The interior structure of the earth. Part 2. The bulk silicate earth (crust and mantle).

Reading part 1 about the core is recommended as some of the preliminary remarks there are also relevant to this discussion. It must be stressed that much of the information about the earth’s interior is inferred as direct observation is not possible.

The outer region of our planet, the Bulk Silicate Earth (BSE), is thought to to be composed of layers. The stratification or differentiation of this region of the earth is a major feature and forms the basis of descriptions of its morphology, that is, its form and structure.

The reasons for this layered structure are much discussed and investigated. Note also that layered structures appear over a wide range of scales. As the name suggests, silicate is the main component and oxygen is the main component of the silicate.

These layers are preceded by the hydrosphere/atmosphere and followed by the inner/outer core. The layers are distinguished by variations in one or more of the following categories.

1. Elemental composition.  Elements are classified according to the number of protons in their atomic structures, the protons reside in the nucleus of the atom along with neutrons. There are about one hundred known elements and the earth contains all of the naturally occurring ones in varying proportions. Examples are hydrogen, oxygen, iron, carbon, mercury and so on.

2. Mineral content.  The elements are combined into various chemical compounds called  minerals which, nearer the surface at least, are generally crystalline in nature (that’s where the mineral components have an ordered periodic structure), but occasionally they are amorphous (i.e, no crystal structure, like for example, glass, or indistinct, like talc). The size of the crystals can vary from very large to microscopic. There are about 5000 different minerals. They can occur as separate deposits or combined with other minerals as rocks. And it just so happens that the majority of  the mineral volume of the BSE is composed of various types of silicates (silicon oxides which are a combination of the metal Silicon and the gas Oxygen) with other metals, for example, magnesium silicate or aluminium silicate.

3. Rock types. Rocks consist of different combinations of minerals. Examples of rock basalt or sandstone. The minerals that constitute the rocks are generally crystalline. The rocks are divided into three separate groups, igneous (intrusive or extrusive), metamorphic and sedimentary. These categories are based on what are thought to be varying formation conditions and origins. However, like the layers of the earth themselves, the boundaries between these rock groups are sometimes blurred. There are a few hundred different rock types spread over the three principal classes.

The most abundant rock types are the igneous rocks which are presumed to have solidified from a magma (a body of molten rock). The magma can take the form of subterranean lakes, layers, plutons, plumes, pipes and dykes. Magma appears on the surface as sheets of basalt (usually), as exposed dykes or as volcanic ejecta.

The magma interacts with the rocks with which it comes into contact, to the extent that some of the host rock becomes incorporated into the magma and transported elsewhere. The interaction can also modify the mineral structure and content of the host rock.

The magma composition can vary depending on the source of the magma itself. Most is considered to have a mantle zone origin (often influenced by subducted crust and its accompanying sediment and water – the water enables the components to liquefy at a lower temperature than would otherwise be the case), while others such as granite magmas may have their origin in reworked sediments. The magma is thought to be enriched in lower melting point elements as it solidifies leaving heavy metal rich silica structures either as disseminated or veined deposits in the solidified magma or the surrounding host rock. Diffusion and fracking are the operative processes.

The other rock types present in the crust  are:

Sedimentary, (includes evaporites and hydrothermal limestone and metal sulphide deposits). They usually have a layered structure and often display varves which suggest an advancing front of deposition which is typical of deltaic sediments. Sedimentary deposits can be regionally extensive and thousands of meters thick. The majority of sedimentary rocks are considered to have a detrital origin, i.e. transported by water (including glacial activity), wind or gravitational collapse to the site of deposition.

Others are thought to have been deposited by evaporation or by hydrothemal activity. The carbonates (e.g. limestone) are considered to have a biological origin, however recently a hydrothermal carbonate has been identified where the source of the carbon is ocean floor methane seep. These Methane Derived Authigenic Carbonates (MDAC’s) occur as ocean floor structures or as subsurface features that can be identified in seismic surveys.

