highest peaks on earth?
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"hot spot can be formed from a straight plume from the outer core or from curculating convection currents in the mantle or through any number of means of plate tectonic actions that generate pressure and heat."doodah said:This is quite a broad definition of the term "hot spot", which is o.k., but is certainly not the conventional definition used by earth scientists.
Congratulations, you've finally broadened your definition of "magma" to include the crust as a source, not just the mantle.
Ophiolite and Underwhelmed are right- the Earth has three distinct layers- the core, the mantle and the crust. The crust is not part of the mantle. Magma can be generated by partial melting of the crust without involving molten mantle material- and in fact, this is common in areas of mantle convection subsidence (subduction zones).
I have to admit, though, that Ophiolite's challenge was tempting...(just for the fun of it)
Yes, unfortuanately many geologists use the word hot spot in anyone of these ways. That is why I have consistently stating on all threads that the word "hot spot" is such an ambiguous term. It should refer to only a geographic location with an underlying magma chamber. At least that is my personal opinion.
I have not broadened my definition of magma. Apparently what I have done is taught you how the word mantle is used differently. In tectonics the mantle includes the crust, while in structural and compositional geology it does not. We were talking about plate tectonics; therefore, I was using the tectonic definition of mantle, which refers to everything about the outer core.
In tectonics the upper layer of the mantle is the lithosphere:
"On the Earth, the lithosphere includes the crust and the uppermost layer of the mantle (the upper mantle or lower lithosphere) which is joined to the crust. The lithosphere is broken up into different plates as shown by the picture." http://en.wikipedia.org/wiki/Lithosphere
Below the lithosphere is the aesthenosphere.
I have never stated nor denied that magma could not consist of the Earth's crust?
You're a bit confused- the lithospere contains both the crust and the uppermost layer of the mantle- the lithosphere is not part of the mantle. Even your Wikipedia source (and your last post) states: "On the Earth, the lithosphere includes the crust and the uppermost layer of the mantle..." This same Wikipedia article further states:valich said:
"The crust is distinguished from the mantle, and hence the upper mantle, by the change in chemical composition that takes place at the Moho discontinuity." http://en.wikipedia.org/wiki/Lithosphere
What you stated was "Magma is molten mantle." I finally understand this statement and your confusion, since you are adamant (and adamantly wrong) that the crust is part of the mantle.
Again- your definition of "hot spot" is much broader than the conventional definition used by virtually all earth scientists.
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This is the Lithosphere:
"The crust is thin, varying from a few tens of kilometers thick beneath the continents to to less than 10 km thick beneath the many of the oceans. The crust and upper mantle together constitute the lithosphere, which is typically 50-100 km thick and is broken into large plates (not illustrated). These plates sit on the aesthenosphere. The aesthenosphere is kept plastic (deformable)...Very slow convection currents flow in this plastic layer, and these currents provide horizontal forces on the plates of the lithosphere." http://csep10.phys.utk.edu/astr161/lect/earth/tectonics.html
"New crust is created by magma pushing up from the mantle. The magma [rises up from the mantle] and squeezes through the widening cracks, sometimes to erupt and form volcanoes. The rising magma, whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the rift zone."
http://pubs.usgs.gov/publications/text/understanding.html
Magma comes from the mantle not from the cooler crust above. That's why in tectonics we expand the definition to call it the lithosphere. The crust can include mountains 10 km high, yet is less than 10 km thick on the ocean floor. The movement of crust is not plate tectonics, so unless you use the lithosphere as a definition of plate tectonic magma, then how else can you define it's origin.
http://pubs.usgs.gov/publications/text/understanding.html
"The crust is thin, varying from a few tens of kilometers thick beneath the continents to to less than 10 km thick beneath the many of the oceans. The crust and upper mantle together constitute the lithosphere, which is typically 50-100 km thick and is broken into large plates (not illustrated). These plates sit on the aesthenosphere. The aesthenosphere is kept plastic (deformable)...Very slow convection currents flow in this plastic layer, and these currents provide horizontal forces on the plates of the lithosphere." http://csep10.phys.utk.edu/astr161/lect/earth/tectonics.html
"New crust is created by magma pushing up from the mantle. The magma [rises up from the mantle] and squeezes through the widening cracks, sometimes to erupt and form volcanoes. The rising magma, whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the rift zone."
http://pubs.usgs.gov/publications/text/understanding.html
Magma comes from the mantle not from the cooler crust above. That's why in tectonics we expand the definition to call it the lithosphere. The crust can include mountains 10 km high, yet is less than 10 km thick on the ocean floor. The movement of crust is not plate tectonics, so unless you use the lithosphere as a definition of plate tectonic magma, then how else can you define it's origin.
http://pubs.usgs.gov/publications/text/understanding.html
Let me word it another way, exactly the same as above but together"
"I have never stated nor denied that magma could not sonsist of the Earth's crust, BUT magma comes from the mantle."
