In the theory of tectonic plates, at a convergent boundary in between a continental plate and an oceanic plate, the denser plate normally subducts under the less thick plate. That is well known that oceanic plates subduct under continental plates, and therefore oceanic key are much more dense than continental plates.

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My inquiry is why room the oceanic plates always denser 보다 the continental plates. I"m mindful that the difference in thickness can be attributed come the plates differing compositions, however what I"m interested in is why this plates have various composition in the first place offering rise to your relative difference in densities.


$egingroup$ Oceanic late is denser 보다 continental tardy because: it contains silica $endgroup$
Ocean lithosphere (geophysical definition of late + top mantle the acts as a "plate") is generally of basaltic ingredient - the top levels room basalt and the lower levels space gabbro. The height levels have been proven v boreholes, whilst the reduced levels have actually been inferred from transform error sampling and also comparisons through ophiolites. This succession is developed by partial melt of mantle peridotite in ~ a fairly controlled rate. So lot so that basalts created in this means even have actually a particular composition "MORB" (Mid ocean Ridge Basalt).

In contrast, continent lithosphere is more complicated and has tendency to be of a "granitic" composition. This consists of granites but can likewise include a most metamorphic rocks (eg. Gneiss) and also sediments. Sediments are lower thickness anyway (high pore space), but so space quartz-rich rocks such as granites. The various processes that develop continents tend to favour silica rich compositions, leading to this bulk "granitic" composition. For example, limited partial melting will initially develop high silica, high alkali melts. Erosion will tend to malfunction most typical minerals before quartz - leaving quartz-rich sediments (hence sandstone is generally quartz). Metamorphism that pelites (rocks affluent in Al and Si) will have tendency from the early mudstones & basalts v to gneisses & migmatites (which have a lot of quartz and also feldspar). Migmatites are partially melted - and the melted bits are essentially granite.

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Basalt is denser 보다 granite. On heaviness surveys, basalts and gabbros will appear as positive anomalies, whilst granites and sedimentary basins will appear as an unfavorable anomalies.