Some clays have also been shown to have an authigenic origin. Clays usually underlie coal seams. In addition some sandstones are now identified as injectites originating at a lower level and some buried evaporite sediments such as salt occur as buoyant plumes in the surrounding sediment which sometimes break the surface forming a salt glacier. Most sediments form in oxygen rich zones but some such as coal appear to have formed in an environment depleted of oxygen. and reduction

Metamorphic. These are originally igneous or sedimentary rocks that have been altered as a result of exposure to high temperatures and pressures (high energy) environment either as a result of burial by subduction, overturning or sediment overburden accumulation. Metamorphism can also occur as a result of coming into contact with intrusions of some kind either as dykes or plutons (a large subterranean magma blob). Other tectonic action uplifts them to near the surface where they outcrop. They are the source of much of the worlds supply of non-ferrous metals. These rocks, when they return from their journey to the underworld, are highly altered and deformed and sometimes enriched with heavy metal sulphides etc. An example of a common metamorphic rock is slate.

4. Physical conditions including,
a. Temperature,
b. Pressure,
c. Structure,
d. Thermodynamic considerations such as
i.     Energy sources,
ii.    How the energy is transmitted, the nature of these flows/transmissions including electromagnetic and acoustic (seismic) activity, mantle flows/plumes, convective or otherwise.
iii.   Its effects, e.g. tectonic activity/earthquakes, deep earthquakes (non-tectonic),  vulcanism, radioactivity, optical properties.
iv.   The phases or states of the components (solid, liquid/gel, gas, plasma or maybe even some high temperature/pressure analogue the fifth state of matter, Bose-Einstein condensates).

Aspects of these categories are referred to where appropriate.

The following is a list of the layers and discontinuities of the BSE from the surface down. They generally follow one another but sometimes they overlap other recognised boundaries. Estimates of the size of the layers vary but the figures provided give a rough idea of their extent and order in which they occur. Layers and boundaries can be distinguished by variations in seismic wave transmission paths and velocities whereas the composition layers are inferred from a study of what is thought to be mantle type rock exposed on the surface, stony meteorite composition, high temperature/pressure studies of various elements, compounds and minerals, and gravitational surveys.

Crust, there are two types, oceanic crust 5-10 km thick which covers two thirds of the earth’s surface and is generally less than 150 million years old. The remaining one third is continental crust which is up to 70 km thick and up to 4.5 billion years old.

Conrad discontinuity, observed in some continental regions at a depth of 15 to 20 km, however it is absent in some continental and in all oceanic regions. Unlike the more well known Mohorovičić discontinuity, it is  somewhat indistinct, and it may be related to the 19th century notion of dividing the crust into an upper felsic sial (silica-aluminium) region containing rocks such as granite, and the lower magnesium-rich mafic sima, for silica-magnesium rocks consisting of andesiticic type rocks. See below for more on the mafic/felsic classification.

Brittle to ductile transition zone. It is the strongest part of the Earth’s crust. For the felsic rocks in continental crust it is located at an approximate depth of 13–18 km (roughly equivalent to temperatures in the range 250-400°C). At this depth rock becomes less likely to fracture and more likely to deform by creep. Many earthquakes occur in this region.

Mohorovičić discontinuity (Moho), it is located about 5-10 km below the ocean floor, and 20-90 km (average 35 km) beneath continental crusts.  It is characterized by an increase in primary seismic wave (P-wave) velocity from about 7 km/sec to about 8 km/sec. It is suggested that it marks a change in rock type from basalt to peridotite, however it is a very regular interface and other observations suggest that such a variation in rock type would have a more uneven boundary. The Moho is characterized by a transition zone of up to 500 m thick.  The P-wave velocity range for basalt is 6.7–7.2 km/s and for peridotite or dunite it is 7.6–8.6 km/s and as they do not overlap this makes for a sharp boundary.