Again, I ask you, what is your definition of magma? Magma originates in plate tectonic activities in the lithosphere, not the thin outer crust. Are you somehow thinking that I contradicted myself here?
"I have never stated nor denied that magma could not sonsist of the Earth's crust, BUT magma comes from the mantle."
Again, I ask you, what is your definition of magma? Magma originates in plate tectonic activities in the lithosphere, not the thin outer crust. Are you somehow thinking that I contradicted myself here?
Ophiolite
Valued Senior Member
Vallich some magma originates in the crust. It does not come from the mantle. It is derived from partial melting of the crustal rocks at the heart of zones of regional metamorphism. This is taught in high school geology. It is a pretty basic and simple concept. I have stated that at least once in this interminable thread. Just accept you are wrong and move on.
UnderWhelmed
Registered Senior Member
valich, it takes a pretty big person to admit when there wrong...be that guy...I've already shown (as well as a number of other people) that you've contradicted yourself, just leave it at that.
as for the highest peaks:
this site has the highest peaks above sea level:
http://www.peakware.com/highest.html?list=8000
Everest at 8,850 m is number 1
as someone else stated the highest peak below sea level (or trench) would be the Mariana Trench at 10,911 m
http://en.wikipedia.org/wiki/Mariana_Trench
as for the highest peaks:
this site has the highest peaks above sea level:
http://www.peakware.com/highest.html?list=8000
Everest at 8,850 m is number 1
as someone else stated the highest peak below sea level (or trench) would be the Mariana Trench at 10,911 m
http://en.wikipedia.org/wiki/Mariana_Trench
No, the highest peak on Earth measuring from the base up is Mauna Lae in Hawaii (10,203 meters, 33,474 ft.). Although I have heard that Mauna Kea may have just surpassed that?
Explain my contradictions. When I studied geology ten years ago, the lithosphere didn't even exist in textbooks. I think we are undergoing a revolutionary redifinition in what we consider as the mantle level since the now-certain acceptance of plate tectonics has redifined the layers of the Earth. Originally, the crust was defined as the layer of of Earth going down to the MOHO (Mohorovičić discontinuity) between 10-80 km. beneath the surface, yet plates may be over 100 km. thick. So now we have redifined the layers to include what we now call the lithosphere in order to account for plate tectonics. With this definition, the crust has been reduced from 50-100 km. to 5-20 km. and has no - or very little - effect on plate tectonics. In today's modern theory, magma is not produced by crust activity: it is produced by plate tectonic activity.
If I have contradicted myself, perhaps it is due to these continuing differing definitions that are constantly being found in the geological scientific literature. So how can I get around this? Without still stating scientific research?
Today the crust is defined as extremely thin compared to what it was ten years ago and geologists tend to see that it does not produce magma: the underlying plate tectonic active does, or the plumes. Do you disagree with this?
Explain my contradictions. When I studied geology ten years ago, the lithosphere didn't even exist in textbooks. I think we are undergoing a revolutionary redifinition in what we consider as the mantle level since the now-certain acceptance of plate tectonics has redifined the layers of the Earth. Originally, the crust was defined as the layer of of Earth going down to the MOHO (Mohorovičić discontinuity) between 10-80 km. beneath the surface, yet plates may be over 100 km. thick. So now we have redifined the layers to include what we now call the lithosphere in order to account for plate tectonics. With this definition, the crust has been reduced from 50-100 km. to 5-20 km. and has no - or very little - effect on plate tectonics. In today's modern theory, magma is not produced by crust activity: it is produced by plate tectonic activity.
If I have contradicted myself, perhaps it is due to these continuing differing definitions that are constantly being found in the geological scientific literature. So how can I get around this? Without still stating scientific research?
