Earth probably began with a solid shell

Plate tectonics — a defining feature of modern Earth and the driving force behind earthquakes, volcanoes and mid-ocean spreading ridges — did not start until later in Earth’s history, new research suggests. The work is the latest salvo in a long-standing geological debate: did plate tectonics start right away, or did Earth begin with a solid shell covering the entire planet? The new results suggest the latter.
Geology News — ScienceDaily