Lithosphere, defined to be the rigid outer layer, it  includes both the crust and the upper portion of the mantle, and is about 150 km thick. This identity, like the asthenosphere mentioned below, is a mechanical classification. The lithosphere is the province of tectonic plate activity where the layer of ocean basin rocks, down to the base of the lithosphere, are thought to be continually recycled via a process which is a combination of subducting old cold ocean crust and its replacement with new material introduced via mid ocean ridge spreading. The energy source for this process is not well understood.

Lehmann discontinuity, like the Conrad discontinuity, a somewhat ephemeral feature that manifests as an increase in P and S wave velocities at 220±30 km. It is present below continental crust, but is much rarer beneath oceanic crust, and is not readily apparent in globally averaged studies.

Upper mantle, the first 400 km of the mantle. It is composed of the magnesium-iron silicate variants, olivine, pyroxenes and garnet. It plays host to magma formations and is believed to be penetrated by more dense sinking cold oceanic crust/top of the upper mantle combination (lithosphere). The regions where this occurs are located at plate boundaries and are called Wadati-Benioff or subduction zones. This process introduces water to the inner region and so enables the creation of shallow magma bodies that can make their way to the surface and manifest as volcanic activity. Volcanic activity that is not related to subduction zones, for example in the Hawaiian islands, produces only  the mafic basalt and has a much smaller amount of volatiles. These intra-plate volcanos are thought by some to be the result of plumes of positively buoyant mantle material ascending from deeper in the mantle.

Middle mantle or Transition layer,  about 250 km thick, it extends down to about 650 km. It is thought to be composed of yet more magnesium-iron silicate variants, spinel and majorite. The lower boundary roughly coincides with the limit of deep earthquake activity. The boundaries between the layers is usually thought to be uneven, rising here and falling there as conditions dictate. The deep earthquakes may be related to sudden alterations in the location or condition of the boundary layer.

Asthenosphere, this is the other mechanical layer classification, it extends from the lower boundary of the lithosphere down to the lower boundary of the middle mantle which, as stated above, is at a depth of about 650 km. It thought to be a region composed  of very viscous, mechanically weak, ductile mafic material. And its lower boundary roughly coincides with the depth limit of deep earthquakes.

Lower mantle,  extends from about about 670 km to 2,700 km. Thought to be composed of yet another magnesium-iron silicate variant, silicate perovskite (Mg,Fe)SiO3 (also known as bridgmanite) combined with calcium silicate (CaSiO3) and arranged in a perovskite structure. Together with ferropericlase, which is the oxide of the ubiquitous magnesium and iron ((Mg,Fe)O), these minerals are thought to be the stable mineral phases of this region.

Spin transition zone. … Changes in the spin state of electrons in iron in mantle minerals have been studied experimentally in ferropericlase. … Results indicate that the change from a high to low spin state in iron occurs with increasing depth over a range from 1000 km to 2200 km.The change in spin state is expected to be associated with a higher than expected increase in density with depth. …

‘D’ layer, the last 150-200 km of mantle, between 2700 and 2900 km with a base temp of 2200 C, thought to be composed of a another in the perovskite sequence, post-perovskite and ferropericlase which is the stable phase at the temperatures and pressures encounted at those depths.

Gutenberg discontinuity delineates the core mantle boundary

Note: The seismically reflective inner core boundary, known as the Bullen discontinuity, is also sometimes referred to as the Lehmann discontinuity as it was first observed by Lehmann. Confusing, no? 🙂

This diagram, courtesy of Britannica, shows how the mechanical classifications overlap the compositional layers.

bse

Reiterating a couple of points mentioned above, the bulk silicate earth includes both the mantle and the crust. It extends from the surface to the core/mantle boundary. And the seismically identified boundary region between the mantle and the core is found at about 2600 km below the surface and is known as the ‘D’ layer.

In contrast to the mainly iron core, the BSE consists mostly of oxygen, silicon and magnesium, combined with lesser amounts of iron, aluminium and calcium along with even smaller amounts of sodium and potassium, and more controversially, with perhaps significant amounts of hydrogen, carbon or sulfur (see below for a more precise accounting of proportions).

