New experiments have called into question a popular explanation for the properties that give rise to dry land. Despite being a crucial factor in making Earth a hospitable place for life compared to other planets in the solar system, the origins and unique characteristics of continents, massive segm
, has advanced our knowledge of Earth’s crust by testing and disproving a widely held theory regarding the lower iron content and higher oxidation levels of continental crust compared to oceanic crust.
One of the key consequences of Earth’s continental crust’s low iron content relative to oceanic crust is that it makes the continents less dense and more buoyant, causing the continental plates to sit higher atop the planet’s mantle than oceanic plates. This discrepancy in density and buoyancy is a major reason that the continents feature dry land while oceanic crusts are underwater, as well as why continental plates always come out on top when they meet oceanic plates at subduction zones.
In 2018, Cottrell and her colleagues set about finding a way to test whether the crystallization of garnet deep beneath these arc volcanoes is indeed essential to the process of creating continental crust as is understood.
In 13 different experiments, Cottrell and Holycross grew samples of garnet from molten rock inside the piston-cylinder press under pressures and temperatures designed to simulate conditions inside magma chambers deep in Earth’s crust. The pressures used in the experiments ranged from 1.5 to 3 gigapascals—that is roughly 15,000 to 30,000 Earth atmospheres of pressure or 8,000 times more pressure than inside a can of soda.
The results of these tests revealed that the garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels of iron depletion and oxidation present in the magmas that are the building blocks of Earth’s continental crust.
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