Earth/Geognosy

Geognosy is the science and theory of the constitution of the Earth.

Theoretical geognosy
Def. the "geological study of the Earth's structure and composition" is called geognosy.

The planet Earth is made up of three main shells: the very thin, brittle crust, the mantle, and the core; the mantle and core are each divided into two parts. These are illustrated accurately to scale in the image on the right.

Geoseismology
"Evidence from geoseismology, heat flow at the surface, and mineral physics is combined with the Earth's mass and moment of inertia to infer models of the Earth's interior - its composition, density, temperature, pressure. For example, the Earth's mean specific gravity (5.515) is far higher than the typical specific gravity of rocks at the surface (2.7–3.3), implying that the deeper material is denser. This is also implied by its low moment of inertia (0.33 M R2, compared to 0.4 M R2 for a sphere of constant density). However, some of the density increase is compression under the enormous pressures inside the Earth. The effect of pressure can be calculated using the Adams–Williamson equation. The conclusion is that pressure alone cannot account for the increase in density."

"Reconstruction of seismic reflections in the deep interior indicate some major discontinuities in seismic velocities that demarcate the major zones of the Earth: inner core, outer core, mantle, lithosphere and crust."

"The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model of the Earth, mineral physics is needed to interpret seismic velocities in terms of composition. The mineral properties are temperature-dependent, so the geotherm must also be determined. This requires physical theory for thermal conduction and convection and the heat contribution of [radionuclides] radioactive elements. The main model for the radial structure of the interior of the Earth is the Preliminary Reference Earth Model (PREM). Some parts of this model have been updated by recent findings in mineral physics (see post-perovskite) and supplemented by seismic tomography."

Crusts
"In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to build." Putting these two words together, we get the term plate tectonics, which refers to how the Earth's surface is built of plates. The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material."

On the right is a subsurface map portraying the thickness of Earth's crust in kms.

Lithospheres
Def. "(cold) electrically resistive, seismically fast material with an anisotropy direction often different from present-day absolute plate motion (APM)" is called the lithosphere.

"Between the crust and the mantle is the Mohorovičić discontinuity. "

"Between 100 and 200 kilometers below the Earth's surface, the temperature of the rock is near the melting point; molten rock erupted by some volcanoes originates in this region of the mantle. This zone of extremely yielding rock has a slightly lower velocity of [seismic] waves and is presumed to be the layer on which the tectonic plates ride. Below this low-velocity zone is a transition zone in the upper mantle; it contains two discontinuities caused by changes from less dense to more dense minerals. The chemical composition and crystal forms of these minerals have been identified by laboratory experiments at high pressure and temperature. The lower mantle, below the transition zone, is made up of relatively simple iron and magnesium silicate minerals, which change gradually with depth to very dense forms. Going from mantle to core, there is a marked decrease (about 30 percent) in [seismic] wave velocity and a marked increase (about 30 percent) in density."

Def. "the depth at which seismic anisotropy direction changes from a lithospheric "fossil" direction to an asthenospheric plate-flow direction parallel to APM" is called the lithosphere-asthenosphere boundary.

Asthenospheres
Def. "the ductile part of the earth" is called the asthenosphere.

Def. "(hot) electrically conductive, seismically slow material with an anisotropy direction parallel to [absolute plate motion] APM" is called an asthenosphere.

"The asthenosphere is about 180 km thick."

"The hypothesis that the Earth has an asthenosphere can be tested by searching experimentally for a layer with physical properties attributable to its low strength. Since the shear modulus of a material reduces as its melting temperature is approached the asthenosphere should retard the passage of earthquake S-waves, whose velocity is directly proportional to the shear modulus of the material through which it is travelling. The presence of a seismological low velocity layer (LVL) or zone (LVZ) near the top of the mantle thus provides evidence for the asthenosphere. The evidence is particularly convincing since S-waves, which are more sensitive to the prevailing shear modulus than P-waves, are slowed down to a greater extent than the latter. The low velocity zone is much better developed under ocean basins than under continental shield areas where it sometimes barely developed. Hence, oceanic lithosphere is much better defined seismologically than continental lithosphere."

Def. an asthenosphere with a "rapid increase in electrical conductivity, at upper mantle depths" is called an electrical asthenosphere.

