the interior structure of the earth. part 1. the core

Our understanding of the interior structure and composition of the earth and its thermal characteristics is limited by the fact that we can’t access most of it directly, and so most of what we think we know about the subject is inferred and subject to revision. The deepest drill hole only penetrates about 10 km which leaves us thousands of kilometers short of the center. Due to its inaccessibility we must of necessity rely upon mainly indirect methods to discern its properties, and this leaves plenty of room for speculation and reinterpretation of the narrative in the light of new information, …of which there is plenty. Matters are further complicated by trying to distinguish between processes that occurred during the development of the planet over time to bring it to its present state and what is currently happening down there. What follows is a slightly haphazard review of what we think we know about the world beneath our feet.

Taking the earth overall, it is thought that 90% of the Earth’s mass is composed of iron, oxygen, silicon and magnesium.

The two main regions of the earth’s interior are the central innermost region, the core,  and an outer layer called the bulk silicate earth (BSE), The BSE includes both the mantle (99%) and the crust (1%). Both core and mantle are, to varying degree, high temperature/pressure environments where strange elemental forms and associations can exist. The core has a radius of about 3850 km, which although more than half of the total radius, constitutes just 15 percent of the Earth’s volume. The radius of the whole earth is about 6350 km, with the added 2500 km from core/mantle boundary to the surface making up 85% of the volume.

Our understanding of the interior is based on various forms of remote sensing and inference including density considerations, corresponding meteorite composition, seismic data, electromagnetic and geological observations together with high temperature/pressure experiments on various elements and compounds.

Due mainly to the magnitude of the bulk earth density, 5.5 g/cm3 , …about twice the density of the crust and mantle, and the composition of some iron-rich meteorites, the central core is thought to consist mainly of metallic iron (Fe), together with some nickel (Ni), silicon (Si) and sulfur (S). Recent suggested additions include hydrogen, oxygen and oxides of magnesium or silicon or even hydrides of the heavier elements, (see below).

Although even stony meteorites contain 10-30% iron (rare lunar meteorites would be an exception), about 5% of located meteorites have a much higher nickel-iron alloy content. Contrast this proportion with the estimated earth’s core volume of 15% which suggests that core type material may be underrepresented in the meteorite population.

The core is in turn divided into an inner and outer region, the inner core being solid while the outer core is thought to be mobile with convection currents radiating away from the center. These convection currents in the outer core (which are powered by exothermic or heat producing activity such as radioactive decay or even oxidation) are thought to be the main source of that feature of the core with which we are most familiar, i.e. the magnetic field that extends well beyond the surface of the earth and without which the surface would be much less habitable.

From a chemical point of view, the core elements are generally present in their reduced form which means that they have neither a shortage or excess of electrons. This implies that the elements coexist as a metal alloy which is a mixture of the metallic components. In a metal  the electrons are shared between many atoms and these electrons can undergo something called gapless excitations (in a gapless system the energy state of the electron can change continuously, …like kinetic energy or even electrical energy) and by which means the metal will act as a conductor. This contrasts with elements in the form of chemical compounds where some electrons are shared between the elements to form the molecules that make up the compound. These compounds are generally gapped systems where the electrons are constrained to to discrete energy levels that can store or release chemical energy.

Note that it has recently been suggested that the reduced metallic alloy model may not be the whole story as some metal oxides, either silicon dioxide (quartz) or magnesium oxide may be present and that the ongoing production of the oxides may add to the thermal budget. Just how this oxidation reaction might proceed in the presumed strongly reducing environment of the outer core remains to be explained. Others speculate that some of the metals exist as sulphides (e.g pyrite, FeS2), silicides (e.g. nickel mono-silicide, NiSi) or, if hydrogen in significant amounts, hydrides. This idea seems to be a better fit to the existing chemical model.

