The cause why continents mostly stand above sea level is believed to be that continental incrustation is scurvy in iron and more oxidize than pelagic crust . This makes continents more buoyant over the Earth ’s mantle . The dispute in composition is poorly understand , but a popular hypothesis indicate that garnet formation was key to this . Now , new experimentation are call that into question .
sour publishedfive years agoproposed the crystallizing of garnet as the crucial mechanism . The location of this event is the continental arc vent , where pelagic crust sinks beneath continental Earth’s crust . The idea is that garnet formation there murder non - oxidized iron from the magma – in one go , the magma that ended up on the continent was at the same time poorer in branding iron and copious in oxidized minerals . A compelling account , but some scientists saw potential limitations that needed examination .
“ You need in high spirits pressures to make garnet unchanging , and you find this low - iron magma at place where crust is n’t that thick and so the air pressure is n’t extremely in high spirits , ” senior source Elizabeth Cottrell , enquiry geologist and curator of rock at the Smithsonian ’s National Museum of Natural History , order in astatement .
Cottrell ’s team createdgarnetin the research laboratory from molten rocks , trying to simulate what happens when these gems form in those piazza where the insistency is not exceedingly high . They placed the molten rock in special cylinders that could model 15,000 to 30,000 Earth ambience of imperativeness and got garnet out of them . They then look at how much unoxidized iron was remove from the rock-and-roll . And it was n’t enough to explicate the make-up of the continental crust .
“ These results make the garnet crystal model an highly unlikely explanation for why magmas from continental arc volcano are oxidize and Fe - depleted , ” Cottrell continued . “ It ’s more probable that conditions in Earth ’s mantle below continental impertinence are setting these oxidate conditions . ”
The finding obviously bring back the crucial interrogation about whycontinentsexist and what has caused their composition to be special .
“ What is doing the oxidizing or Fe eat up ? ” Cottrell require . “ If it ’s not garnet crystallization in the crust and it ’s something about how the magmas get in from the mantle , then what is happening in the mantle ? How did their composing get change ? ”
The team is now investigating whether oxidized sulfur could be oxidize the Fe , and therefore explain the disagreement between continental and oceanic crust .
The findings were issue in the journalScience .
