Earth
“Those who dwell among the beauties and mysteries of the Earth are never alone or weary of life.” ~ American marine biologist Rachel Carson
Life may have found a precarious hold on a few orbs in the solar system. Venus might have once supported life. Mars almost certainly did and may still.
Mars once had regular wet seasons, rivers, and lakes. Even now there are discharges of methane from the surface of Mars into the atmosphere. This may be from serpentinization: a geological process of rock oxidation and hydrolysis via heat and water. Or it may be microbial methanogens.
Europa is another candidate for life. Even nominally hellish Mercury may harbor microbes, nestled in lakes which never see the Sun.
Earth was something special. Its ability to hold an atmosphere made a difference.
4.55 BYA, Earth came into being by accretion: currents of particles swirling around the Sun collided and coalesced, as with other planets. Molten iron sank to the center, forming the planetary core.
Earth’s continued formation was by violence for at least 800 million years, pilloried by 2018 tonnes of cosmic debris. Enormous impacts occurred as recently as 1.8 BYA.
Impacting meteorites were stirred into Earth’s mantle by massive convection processes. The vast bulk of the planet’s precious metals, including gold, came from space after its formation.
The bombardment continues to this day, but it has been reduced to a fine drizzle. Over 3,600 tonnes of extraterrestrial dust a year – 9 tonnes a day – settle on Earth’s surface.
By 4.4 BYA a crust had formed. A 100 million years later vast oceans covered the surface. Life made its debut on Earth ~4.1 BYA.
Dechlorination
Chlorine is an extremely reactive element – a blatant oxidizing agent that readily strips electrons from those elements it deals with. Though chlorine is an essential dietary element in minute quantities, it is not biologically friendly.
The composition of ancient meteorites indicates that Earth should have 10 times the chlorine that it does. Mars has more than twice the chlorine of Earth despite having suffered much less cosmic assault.
The 4 halogen elements, including chlorine, do not readily dissolve in metals; nor do they often combine with other elements to form rock minerals. Hence, chlorine is concentrated on the surface. Much of Earth’s chlorine that is not in the ocean lies in salt deposits and brines.
The relentless bombardment of early Earth engendered life later by scouring much of the chlorine off the planet. If not, the world’s oceans would have been too salty for complex life to evolve.
Chlorine-rich seas would have reduced precipitation. With less rain, there would have been less erosion, and fewer nutrients washing into the ocean to foster life.
The Moon
“When a finger points to the Moon, the imbecile looks at the finger.” ~ Chinese proverb
~4.51 bya, much of Earth’s iron had sunk towards the core when a planetesimal the size of Mars – Theia – smacked the Earth at an oblique angle. Theia was the goddess who gave birth to Selene, the Moon.
Like a caroming billiard ball, Theia rebounded, but was captured within Earth’s gravitational pull. Theia took with it a divot from Earth: adding a clump to what would become the Moon.
On its bounce back into space 4.5 bya, the Moon was, for a while, but 20,000 kilometers from Earth: exerting tremendous pull that buckled Earth’s crust with each lunar rotation. Earth spun on its axis in a 5-hour day. The solar cycle was the same, but there were over 1,750 days per year. Meanwhile, the lunar cycle – from one full moon to the next – was a mere 1.5 days.
Early on, the Earth and Moon loomed large in each other’s skies. Because the Earth and Moon were tidally locked from the beginning, the still-hot Earth radiated its heat on the near side of the Moon. While the far side cooled, the Earth-facing side remained molten. This temperature gradient crucially affected crustal formation on the Moon.
The 2 sides of the Moon are strikingly different. The near side is low and flat, rich in rare earth elements. The far side is mountainous and heavily cratered. The evolution of the Moon accounts for the bifurcation.
“The thermal gradient created by Earthshine produced the chemical gradient responsible for the crust thickness dichotomy that defines the lunar highlands.” ~ American astronomer Jason Wright et al
Early meteoroid impacts on the Moon’s near side punched through the crust, releasing vast lakes of basaltic lava that crafted the large, dark plains which form the Moon’s visage. These basins were dubbed maria – Latin for “seas” – by early astronomers who mistook them for actual oceans. Their darkness owes to heavy iron concentration, which is less reflective than surrounding crust. Maria cover 16% of the lunar surface, almost all on the visible side of the Moon.