Unlike the core, where the elements exist in their reduced state, the metallic BSE elements are generally combined with lighter elements (oxygen, sulfur, hydrogen) in what is termed an oxidized state where the metal atoms are depleted of one or more electrons. These depleted electrons are shared or appropriated to some degree by the other atoms in the molecule (a combination of atoms) and serve as part of the binding energy that holds the molecules together. These combinations of elements are known as chemical compounds or minerals. For example most of the mantle consists of some form of magnesium or ferro-magnesium silicate (together with calcium silicate) which, as the name suggests, is a combination of the three principal mantle elements, silicon, oxygen and magnesium.

Combinations of these compounds/minerals are the main components of rocks (they can also contain smaller amounts of mineral liquids and gases). Although the BSE contains a significant amount of metallic elements, they are generally combined with silicon and oxygen to form rock minerals and rock minerals are the components of rocks. Hence we deduce that, in contrast to the metallic core, the BSE is rocky.

One of the pieces of data that assist in our understanding of the earth’s interior is the estimate of the earth’s bulk density. The bulk density of the earth is calculated to be about 5.5 gm/cm3 (in comparison water is 1gm/cm3) but the density of the crust and exposed mantle rock is usually less than 3 gm/cm3, so a dense core is required to reach the overall density of 5.5 gm/cm3 , hence the iron core model. In addition, mantle seismic data reveals numerous discontinuities where the wave velocity steps up or down suggesting a change in density or composition at those levels.

The composition of stony meteorites is also taken into account when modelling mantle composition. At normal temperature and pressure, rocks display a very wide range of densities. They can vary from about 0.3 gm/cm3 for calcium silicate to 3.4 gm/cm3 for gabbro. The first of these, calcium silicate (along with magnesium-iron silicate) features prominently in the lower mantle while gabbro is thought to constitute up to 90% of the ocean crust.

The most significant elemental difference between the rocks/minerals of the crust and those of the mantle is their magnesium content. The chart below shows the composition of the mass of the bulk silicate earth (BSE) and the crust. The crust is about 1% of the BSE, so the BSE estimate can be taken as an analogue for mantle composition. The most abundant elements are oxygen and silicon.

bse1

Note how oxygen is main component of both the crust and the mantle, it accounts for nearly half of the whole BSE by mass, and a surprising 94% of it by volume. Note also the large variation in magnesium (the red wedge is magnesium) between the mantle and the crust. There is a lot in the mantle, 22.8% and much less in the crust, about 2%. Note also that magnesium and silicon are very close in the periodic table of elements (separated only by aluminium) with atomic numbers of 12 and 14 respectively. The role of magnesium in mantle rocks is reflected in the names given to that class of rocks, the terms are mafic and ultramafic (see discussion below). 

As well as magnesium, the other minor elements of the crust are, aluminum – 8.1%, iron – 5.0%, calcium – 3.6%, sodium – 2.8% and potassiun – 2.6%

One can roughly divide igneous rocks (rocks that have a magmatic origin) into two types, 1. Felsic which combines the words feldspar and silica or
2. Mafic which combines the words magnesium and ferric). Mafics also contain silicate but in smaller proportion to the felsic rocks.

A similar classification divides rocks according to their silica content. Those with high silicate content are labelled acid while low silicate rocks are labelled basic.  The in-between ones are referred to as intermediate rocks.

The igneous continental crust rocks are generally felsic while the ocean crust and the mantle rocks are generally mafic. But mafic rocks are also present in the continental crust as intrusions or ocean crust fragments or volcanics or even as exposed metamorphics.

The magnesium and iron rich mafic minerals that are encountered on or near the surface are usually dark coloured, with densities often greater than 3 gm/cc. Some common types include olivine, pyroxene, amphibole, and biotite.

Another mafic mineral, serpentine, is found as an alteration product (it is a hydrated form of the original mineral) of other mafic minerals like pyroxene and olivine. As such it occurs as a pseudomorph (meaning it retains the external shape of the original mineral) of the crystals it replaces while the actual crystal structure of serpentine itself is indistinct as it is composed of a fine admixture of about twenty different varieties.

