Lithosphere and Tectosphere
Saturday, 28. February 2009, 09:47:12
Lithosphere is a new journal from GSA. Its focus is the tectonic processes that affect Earth's crust and upper mantle. One of the papers in the first issue (of February 2009) is “A fluid factory in solid Earth”. The text contains terms like “slab graveyards”, “superplumes” and “hot spots” - at a time when many geologists still wonder whether these things really exist.
But first a few words about the tectosphere. I find the tectosphere a useful term, if everybody could just agree on what they understand by tectosphere. The term has been in use for at least 20 years. It was a.o. used by Jordan in 1978. Here are a few, different, definitions: The tectosphere is the portions of Earth's mantle lying below cratons. The tectosphere is the region of the earth's crust occupied by the tectonic plates. It is the continental roots. It is the continental keels. The tectosphere is the part of the crust and uppermost mantle, that moves laterally as a plate. Oceanic tectosphere is identical with Lithosphere, but Continental tectosphere is considerably thicker, 400 km, or even more. Apart from that some people may confuse it with the tectonosphere (The tectonosphere seems to include all outer layers of the Earth down to the level where there is a hydrostatic gravitational equilibrium and is about 120 km thick - but that again, of course, is arguable).
Anyway here is how the authors of the mentioned paper understand the term:
“The tectosphere, also referred to as continental “keels,” is considered to be essentially rigid and cold, representing a chemically distinct raft supporting the bulk of the continental crust (Jordan, 1988). The tectosphere appears to be confined to continental cratonic regions, formed before 2.0 Ga, the origins of which are controversial.“
A cartoon (fig 3) illustrates how the Earth’s interior is seen by some geologists - I would like to stress that some of the elements in the cartoon still are more or less controversial, while others are covered by a greater consensus.
MOR is an abbreviation for mid-ocean ridge, and MORB for mid-ocean-ridge basalt. The green line to the far right shows how an ocean plate is subducted along an oblique Benioff zone down to about 660 km, this zone can be detected by earthquakes. The deepest earthquakes practically never occur lower than 700 km. The lower mantle may be a “plate graveyard” (in the recycled MORB area in the cartoon). Although earthquakes cease below 660 km, plate descent may continue through the middle mantle (there is still some disagreement on this point). I have mentioned the D“ layer in several posts (a.o. here).
If we assume that the subducted slab material reach the core-mantle boundary, horizontal movements must take place at the base of the mantle, and these movements have been referred to as “anti–plate tectonics”. They are in many respects analogous to lithospheric plate tectonic processes operating in near-surface regions.
From here vertically rising “superplumes” enters into the upper mantle, transforms to horizontal, and branches out into several “hot spots”. These hot spots cause the rifting of the continent and deliver the mantle fluid to the surface.
I recommend the paper. The full text (pdf) is free!
Reference:
M. Santosh, S. Maruyama and S. Omori
A fluid factory in solid Earth
Lithosphere 2009;1;29-33
doi:10.1130/L2.1
• http://lithosphere.gsapubs.org/cgi/content/abstract/1/1/29
• http://lithosphere.gsapubs.org/current.dtl
But first a few words about the tectosphere. I find the tectosphere a useful term, if everybody could just agree on what they understand by tectosphere. The term has been in use for at least 20 years. It was a.o. used by Jordan in 1978. Here are a few, different, definitions: The tectosphere is the portions of Earth's mantle lying below cratons. The tectosphere is the region of the earth's crust occupied by the tectonic plates. It is the continental roots. It is the continental keels. The tectosphere is the part of the crust and uppermost mantle, that moves laterally as a plate. Oceanic tectosphere is identical with Lithosphere, but Continental tectosphere is considerably thicker, 400 km, or even more. Apart from that some people may confuse it with the tectonosphere (The tectonosphere seems to include all outer layers of the Earth down to the level where there is a hydrostatic gravitational equilibrium and is about 120 km thick - but that again, of course, is arguable).Anyway here is how the authors of the mentioned paper understand the term:
“The tectosphere, also referred to as continental “keels,” is considered to be essentially rigid and cold, representing a chemically distinct raft supporting the bulk of the continental crust (Jordan, 1988). The tectosphere appears to be confined to continental cratonic regions, formed before 2.0 Ga, the origins of which are controversial.“
A cartoon (fig 3) illustrates how the Earth’s interior is seen by some geologists - I would like to stress that some of the elements in the cartoon still are more or less controversial, while others are covered by a greater consensus.
MOR is an abbreviation for mid-ocean ridge, and MORB for mid-ocean-ridge basalt. The green line to the far right shows how an ocean plate is subducted along an oblique Benioff zone down to about 660 km, this zone can be detected by earthquakes. The deepest earthquakes practically never occur lower than 700 km. The lower mantle may be a “plate graveyard” (in the recycled MORB area in the cartoon). Although earthquakes cease below 660 km, plate descent may continue through the middle mantle (there is still some disagreement on this point). I have mentioned the D“ layer in several posts (a.o. here).
If we assume that the subducted slab material reach the core-mantle boundary, horizontal movements must take place at the base of the mantle, and these movements have been referred to as “anti–plate tectonics”. They are in many respects analogous to lithospheric plate tectonic processes operating in near-surface regions.
From here vertically rising “superplumes” enters into the upper mantle, transforms to horizontal, and branches out into several “hot spots”. These hot spots cause the rifting of the continent and deliver the mantle fluid to the surface.
I recommend the paper. The full text (pdf) is free!
Reference:
M. Santosh, S. Maruyama and S. Omori
A fluid factory in solid Earth
Lithosphere 2009;1;29-33
doi:10.1130/L2.1
• http://lithosphere.gsapubs.org/cgi/content/abstract/1/1/29
• http://lithosphere.gsapubs.org/current.dtl
WIDGETIZE
dortejakobsen # 28. February 2009, 17:59
nielsol # 28. February 2009, 19:38
Der har for øvrigt været et nyt jordskælv i SVERIGE for nogle timer siden - lige norden for Vättern.
http://www.emsc-csem.org/index.php?page=current&sub=detail&id=120242
dortejakobsen # 1. March 2009, 20:25
Og jordskælv, de er bare ikke relevante i Danmark i dag, for regeringen har lige indgået et skatteforlig.
nielsol # 1. March 2009, 20:38
Heldigvis er der en del krimier der indeholder lidt om geologi
dortejakobsen # 2. March 2009, 20:18