Understanding Continental Crust Composition and Formation

The distinctive positioning of continents above the sea level can be attributed to their crust composition, which is significantly less iron-rich and more oxidized than the oceanic crust. This compositional dissimilarity facilitates the floating of continents on the Earth's mantle. Previously, the formation of garnets was deemed to be integral to this difference, however, contemporary experiments are putting this theory into question.

A scientific study, dating back to five years ago, proposed that the crystallization of garnets played a pivotal role in this process. As per this proposition, the formation of garnets in continental arc volcanoes would extract unoxidized iron from magma, leading to a magma composition on the continents that is iron-deficient but rich in oxidized minerals. While this theory initially appeared persuasive, it was met with skepticism from some scientists, prompting further investigation.

One of the key uncertainties arose from the fact that garnet stability requires high pressures, yet low-iron magma is often found in locations with thinner crust and consequently, lower pressure. To probe this theory, researchers attempted to create garnets from molten rock in a lab setting, in an effort to mimic the conditions that allow these precious stones to form in low-pressure environments. Interestingly, they found that the amount of unoxidized iron removed from the rocks was insufficient to account for the continental crust's composition.

Such findings cast a shadow of doubt over the garnet crystallization theory as the explanation for the oxidized, iron-depleted composition of magmas in continental arc volcanoes. It seems more plausible that the conditions within the Earth's mantle beneath the continental crust might be fuelling these oxidized conditions. This raises intriguing questions about the factors influencing the unique composition of continents. The research team is now delving into the possibility that oxidized sulfur could be oxidizing iron, thereby providing an explanation for the compositional discrepancy between continental and oceanic crust.

Published in the esteemed Science journal, these groundbreaking findings have paved the way for new research directions to gain deeper insights into the formation and unique traits of continents.

(Glass (brown), large garnets (pink) and other small mineral crystals observed in an area the size of a sugar crystal. Image credit: G. Macpherson and E. Cottrell, Smithsonian)