The oceanic asthenosphere is apparently at a much lower depth than the asthenosphere is detected at beneath continents.

Conrad discontinuity
"While most impact craters are characterised by negative magnetic anomalies over their central regions, aeromagnetic surveys over the Vredefort meteorite impact crater reveal multiple concentric magnetic patterns with no significant anomaly at its centre. [A] prominent negative magnetic anomaly... extends in a broad semicircular belt about half way into the basement floor of the crater. Magnetic anomalies defined by our data are most often negative and occur over a wide range of wavelengths. The longest wavelength negative anomaly coincides well with aeromagnetic data. [This] feature is centred over the amphibolite to granulite metamorphic facies transition exposed in the basement floor. The transition zone is analogous to the Conrad discontinuity, observed at depths of about 20 km elsewhere in the Kaapvaal craton. Petrographic studies show a marked increase in the intensity of the impact-related thermal and shock metamorphism at this transition, which we explain by the focusing and defocusing of shock waves at a rheologic interface during impact. We therefore suggest that the magnetic signature at this boundary is caused by a combination of both thermal and shock effects related to the impact event. A numerical model of the long wavelength anomaly suggests that it is underlain by a body of coherently magnetised rock whose direction and intensity are similar to those found in pseudotachylites and impact melts that formed during impact. On the other hand, negative anomalies occurring over smaller (100 to 20 m) wavelengths often do not coincide with the surface geology. These features cannot be modeled using the same criteria as that for the long wavelength anomaly."

Mohorovicic' discontinuity
Def. "a depth where seismic waves change velocity and there is also a change in chemical composition" is called the Mohorovicic' discontinuity.

"The Moho is the boundary between the crust and the mantle in the earth."

"The boundary is between 25 and 60 km deep beneath the continents and between 5 and 8 km deep beneath the ocean floor."

Mantles
"The mantle is mainly composed of silicates, and the boundaries between layers of the mantle are consistent with phase transitions. "

"The mantle acts as a solid for seismic waves, but under high pressures and temperatures it deforms so that over millions of years it acts like a liquid. This makes plate tectonics possible. Geodynamics is the study of the fluid flow in the mantle and core."

"The mantle itself is divided into the upper mantle, transition zone, lower mantle and D′′ layer."

"The deep Earth holds about the same amount of water as our oceans. That’s the conclusion from experiments on rocks typical of those in the mantle transition zone, a global buffer layer 410 to 660 kilometres beneath us that separates the upper from the lower mantle."

"If our estimation is correct, it means there’s a large amount of water in the deep Earth. The total amount of water in the deep Earth is nearly the same as the mass of all the world’s ocean water."

There "is much more water than expected beneath us, mostly locked up within the crystals of minerals as ions rather than liquid water."

"[W]ater-rich rock fragments [have been discovered] in volcanic debris originating from the mantle."

"The vast amount of water locked inside rocks of this deep region of the mantle will certainly force us to think harder about how it ever got there, or perhaps how it could have always been there since solidification of the mantle. It’s a key question about the evolution of the Earth, which extends to extrasolar planets as well."

"Already, real-world geophysical and seismic measurements have revealed that the viscosity of the mantle transition zone is lower than that of the upper mantle above and the lower mantle below, which extends as deep as 2900 kilometres, right down to Earth’s core."

"We determined the relationship between water content and dislocation mobility [in synthetic versions of ringwoodite from the transition zone], then used this to estimate the water content in the transition zone."

"These very difficult and very well-performed experiments are part of a growing picture emerging from laboratory and field observations via geophysics and natural studies that indicate that the mantle transition zone is likely to host significant water."

Cores
"The core was the first internal structural element to be identified. It was discovered in 1906 by R.D. Oldham, from his study of earthquake records, and it helped to explain Newton's calculation of the Earth's density. The outer core is presumed to be liquid because it does not transmit shear (S) waves and because the velocity of compressional (P) waves that pass through it is sharply reduced. The inner core is considered to be solid because of the behavior of P and S waves passing through it."