Serpentine is one of an important group of minerals called phyllosilicates that also includes the metamorphics mica, chlorite, talc, as well as the clay minerals. Phyllosilicates can be either mafic or felsic, their distinguishing feature being a silicon oxide ring structure, a bit like carbon’ benzine rings. Clay is one of the major constituents of sedimentary rocks and much of it has an authigenic origin.

Serpentine also features of in a group of metamorphosed ultramafic (extra high magnesium), high melting point) extrusives (lavas) and sub-volcanics called komatites. They are thought to originate deep within the mantle and somehow manage to become enriched with heavy metals as the buoyant magma ascends to the surface (how they maintain buoyancy as the become enriched with metal is something of a mystery). They are restricted to the older shield regions of the crust and are generally adjacent to granite pluton material and show typical lava characteristics such as pillow structure and evidence of surface cooling.

All komatite formations have experienced some degree of metamorphism so strictly speaking they are metakomatites. Serpentinization (hydration) and carbonation are defining features of all komatites. The alteration is a function of the carbon dioxide and water content of the fluids encountered, producing respectively carbonates or serpentine. Komatites are very rich in magnesium and depleted in silicon, and as such the liquid phase is very mobile having the consistency of water. They are often differentiated to the extent that the base of the flows sometimes contain channels that contain a layer of metal sulphides overlain by enriched lava. The altered lava generally consists of serpentine pseudomorphs of olivine or other magnesium rich silicate minerals and/or magnesium carbonate. In some places the mineralization levels can be exploited/mined for the production of the metals such as nickel and uranium.

Kimberlites (CO2-rich ultramafic potassic mantle-derived volcanic and subvolcanic rocks, often brecciated (fragmented) dominated by olivine/serpentine and carbonate minerals), and lamproites  (ultrapotassic mantle-derived volcanic and subvolcanic rocks with high potassium and high magnesium content) are another manifestation of komatite rocks originating in the mantle. They occur as pipes and dykes and some of them contain diamonds.

While on the subject of serpentine there are a couple of other points worth noting. First, it has an odd crystal habit where the crystal structure has either a fibrous structure (somewhat polymer like) or a fine aggregate or admixture of closely related crystal structures. The fibrous structure consists of a series of curved plates of serpentine crystal matrix joined together in one-up one-down fashion so as to form a corrugated or crimped fibre. The plates are curved due to different crystal structures on either side of the plates. Serpentine itself is actually a combination of about 20 different varieties or polymorphs with the three most important being antigorite, chrysotile and lizardite. Antigorite is especially important as it is about 13% by mass water.

The metamorphics in general are inhospitable to plant life due to a lack of phosporus (P) and potassium (K) and no doubt the toxic metals, e.g. nickel (Ni) , chromium (Cr), cobalt (Co) they sometimes contain also take their toll on plant life. It is also somewhat ironic that a polymorph (same chemical composition, magnesium silicate, but different crystal structure) of what is thought to be the most abundant mineral in the mantle, the metamorphic mineral asbestos, is now shunned in the west as a dangerous carcinogen. Man made magnesium silicate is amorphous (meaning it has no crystal structure and is porous giving it a very high surface area to mass ratio, i.e it is very absorbent. It is considered to be biologically inert which is a good thing as it is ubiquitous as an anti-caking agent in salt.

Common mafic rocks include basalt, dolerite and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar. Mafic lava  has a low viscosity due to the lower silica content in mafic magma whereas felsic lava, with a higher silicate content has a higher viscosity. At the surface this difference manfests as extensive sheets of mafic rocks like basalt.

Another mafic mineral, silicate perovskite, is thought to be the main constituent of the lower mantle, possibly up to 93% by volume.

Felsic minerals are silicates of the lighter elements such as aluminium, sodium, and potassium. Common felsic minerals include quartz,  muscovite commonly known as mica, potassium-rich orthoclase feldspar, and the sodium-rich plagioclase feldspars. In some cases, felsic volcanic rocks may contain phenocrysts (inclusions) of mafic minerals. They are usually light in color and have densities less than 3 gm/cm3, generally about 2.6 gm/cm3.

