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How old are they ?
The fate of oceanic crust (oc), which is denser than the continental crust (cc), is to come back into the mantle. The oldest oc is 180 Ma old (Figure). However, exceptionally, pieces of oc (called ophiolites) are preserved because they were tectonically obducted on top of the cc.
By contrast, the lighter cc (almost) never sinks in the mantle, it remains close to the surface. Therefore cc may be young or old … The oldest cc is about 4.03 Ga (enclave in the Acasta gneissic formation, Canada). Such measurements are performed on zircons (the most inert, resistant to alteration, mineral) using radioactive elements having long periods, Uranium (U) being the best suited element (U/Pb technique). Still older dates have been found froms isolated zircons in younger rocks, proving that such rocks had existed at least up to 4.4 Ga.
Map of the ocean-floor ages. In purple, the oldest (144-160 Ma) ; in red the youngest (ocean ridges)
The origin of plate mouvements ? the thermal engine …
- Plate mouvements, and more generally tectonics (mountain building etc …), are triggered by the internal heat of the Earth : the Earth is a thermal machine. Internal heat is no more residual (i.e., coming from its early formation by accretion, in which kinetic energy was transformed into heat …) but created by disintegration of the radio-elements. Presently, the main heat-producing elements are U, Th and K (elements having very long disintegration periods : lU238 = 4.5 109 years ; lTh232 = 14 109 years ; lK40 = 1.25 109 years) … heat-producing elements having short periods having disappeared (example Al26, lAl26 = 740 000 years).
- Since heat is produced continuously, it cannot accumulate indefinitely … heat must be released at the surface of the Earth (there is a thermal gradient between the inside and the outside of the Earth !!! hence a flux). Indeed heat is released through the plates, it is reponsible for the thermal flux (a few tens to a few hundreds of mW/m2 : Figure : Surface heat Flow. Please comment).
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Note that the flux ix maximum at the ridges (red to yellow ; because of mantle upwell). Below 85 mW/m-2 the distribution of the flux is complex. This is related to the distribution of the heat-producing elements (hpe) : reach in hpe in young continental crust ; moderate in old continental crust, and poor in the oceanic crust.
Date: 2015-01-29; view: 960
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