"Data from [seismic] waves, rotations and inertia of the whole Earth, magnetic-field dynamo theory, and laboratory experiments on melting and alloying of iron all contribute to the identification of the composition of the inner and outer core. The core is presumed to be composed principally of iron, with about 10 percent alloy of oxygen or sulfur or nickel, or perhaps some combination of these three elements."

Potassium in the cores
"Radioactive potassium [...] appears also to be a substantial source of heat in the Earth's core"

"Radioactive potassium, uranium and thorium are thought to be the three main sources of heat in the Earth's interior, aside from that generated by the formation of the planet. Together, the heat keeps the mantle actively churning and the core generating a protective magnetic field."

Much "less potassium [occurs] in the Earth's crust and mantle than [is] expected based on the composition of rocky meteors that supposedly formed the Earth. If, as some have proposed, the missing potassium resides in the Earth's iron core, how did an element as light as potassium get there, especially since iron and potassium don't mix?"

At "the high pressures and temperatures in the Earth's interior, potassium can form an alloy with iron never before observed. During the planet's formation, this potassium-iron alloy could have sunk to the core, depleting potassium in the overlying mantle and crust and providing a radioactive potassium heat source in addition to that supplied by uranium and thorium in the core."

The "new alloy [is created] by squeezing iron and potassium between the tips of two diamonds [a diamond anvil] to temperatures and pressures characteristic of 600-700 kilometers below the surface - 2,500 degrees Celsius and nearly 4 million pounds per square inch, or a quarter of a million times atmospheric pressure."

"Our new findings indicate that the core may contain as much as 1,200 parts per million potassium -just over one tenth of one percent."

"This amount may seem small, and is comparable to the concentration of radioactive potassium naturally present in bananas. Combined over the entire mass of the Earth's core, however, it can be enough to provide one-fifth of the heat given off by the Earth."

"With one experiment, Lee and Jeanloz demonstrated that potassium may be an important heat source for the geodynamo, provided a way out of some troublesome aspects of the core's thermal evolution, and further demonstrated that modern computational mineral physics not only complements experimental work, but that it can provide guidance to fruitful experimental explorations,"

"More experiments need to be done to show that iron can actually pull potassium away from the silicate rocks that dominate in the Earth's mantle."

"They proved it would be possible to dissolve potassium into liquid iron."

"Modelers need heat, so this is one source, because the radiogenic isotope of potassium can produce heat and that can help power convection in the core and drive the magnetic field. They proved it could go in. What's important is how much is pulled out of the silicate. There's still work to be done."

"If a significant amount of potassium does reside in the Earth's core, this would clear up a lingering question - why the ratio of potassium to uranium in stony meteorites (chondrites), which presumably coalesced to form the Earth, is eight times greater than the observed ratio in the Earth's crust. Though some geologists have asserted that the missing potassium resides in the core, there was no mechanism by which it could have reached the core. Other elements like oxygen and carbon form compounds or alloys with iron and presumably were dragged down by iron as it sank to the core. But at normal temperature and pressure, potassium does not associate with iron."

"Early in Earth's history, the interior temperature and pressure would not have been high enough to make this alloy."

"But as more and more meteorites piled on, the pressure and temperature would have increased to the point where this alloy could form."

"The Earth is thought to have formed from the collision of many rocky asteroids, perhaps hundreds of kilometers in diameter, in the early solar system. As the proto-Earth gradually bulked up, continuing asteroid collisions and gravitational collapse kept the planet molten. Heavier elements - in particular iron - would have sunk to the core in 10 to 100 million years' time, carrying with it other elements that bind to iron."

"Gradually, however, the Earth would have cooled off and become a dead rocky globe with a cold iron ball at the core if not for the continued release of heat by the decay of radioactive elements like potassium-40, uranium-238 and thorium-232, which have half-lives of 1.25 billion, 4 billion and 14 billion years, respectively. About one in every thousand potassium atoms is radioactive."

"The heat generated in the core turns the iron into a convecting dynamo that maintains a magnetic field strong enough to shield the planet from the solar wind. This heat leaks out into the mantle, causing convection in the rock that moves crustal plates and fuels volcanoes."