As one would expect given the proposed presence of fluid metal, the core is a high energy region, it’s hot. The science is far from settled but popular estimates range from about 3,000 °C for the outer core to around 6,000 °C or more for the inner core. We base these estimates of the core temperatures on the melting behaviour of iron at very high pressures. Core pressure estimates vary from +1 million to around 3.5 million atmospheres (one atmosphere of pressure is also called 1 bar and is around 15 psi)  which roughly translates to anywhere between  15 and 55 million psi.  At these pressures, iron melts somewhere between about 4,200 °C to about 7,200 °C.

The source of this energy/heat is thought to consist of residual primordial accretion energy, gravitation derived friction generated by descending heavy elements and a current exothermic or energy releasing process, the main contender being radioactive decay, and possibly accompanied by an exothermic chemical process like oxidation. Estimates of the energy contribution of radioactive decay  vary widely as there is uncertainty about the amount of radioactive material present in the core. The convection process may also result in electrostatic charge separation similar to charge buildup in a thunder cloud.

As mentioned above, the core, or more specifically, the state and motion of the outer core is thought to be the principal source of the earth’s magnetic field which has two main poles (North and South) like a permanent bar magnet (note that the mantle, crust, oceans, ionosphere and magnetosphere also contribute to the overall field) . But temperature levels in the core would seem to rule out a permanent magnet as the source, …molten metal is generally not magnetic (we believe that for a substance to be magnetic many of the the particles must be aligned, …a  difficult feat to achieve for metal in a mobile liquid state). Permanent magnets generally lose their magnetic field  at about 500°C (for the technically minded :-), the temperature at which some materials lose their permanent magnetic properties or their ability to be attracted by a magnet is known as the Curie point).

We  know of only three ways to create a magnetic field, firstly via these natural or man made permanent magnets, next, and this is the favoured dynamo model, by utilizing an electrical circuit where the current, in addition to creating an electric field, creates a magnetic or B field. The relative stability of the earth’s magnetic field suggests a similarly stable, solenoid type circuit. The third possibility involves  a state of matter known as plasma where atoms experience some degree of charge separation which creates both unbound electrons and ions. This plasma state produces its own static? magnetic field which will induce currents in molten material. In addition although the magnetic field is stable over shorter time scales, we observe that the poles wander over time and sometimes reverse polarity. This dynamic nature of the field suggests that it is current/circuit related. We also find the field to be asymmetric. However this characteristic doesn’t readily lend themselves to a rotating or convecting  liquid outer core model.

As discussed above, it is likely that some proportion of the elements of the alloy are in an ionized, plasma-like state, where one or more electrons are separated from their atoms. These relatively unbound electrons and the corresponding electron deficient particle result in dynamic charge separation of the moving atoms which creates energy gradients/transmissions, which in their turn produce a dynamic magnetic field, which then induces eddy currents in the surrounding material, (and the possibility of superconducting conditions at some scale cannot be ruled out, metallic hydrogen is a superconductor). A picture emerges of a chaotic electrical milieu. This is of more than academic interest as the small scale charge separated plasma component of the alloy (and the resulting electromagnetic/electrostatic mismash) is, as mentioned above, thought to generate the large scale magnetic field that pervades the solid earth and near space. Somehow or other, a small scale charge separation process is able to generate a global field. As mentioned above, this core related magnetic field and the associated magnetosphere is one of the components that makes the surface of the earth habitable as it deflects most of the harmful high energy cosmic rays that arrive from the sun and the wider cosmos. It is somewhat ironic that, far from being a somewhat distant and irrelevant hot blob :-), life on the surface depends in part on the behaviour of the distant and only indirectly accessible core.

Other more remote possibilities for the source of the field include, a. the presence of a permanently magnetic section of the core that is a remnant of past events or b. present day electrical activity above or below the earth’s surface or c. the convecting/moving fluid may induce electrostatic charge separation similar to charge buildup in a thunder cloud. Static charge accumulation does not generate a magnetic field but dielectric failure will create a circuit of plasma which will then discharge and in the process generate a magnetic field.

a special type of relativity

out of the eerie dawn
appears a heralding glow
of variable hue.
and then glinting rays
radiate
and start the day
light
it’s no longer night
when sleepers dream
the’re in a dream
but awaken to another
dream sodden day…
the encyclemental sun
begins its daily journey across the sky.
…well from our shoes at least.
but if one zooms out, so to speak
we encounter
an enlightened perspective.
it becomes apparent that
we are just going round in circles
and see the steady sun
pass by
on each and every one
of our daily revolutions
p.s. encyclemental is not a real word but i thought it fitted in ok

methane in the mantle?

