Did the oceans cause plate tectonics? That’s the question that a new study in Earth, Planets and Space titled “Evolution of the Earth as an andesite planet: Water, plate tectonics, and delamination of anti-continent” sets out to answer. Or, rather, Yoshiyuki Tatsumi, Takeshi Sato, and Shuichi Kodaira at Kobe University and the Japan Agency Marine-Earth Science and Technology set out to look at why the Earth, alone among the solar system’s terrestrial planets, has plate tectonics. That is to say, that the other terrestrial planets—Mercury, Venus, and Mars—lack our familiar plate tectonics; and while they have (or had) volcanism, they don’t have continental plates or plate-derived quakes.
And as the Earth is also alone among the terrestrial planets in having surface liquid water, they chose to investigate the possible influence of this surface water (i.e., the oceans) on the Earth’s crust formation. Especially as much of the crust formation happens at the ocean floor—in mid-oceanic ridges and also subduction zones.
So on one hand, it’s reasonable to hypothesize about the impact of the oceans on the mechanics of the crust underneath them. And on the other, it’s also reasonable to look at two things that make the Earth geologically different from the solar system’s other terrestrial planets.
A “challenging” hypothesis
So how would one go about testing this hypothesis? Tatsumi, Sato, and Kodaira chose to investigate the workings of the Izu-Bonin-Mariana (IBM) arc, a large subduction zone under the western Pacific Ocean. They investigated the geodynamics, structure, and geochemistry of the crust being created at that zone, and they found that as new crust is created, it evolves into three zones. As these zones evolve, their chemical composition changes, due in part to changes, caused by the ocean water, in the basalt’s solidification temperature, and the result is minerals called andesites and other plutonic minerals.
This becomes important because the Earth’s crust has another unique feature—these non-basalt intermediate composition minerals. These minerals contain 52-63% by weight of silicone dioxide (SiO2), as compared with the basalt the composes the crusts of the other planets. Tatsumi et al. found andesitic compositions in the middle crust layer of the IBM arc subduction zone, leading them to conclude that this line “of evidence provide[s] compelling support for a challenging hypothesis; the ingredients for continental crust are created at modern intra-oceanic arcs. Thus the continents are born in the oceans.” They conclude, “The Earth has been simultaneously creating continents at the top of the mantel and the… base of the mantle, and consequently has evolved into a unique planet.”
You can read the entire study here.