Pure "iron and pure potassium [combined] in a diamond anvil cell [that] squeezed the small sample to 26 gigapascals of pressure while heating the sample with a laser above 2,500 Kelvin (4,000 degrees Fahrenheit), which is above the melting points of both potassium and iron. [Repeat] six times in the high-intensity X-ray beams of two different accelerators - Lawrence Berkeley National Laboratory's Advanced Light Source and the Stanford Synchrotron Radiation Laboratory - to obtain X-ray diffraction images of the samples' internal structure. The images confirmed that potassium and iron had mixed evenly to form an alloy, much as iron and carbon mix to form steel alloy."

"In the theoretical magma ocean of a proto-Earth, the pressure at a depth of 400-1,000 kilometers (270-670 miles) would be between 15 and 35 gigapascals and the temperature would be 2,200-3,000 Kelvin."

"At these temperatures and pressures, the underlying physics changes and the electron density shifts, making potassium look more like iron."

"At high pressure, the periodic table looks totally different."

"The work by Lee and Jeanloz provides the first proof that potassium is indeed miscible in iron at high pressures and, perhaps as significantly, it further vindicates the computational physics that underlies the original prediction."

"If it can be further demonstrated that potassium would enter iron in significant amounts in the presence of silicate minerals, conditions representative of likely core formation processes, then potassium could provide the extra heat needed to explain why the Earth's inner core hasn't frozen to as large a size as the thermal history of the core suggests it should."

There are three requisites for a dynamo to occur and subsequently operate:


 * An electrically conductive fluid medium such as a plasma or liquid iron
 * local magnetohydrodynamic instabilities
 * An energy source to create the local magnetohydrodynamic instabilities and to drive mechanical turbulence, motion, or shear within the fluid.

In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core. A requirement for the induction of field is a rotating fluid. Rotation in the outer core is supplied by the Coriolis effect caused by the rotation of the Earth. The Coriolis force tends to organize fluid motions and electric currents into columns aligned with the rotation axis. Induction or creation of magnetic field is described by the induction equation:


 * $$\frac{\partial \mathbf{B}}{\partial t} = \eta \nabla^2 \mathbf{B} + \nabla \times (\mathbf{u} \times \mathbf{B}),$$

where u is a velocity, B is the magnetic field, t is time, and $$\eta=1/\sigma\mu$$ is the magnetic diffusivity with $$\sigma$$ electrical conductivity and $$\mu$$ permeability. The ratio of the second term on the right hand side to the first term gives the Magnetic Reynolds number, a dimensionless ratio of advection of a magnetic field to diffusion.

Tidal forces between celestial orbiting bodies causes friction that heats up the interiors of these orbiting bodies. This is known as tidal heating, and it helps create the liquid interior criteria, providing that this interior is conductive, that is required to produce a dynamo.

Outer cores
"Reconstructions of seismic waves in the deep interior of the Earth show that there are no S-waves in the outer core. This indicates that the outer core is liquid, because liquids cannot support shear. The outer core is liquid, and the motion of this highly conductive fluid generates the Earth's field (see geodynamo)."

A "kind of "jet stream" - a fast-flowing river of liquid iron [depicted with an artist's impression in the image on the right] is surging westwards under Alaska and Siberia."

"The moving mass of metal has been inferred from measurements made by Europe’s Swarm satellites. [...] the jet is the best explanation for the patches of concentrated field strength that the satellites observe in the northern hemisphere."

"This jet of liquid iron is moving at about fifty kilometres per year. That might not sound like a lot to you on Earth's surface, but you have to remember this a very dense liquid metal and it takes a huge amount of energy to move this thing around and that's probably the fastest motion we have anywhere within the solid Earth.”

The "jet [is] about 420 km wide, and [...] wraps half-way around the planet."

“It's likely that the jet stream has been in play for hundreds of millions of years."

"It currently wraps about 180 degrees around the tangent cylinder [a boundary between two different regions in the core, a tube sitting around the solid inner core, running along Earth’s rotation axis]. Although observations only constrain the jet stream on the edge of the core, our theoretical understanding suggests that the jet could in principle go very deep indeed - possibly in fact all the way down to the edge of the core in the southern hemisphere (i.e. at the other end of the tangent cylinder)."

"When liquid iron approaches the boundary from both sides, it gets squeezed out sideways to form the jet, which then hugs the imaginary tube."

