c. 4–3 Billion BCE?

Plate Tectonics

Earth’s crust and upper mantle in the Hadean eon (4.5–4 billion years ago) were hot, mostly molten, violent, and unstable places. As the planet cooled and the seemingly endless rain of asteroid and comet impacts slowed considerably in the Archean (4–2.5 billion years ago), the outer layers of our planet began to take on their more familiar appearance. This included the formation of the oceans, the formation of the first pieces of lower-density continental crust that could “float” on the higher-density volcanic lavas that make up the seafloor, and the division of the upper mantle into a rigid, cooler outermost section called the lithosphere, and a hotter region just beneath that called the asthenosphere.

The asthenosphere (Greek for “weak” and “sphere”) starts somewhere on average about 30 to 60 miles (c. 50 to 100 kilometers) beneath the surface, and varies in thickness from around 10 to more than 300 miles (a few tens to more than 500 kilometers), depending on temperature. The rocks there are ductile, meaning that they can easily deform or even slowly flow, unlike the colder, stiffer lithosphere above. Rocks in the warm asthenosphere are compelled to move by enormous convection plumes that carry molten rock and heat from the deep interior of the Earth up toward the surface. “Blobs” of hotter (or even molten) mantle rocks cause the asthenosphere to bulge, bend, and move laterally as the plumes ascend. This places enormous stress on the rigid rocks of the lithosphere.

During the Archean, sometime between 4 and 3 billion years ago (the timing is controversial and the subject of much active research), the rigid lithosphere fractured under the stress and broke into numerous (possibly hundreds or thousands) of individual plates, each of which remained semi-anchored to the moving asthenosphere below. These puzzle pieces were then free to move about, crashing into one another to create early mountain belts, or one diving underneath the other to create enormous trenches.

As the continents grew into larger plates, they became more formidable obstacles to the denser seafloor volcanic plates, which are also continuously growing at mid-ocean ridges. Earth now has about two dozen of these large lithospheric plates, and many of their boundaries mark zones of strong earthquakes and extensive volcanic eruptions.

The famous San Andreas fault, seen here running through southern California, is one of the most famous boundaries between Earth’s lithospheric plates (the Pacific and North American).