Era· The columns of time

Mesoarchean

3.2 Ga → 2.8 Ga

The Mesoarchean, from 3.2 to 2.8 billion years ago, is the era in which the Earth's crust may have begun to work as it does today, one plate sinking beneath another and building continents.

Era in which the crust of the Earth may begin to operate under the present regime, one plate descending beneath another and driving the processes that build continents.

What the evidence shows

The date at which plate tectonics began is among the least resolved questions in the discipline. Several lines of evidence converge on a change in the behaviour of the crust between 3.2 and 2.5 billion years, an interval that opens with this era. The transition appears gradual rather than instantaneous, subduction operating locally before becoming general. The consequences extend beyond geology: subduction drives the long-term carbon cycle, which returns to the atmosphere through volcanism the carbon removed by rock weathering, builds continents and holds climate within habitable bounds over billions of years. A planet lacking that mechanism follows a quite different course.

Bounds and conventions

Neither limit of the era is marked in the rocks; 3.2 and 2.8 billion years are conventional round numbers. The phenomenon that characterises it, the onset of plate tectonics, is dated to neither and may not be a single moment. The evidence is indirect, the criteria disputed, and the estimates proposed range from four billion years to less than one.