"Of course, you need a force to move fluid towards the tangent cylinder. This could be provided by buoyancy, or perhaps more likely from changes in the magnetic field within the core."

Inner cores
We know that the Earth's core is composed of an alloy of iron and other minerals.

"A PKJKP [P wave, traversing the outer core K, and the inner core J, to emerge again as the P wave] traverses the inner core as a shear wave, so this is the direct evidence that the inner core is solid, because only in the solid material the shear wave can exist. In the liquid material, say water, only the compressional wave can travel through."

Studying "archived data from about 20 large earthquakes, all monitored by an array of German seismic detectors back in the 1980s and '90s" has "reliably detected" a PKJKP wave in 2005, demonstrating that the inner core is solid.

"The inner core, however, is solid because of the enormous pressure. "

The inner core "is a solid ball of superhot iron and nickel alloy about 760 miles (1,220 kilometers) in diameter. ... the inner core is, at 10,800 degrees Fahrenheit (6,000 degrees Celsius), as hot as the surface of the sun."

"We know the Earth's inner core is composed mostly of iron".

"The metal [iron] was subjected to more than 200 billion pascals of pressure".

"[M]aterial within Earth's inner core is apparently distributed in a lopsided way ... The weakness of iron might lead crystallites in the inner core to flow and line up a certain way".

"[T]he speed at which the inner core spun apparently fluctuated over the course of approximately decades between 1961 and 2007."

"As the inner core cools, crystallizing iron releases impurities, sending lighter molten material into the liquid outer core. This upwelling, combined with the Earth's rotation, drives convection, forcing the molten metal into whirling vortices. These vortices stretch and twist magnetic field lines, creating Earth’s magnetic field. Currently, the center of the field, called an axis, emerges in the Arctic Ocean west of Ellesmere Island, about 300 miles (500 kilometers) from the geographic North Pole."

"In the last decade, seismic waves from earthquakes revealed the inner core looks like a navel orange, bulging slightly more on its western half. Geoscientists recently explained the asymmetry by proposing a convective loop: The inner core might be crystallizing on one half and melting on the other."

"The lopsided growth of the inner core makes convection in the outer core a little bit lopsided, and that then induces the geomagnetic field to have this lopsided or eccentric character too".

"Magnetic particles trapped and aligned in rocks reveal that the magnetic north pole wandered around the Western Hemisphere over the past 10,000 years, and circled the Eastern Hemisphere before that — a result mirrored by the numerical test."

"The key question for interesting ideas like translational instability is, 'Can we test it?' ... What we're doing is proposing a test, and we think it's a good test because people can go out and look for eccentricity in the rock record and that will either confirm or shoot down this idea."

"Within less than 100 million years, everything that has been crystallized on the west will have melted on the east"

Seismic "waves appear to travel faster through the inner core from north to south than from west to east. Seismic properties also seemed to vary between the Eastern and Western hemispheres of the globe."

There is a "124-mile (200-km) thick layer of dense material detected on its surface."

"[T]he inner core [may be] shifted slightly off-center, just to the east. This would put more pressure on the western side, where it would be closer to the center of the planet, and less pressure on the eastern side. The result could be a perpetually denser Western hemisphere and a continual flow of dense fluid from the east that eventually spreads out atop the entire inner core."

"The inner core is basically regenerating itself. And superimposed on that is this overall cooling that makes the inner core bigger and bigger over time".

"It is the first observational evidence that the inner core rotates at a variety of speeds with respect to the mantle...It also reconciles old discrepancies".

"The inner core, on average, rotates eastward. At the speeds it travels, it might, on average, complete a revolution every 750 to 1,440 years. However, these speeds appear unstable, which makes it uncertain just how long it actually takes to finish a turn on its axis".

"Earth’s magnetic field [...] is powered by circulation of iron-rich fluid in the core. [...] Earth’s solid inner core [may have] formed after 565 million years ago, saving a weakening magnetic field from collapse."

"We don’t have many real benchmarks for the thermal history of our planet."

"We know the interior was hotter than today, because all planets lose heat. But we don’t know what the average temperature was a billion years ago, compared with today. Pinning down when iron in the inner core began to crystallize could offer a window into how hot the interior of the planet was at the time."

"Proposed ages have been anywhere from 500 million years ago to older than 2.5 billion years."

