The first ruptures in early Earth’s skin formed because of the weakness of rock minerals merely a millimeter wide, two scientists propose. The small minerals’ behavior created boundaries defining Earth’s first crustal plates and set the stage for plate tectonics, according to a new computer simulation appearing April 6 in Nature.
Plate tectonics is special to Earth: The planet’s crust is divided into giant, mobile plates. A plate can bump up against another plate at a fault zone, or dive beneath one at a subduction zone. The outcome can be an earthquake or volcano. Where plates split apart, new crust forms. This occurs, for example, at the rift in the seafloor below the Atlantic Ocean. Venus, Earth’s near twin in size and composition, may once have had the conditions to start plate tectonic processes, but it didn’t.
Scientists have long wanted to know what made Earth different from its rocky neighbors and how plate tectonics began. After the planet formed 4.5 billion years ago, its innards settled into a dense iron core surrounded by a mantle of slowly flowing rock. That mantle was enveloped by a harder skin on top, called the lithosphere. Hot magma rises, while cold crust sinks, and scientists think that early on, cold chunks of rock from the lithosphere periodically sank or “dripped” into the warm, churning mantle below.
“This intermittent dripping would have caused damage in the lithosphere,” says David Bercovici, a geophysicist at Yale University, who created the new simulation with geophysicist Yanick Ricard of the University of Lyon in France. The recurring mechanical damage would have made the minerals that compose the rocks there smaller and smaller, and ever easier to grind. The scientists used lab observations of millimeter and submillimeter crystals to model the behavior of crystals from common lithospheric minerals. The results suggest that the tiny crystals deform more with decreasing size. The tinier the size of the mineral grains that make up the rocks, the more easily deformed the rocks are at a larger scale.
Because of the mineral grains’ increasing damage, a narrow corridor of weakness in the lithosphere would get still weaker with further dripping and mechanical damage. Bercovici compares the Earth’s early skin to a paperclip: “Bend a paperclip back and forth often enough, it will develop enough cracks and weakness that it will crack and break.”
Stresses of the convecting mantle would have started to tear at these lithospheric weak spots, and movement would create proto-subduction, the researchers think. Repeated intermittently over a very long time, perhaps a billion years, the dripping and tearing could “build up enough to get plate tectonics,” Bercovici suggests.
The key to the model’s view of plate tectonics is whether the minerals in lithospheric rocks got damaged or “healed,” merging with other rocks to form larger grains. In Earth’s case, they didn’t heal: In the weak zones, the minerals kept breaking into smaller bits, according to the model. But in a simulation using Venus, which has always had a lithosphere that is 200 to 400 degrees Celsius warmer than Earth’s, the minerals heal and grow.
The model shows one possible scenario for the origins of plate tectonics, but it lacks a mechanism for creating new lithosphere, as happens at the rift in the middle of the Atlantic, says geochemist Kent Condie of New Mexico Tech in Socorro. “Nobody has come up with a satisfying answer yet on how plate tectonics started,” he says. One problem is the lack of physical record from the era: Aside from a handful of ancient deposits, most of the rocks that were around 4 billion to 3 billion years ago have long since dived deep into the Earth, thanks to plate tectonics.