Meanwhile, bolides that struck on the far side hit crust too thick to puncture, making their mark with craters and highlands, but scant maria.
The Moon is now a bit squashed, with an equatorial bulge. Tidal heating during Moon formation thinned the polar crust while thickening crust in regions in line with the Earth. That, along with rotational forces, left a lemon-shaped bulge on the side facing the Earth, and a counterpart distention on the opposite side.
Whereas the combined mass of the outer planets’ satellites is less than 0.1% of their parents, the Moon is ~1% of Earth’s mass. Even more important, the Moon contributes 80% of the angular momentum of the Earth–Moon system. For the outer planets, this figure is less than 1%. In sum, the Moon’s effect on Earth is much greater than other moons in the solar system.
The Moon-forming event birthed Earth’s seasons, facilitating the equator-to-pole heat conduit that rendered the planet more habitable.
The impact from the emergent Moon blew away much of Earth’s early atmosphere, which had been captured by gravity from the solar nebula before the nebular cloud dissipated.
Gases were released from Earth’s mantle into the atmosphere. Nitrogen, which is relatively unreactive, outgassed early, and remains the predominant atmospheric gas (78%).
Earth’s atmosphere had enough carbon dioxide to attenuate solar infrared radiation. This helped stabilize Earth’s surface temperature.
As the Moon receded from the Earth, and its orbit stabilized, Earth’s day length increased slightly, and the amplitude of Earth’s ocean tides lessened. Ocean tides are produced by a combination of the Sun and the Moon’s gravity, along with Earth’s rotation, creating bulges of water on opposite sides of the planet.
Collisions like the pairing of the Moon and the Earth are cosmically common. 8% of Earth-sized planets may have captured a moon. These typically occur as a star system is forming, as happened here.
The gravitational pull of a big nearby satellite keeps a planet from tilting too much on its axis, and so helps stabilize planetary rotation. The tidal effects from a moon’s orbit may also be beneficial, as they are on Earth. A moon may encourage the prospects of a planet birthing life and sustaining it.
The Moon does more than sway ocean tides. It also tugs on Earth’s crust, triggering massive earthquakes along fragile fault lines, especially during new and full moons, when the Earth, Moon and Sun are aligned. Thus, the Moon has contributed to the Earth’s geophysical evolution.
The gravitational field of the Moon is the most varied in the solar system. Gravity anomalies, termed mascons, come from matter compression – often caused by meteorite impacts, but sometimes due to dense basaltic lavas.
The Moon was long thought dry, but water molecules are widely distributed over the lunar surface, as well as locked up in icy crust enclaves within, bound to phosphates in volcanic rocks. The Moon’s mantle has much water locked within, as Earth does.
The Moon would long ago have become a cold rock if not bound to the Earth. Surrounding the metal core of the Moon is a slightly viscous deep mantle, kept warm by Earth’s gravity. Tidal heating occurs via viscous dissipation.
Early in the Moon’s history, its rotation slowed; becoming locked into synchronous rotation by frictional gravitational forces, caused by tidal effects from Earth (tidal periodicity). This tidal interaction pulls the Moon slightly along its orbit, causing it to move further away from Earth 3.8 cm per year. The Moon is now 384,400 km away from Earth.
The lunar cycle – from one new moon to the next – is 29.53 days. The Moon constantly shows the same face because of its synchronous rotation about the Earth. It takes precisely as long for the Moon to orbit the Earth as it does to revolve.
3,475 km in diameter, the Moon is the 5th largest satellite in the solar system, but the largest relative to its parent planet. Earth’s diameter is 12,756 km; only 3.67 times that of the Moon.
In the last billion years, the Moon has shrunk by 200 meters in diameter. Why the Moon is a prune is not yet known.
The Moon has been cooling since its fiery birth but may not be dead yet. Explosive releases of underground gas have occurred within the past 10 million years.
The Moon dramatically shaped the Earth on its way to being a lasting rhythmic influence: the lunar cycle to which much life, especially nocturnal creatures, respond.