Today the crust is defined as extremely thin compared to what it was ten years ago and geologists tend to see that it does not produce magma: the underlying plate tectonic active does, or the plumes. Do you disagree with this?
For example:
"Oceans and continents are now considered to be mobile and interconnected. The paper discusses heat flow through the ocean floor, continental heat flow, heat loss of the earth, thermal structure and thickness of the lithosphere, as well as convection in the mantle and the thermal structure of the lithosphere, within the framework of the theory of plate tectonics. It is concluded that the observed subsidence of the ocean floor and the measured decrease of heat flow with age are accounted for by the creation of lithospheric plate. Furthermore, the marginal basins exhibit the same relation between heat flow and age as the deep ocean floor."
Source: "The heat flow through oceanic and continental crust and the heat loss of the earth," by Sclater, J. G.; Jaupart, C.; Galson, D., Reviews of Geophysics and Space Physics, vol. 18, Feb. 1980, p. 269-311.
"Hotspots – large volcanic provinces – such as Iceland, Hawaii and Yellowstone, are almost universally assumed to come from plumes of hot mantle rising from deep within the Earth. At Iceland, perhaps the best-studied hotspot on Earth, this hypothesis is inconsistent with many first-order observations, such as the lack of high temperatures, a volcanic track or a seismic anomaly in the lower mantle."
Source: "Hotspots: Plumes, or plate tectonic processes?," by G R Foulger, Astronomy & Geophysics, Vol. 43, Dec. 2002, Page 6-19.
"Fluid inclusions of minerals in mantle-derived xenolithsand demonstrated that the mantle xenolith of the Far Eastern Russia originated from the depth up to 40 km corresponding tothe uppermost mantle."
Source: "Spectroscopic estimation of the Mohodepth from residual pressures of CO2 fluid inclusions," by Y. Kawakami, et. al. 2003.
http://www.the-conference.com/2003/Gold2003/abstracts/A207.pdf.
Getting more complex:
"The reconstructed lithostratigraphy reveals a simple pattern, in ascending order, of greenstone with low-K tholeiitic composition with or without pillow lava structures, chert/banded iron-formation, and turbidites. The cherts and underlying low-K tholeiites do not contain continent- or arc-derived material. The lithostratigraphy is quite similar to Phanerozoic "oceanic plate stratigraphy," except for the abundance of mafic material in the turbidites. The evidence of duplex structures and oceanic plate stratigraphy indicates that the Isua supracrustal belt is the oldest accretionary complex in the world. The dominantly mafic turbidite composition suggests that the accretionary complex was formed in an intraoceanic environment comparable to the present-day western Pacific Ocean. The duplex polarity suggests that an older accretionary complex should occur to the south of the Isua complex. Moreover, the presence of seawater (documented by a thick, pillow, lava unit at the bottom of oceanic plate stratigraphy) indicates that the surface temperature was less than ca. 100°C in the Early Archean. The oceanic geotherm for the Early Archean lithosphere as a function of age was calculated based on a model of transient half-space cooling at given parameters of surface and mantle temperatures of 100° and 1450°C, respectively", suggesting that the Archean oceanic lithosphere was rigid. These conclusionsrigidity and lateral plate movementsupport the idea that the modern style of plate tectonics was in operation only 0.70.8 G.yr. after the formation of the Earth."
Source: "Plate Tectonics at 3.83.7 Ga: Field Evidence from the Isua Accretionary Complex, Southern West Greenland," by Tsuyoshi Komiya, et. al., The Journal of Geology, vol. 107, 1999, pages 515–554.
For further reference see:
"Self-Consistent Generation of Plate Tectonics in Mantle convection Models." http://artemis.ess.ucla.edu/~pjt/papers/Chapman_preprint.pdf
"Generation of Plate Tectonics from Lihoshere-Mantle Flow," by David Bercovici, Department of Geology and Geophysics, University of Hawaii, 1997.
http://earth.geology.yale.edu/~dberco/papers/1998/void-vol_EPSL98.pdf
"Oceans and continents are now considered to be mobile and interconnected. The paper discusses heat flow through the ocean floor, continental heat flow, heat loss of the earth, thermal structure and thickness of the lithosphere, as well as convection in the mantle and the thermal structure of the lithosphere, within the framework of the theory of plate tectonics. It is concluded that the observed subsidence of the ocean floor and the measured decrease of heat flow with age are accounted for by the creation of lithospheric plate. Furthermore, the marginal basins exhibit the same relation between heat flow and age as the deep ocean floor."
