Today, the Earth’s surface is made up of continental and oceanic crust. Continental crust makes up the land we live on, and oceanic crust makes up the ocean floors. One of the major differences between oceanic crust and continental crust is that continental crust incorporates water into its minerals as it forms.
Nowadays, when continental and oceanic crust collides, the heavier, denser oceanic crust gets thrust down under the continental crust. As it plunges down into the Earth, the oceanic crust takes ocean water with it. Eventually, the heat and pressure cause this oceanic crust to melt and combine with the water, forming a lighter weight magma that rises and cools to form new continental crust. This process is called subduction. However, when the earth first formed there were no continents and therefore no subduction, so where did the water needed to create the earliest continental crust come from?
New research by a team from the Geological Survey of Western Australia and Curtin University led by Dr Hugh Smithies has uncovered the source of this water. The team studied tiny crystals, called zircons, collected from some of Earth’s earliest continental crust located in the Pilbara, Western Australia. Compositional analysis of these zircons at Microscopy Australia’s University of Western Australia facility allowed the team to determine both the source of the water that formed them, and the date at which they formed. It also revealed that early continental crust did not form from surface water as it does today, but instead from primordial water, that already existed deep within the Earth.
This implies that the Earth contained far more primordial water than previously thought and will alter how scientists think about how our planet formed. It also challenges one of the central tenets of physical sciences: uniformitarianism, or the idea that physical processes in the present reflect the physical processes of the past.
R. H. Smithies et al., Nature 2021