Life Under the Moon
“Ecologists have long viewed the darkness of a moonless night as a protective blanket for nocturnal prey species. Moonlight alters predator-prey relations in more complex ways than previously thought.” ~ American biologist Laura Prugh
At full moon the Earth stands between the Moon and the Sun. The view of the Moon is like a brightly lit coin in the sky.
In the following nights, as the Moon circles back toward the Sun, that coin slowly shrinks. Yet the sky seems darker than just dwindling light would allot; and it is. The Moon rises 50 minutes later each evening, carving a channel of darkness between the Sun dropping below the horizon and the Moon appearing.
Predators ply that channel. In doing so, the early waning Moon instilled innate fears in potential prey, where darkness spells danger.
During the full moon and days thereafter, few nocturnal reef fish are to be found, as they are more easily spotted by those that would make a meal of them. In contrast, the dark nights around the new moon cue fish that swimming about is safer.
Rabbits stay close to their burrows during the full moon and the days that follow. The darkness of the new moon lets them travel long, exposed distances.
Conversely, cheetahs and wild dogs in Africa have more active nights once the lunar cycle has waxed past half-full. Illuminating the hunt raises the odds of a kill.
Eagle owls and other avian predators take advantage of the Moon toward fullness to vigorously hunt and seek out new territory.
Lunar favor depends upon an animal’s senses. Nearly half of all mammals are nocturnal, experiencing lunar cycles with light levels that change 3 orders of magnitude every month.
Animals active at night are adapted to the lifestyle. Moonlight benefits visually oriented prey. The prospects for lurking predators are lessened under the Moon’s glow.
Many bat species become less active as the moon waxes full. Nocturnal insect prey have a better chance of spotting a threat, and echolocation yields no edge for luminosity.
Many marine organisms move up and down in the sea depending on the level of moonlight, to keep their light level constant.
The Moon is an environmental cue to many species, providing coordination with an animal’s innate circadian rhythm. Coral synchronously spawn on full moon nights, their clocks aligned by cryptochrome: a protein sensitive to blue light. Galápagos marine iguanas travel for hours to arrive at the shoreline in time to graze on algae at low tide.
Gardening folklore suggests that planting crops according to the phases of the Moon yields a better harvest. The gravitational tug of the Moon does affect plants slightly. Plants can feel the Moon via the water that runs through them, most sensitively in the pulvinus: the joint where leaf meets stem.
Final Formation
4 BYA, the Sun brightened to 70% of its current light level, while the intense solar ultraviolet output dropped dramatically: by more than 30 times.
By absorbing more of the Sun’s energy, Earth failed to ice over when the Sun was dimmer. Earth’s surface was darker. The continents were much smaller, so the oceans, which are typically much darker than land masses, absorbed more heat.
Earth’s early atmosphere was a brew of greenhouse gases that helped stabilize global temperature. Carbon dioxide (CO2) and methane (CH4) prevented the planet from freezing and triggered synthesis of a rich variety of organic molecules via ultraviolet radiation in the upper atmosphere.
Bombardment from space continued after Earth was moonstruck, cratering both Moon and Earth. The celestial siege of Earth eased somewhat after practically sterilizing the planet’s surface, but bringing water, hydrogen, nitrogen, and a wealth of minerals and organic compounds that would transform the planet.
Jupiter was instrumental in both seeding Earth and in sweeping up errant projectiles, some of which formed the array of moons and asteroids coming under Jupiter’s sway.
“Discount the “Jupiter as shield” concept. Jupiter was responsible for the vast majority of the encounters that “kicked” outer planet material into the terrestrial planet region, delivering the volatile-laden material required for the formation of life. Saturn assisted in the process far more than has previously been acknowledged.” ~ American planetary physicist Kevin Grazier
To this day, Jupiter is Janus-faced toward Earth. While it does vacuum some debris, it also sometimes hurls objects Earth’s way.
In 1770, a large comet whizzed by, missing Earth by a mere million miles. The comet had come into the outer solar system 3 years earlier, its path determinedly far from Earth. But the comet passed close to Jupiter, which diverted it to a new course: a cosmic whisker away from collision with the blue planet that Jupiter only sometimes protects.
The comet made 2 passes around the Sun before heading out. In 1779, the comet again passed close to Jupiter, which summarily slung it out of the solar system.