Source: "The heat flow through oceanic and continental crust and the heat loss of the earth," by Sclater, J. G.; Jaupart, C.; Galson, D., Reviews of Geophysics and Space Physics, vol. 18, Feb. 1980, p. 269-311.
"Hotspots – large volcanic provinces – such as Iceland, Hawaii and Yellowstone, are almost universally assumed to come from plumes of hot mantle rising from deep within the Earth. At Iceland, perhaps the best-studied hotspot on Earth, this hypothesis is inconsistent with many first-order observations, such as the lack of high temperatures, a volcanic track or a seismic anomaly in the lower mantle."
Source: "Hotspots: Plumes, or plate tectonic processes?," by G R Foulger, Astronomy & Geophysics, Vol. 43, Dec. 2002, Page 6-19.
"Fluid inclusions of minerals in mantle-derived xenolithsand demonstrated that the mantle xenolith of the Far Eastern Russia originated from the depth up to 40 km corresponding tothe uppermost mantle."
Source: "Spectroscopic estimation of the Mohodepth from residual pressures of CO2 fluid inclusions," by Y. Kawakami, et. al. 2003.
http://www.the-conference.com/2003/Gold2003/abstracts/A207.pdf.
Getting more complex:
"The reconstructed lithostratigraphy reveals a simple pattern, in ascending order, of greenstone with low-K tholeiitic composition with or without pillow lava structures, chert/banded iron-formation, and turbidites. The cherts and underlying low-K tholeiites do not contain continent- or arc-derived material. The lithostratigraphy is quite similar to Phanerozoic "oceanic plate stratigraphy," except for the abundance of mafic material in the turbidites. The evidence of duplex structures and oceanic plate stratigraphy indicates that the Isua supracrustal belt is the oldest accretionary complex in the world. The dominantly mafic turbidite composition suggests that the accretionary complex was formed in an intraoceanic environment comparable to the present-day western Pacific Ocean. The duplex polarity suggests that an older accretionary complex should occur to the south of the Isua complex. Moreover, the presence of seawater (documented by a thick, pillow, lava unit at the bottom of oceanic plate stratigraphy) indicates that the surface temperature was less than ca. 100°C in the Early Archean. The oceanic geotherm for the Early Archean lithosphere as a function of age was calculated based on a model of transient half-space cooling at given parameters of surface and mantle temperatures of 100° and 1450°C, respectively", suggesting that the Archean oceanic lithosphere was rigid. These conclusionsrigidity and lateral plate movementsupport the idea that the modern style of plate tectonics was in operation only 0.70.8 G.yr. after the formation of the Earth."
Source: "Plate Tectonics at 3.83.7 Ga: Field Evidence from the Isua Accretionary Complex, Southern West Greenland," by Tsuyoshi Komiya, et. al., The Journal of Geology, vol. 107, 1999, pages 515–554.
For further reference see:
"Self-Consistent Generation of Plate Tectonics in Mantle convection Models." http://artemis.ess.ucla.edu/~pjt/papers/Chapman_preprint.pdf
"Generation of Plate Tectonics from Lihoshere-Mantle Flow," by David Bercovici, Department of Geology and Geophysics, University of Hawaii, 1997.
http://earth.geology.yale.edu/~dberco/papers/1998/void-vol_EPSL98.pdf
My point is in reply to the post above: "correct me if i am wrong but isn't mt. everest the highest mountain on the planet?" ???leopold99 said:
And your point is? And yes I know that measured from the center of the Earth it is Mount Chimborazo (6,267 meters, 20,561 feet ) above sea level.
Still. Again. And your point is?
Ophiolite
Valued Senior Member
There are two possibilities vallich. I believe I have previously posited them. They bear repeating.valich said:
Option 1: You have significant cognitive difficulties - in common parlance, you are thick as three short planks nailed together.
Option 2: You are a despicable, chronic liar, with immense powers of self delusion.
Ten years ago, when you studied geology, the term lithosphere did not exist. Yeah, right. How about this then Valiarich? From 1979. Gosh, that's twenty six years ago.
http://www.pnas.org/cgi/content/abs...136016177648_59&FIRSTINDEX=0&journalcode=pnas
Plate tectonics offers an explanation for the present motions and heterogeneity of the rocks that form the external part of the Earth. It explains the origin of the first-order heterogeneity of oceanic and continental lithospheres. Furthermore, it explains the youth and simplicity of the oceanic lithosphere and offers the potential to explain the antiquity, complexity, and evolution of the continental lithosphere.
