Noun
the movement of continents resulting from the motion of tectonic plates.
Encyclopedic Entry: continental drift convection current Noun
movement of a fluid from a cool area to a warm area.
the extremely hot center of Earth, another planet, or a star.
Encyclopedic Entry: core correspond Verb
to match or be similar to.
rocky outermost layer of Earth or other planet.
Encyclopedic Entry: crust crystal Noun
type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.
deepest layer of Earth's mantle, about 2,700 kilometers (1,678 miles) beneath Earth’s surface.
having parts or molecules that are packed closely together.
dome-shaped geologic feature (intrusion) where more fluid material is forced into brittle overlying rock.
dome-shaped geologic feature (intrusion) where more fluid material is forced into brittle overlying rock.
individual or distinct.
to fall apart and disappear.
unique or identifiable.
to differentiate or recognize as different.
shape that is half of a sphere.
Encyclopedic Entry: dome dominate Verb
to overpower or control.
ductile Adjective capable of withstanding a certain amount of force by changing form before fracturing or breaking. dwarf Verb
to make something appear small by having it appear next to something much larger.
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
Earth's heat budget Noun
measurement of the flow of thermal energy from the core to the atmosphere, dominated by absorbed and reflected solar radiation.
tools and materials to perform a task or function.
to explode or suddenly eject material.
narrow opening or crack.
material that is able to flow and change shape.
rate of occurrence, or the number of things happening in a specific area over specific time period.
person who studies the physical formations of the Earth.
geothermal gradient Noun
gradual change in temperature from the Earth's core (hot) to its crust (cool), about 25° Celsus per kilometer of depth (1° Fahrenheit per 70 feet of depth).
soft, common allotrope of carbon that is the highest rank of coal. Also called black lead.
Gutenberg discontinuity Noun
point between the Earth's mantle and the core below.
hot spot Noun
intensely hot region deep within the Earth that rises to just underneath the surface. Some hot spots produce volcanoes.
Encyclopedic Entry: hot spot hydroxide Noun
chemical compound containing the OH‾ ion.
igneous intrusion Noun
rock formation created by magma as it is pushed from the Earth's mantle into cracks or holes in the crust.
force that effects the actions, behavior, or policies of others.
electrically charged atom or group of atoms, formed by the atom having gained or lost an electron.
equilibrium of Earth's crust, where the forces tending to elevate landmasses balance those tending to depress them. Also called isostatic equilibrium.
layered-mantle convection Noun
process in which mantle convection is primarly carried out by activity in the upper mantle.
outer, solid portion of the Earth. Also called the geosphere.
Encyclopedic Entry: lithosphere lithosphere-asthenosphere boundary (LAB) Noun
chemical and mechanical distinction between the cool, rigid lithosphere and the warmer, more ductile asthenosphere.
(large low shear velocity province) seismically anomalous region at the deepest part of Earth's mantle. Also called a superplume or thermo-chemical pile.
lower mantle Noun
bottom layer in Earth's mantle, closest to the core.
molten, or partially melted, rock beneath the Earth's surface.
Encyclopedic Entry: magma magma Noun
molten, or partially melted, rock beneath the Earth's surface.
Encyclopedic Entry: magma magnitude Noun
intensity of an earthquake, represented by numbers on a scale.
flexible and capable of reforming itself without breaking when under stress.
middle layer of the Earth, made of mostly solid rock.
Encyclopedic Entry: mantle mantle convection Noun slow movement of Earth's solid mantle caused by convection currents transferring heat from the interior of the Earth to the surface. mantle plume Noun
upwelling of magma within Earth's mantle.
liquid part of magma.
inorganic material that has a characteristic chemical composition and specific crystal structure.
Mohorovicic discontinuity Noun
point between Earth's crust and the mantle below. Also called the Moho.
solid material turned to liquid by heat.
oceanic crust Noun
thin layer of the Earth that sits beneath ocean basins.
fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.
the way mountains are formed.
outer core Noun
liquid, iron-nickel layer of the Earth between the solid inner core and lower mantle.
to release a gas that was dissolved, trapped, frozen or absorbed in another material.
allowing liquid and gases to pass through.
at a right angle to something.
large, spherical celestial body that regularly rotates around a star.
Encyclopedic Entry: planet plasma Noun
state of matter with no fixed shape and molecules separated into ions and electrons.
in a moldable, alterable manner.
plate tectonics Noun
movement and interaction of the Earth's plates.
force pressed on an object by another object or condition, such as gravity.
seismic shock wave that represents longitudinal motion. Also called a primary wave or pressure wave.
extreme or drastic.
to identify or acknowledge.
to bend or change the path of something.
any area on Earth with one or more common characteristics. Regions are the basic units of geography.
Encyclopedic Entry: region reservoir Noun
large, concentrated supply or reserve.
scientist who studies the flow and shape-changing (deformation) of matter.
natural substance composed of solid mineral matter.
seafloor spreading Noun
rift in underwater mountain range where new oceanic crust is formed.
Encyclopedic Entry: seafloor spreading sediment Noun
solid material transported and deposited by water, ice, and wind.
Encyclopedic Entry: sediment seismic wave Noun
shock wave of force or pressure that travels through the Earth.
person who studies earthquakes.
shock wave Noun
moving, measurable change in pressure and density of a material.
most common group of minerals, all of which include the elements silicon (Si) and oxygen (O).
flat, thick piece of material such as earth or stone.
solar radiation Noun
light and heat from the sun.
solar radiation Noun
light and heat from the sun.
knowledgeable or complex.
to cause to slow or come to a stop.
to pull downward or beneath something.
process of one tectonic plate melting, sliding, or falling beneath another.
seismic shock wave that represents perpendicular motion. Also called a secondary wave or shear wave.
the science of using tools and complex machines to make human life easier or more profitable.
tectonic activity Noun
movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.
tectonic plate Noun
massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
degree of hotness or coldness measured by a thermometer with a numerical scale.
Encyclopedic Entry: temperature thermal Adjective
having to do with heat or temperature.
to change from one form into another.
transition zone Noun
areas in the Earth's interior between the upper mantle, near the Earth's crust, and the lower mantle, near the Earth's core.
upper mantle Noun
layer in Earth's mantle between the asthenosphere (above) and the lower mantle (below).
to rise, grow, or gush upward.
visible liquid suspended in the air, such as fog.
measurement of the rate and direction of change in the position of an object.
measure of the resistance of a fluid to a force or disturbance.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
Encyclopedic Entry: volcano volume Noun
space an object occupies.
whole-mantle convection Noun
process in which mantle convection is a result of activity in both the upper and lower mantle.
piece of rock embedded in another type of rock, usually igneous.
Encyclopedic Entry: xenolith