Though still subject to upheaval, Earth’s crust was complete within 100 million years after its birth: a solid but deformable shell.
Nearly half of the crust’s mass is made up of oxygen and over 27% silicon. Both are major components of rocks. Metals used in manufacture, such as iron and aluminum, comprise ~18% of the crust.
The lithosphere sorted itself into continents above sea level, resulting in land surface. Volcanism accomplished this. Frequent eruptions subducted hot surface materials, eventuating in a cool, thick crust by altering convection dynamics between Earth’s crust and mantle.
“The mantle’s viscosity is extremely dependent on its temperature.” ~ Australian geophysicist Craig O’Neill et al
An abrupt transition to tectonics began ~3 BYA, once the lithosphere had sufficiently cooled. Before that, the upper mantle was too hot to convey rock without melting it.
Despite voluminous bombardment, early Earth mineral variety was quite limited. Of the 4,500 chemical species on Earth today, up to 2/3rds are attributable to biological activity. The earliest life engendered mineral evolution.
Late arrivals from space added to land mass. Meteorite impacts shifted mantle convection patterns, triggering plumes that heated the crust from below. Continents evolved.
In chewing rocks for sustenance, the earliest microbes were instrumental in creating the continents. Their waste products – sedimentation – acted as a viscous lubricant for tectonic plate subduction, thereby facilitating the rise of vast land masses. Without the lubricating sediment, Earth might have been a water world, dotted with small volcanic islands.
Earth’s oldest rocks were volcanic artifacts (igneous). As the surface cooled, torrential storms ensued, begetting erosion. From surface debris emerged the 2nd great family of rocks (sedimentary).
The heat and crushing turmoil of tectonics led to melting and recrystallization of older rocks, producing a 3rd rock family (metamorphic). From these mountains were made.
The atmosphere in the late Hadean eon comprised gases released by volcanic activity, primarily large volumes of carbon gases (CO, CO2, CH4) which helped keep the surface warm.
Water vapor was in the air, as oceans had already formed; but free oxygen of any form (O2, O3 (ozone)) was entirely absent.
Tilt & Spin
Thanks to being whacked by its soon-to-be Moon, Earth’s rotational axis tilts at 23.4°. This tilt brought seasonal variations.
The obliquity of Earth – the orientation of its spin axis to solar orbital plane – has changed over time. Even minor changes in obliquity cause major climatic shifts. The tilt of the Earth’s axis as it spins gives rise to the seasons. The aspects of periodicity in Earth’s spatial and orbital changes are known as Milankovitch cycles.
Earth’s shape and spin result in a difference in gravitational pull between the poles and the equator, with equatorial objects lighter by 0.6%. Even at the same latitude, gravity varies from place to place because of several factors, including the bulge about the Earth, elevation, such as mountain ranges, and the moon’s gravitational influence.
Earth’s rotation has a gravitational effect upon itself, causing the diameter at the equator to be 27 km greater than its diameter through the poles.
Earth spins about its axis at a rate of 0.5 km per second. Its revolution about the Sun moves at 30 km/sec.
Water Supply
The solar system’s water supply was inherited as ice from interstellar space. The water included prebiotic matter that would later integrate into life on Earth. Similar ices are likely to be found around other protoplanetary disks. There’s a surprising amount of cosmic water.
“Water is pervasive throughout the universe, even at the very earliest times.” ~ American astrophysicist Matt Bradford
The solar system has a planetary snow line: the zone beyond which ice could have condensed on emergent planets.
Although water covers 70.9% of Earth’s surface, it accounts for far less than 1% of the planet’s mass. Uranus and Neptune, which formed well past the snow line, are loaded with tens of percents of water by mass.
During Earth’s birth, the inner solar system was hot enough to melt lead. The inner planets – those as far out as Mars – would have been born dry, had they started out where they are now; which was certainly not the case for Mercury. If Earth did not start out as a hot dry rock, then it either moved into its current orbit after formation or made much of its water itself.
Earth looks to have been born wet, not dry. The chemical signature for water found deep within the mantle suggests that much of the planet’s water was primordial. Water was generated within the mantle by combining fluid hydrogen and the silica in quartz, both of which would have been abundant in Earth’s early mantle. Earth’s crust is now 59% silica.