Although density is mostly related to the atomic mass of the atoms themselves, the density of a body is also affected by its electron structure. For instance, it is known that increased density is associated with the state of the spin structure of the electrons, in iron at least.

The most familiar felsic rock is granite while another felsic rock, andesite is thought to form the majority of the base of the continental crust.

As mentioned above, the crust consists of two main types of rock, continental crust and ocean crust. they differ markedly in age and composition and thickness, chalk and cheese really. The majority of ocean crust is thought to be less than 150 million years old while we measure the ages of crustal rocks in the hundreds of millions to billions of years old. It is thought that the older ocean crust has been replaced by more recent magma upwellings. The older crust, by dint of its slightly higher density and lower temperature is thought to sink into the depths of the mantle with some being reduced to magma and eventually be recycled to the surface via vulcanism and other types of intrusion.

There is now general agreement that the surface (down to an uncertain depth but including both the crust and an upper portion of the mantle) rearranges itself over time. Crustal displacement or tectonics is thought to be powered by convective heat flows in the mantle with the necessary energy being transferred from the presumed energy source, the lower mantle. However as mentioned above the energy budget for the process is not well understood. And again as mentioned above, the crust is a but very small proportion of the overall bulk silicate earth. As one observer puts it,

“This narrow stratum as compared with the diameter of the Earth is but one half the thickness of one leaf of a thousand-page book.”

With the average crust thickness being measured in tens of kilometers and the mantle in thousands, a picture emerges of a thin skin covering a generally more pliable mantle, and compared to the interior this skin or crust is highly depleted in magnesium, …but enriched in silicon and with extra aluminium, sodium and potassium.

The rocks of the crust can be roughly divided into the the silicate rich, felsic continental crust which has a diverse array of rock types, and the more homogeneous mafic ocean crust, 90% of which is composed of the mafic rock gabbro. The age of the oldest regions of the continental crust is measured in billions of years while the ocean crust is generally less than 150 million years old.

The following chart shows how the elemental composition changes as one transitions from mantle rocks to ocean and continental mantle rock derivatives. Note that continental crust rock is enriched in silica and to a lesser extent aluminium but depleted in magnesium. The chart below shows how the silicate content increases and the magnesium content decreases as one progresses from the mantle to the crust. The proportions of oxides of metals generally reflects the proportions of the metallic elements.

mantlecrust1

The next diagram shows how rocks vary in mineral content and density. From left to right, the silicate content decreases while the density increases.

Paul R Pinet. Oceanography, an introduction to Planet Oceanus. 1992

There is evidence for the presence of water rich minerals in the upper mantle. It comes, somewhat ironically, from meteorite material which is rich in magnesium silicate. As already mentioned (but it bears repeating 🙂 ), the mantle is thought to be rich in magnesium whereas the crust and the core are depleted of magnesium, so the magnesium rich minerals in the meteorite are taken to be analogues of some of the mantle minerals. The water and other volatiles are thought to be introduced to the upper mantle via transport from the surface by subducted ocean crust. In fact all of the hydrogen and carbon in the mantle are thought to have their origin on the surface.

This idea, that subducted or infiltrated surface water is the sole source of inferred mantle water, is being challenged. It has recently been suggested that water may be present in large volumes, in the upper mantle at least, and computer modeling suggests that some of the water may be authigenic in that it is formed in situ from a reaction that reduces silicate (an oxide of the metal silicon) thereby releasing oxygen which then combines with free hydrogen or reacts with a metal hydride to produce water. There is not much discussion about a possible source of hydrogen in the mantle. Mantle water and mantle chemistry in general is currently a very active field of research.

The lower mantle is thought to be mainly composed of various analogues of magnesium silicate that can exist in the extreme temperatures and pressures of the region.

The following quote gives a description of how the various stables phases of magnesium silicate analogues progress from one phase to the next.

