Earth's Interior
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

Layers or spheres and boundaries from top to bottom: Crust - two sorts: continental crust and oceanic crust Mohorovicic discontinuity (Moho for short) is the boundary between the crust and the mantle Mantle - The mantle forms three quarters of the volume of the Earth and two thirds of its weight. It can be divided into four spheres: lithosphere (cool and rigid) asthenosphere (hot, partially melted) 150 km thick on average mesosphere (transition region or middle mantle, but sometimes used for the rest of the mantle = deep mantle)) lower mantle D" layer (pronounced "dee double prime").This may be the most dynamic and active zone, although it is very thin and the thickness is extremely variable. Core - The core can be divided into a liquid outer core and a solid inner core

Approximite figures:

Core-Mantle Boundary

Most of our earlier knowledge of the Earth's interior came from studying seismic waves (after earthquakes). It became clear that the boundary between the core and the mantle was something special. S-waves (secondary wave or shear wave) stopped. They cannot propagate through liquids. Seismic velocity is linked to the density of the medium through which the waves travel. P-wave (primary wave) velocity in general increases with increasing density - In liquids, however, the speed will be less. The core is much denser than the mantle.

When it was discovered that a thin layer directly above the core-mantle boundary had some mysterious properties it had to get a name consistent with the naming of the earth’s layers in the middle of last century. It is now still referred to as the D" ("D double-prime" or "D prime prime"). The D" name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his "D" layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D’ (D prime) and the lower part (the bottom 200 km) was named D" (pronounced "dee double prime"). A layer that produces strange seismic properties.

http://my.opera.com/nielsol/blog/2008/11/13/core-mantle-boundary

Crust

Oceanic crust:

is about 6-11 km thick. The material of which the oceanic crust consists is for the greater part tholeiitic basalt (this is basalt without olivine). Basalt has a dark, fine and gritty volcanic structure. It is formed out of very liquid lava, which cools off quickly. The grains are so small that they are only visible under a microscope. The average density of the oceanic crust is 3g/cm³. The oceanic crust is constructed of melt from the mantle which is intruded into pre-existing crust and erupted onto the seafloor.

Most of what we know today about oceanic crust, we know from ophiolites. When the term was cornered in 1813, it was used about a series of rocks in a certain assemblage. With the knowledge gained about plate tectonics in the late 1950's to early 1960's it was recognised that this assemblage represented oceanic crust created through the process of seafloor spreading. In 1972 the term ophiolite was redefined to include only the sequence that I try to describe here.

At a mid ocean ridge fluid rises from the upper mantle (of peridotite). The rising fluid is rich in magnesium and iron. It is said to be mafic. Mafic is a combination of "magnesium" and "ferrum", the Latin word for iron [ma(gnesium) + f(errum) + ic]. Mafic rocks are always dark because of the high content of magnesium and iron. In a magma chamber below the sea floor peridotite layers settle out at the bottom of the magma chamber, but most of the fluid magma crystallises progressively to form gabbro, During the crystallisation the gabbro becomes layered. Higher up gabbro forms (as gabbro dykes or sheets) in the vertical fissures leading to the sea bed where the fluid finally erupts as basalt lava. When lava cools in the sea it forms pillow like structures known as pillow lava. Later the new formed ocean crust will be covered by sediments.

Another approach to studying the oceanic crust is seismology or seismic exploration.

In part because of their different perspectives and techniques, geologists (studying ophiolites) and geophysicists/seismologists (studying seismograms) have been at odds over the basic definitions of oceanic crustal structure.

Geologists typically describe an upper layer of basaltic lavas, a middle layer of basaltic intrusive rock units, known as dykes, and a lower layer of gabbroic rocks. The top layer of lavas formed when magma, or molten rock, erupted onto the seafloor.

The middle layer of dykes were created as molten rock from the underlying magma chamber intruded into incrementally opening cracks at a spreading center, with younger dykes cross-cutting older dykes, eventually creating a massive collection of rock units referred to as a sheeted dyke complex. The incremental in-filling of cracks above a magma chamber is the essence of seafloor spreading.

Geophysicists divide oceanic crust (beneath any sedimentary material) into two basic layers, layer 2 and layer 3. Layer 2 is typically subdivided further into layers 2A and 2B. Layer 2A is a commonly imaged horizon in the seismic data, known as the 2A reflector, which numerous studies have mapped over extensive regions of young oceanic crust.