Fluid hydrogen and silica form water at 1700 K and pressure 20,000 times that of Earth’s atmosphere. These requirements were easily met in the planet’s mantle.
The swirling in the solar system that caused the planets to coalesce from bits of cosmic dust dragged an emerging Jupiter about before it settled into its current orbit. Earth’s primordial water supply suggests that Earth, like Jupiter, came toward the Sun during its formation, likely in tow of Jupiter (and Saturn).
A chondrite is a stony meteorite, formed by accretion from dust and small grains. Many chondrites in the early solar system acquired a coating of ice.
Jupiter’s promenade dragged chondrites into collision with Earth, seeding it and the Moon with water. Hence, the bombardment of Earth – that largely started and abated by Jupiter’s planetary evolution – supplemented Earth’s water supply. The leftovers which did not rain down form the asteroid belt that ranges between the orbits of Mars and Jupiter, where the solar system’s snow line is situated.
As Earth cooled, a crust formed, as well as an atmosphere bearing water; bringing rain, and in time, oceans. This allowed surface temperatures to drop to less than 102 °C as early as 4.4 bya.
Earth may have sported its first ocean at this time, when the planet was but 150 million years old. The Sun’s evaporative blaze was 30% less than now.
3 billion years ago, the ocean was 67 °C. By 1.5 bya, the ocean had cooled to 27 °C; a warm soup supporting life.
Sponged up in the Earth’s interior, 410–600 km down, is at least 25 times the water in the oceans. If not for this lubrication, there would be no plate tectonics and no continents. Without continents, there would be no transport of life-sustaining nutrients from rivers into the oceans.
Structure
“Earth is a thermodynamic engine powered by its own finite internal reserves of heat that are gradually brought to the surface and radiated to space.” ~ American geophysicists Don Anderson & Scott King
The Earth is a geologic onion of layers, alternately described by chemical or rheological properties. Rheology is the study of matter flow.
There are at least 2 main core layers, multiple layers of mantle, with a transition zone between the upper and lower mantle, and a 2-part outer layer, capped by a crust.
Owing to gravitational force, the planet gets denser further down. The center of the Earth is 6,370 km below sea level.
Core
Earth’s cores are nested layers. The outer core is a molten metal sea, floating over a solid inner core that is roughly the size of the Moon, and is almost entirely iron.
The viscous outer core is mostly iron, with some nickel. 10% of outer core material comprises light alloys, made of silicon, oxygen, sulfur, carbon, and other elements.
Radioactive material within the core decays, giving off heat. Earth’s core has cooled by some 1000 degrees (K) since its formation. Such cooling is necessary to sustain the planet’s geomagnetic field.
At the heart of the inner core lies the innermost core: a solid ball of pure iron 600 km in diameter. Everything else got squeezed out of the innermost core.
The innermost core may be as hot as 6,923 K; yet the otherwise liquid iron is frozen because of the extreme pressure.
Earth was initially a growing ball of molten rock. As the orb grew, the heavy metals in the rocks descended into the planet’s interior. The frictional heat from sinking made for a relatively chilly metallic center surrounded by hot viscous rock.
Rising pressure inside the planet caused the cool core to condense. The innermost core solidified ~100,000 years after Earth’s accretion began.
Altogether, Earth’s cores comprise a turbulent engine: generating over 15 terawatts of heat energy at the core-mantle boundary. All this is fueled by energy left over from the cosmic collisions that formed the planet.
Earth’s Magnetic Field
Earth’s inner core rotates eastward, slightly faster than the rest of the planet. Outside the spinning inner core, flows of electrically conductive liquid-metals near the boundary of the outer core and mantle fashion fluctuations that create massive, shifting electromagnetic currents.
This geodynamic generates Earth’s magnetic field, which first developed 4.2 BYA. The timing of the core condensing and rotation was critical to propagating a strong magnetic field. The magnetic field extends for several thousand kilometers outside of Earth, creating a protective blanket that deflects much of the solar wind.
Geophysicists long had a hard time explaining how Earth’s magnetic bodyguard came on duty so early in the planet’s history. The answer lies in the dynamics of hot metal under pressure.