All of your dissembling, all of your wriggling, all of your direct lying, does not alter the fact, visible to all that, you were wrong, but instead of simply acknowledging a small mistake, and moving on, you resort to lying. You turn a minor point into a major issue with your dishonesty as you wriggle unattractively at the end of your lies. Vallich, you are a persistent, despicable, snivelling weasel of a liar. Do all of us a favour and piss off permanently.
Yours very sincerely,
Ophiolite
Ophiolite
Valued Senior Member
On reflection, I probably owe vallich an apology. He is probably not a liar, but just really thick. I was curious as to when the term lithosphere had come into use. I could certainly recall it being current when I did my undergraduate geology in the 1960s. [Remember that Vallich says, When I studied geology ten years ago, the lithosphere didn't even exist in textbooks.]
I mean, you have to laugh, or at least smile, at the inanity of him. He keeps asking for respect, then displaying (and defending) his ignorance in a way that merits not an iota of respect.
So, I recall the lithosphere being a standard part of the geologist's vocabulary in the 1960s. Vallich assures us it didn't even exist in textbooks ten years ago.
You know, he is partly correct. No, really he is. I have consulted several textbooks, on biochemistry, pediatrics, Roman history, and politics, and none of them make even passing reference to the lithosphere. Remarkable.
I'm still not quite sure when it was first used, but I did come across this paper, Discoidal Structure of the Lithosphere, by Bailey Willis.
http://www.pnas.org/cgi/reprint/5/9...136016177648_59&FIRSTINDEX=0&journalcode=pnas
And the date? 1916. That's almost ninety years ago. I'm pretty certain it is over a century old in routine usage. But clearly there will be an explanation.
Vallich, will doubtless explain that the term was used in a different way then. It is not what is now meant by lithosphere. He will dissemble in some way.
Of course he could make me look a bit like a vindicitive ass-hole by fronting up and saying that he was mistaken, yet again. But that is one of the differences between me and vallich. He has routinely denied being mistaken or lying. I have never denied being a vindictive ass hole.
All that said I shall offer vallich this sincere apology. Vallich, I am truly sorry you are such a lying, delusional cretin. Have a very good New Year. May I wish you everything you wish me.
I mean, you have to laugh, or at least smile, at the inanity of him. He keeps asking for respect, then displaying (and defending) his ignorance in a way that merits not an iota of respect.
So, I recall the lithosphere being a standard part of the geologist's vocabulary in the 1960s. Vallich assures us it didn't even exist in textbooks ten years ago.
You know, he is partly correct. No, really he is. I have consulted several textbooks, on biochemistry, pediatrics, Roman history, and politics, and none of them make even passing reference to the lithosphere. Remarkable.
I'm still not quite sure when it was first used, but I did come across this paper, Discoidal Structure of the Lithosphere, by Bailey Willis.
http://www.pnas.org/cgi/reprint/5/9...136016177648_59&FIRSTINDEX=0&journalcode=pnas
And the date? 1916. That's almost ninety years ago. I'm pretty certain it is over a century old in routine usage. But clearly there will be an explanation.
Vallich, will doubtless explain that the term was used in a different way then. It is not what is now meant by lithosphere. He will dissemble in some way.
Of course he could make me look a bit like a vindicitive ass-hole by fronting up and saying that he was mistaken, yet again. But that is one of the differences between me and vallich. He has routinely denied being mistaken or lying. I have never denied being a vindictive ass hole.
All that said I shall offer vallich this sincere apology. Vallich, I am truly sorry you are such a lying, delusional cretin. Have a very good New Year. May I wish you everything you wish me.
here's a sidebar on the lithosphere:
It's the lithosphere that is moving towards Siberia, not the geomagnetic pole
which some people believe. LITHOSPHERE. heh, makes sense to me.
Think about all the "outgassing" happening in the USA. These old gasses and materials making their way to the crust. What's pushing it up? The N. Amer Plate thats what. If people knew that Americans were sittin' on a "powder keg" they'd freak. The gasses from deep deep farther down than the crust is pushing its way up. Imagine that..,
It's the lithosphere that is moving towards Siberia, not the geomagnetic pole
which some people believe. LITHOSPHERE. heh, makes sense to me.
