18-Jul-2018: Meghalayan Age: A new phase in the Earth’s history named after Meghalaya rock

Geologists have classified the last 4,200 years as being a distinct age in the story of our planet and named it the Meghalayan Age, the onset of which was marked by a mega-drought that crushed a number of civilizations worldwide. The International Chronostratigraphic Chart, the famous diagram depicting the timeline for Earth's history will be updated.

The Meghalayan, the youngest stage, runs from 4,200 years ago to the present. It began with a destructive drought, whose effects lasted two centuries, and severely disrupted civilizations in Egypt, Greece, Syria, Palestine, Mesopotamia, the Indus Valley, and the Yangtze River Valley. It was likely triggered by shifts in ocean and atmospheric circulation.

The Meghalayan Age is unique among the many intervals of the geologic timescale in that its beginning coincides with a global cultural event produced by a global climatic event.

To win a classification, a slice of geological time generally has to reflect something whose effects were global in extent, and be associated with a rock or sediment type that is clear and unambiguous.

For the famous boundary 66 million years ago that marks the switch in period from the Cretaceous to the Palaeogene, this "golden spike" is represented by traces in sediments of the element iridium. This was spread across the planet in the debris scattered by the asteroid that wiped out the dinosaurs.

For the Meghalayan, the spike is epitomised in a perturbation in the types, or isotopes, of oxygen atoms present in the layers of a stalagmite growing from the floor of Mawmluh Cave in the northeastern state of Meghalaya in India. This two-step change is a consequence of weakening monsoon conditions. The isotopic shift reflects a 20-30% decrease in monsoon rainfall.

The two most prominent shifts occur at about 4,300 and about 4,100 years before present, so the mid-point between the two would be 4,200 years before present, and this is the age that geologists attribute to the [Meghalayan golden spike].