Thermal energy nominally transfers freely from atom to atom via conduction. The atoms are unmoved. But when the heat flow exceeds what a material can handle through conduction, atoms become restless. Convection emerges.
In metals such as iron, free-moving electrons ferry electromagnetic charge as well as heat. How readily they do so depend upon how much resistance they encounter.
Earth’s early core would have been more conductive and less convective if not for the tremendous pressure involved. Containment built both heat and resistance.
Pressure in the core squeezes the iron and nickel to more than 1.6 times its normal density. The electrons within are especially excited.
Above 1,970 K, thermally energized electrons more than scatter off vibrating atoms: they increasingly collide with each other. This electron–electron bashing drives electromagnetic generation.
Resistivity doubles while thermal conductivity drops. Pressure amplifies the electromagnetic effect of convection. Hence, the early emergence of a powerful magnetic field from the core.
Though predominantly iron, Earth’s core is almost 20% nickel, which plays a crucial role in generating the magnetic field.
“Under pressure, nickel behaves differently from iron. At high pressure, the electrons in nickel tend to scatter much more than the electrons in iron. As a consequence, the thermal conductivity of nickel and, thus, the thermal conductivity of Earth’s core, is much lower than it would be in a core consisting only of iron.” ~ Italian physicist Alessandro Toschi
“If Earth’s core consisted only of iron, the free electrons in the iron could handle the heat transport by themselves, without the need for any convection currents. Then, Earth would not have a magnetic field at all.” ~ Austrian physicist Karsten Held
By sheltering the planet from high-energy solar radiation and wind, the magnetic field helped preserve early Earth’s oceans from evaporation and provided some protection for nascent life. By lessening ionization from the solar wind, the magnetic field also kept atmospheric nitrogen from escaping.
Other rocky worlds in the solar system have not been so fortunate. Lacking a magnetic shield, the Sun has stripped away their atmospheres.
Van Allen Belts
From 1,000 km above Earth’s surface stretches layered belts of highly charged particles. Via the first artificial satellites, the belts were first noticed in 1958, and named after their discoverer: American astrophysicist James Van Allen.
This plasma zone arises from the planet’s magnetic field, which holds the belts in place.
The Van Allen belts are 2 pronounced concentric doughnut-shaped rings that strip atoms of their electrons and accelerate the subatomic particles to near lightspeed. No real gap exists between the 2 zones; simply gradations in radiation intensities.
As a particle approaches a magnetic pole, the increased field strength bounces it back to the other pole. Hence the belts are most intense over the equator, and effectively absent above the poles. Over time, particles collide with atoms in the atmosphere and are knocked out of the belt.
The inner belt (1,000–6,000 km) largely comprises protons, energized to 30 million electron volts (MeV). Many of the protons are produced by decay of neutrons, which wither from the intense radiation.
The outer belt (13,000+ km), fed from particles both atmospheric and solar in origin, has lower-energy protons. The most energetic particles in the outer belt are electrons, reaching several hundred MeV.
There is also a 3rd belt between the inner and outer belts, which modulates the activity of the outer belt.
Particles in the belts stream in spiral paths along the force lines of Earth’s magnetic field. Synchronicity in the frequency of electromagnetic waves and electrons in transit keeps the belts enlivened.
At 11,600 km altitude, there is an extremely sharp boundary at the inner edge of the outer belt that acts as a shield, blocking ultrarelativistic electrons from whizzing closer to Earth’s atmosphere. This boundary is a mystery, as Nature typically abhors strong gradients.
“It’s almost like these electrons are running into a glass wall in space. The invisible shield blocking these electrons is an extremely puzzling phenomenon.” ~ American astrophysicist Daniel Baker
Solar flares disrupt the belts, which in turn invokes auroras and magnetic storms. Even during less turbulent times, the Van Allen belts endanger man-made satellites with their intense, fluctuating radiation.
Polarity Reversal
“Sometimes you won’t have a flip for about 40 million years; other times there will be 10 flips in 1 million years. On average, the duration between two flips is a few hundred thousand years. The last flip was around 780,000 years ago, so we are actually overdue for a flip.” ~ Chinese geophysicist Huapei Wang
Earth’s magnetic field sporadically reverses polarity. The duration of continued polarity (a chron) varies by tens of millions of years, with an average of 450,000 years.
