“Two great tastes that taste great together.” So goes the jingle for Reese’s peanut butter cups, where a layer of smooth milk chocolate coats a creamy peanut butter center. Take either away and you don’t have a Reese’s. The same seems to be true for a planet, at least one capable of sustaining life. Liquid water lubricates the tectonic activity that forms islands and builds continents. However, without the weathering of those landmasses—and the ensuing alteration of the atmosphere—any planet starting with liquid water would quickly lose it as the output of the host star steadily increases. Abundant evidence shows that water has covered Earth’s surface since the late heavy bombardment. However, scientists are still seeking an answer to when tectonic activity started.
In Earth’s recent history, the operation of plate tectonics requires at least two different processes. First, as the plates move across the mantle, new crust must form from magma welling up from the underlying mantle. Second, the motion of the plates will eventually cause collisions, which will then cause subduction (where one crustal plate descends below the edge of another). Both of these processes result in volcanic activity. Fortunately for scientists looking to find evidence of both processes, the rocks formed from each exhibit different chemical signatures.
Previous studies of formations in the Isua Supracrustal Belt, Greenland, (with rocks as old as 3.8 billion years) uncovered evidence of subduction volcanism. However, scientists found none of the chemical signatures of mid-ocean ridge or ocean-island production in similarly old rocks. Without such production, it is unlikely that subduction actually occurred. It is akin to finding the steps of an escalator disappearing, but without any signs of new steps appearing.
Recent work changes the picture. Another set of rocks from Greenland, specifically Innersuartuut Island, also date older than 3.7–3.8 billion years. Analyses of these rocks show no contamination from continental crust (so they are volcanic in origin) and exhibit chemical signatures distinct from subduction volcanism. Furthermore, the chemical signature compares favorably to volcanic magma from ocean island production.1 Thus, scientists now have evidence for the two key components of modern plate tectonics—both ocean-island and subduction volcanism—from an era nearly four billion years ago.
Plate tectonics influences two events recorded in the biblical creation accounts. First, active plate tectonics ensures that Earth forms a stable water cycle (creation day two) and maintains it for billions of years. Second, over time this tectonic activity results in the formation of islands and, more importantly, continental landmass (creation day three). Earth’s habitability relies on both of these conditions. Consequently, it comes as no surprise that plate tectonics extends all the way back to the planet’s formative stages. The big question that remains is whether any of the recently found “habitable” exoplanets show similar longevity.