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If Earth never had life, continents would be smaller

April 16, 2015

VIENNA—It may seem counterintuitive, but life on Earth, even with all the messy erosion it creates, keeps continents growing. Presenting here this week at the annual meeting of the European Geosciences Union, researchers say it's the erosion itself that makes the difference in continental size. Plant life, for example, can root its way through rock, breaking rocks into sediment. The sediments, like milk-dunked cookies, carry liquid water in their pores, which allows more water to be recycled back into Earth’s mantle. If not enough water is present in the mantle about 100 to 200 km deep to keep things flowing, continental production decreases. The authors built a planetary evolution model to show how these processes relate and found that if continental weathering and erosion rates decreased, at first the continents would remain large. But over time, if life never evolved on Earth, not enough water would make its way to the mantle to help produce more continental crust, and whatever continents there were would then shrink. Now, continents cover 40% of the planet. Without life, that coverage would shrink to 30%. In a more extreme case, if life never existed, the continents might only cover 10% of Earth. When it comes to a habitable planet, life even plays a role in building the habitats.

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Geophysical Research Abstracts

Vol. 17, EGU2015-13398, 2015

EGU General Assembly 2015

Feedback cycles in planetary evolution including continental growth and mantle hydration, and the impact of life

Dennis Höning and Tilman Spohn

German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany (Dennis.Hoening@dlr.de)

"The Earth’s evolution is significantly affected by several intertwined feedback cycles. One of these feedback loops describes the production and erosion of continental crust. Continents are produced in subduction zones, whose total length in turn is determined by the fraction of continental crust. Furthermore, the fraction of continental crust determines the amount of eroded sediments. These sediments eventually enter subduction zones and affect the water transport into the mantle. As the biosphere enhances weathering and erosion of continental crust, we show how life on Earth can enter this feedback cycle and stabilize the present day state of the Earth. A second feedback loop – coupled to the first one – includes the mantle water cycle. Water in the Earth’s mantle reduces its viscosity, and therefore increases the speed of mantle convection and plate subduction.

Here, we present a thermal evolution model of the Earth which reproduces the present day observations. We investigate the influence of the biosphere during the Earth’s evolution on continental growth and mantle hydration. Finally, we discuss implications on the evolution of plate-tectonics planets beyond our solar system."