A recent report describes the discovery of methane seeps and unusual features called carbonate chimneys off the west coast of North America,

… “It appears that the entire coast off Washington, Oregon and California is a giant methane seep,” … a Lost City of carbonate chimneys towering 60 meters …

They don’t mention it in the article but the carbonate chimneys that were discovered are directly related to the methane seeps in that the the methane is the actual source of the carbon in the carbonate.

The carbonate chimneys are probably what is known in the trade as Methane Derived Authigenic Carbonates or MDAC’s for short. Authigenic is the term used to describe sediments that are generated in situ as opposed to being transported from elsewhere.
The carbonate is precipitated when the methane reacts with the hydrothermal mileu which includes both chemical and metabolic/biological activity.

The precipitation of authigenic carbonates at fluid seepage sites is a common phenomenon that can be triggered by the activity of a consortium of archaea and bacteria that oxidize methane close to the seafloor … or that can occur due to chemical reactions (i.e. without microbial mediation). Such carbonate deposits reveal different morphologies depending on combined internal (seepage-related) and external (setting-related) factors.
http://folk.uio.no/hensven/Mazzini_etal_MarGeol_06.pdf

Being associated with methane, the carbonate features are widely distributed and occur in a variety of forms. The carbonates can be formed in both shallow and deep water and can manifest as both surface (chimneys) and subsurface features such as nodules or column/cone shaped features which go by the rather unwieldy name of Positive High Amplitude Anomalies (PHAA’s), a term which refers to their seismic characteristics (more consolidated than the background sludge).

… The carbonate morphologies include thin (∼1 cm) platy carbonate crusts, blocky and massive carbonate ridges up to several metres in size, and irregularly shaped carbonate deposits consisting of interconnected tubular and uneven intervals displaying high porosity. …
http://folk.uio.no/hensven/Mazzini_etal_MarGeol_06.pdf

This carbonate formation is the opposite of the process needed to convert carbonate to methane or other hydrocarbon. Methane to carbonate is an oxidation reaction whereas converting carbonate to methane is a reduction or de-oxidation reaction. In chemical lingo it is called a redox reaction. Depending on energy and catalytic conditions, and ability to remove product, the reaction can be pushed in the opposite direction.

So there may be both chemical and/or biological pathways available via hydrothermal/catalytic (with generous amounts of finely divided metals in the vicinity the potential for catalytic activity is good) or biological activity that convert carbonate to methane/hydrocarbon. And although the presence of hydrocarbon is attributed to “cooking” of organic matter, the possibility that this reverse reaction may occur can’t in my opinion be summarily dismissed at present.

… The geologists also noticed that their [carbonate] rock samples smelled like diesel. They hypothesize that hot hydrothermal fluids migrating upward through the thick sediments of the Pescadero Basin ‘cook’ organic matter in the sediment, converting it into petroleum-like hydrocarbons — a process that has been observed at several other vents in the Pacific. Hydrocarbons may provide nutrition for the unusual microbes that thrive at these vents. …
http://www.mbari.org/mbari-researchers-discover-deepest-known-high-temperature-hydrothermal-vents-in-pacific-ocean/

This says nothing about the origin of the carbon in the geological column or whether the carbon in source rocks is a product of methane or vice versa, but it suggests to me at least that the idea of surface derived and subsequent burial of biological carbon needs to be kept under review.

Nor can one rule out the possibility that the methane (and its carbon component) is the product of some as yet unknown process operating in the upper portion of the mantle (perhaps the region which has access to water). There is some recent evidence that adds support to this possibility by way of the identification of methane in the inclusions of large diamonds which are thought to originate in the mantle.

