The likelihood that other technologically sophisticated societies exist is smaller than previously thought, because basic amenities we take for granted on Earth—continents, oceans, and plate tectonics—are cosmically rare.
Most geologists will agree with Stern’s and Gerya’s argument that plate tectonics should be included as a criterion for long-term planetary habitability. Earth’s tectonic system allows the planet’s atmosphere and hydrosphere to remain in communication with its interior, in a remarkable, self-perpetuating cycle. Subducted ocean crust—seafloor that slips down into Earth’s interior—carries water back into the mantle, and at shallow depths, this water lowers the melting temperature of mantle rock, giving rise to unusual magmas that create the continental crust—what we surface dwellers live on—which is rich in rare elements, like phosphorus, that are critical to life.
At greater depths, subducted water acts to decrease the viscosity of the mantle, allowing it to churn, or convect, more vigorously—which in turn drives plate motion. When the Earth’s mantle exports heat via convection, it encourages the liquid iron outer core to convect as well, and this generates Earth’s protective magnetic field, which shields the surface environment from harmful cosmic radiation. Without plate tectonics, continents would quickly be eroded to sea level. But tectonic collisions continuously rejuvenate Earth’s topography, providing rivers with more energy to transport nutrient-rich sediments to shallow marine environments. In other words, plate tectonics is entangled with all the phenomena that support life on Earth.