"The interplay of the two layers drives the geodynamo, the circulation of iron-rich fluid that powers the magnetic field. That field, surrounding the planet, protects Earth from being battered by the solar wind, a constant flow of charged particles ejected by the sun. As the inner core cools and crystallizes, the composition of the remaining fluid changes; more buoyant liquid rises like a plume while the cooling crystals sink. That self-sustaining, density-driven circulation generates a strong magnetic field with two opposing poles, north and south, or polarity."

"Traces of magnetism in ancient rocks suggest that Earth had a magnetic field as far back as 4.2 billion years ago. That earlier field was likely generated by heat within the planet driving circulation within the molten core. But over time, computer simulations suggest, the heat-driven circulation wouldn’t have been strong enough alone to continue to power a strong magnetic field. Instead, the field began to shut down, signaled in the rock record by weakening intensities and rapid polarity reversals over millions of years. And then, at some point, Earth’s inner core began to crystallize, jump-starting the geodynamo and generating a new, strong magnetic field."

Magnetic "inclusions within a suite of rocks in Quebec, Canada, dating to about 565 million years ago [...] — needlelike iron-rich grains that align themselves with the orientation of the magnetic field that existed when the rocks formed — show that the planet’s magnetic field was extremely weak at that time. These paleo-intensity values were 10 times less than the present magnetic field, lower than anything observed previously. It suggested there’s something fundamental going on in the core."

"Combined with previous studies that have found that the magnetic field was also rapidly reversing polarity during that time period, the new result indicates that Earth’s field may have been on the point of collapse about 565 million years ago. That suggests that the inner core hadn’t yet solidified."

"Because the rocks bearing the magnetic grains didn’t cool instantaneously but over a long time, the data represent an average field intensity for about a 100,000-year period. [A] true, persistent signal [was found]. Computer simulations have suggested that the weak field phase may have lasted much longer, from about 900 million to 600 million years ago."

If "the core is cooling quickly, that means it was very hot in the recent past, and that the lower mantle was very hot in the recent past — so hot that both were molten just 1 billion to 2 billion years ago. We absolutely do not see that in the rock record."

Palaeointensity "data from the Ediacaran (~565 million years old) Sept-Îles intrusive suite measured on single plagioclase and clinopyroxene crystals that hosted single-domain magnetic inclusions [indicates] a time-averaged dipole moment of ~0.7 × 1022 A m2, the lowest value yet reported for the geodynamo from extant rocks and more than ten times smaller than the strength of the present-day field."

"Palaeomagnetic directional studies of these crystals define two polarities with an unusually high angular dispersion (S = ~26°) at a low latitude. Together with 14 other directional data sets that suggest a hyper-reversal frequency, these extraordinary low field strengths suggest an anomalous field behaviour, consistent with predictions of geodynamo simulations, high thermal conductivities and an Ediacaran onset age of inner core growth."

Siderophiles
Def. "an element that forms alloys easily with iron and [may be] concentrated in the Earth's core" is called a siderophile.

Siderophile (metal-loving) chemical elements include W, P, Co, Ni, Mo, Re, and Ir.

"The platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, and Pd) and Re are highly siderophile elements (HSE)".

"We believe that silicon is a major element - about 5% [of the Earth's inner core] by weight could be silicon dissolved into the iron-nickel alloys."

"The innermost part of Earth is thought to be a solid ball with a radius of about 1,200 km (745 miles)."

"It is mainly composed of iron, which makes up an estimated 85% of its weight, and nickel, which accounts for about 10% of the core."

"These difficult experiments are really exciting because they can provide a window into what Earth's interior was like soon after it first formed, 4.5 billion years ago, when the core first started to separate from the rocky parts of Earth."

"But other workers have recently suggested that oxygen might also be important in the core."

"In a way, these two options [oxygen was sucked into the core that would leave the rocky mantle surrounding the core depleted of the element or a larger amount of silicon had been incorporated in Earth's core more than four billion years ago, that would have left the rest of the planet relatively oxygen rich] are real alternatives that depend a lot on the conditions prevailing when Earth's core first began to form."

Hypotheses

 * 1) The core of the Earth is not the origin of the Earth's magnetic field.