… Here we report that large, exceptional gem diamonds like the Cullinan, Constellation, and Koh-i-Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle. These sublithospheric diamonds contain inclusions of solidified iron-nickel-carbon-sulfur melt, accompanied by a thin fluid layer of methane ± hydrogen, and sometimes majoritic garnet or former calcium silicate perovskite. The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated conditions. We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen.
http://science.sciencemag.org/content/354/6318/1403

Finally, one can’t rule out the possibility that the methane generation process includes some type of mantle metabolic activity as some lifeforms may be present even in high temperature/pressure environments.

 

sea level rise or…

Despite previous assurances^ about the rock-solid stability of the earth’s dimensions, further analysis of satellite altitude data suggests that there may still be a bit of life in the old girl (Mother Earth) yet.* :-)

The following abstract suggests that some minimal expansion activity can still be discerned today insofar as some of the observed satellite altitude variation that is usually explained as sea level rise is actually the result of earth expansion.

Evidences of the expanding Earth from space-geodetic data over solid land and sea level rise in recent two decades
Wenbin Shena, Ziyu Shena, Rong Sunc, Yuri Barkind
30 June 2015

According to the space-geodetic data … vertical variation of the Earth’s solid surface suggests that the Earth’s solid part is expanding at a rate of 0.24 ± 0.05 mm/a in recent two decades.

In another aspect, the satellite altimetry … demonstrate the sea level rise (SLR) rate 3.2 ± 0.4 mm/a, of which 1.8 ± 0.5 mm/a is contributed by the ice melting over land. This study shows that the oceanic thermal expansion is 1.0 ± 0.1 mm/a due to the temperature increase in recent half century, which coincides with the estimate provided by previous authors. The SLR observation by altimetry is not balanced by the ice melting and thermal expansion, …, in this study we infer that the oceanic part of the Earth is expanding at a rate about 0.4 mm/a.

Combining the expansion rates of land part and oceanic part, we conclude that the Earth is expanding at a rate of 0.35 ± 0.47 mm/a in recent two decades. If the Earth expands at this rate, then the altimetry-observed SLR can be well explained. …

^ From the linked report, … The scientists estimated the average change in Earth’s radius to be 0.004 inches (0.1 millimeters) per year, or about the thickness of a human hair, a rate considered statistically insignificant. …

* By way of explanation, my current view is that most of the expansion occurred in the past when mantle conditions were radically different to present day norms, …and that the activity was both rapid and episodic.

crust mobility conundrum. how does the earth move?

Until comparatively recently, the only crustal movements worth the name were thought to be either post glacial rebound, where surface ice melting induces crust rebound until a new equilibrium is established, or gravitational collapse, such as weathering  and erosion. Both are gradual, isostatic processes.

The many extensional features such as rifts, horst/graben faulting, uplift, tilted blocks and basin formation were seen to be part of these isostatic processes, occurring over geologic time scales and with any lateral movement being a localized event.

However mounting geographical, geological, paleontological and botanical evidence was pointing to continental mobility on a global scale, and by the sixties the Isostatic Tectonics view was finally rejected in favour of a globally mobile surface with most opting for a surface plate model driven by mantle convection currents, …Plate Tectonics.

Others concluded that the abundant extensional features, the spreading ridge that surrounds Antarctica being one of many such examples, …that such features could only be accounted for by an increase in the surface area, and thus radial expansion, of the earth, …Expansion Tectonics.

‘Surface’ mobilists espoused Plate Tectonics while the ‘surface area’ mobilists advocated Expansion Tectonics. The ‘facts’ could be made to fit both explanations. But the absence of a plausible expansion mechanism was an insurmountable problem for Expansion Tectonics, and Plate Tectonics was adopted to replace the Isostatic model.

To recap, Plate Tectonics prevailed over Expansion Tectonics because Plate Tectonics requires less energy, no mass/volume variation, and, more importantly, it has a plausible mechanism to drive the process, i.e. convection. Earth expansion was a contender process in the sixties but is now considered a discredited theory.  It has been considered, and, largely due to the absence of any causal mechanism, rejected by most, but not all,  right-thinking geo-folk. This is despite the fact that much of the geological/geographical/botanical/fossil/seismic evidence for plate tectonics can be used to support the expansion model. The evidence is also consistent with a sub-crustal increase in mantle or core/mantle volume so that the increased surface area consists of primordial crust separated by recent ocean crust.

