I knew so far that, the classification of crust as SiAl-SiMa, vs. classification of crust as Continental-Oceanic; are completely different, as told in my school geography and university paleobiology class. (However I'm not a geology student)


the 2 concept

Where Continental crust and oceanic crust placed side-by-side, so, no-part of continental crust can cover-up oceanic crust. Also, in the places of continental crust; both SiAl and SiMa could be found

Now a geology student is telling me, that is wrong. SiAl is Same as Continental crust, and SiMa is same as Oceanic crust. He's Also telling, "SiAl or Continental crust lie upon Sima or Oceanic crust". That implies at the same-place on Earth, continental and oceanic crust occur together, one upon other.


The SiAl-SiMa classification is very general. The continental crust is much more varied than that, but it is ok for a first approximation.

The oceanic crust is dominantly mafic (aka SiMa), composed of basalt and gabbro. The continental crust is much more complex than that. The upper layer of the continental crust is felsic (aka SiAl), mostly of a granite or a granodiorite composition. The lower part of the continental crust is much more mafic (again, aka SiMa) and the composition of that lower layer has similarities to the oceanic crust. However, it is not the same to oceanic crust. Whereas the mafic rocks in the oceanic crust form by partial melting of mantle ultramafic rocks, the mafic rocks of the lower continental crust form by repeated metamorphic processes and extraction events of felsic material. Yes, there are gabbros in there but most of the lower continental crust is composed of high-T restite rich granulites and eclogites, rather than the gabbro you see in the oceanic crust.

Here's a picture to help you visualise it, with slight editing by myself:

enter image description here

So in short, the lower continental crust is compositionally similar to the oceanic crust, but it has different origins and it is not an extension of the oceanic crust.

  • $\begingroup$ Thanks for clarifying "the lower continental crust is compositionally similar to the oceanic crust, but it has different origins and it is not an extension of the oceanic crust." $\endgroup$ – Always Confused Aug 7 '16 at 11:42
  • $\begingroup$ btw Wikipedia yet doesn't contain any image of such sectional view on pages for crust (en.wikipedia.org/w/… ), continental crust (en.wikipedia.org/w/… ) and oceanic crust ( en.wikipedia.org/w/…) (links to latest editions). So anyone can add informations and improve those articles in wikipedia. $\endgroup$ – Always Confused Aug 7 '16 at 14:50
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    $\begingroup$ @AlwaysConfused try the page on Subduction $\endgroup$ – Gimelist Aug 7 '16 at 19:50
  • $\begingroup$ hmm its a very good image. it also explains "buoyant magma diapirs" and "water vapour". I've many-times seen them in diagrams, but never seen them with pointing, so looked mysterious. (it solved another question that i could ask at near future). so thanks. $\endgroup$ – Always Confused Aug 7 '16 at 19:57

Michael is correct. SIAL and SIMA are very vague and over-simplified summaries of the continental and oceanic crustal compositions. These terms may be OK for school introductions to geology, but you don't have to look far to find many exceptions. There are eroded and fragmented continental remnants, of nominally SIALic composition, in every ocean. Indeed, 100% oceanic crust may include small amounts of highly differentiated plagiogranite (technically SIALic in composition). Then there are bits of oceanic crust, called ophiolites (SIMA) incorporated into continental crust through processes of obduction and terrain accretion, not to mention the many flood basalt terrains (SIMA), which can occur on either continental or oceanic crust. Whether crust is oceanic or continental is a matter of overall rock density, composition and crustal thickness.


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