Field reversal typically takes 4,000 years, though it may occur in as little as a decade. Changing continental configurations via tectonics may trigger geomagnetic field reversal.
The Earth’s magnetic field is currently weakening 5% each decade. Magnetic north is moving toward Siberia. Current field intensity is twice the historical average, so polarity reversal is not likely for many millennia.
Mantle
2,900 kilometers thick, the mantle is the thickest layer, comprising 68% of Earth’s mass and 84% of its volume. The mantle is dense and rigid at depth.
The predominant mineral in the lower mantle is bridgmanite ((Mg,Fe)SiO3): a ferromagnesian silicate mineral with different phases at different depths, with more or less iron. 38% of the Earth’s volume is bridgmanite.
Closer to the surface, the mantle becomes increasingly viscous. Toward its upper boundary, there is a sharp increase in energy (seismic) waves.
The mantle is mostly iron-magnesium silicate rock but mixed with many other minerals. Although the mantle is of solid stuff, the high temperatures within render the silicate sufficiently ductile to deform and flow, albeit on a geologic time scale, where a millennium is a New York minute.
The upper and lower mantle are separated by a 250 km transition zone (410–660 km down), where mineralogy modes change. This transition zone has a major role in Earth’s geodynamics, particularly influencing mantle convection: slowing slab subduction and plume ascent.
Water is introduced into the mantle where oceanic plates spread apart and new ocean bedrock forms. This water may make its way to great depths, where its pressurized presence fuels partial melting of mantle rocks, creating magmas laden with water. Thusly water recycles through the mantle.
Water ordinarily has an ordered, polymerized structure. Pressure and heat within the mantle create supercritical conditions which disorganize water molecules. The flowing hydrogen-bond network that exists in surface water is literally crushed. Supercritical water turns into an aggressive solvent. This property of water is crucial in promoting the geodynamics of Earth’s crust and mantle.
Crust
Earth’s relatively rigid outer shell, the lithosphere, is made up of the crust and uppermost mantle layer. Oceanic crust is thinner (5–10 km) but denser than continental crust (20–90 km; average 35 km). Currently, 1/3rd of the crust is continental, 2/3rds oceanic.
Crust composition differs. Oceanic crust is rich in iron and magnesium. Continental crust, derived from oceanic crust over eons, and fed by volcanoes, has more granite.
The continental crust contains the oldest rocks: up to 4 billion years. Nevertheless, continental crust changes constantly, due to erosion, sedimentation, volcanic activity, and tectonics.
Oceanic crust is constantly recycled by tectonic plate subduction and regenerated by magma plumes rising from the bottom of the mantle. No location in the oceanic crust is older than 200 million years.
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Mantle plumes are not rapidly rising jets of magma. Instead, they are broad upwellings – thousands of kilometers across – of magma and hot rock.
“Surface plates, their motions, and their return to the mantle via subduction control mantle dynamics and heterogeneity. Ultimately, it is the cooling of the Earth, modulated by internal heating, that provides the energy for convection.” ~ Don Anderson & American marine geologist James Natland
Little violent mixing of materials occurs deep within Earth. Subducted volcanic rock may travel through the mantle and resurface, largely intact, after billions of years. The deep mantle is a graveyard of ancient tectonic slabs.
From planetary wear and tear over the course of billions of years, rocks and minerals ground to a powder on the surface, mixing with moisture and microbes to become dirt. Life was well on its way by the time dirt was young.
Day Length
300 million years ago a day was 21 hours, while a year was 450 days. Since then the Earth’s rotation has slowed, lengthening the day.
The fluid outer core and solid mantle create an angular momentum that largely determines the rate of Earth’s rotation. There is a persistent wobble every 5.9 years that stutters day length. Other jitters happen when a patch of the molten outer core temporarily gets stuck to the mantle, ratcheting angular velocity. This also affects the Earth’s magnetic field.
The oceans, land, and atmosphere also have some significance in rotational fluctuations. For example, the force of the wind against mountain ranges can change the length of a day by a millisecond or so over the course of a year.