…At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about 410 km, olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite and, at about 520 km depth, wadsleyite transforms exothermically into ringwoodite, which has the spinel structure. At a depth of about 660 km, ringwoodite decomposes into silicate perovskite ((Mg,Fe)SiO3) and ferropericlase ((Mg,Fe)O) in an endothermic reaction. These phase transitions lead to a discontinuous increase in the density of the Earth’s mantle that can be observed by seismic methods. …

Here are the chemical formula of some typical mafic mantle minerals.

Olivine, an upper mantle mineral, – a solid solution of Mg2SiO4 <-> Fe2SiO4. in variable ratios.
Serpentine, a derivative of an upper mantle mineral,  – Mg3Si2O5(OH)4
Perovskite, a lower mantle mineral which, when combined calcium silicate may constitute 93% of this region – (Mg,Fe)SiO3.

Due to its unusual properties perovskite is presently a very hot research topic. Another quote from Wikipedia explains the reasons for the attention:

… Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view. Colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermopower and the interplay of structural, magnetic and transport properties are commonly observed features in this family. These compounds are used as sensors and catalyst electrodes in certain types of fuel cells and are candidates for memory devices and spintronics applications.

Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures, often with 3 or more metals including copper, and some oxygen positions left vacant. One prime example is yttrium barium copper oxide which can be insulating or superconducting depending on the oxygen content. …

It is apparent that doped perovskite-like compounds have all the makings of the being next big thing in semi-conductor technology.

The next diagram shows the mantle from a mineralogical perspective.

 

minerals

Energy considerations have hardly been mentioned, but this is already too long so apart from this quote about energy matters in outer space, discussion here on the subject will have to wait.

… Here we demonstrate that escape of positrons released in the decay of the short-lived radionuclide 26-Al leads to a large-scale charging of dense pebble structures, resulting in neutralisation by lightning discharge and flash-heating of dust and pebbles …
-Harvesting the decay energy of 26-Al to drive lightning discharge and chondrule formation. A. Johansen and S. Okuzumi Lund Observatory, Lund University

After saying that here is a bit of speculation about possible mass/energy processes that could occur in the underworld. 🙂

The prospect of a large proportion of mantle being composed of magnesium and calcium silicates and oxides doesn’t immediately bring accretion to mind, well not in my opinion at least, but rather some bulk production process. We presume phase transitions takes place from the outer to the inner layer as per the gravitational accretion model.

Accretion requires that we think of the mantle processes as changes from the top down but a bulk authigenic process is more suggestive of a radial or centre out/bottom up process. In this view the crust becomes the first and oldest growth/inflation phase. A centre-out formation has a progression from more to less homogeneous material and the exothermic boundary reactions become endothermic and vice versa.

The centre-out view creates the prospect of some bulk conversion process where the oxygen, silicon and magnesium originate from the iron that is presumed to be in the core. Iron (26) can yield silicon (14) and magnesium (12) or three oxygen’s (8) with room to spare. Transformations may be sufficient to account for the significant volumetric increase/density decrease required.  

Note too that magnesium (12), aluminium (13) and silicon (14) follow one another in proton number. So one can imagine another possibility whereby silicon releases one or two protons (as hydrogens (1) or helium (2) ) and transmutes into magnesium (12) or aluminium (13).  And considering that oxygen is 93% by volume it is hard to avoid speculation about a variation in this ratio in an inflation process. Of course the possibility remains that such a process developed after accretion reached a critical mass that initiates the process.

Finally the large variation in magnesium content between the crust and the mantle might provide an explanation as to why, if expansion has actually occurred, the mantle inflated while the crust cracked and split. Perhaps whatever it was that caused the inflation had something to do with the presence of, or production of, magnesium. and the cracking of the crust is due to no production or presence.  This hypothetical process may initiate at different regions of the mantle at different times. We also have to consider that conditions may change over time, and we can’t altogether discount the existence of voids at some scale.

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 about 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 itself 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. 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.