The continental crust of the Earth could have emerged 500 million years earlier than scientists expected. Determining the moment of the appearance of land on the planet will help to understand the conditions in which primitive life originated.
The first appearance and preservation of the continental crust on Earth during the Archean (4–2.5 billion years ago) has important implications for plate tectonics, ocean chemistry, and biological evolution. This happened about half a billion years earlier than previously thought, according to a new study presented at the General Assembly of the European Geosciences Union (EGU) in 2021.
Today, oceanic crust rises on the mid-ocean ridge where tectonic plates diverge. In turn, the continental crust is much older and was formed as a result of volcanism, when plates slammed into each other, raising a thicker and less dense layer of rock above sea level. It is contrasted with the oceanic crust, which is different in structure and composition.
The weathering process of the continental crust added nutrients to the ocean. It was this process that may have played a role in maintaining primitive life. The question remains – when did the Earth’s crust begin to form?
To try to answer this question, Desiree Roerdink of the University of Bergen in Norway and her colleagues analyzed 30 ancient rock samples from six locations in Australia, South Africa, and India. They contain barite, which can form in hydrothermal vents – cracks in the ocean floor where warm, mineral-rich waters react with seawater.
“Barites don’t really change, their chemical composition contains an imprint of the environment in which they were formed,” explains Roerdink.
She and her team used the strontium isotope ratio in the sediment. The goal is to find out when the weathering crust began to enter the oceans. They found that weathering began about 3.7 billion years ago.
When the Earth formed 4.5 billion years ago, its landscape consisted of molten rock. Eventually, the outer layer of the planet cooled so much that it began to form a hard crust covered by the world’s oceans.
This marked the beginning of a new geological aeon about 4 billion years ago, known as the Archean. It was then, according to scientists, that life first appeared. There is compelling evidence for microbial activity at least 3.5 billion years ago, but it is not known exactly when or how life began.
Aaron Satkoski of the University of Texas at Austin says the new study suggests life may have originated on land, not in the oceans.