A new study shows Earth’s first continents, known as cratons, emerged from the ocean between 3.3 and 3.2 billion years ago. Previous estimates suggested that large-scale craton emergence took place roughly 2.5 billion years ago.
The evidence was from sedimentary rocks – which form from the bocken-up bits of other rocks that have undergone erosion and weathering – that date back to that era. Such sedimentary rocks could only form once land had penetrated the surface of the early Earth’s oceans.
The researchers suggest that entire cratons, not just small patches of land, emerged from the oceans 3.3 billions years ago, even though the planet lacked the plate tectonics needed to drive those floating bits of crust upward.
For the new study, the authors trekked to the Singhbhum Craton, located in eastern India. Pockets of ancient sedimentary rocks had previously been found at the craton, and the team wanted to determine their exact ages and how they formed.
To figure out what forces first drove the Singhbhum Craton out of the water, the authors sampled igneous rocks from the cratons, meaning rocks formed through the crystallization of hot magma; these igneous rocks lie just below the sedimentary rocks in the craton, forming a basement.
The model suggests that, about 3.5 billion and 3.2 billion years ago, hot plumes of magma beneath the crust caused portions of the craton to thicken and become enriched with buoyant, lightweight materials, like quartz and silica. This process left the craton chemically light and physically thick, as compared to the denser rock surrounding it, and thus buoyed the land mass up and out of the water.
https://www.livescience.com/earth-first-continents-cratons-study