Plate tectonics likely started when cooler parts of Earth's early crust were pulled downwards into the warmer upper mantle, damaging and weakening the crust, a process that repeated until plate boundaries formed.
The Early Earth and Crustal Instability
The early Earth differed significantly from the planet we know today. It possessed a thinner, more unstable crust and a hotter mantle. This meant that the crust was more susceptible to deformation and sinking.
The Mechanism: Downwelling and Weakening
- Cooling and Sinking: As the Earth's surface cooled, certain regions of the crust became denser than the underlying mantle. This density difference caused these cooler regions to sink or be "pulled downwards" into the mantle in a process called downwelling.
- Crustal Damage: The repeated downwelling events caused significant damage and weakening to the surrounding crust. This constant fracturing and deformation created zones of weakness.
- Formation of Plate Boundaries: Over vast stretches of geological time, these zones of weakness coalesced and evolved into the plate boundaries we observe today. These boundaries mark the edges of tectonic plates and are where most of the Earth's seismic and volcanic activity occurs.
Why This is a Leading Hypothesis
This hypothesis aligns with current understanding of the early Earth's thermal state and crustal properties. The early Earth's hotter mantle would have facilitated the downwelling process, and its thinner crust would have been more vulnerable to fracturing. This repeated cycle of sinking, weakening, and fracturing likely laid the foundation for the development of plate tectonics.