Think about all the "outgassing" happening in the USA. These old gasses and materials making their way to the crust. What's pushing it up? The N. Amer Plate thats what. If people knew that Americans were sittin' on a "powder keg" they'd freak. The gasses from deep deep farther down than the crust is pushing its way up. Imagine that..,
That's news to me that the lithosphere moves the geomagnetic North Pole. Can you give a source on that? I always just assumed the oscillations were due to the core, you know, like eventually leading towards magnetic reversals. It's hard to imagine that the magnetic field could come all the way up through the circulating convection currents in the mantle and then only finally be influenced by the outer lithosphere and plates?
New Facts about Mouna Loa:
"Its height as measured from the seafloor exceeds 30000 ft (9000 m), but the weight of the mountain has depressed the seafloor by perhaps another 25000 ft (8000 m), making a total height of over 55000 ft (16764 m or 17 km). Unlike its slightly higher neighbor Mauna Kea, Mauna Loa is still young and active, erupting large lava flows every few decades which continue to increase its vast bulk."
http://www.skimountaineer.com/ROF/ROF.php?name=MaunaLoa
However, I keep coming across articles that say Mouna Kea is now higher?:
"Highest Mountain: Mauna Kea, Hawaii, rises 33,474 feet (10,203 meters) from its base on the ocean floor." http://www.magazine.noaa.gov/stories/mag26.htm
Mauna Kea is dormant volcano: Mauna Loa is still active. So I don't understand the discrepancy?
As previously stated on the first page, the mid-Atlantic ridge is longest and tallest mountain "range" on Earth (65,000 km or 40,000 miles long).
The deepest trench - if you want to compare the height of a trench to a mountain - is the Challenger Deep, 35,810 feet deep [10915 meters], in the Marianas Trench.
http://pao.cnmoc.navy.mil/pao/Educate/OceanTalk2/indexunderwater.htm
Plate tectonics:
"The rigid lithospheric plates move atop a molten layer, the asthenosphere. New crustal material is added to the lithospheric plates at spreading centers, zones of volcanic activity where the crust is being pulled apart by the plates. Old crustal material is reincorporated into the mantle where lithospheric plates collide. The denser plate is subducted and melts as it is plunged into the asthenosphere. Plumes of molten material from deep in the mantle rise to the asthenosphere and initiate plate movement along zones of sea-floor spreading."
http://www.ggy.uga.edu/courses/geog1111/platec.html
"Its height as measured from the seafloor exceeds 30000 ft (9000 m), but the weight of the mountain has depressed the seafloor by perhaps another 25000 ft (8000 m), making a total height of over 55000 ft (16764 m or 17 km). Unlike its slightly higher neighbor Mauna Kea, Mauna Loa is still young and active, erupting large lava flows every few decades which continue to increase its vast bulk."
http://www.skimountaineer.com/ROF/ROF.php?name=MaunaLoa
However, I keep coming across articles that say Mouna Kea is now higher?:
"Highest Mountain: Mauna Kea, Hawaii, rises 33,474 feet (10,203 meters) from its base on the ocean floor." http://www.magazine.noaa.gov/stories/mag26.htm
Mauna Kea is dormant volcano: Mauna Loa is still active. So I don't understand the discrepancy?
As previously stated on the first page, the mid-Atlantic ridge is longest and tallest mountain "range" on Earth (65,000 km or 40,000 miles long).
The deepest trench - if you want to compare the height of a trench to a mountain - is the Challenger Deep, 35,810 feet deep [10915 meters], in the Marianas Trench.
http://pao.cnmoc.navy.mil/pao/Educate/OceanTalk2/indexunderwater.htm
Plate tectonics:
"The rigid lithospheric plates move atop a molten layer, the asthenosphere. New crustal material is added to the lithospheric plates at spreading centers, zones of volcanic activity where the crust is being pulled apart by the plates. Old crustal material is reincorporated into the mantle where lithospheric plates collide. The denser plate is subducted and melts as it is plunged into the asthenosphere. Plumes of molten material from deep in the mantle rise to the asthenosphere and initiate plate movement along zones of sea-floor spreading."
http://www.ggy.uga.edu/courses/geog1111/platec.html