Trying to reconcile the two models of the structure of oceanic crust is not as easy as it may seem. Many researchers interpret seismic reflector 2A as the geologic boundary between the upper layer of lavas and the underlying sheeted dykes. A study published in the journal Nature of 25 January 2007 shows, however, that you cannot reliably use seismic methods to map the boundary between lavas and dykes in young oceanic crust.

The Mohorovicic discontinuity

The Mohorovicic discontinuity, usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. The Moho serves to separate both oceanic crust and continental crust from underlying mantle. The Moho mostly lies entirely within the lithosphere; only beneath mid-ocean ridges does the Moho also define the lithosphere-asthenosphere boundary. The boundary between oceanic crust and the mantle may cause some confusion. For a petrologist (i.e. one who studies the composition of rocks) the layered peridotite forms the base of the crust. This indeed leaves us with two different moho's. A geophysical or seismic moho based on the original definition, namely a seismic discontinuity situated between the gabbro and the peridotite having different seismic velocities (the mantle is then called: "layer 4" by the seismologists). And a petrological moho between (peridotite as) primary mantle rock and layered peridotite precipitated from magma (ultimately derived from the upper mantle by partial melting of mantle peridotites). You may also see the layered peridotite described as ultramafic cumulates - Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating, the layered gabbros are also cumulates.

Lithosphere

Earth's outermost shell is the strong, solid lithosphere, composed of the crust and the top of the mantle. The division of Earth's outer layers into lithosphere and asthenosphere should not be confused with the chemical subdivision of the outer Earth into mantle, and crust. All crust is in the lithosphere, but lithosphere generally contains more mantle than crust. The lithosphere is defined by physical properties, which can be measured by the speed of earthquake waves. The lithosphere is the stiff part of the earth. Moving tectonic plates are made up of lithosphere (floating on the asthenosphere).

Asthenosphere

The word asthenosphere is derived from an invented Greek ασθενος "a" + ''sthenos "without strength".

Plate tectonics is based on the concept of rigid lithosphere plates sliding on a mechanically weak asthenosphere.
The asthenosphere is the region of the Earth between 100-200 km below the surface — but perhaps extending as deep as 400 km — that is the weak or "soft" zone in the upper mantle. It lies just below the lithosphere, which is involved in plate movements. The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. The asthenosphere is often assumed to roughly coincide with the “low-velocity zone”, a layer of reduced seismic velocities and increased attenuation of seismic waves.

It has often been assumed that the weakness of the asthenosphere is related to the presence of small amounts of hydrous (water containing) melts. However, the mechanism that may cause melting in the asthenosphere is not well understood. A study published in Science show that the asthenosphere coincides with a zone where the water solubility in mantle minerals has a pronounced minimum. The minimum is due to a sharp decrease of water solubility in aluminous orthopyroxene with depth, whereas the water solubility in olivine continuously increases with pressure. (Partial) melting in the asthenosphere may therefore be related not to volatile enrichment but to a minimum in water solubility, which causes excess water to form a hydrous silicate melt.

The high water solubilities in aluminous orthopyroxene at low pressure and temperature will “dry out” olivine, and this may contribute to a stiffening of the lithosphere.

The findings imply that the existence of an asthenosphere—and therefore of plate tectonics as we know it—is possible only in a planet with a water-bearing mantle.

Science 19 January 2007: Vol. 315. no. 5810, pp. 364 - 368
Abstract at http://www.sciencemag.org/cgi/content/abstract/315/5810/364

Earth's Interior treated in Olelog

Crust and Mantle

Posted on Monday, 26 December 2005, 16:30:39

Earth's Interior

Posted on Friday, 16 December 2005, 10:35:20

With the deep-sea drilling vessel Chikyu Japan hopes to dig deeper into the Earth's surface than ever before. Deep drilling has been done before, but so far only into continental or oceanic crust – the skin of the Earth so to say. Now the scientists want to dig into the mantle, something so far generally thought impossible, not only because of the depth but certainly because of the immense heat down there. The research vessel is equipped with a 121-meter drill tower that can dig 7,000 meters below the seabed, nearly three times as deep as its predecessors. Oceanic drilling is preferred over land drilling because oceanic crust is thinner and allows for deeper digs into the crust and mantle. The results won't be ready tomorrow. The first voyage is scheduled for 2007 and I suppose that the digging will go on for many years.

See spacedaily at http://www.spacedaily.com/news/tectonics-05zzzzzh.html

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