Earth’s first continents emerged from the ocean 700m years earlier than thought

A new analysis of ancient rocks shows that the Earth's first continents emerged from the ocean 700m earlier than previously believed.
Researchers who studied eastern Indian rock sediments believe that the discovery could have led to an increase in oxygen levels and the formation glaciers during this period in Earth's history.

Analysing sediments from Singhbhum near Kolkata suggests that the first stable continents, known as cratons, began to emerge above sea level around 3.3 to 3.2bn year ago.

Monash University's Dr Priyadarshi, who was the study's principal author, stated that the team realized the rocks had to have been formed on land due to the presence of ripple marks, which are similar in appearance to how wind and waves leave marks onto sandy beaches.

He said, "We realized these were ancient riverine [rocks], formed by rivers and estuaries."

Chowdhury stated that the first continents were likely to have formed before the existence plate tectonics. This is what drives today's rise in land mass elevation.

To control elevation, we have plate tectonics. He said that when two continents' [plates] collide you get Himalayas and Alps. "That was not the case three billion years ago."

The sandstone horizons are 3.1bn year old. They were formed on top of the crust of Singhbhum craton shortly after it rose above sea level. Photograph: Subhajit Roy / Monash University

Instead, scientists believe that the first continents emerged from the ocean that covered the earth over 300-400m years of volcanic activity.

Chowdhury suggested that the Singhbhum Craton could have been formed from a lava pile over time so that the crust, approximately 50km thick, "becomes too thick and it just rises above the an iceberg floating on the water."

From the Singhbhum sediments, tiny grains of zircon mineral were extracted by the team. The team was able to determine the age of the rocks by shooting lasers at it and measuring the amount of elements that were released.

Geological similarities have connected the Singhbhum Craton to other cratons in South Africa, Western Australia.

Researchers believe that weathering the cratons would have resulted in nutrient runoff, which would have provided the ocean with phosphorus and other essential building blocks for early development.

Chowdhury stated, "Once land is created, you also create shallow seas like lagoons," which could have accelerated the growth of oxygen-producing organisms that may have increased oxygen levels in the atmosphere or ocean.

Chowdhury stated that the emergence of the first continents would have also drawn carbon dioxide from the atmosphere. This would have led to the formation of glaciers and localised areas of cold climate. "This was the first step in making the earth more habitable."

The journal Proceedings of National Academy of Sciences published the study.