My family lives in one of the fastest-rising neighborhoods in the nation—not economically, but topographically. Our home rises by an average of 9 millimeters (1/3 inch) per year. Sometimes the elevation gain (via earthquake) seems a bit disturbing. Sometimes it's destructive. Nonetheless, I tell my wife and sons we should be thankful for all the uplift we get. Specifically, we can thank God for designing Earth for vigorous and virtually constant plate tectonic activity. Why? Because such movement is essential for life.
Earth has experienced robust plate tectonics for four billion years. Without it, our planet would possess no continents, no mountains, no stable water cycle, and nothing like the diversity and abundance of life we enjoy.1 In fact, without tectonic activity, Earth would have no mechanism to compensate for ongoing changes in the Sun's luminosity, and all life would be driven to extinction.2 Without such large-scale motions, nutrient-restoring cycles would fail to provide for life's basic needs3 and humanity would lack the abundant biodeposits (like coal, oil, natural gas) on which civilization depends.4
For some time now scientists have recognized the importance of plate tectonics, but only recently have they discovered the degree to which Earth's tectonics reflect exquisite fine-tuning. This understanding was greatly enhanced when two planetary physicists, Diana Valencia and Richard O'Connell at Harvard University, developed detailed models of the internal structure of massive rocky planets.5
Their research showed that as the mass of a rocky planet increases, the thickness of its crustal plates decreases, and so does its resistance to tectonic motion. Therefore, the greater the mass of a rocky planet, the higher the probability for plate tectonic activity and the more aggressive that activity will be.
Valencia and O'Connell's study helps explain a solar system enigma—why only Earth, of all the planets in our solar system, manifests plate tectonics. Liquid water is the key. If it weren't for Earth's abundant surface water, its crust wouldn't crack and move. Water lowers the yield strength of certain crustal minerals. For example, water cuts in half the yield strength (resistance to crumbling) of olivine, a primary constituent of Earth's crust.
For permanent, strong plate tectonics to be possible on a dry rocky planet, the planet's mass would have to be more than twice that of Earth. (At such a mass, the planet approaches the boundary between rocky planets and gaseous planets.) And even though the presence of liquid water lowers the mass boundary for strong and ongoing tectonics, Earth's mass represents the lower limit, according to the Harvard team's calculations. (Previous studies put the plate tectonics limit at one-third the mass of Earth, but such a low mass allows only for weak or ephemeral plate tectonic activity.)
This finding becomes especially remarkable in light of the fact that from a physiological perspective, Earth's mass could not be any larger and still be suitable for life (specifically for respiration). The problem for a planet more massive than Earth is its atmosphere. The more massive a planet, the thicker the atmosphere it accumulates during its formation. And, the atmospheric thickness rises geometrically with a planet's mass. For example, Venus, with seven times the mass of Mars, has an atmosphere more than 600 times thicker. In fact, Earth's atmosphere would be too thick for breathing if it weren't for its low-velocity collision with a Mars-sized object early in its history, a collision that blew away most of the thickness.6
As Valencia and O'Connell's research points out, a planet's mass must be virtually identical to Earth's for that planet to have a chance at sufficient-for-life tectonics. It also must be as wet as Earth but no wetter. (A wetter planet would lack continents and critical nutrient cycles.) It seems the more researchers learn about planets, the more evidence they find for the purposeful shaping of Earth for life.