Both Plate and Expansion theories assume that the process occurs continuously over geological time spans. Whatever its driving mechanism, tectonic movement is considered to be a slow but inexorable process.

However recent work on the structure of the world that lies beneath our feet, specifically 3-D images of the crust and mantle generated by supercomputer processed, earthquake induced, seismic wave data,  reveals that there is no obvious evidence for the simple convective system that one might expect from the tectonic model. From the link…

… One could a priori expect such [tomographic ] images to reflect a very simple convective system, where the ascending currents are found under mid-ocean ridges, and the descending currents are found at subduction zones, where the tectonic plates slide back into the mantle. In fact, seismic tomography … reveals a more complex situation that cannot yet be very clearly interpreted. …
– Physics of the Earth’s Interior. Barbara Romanowicz.

The images,10060010002800From the maps it is evident that Mid-Ocean Ridge spreading centers do not persist below 600 kms. What emerges at deeper levels are ocean sized blobs located under the southerly region of the Pacific and African plates to create a more or less balanced global distribution of these low density/high temperature features.

This configuration does not support the simple convection model of a continuous column of ascending or descending material originating near the core-mantle boundary and extending to the base of the, …according to the Plate Tectonic model, very mobile and actively subducting crust.

Convection is an important part of the Plate Tectonics narrative in that it is the proposed mechanism that supplies the enormous amount of energy needed maintain the currents necessary to drive the plates across the surface of the earth. Without it, Plate Tectonics has difficulties with its energy budget as it assumes that surface plate mobility is powered by energy from below. Without convection, plate mobility is left without a plausible driving mechanism. It could not be maintained, let alone initiated. Something else would be required to explain the obvious separation of pieces of continental crust that both Plate Tectonics (and Expansion Tectonics) seek to explain.

The lack of  evidence for simple convection in the tomographic data has prompted modifications to the model by way of the introduction of a stratified two-stage convection model. This is roughly analogous to a pot filled with water and oil, with separate convection cells in both oil and water layers and a conductive energy transfer process at the interface.of the liquid layers. The modifying effects of uncontained convection, regional lateral temperature variations and other tweaks are also being considered.

The picture that emerges, …a chemically stratified mantle with variable-depth chemical boundaries near 1,000 and 2,000 km and a lower mantle depleted in radioactive elements appears to satisfy available geochemical and geophysical constraints. … suggests a major revision of the Plate Tectonics energy budget and itself appears to be somewhat at odds with what one might expect were convection currents present. Convection is not generally associated with stratification.

The tomographic picture may offer more support to a model where the plates are relatively static with any relative movement due an increase in the overall surface area of the globe together with accompanying isostatic adjustment and curvature modification.

Separation by surface area expansion still doesn’t have a causative mechanism, but the lack of evidence for simple convection also casts doubt on the viability of the surface plate mobility claimed by Plate Tectonics.

a rapid expansion event?

 

This is an  ESA Rosetta Navcam image of comet 67P/C-G taken on Feb 3 2015.

It may be just a trick of the light, so to speak :-), but from this perspective it sure looks like something popped, …well it does to me at least.

Rosetta_NAVCAM_comet_67P_20150203_mosaic_625g

The current view is that the lobed shape of this and other comets is a product of ‘binary contact’ but this view of the comet seems more indicative of separation than evidence of a contact event.

Picture: Copyright: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0
licensed under a Creative Commons Attribution-ShareAlike 3.0 IGO License.

 

 

 

A close association

The behemoth lies under the lotus tree.

embraced imperceptibly
by the slightest of brushes
with the nebulous aether,
we,
in tidal synchronicity with its inscrutable motions,
…we spin and weave our way,
to come what may.

in all rolls and rotations,
and loops and orbits,
we hum to the tune
of our live companion
via intimate connection
with each tiny spark that makes up our quarks.