Ancient Seawater May Have Helped Build Earth’s First Continents

A research team from the HKU Department of Earth and Planetary Sciences has uncovered new evidence that Earth’s earliest continents may have formed from rocks that had once interacted with ancient seawater and the surface environment, rather than from purely deep, untouched rocks inside the Earth.

The study, published in Nature Communications, focuses on ancient granitoid rocks from the North China Craton, one of the world’s oldest continental regions. These rocks belong to a group known as tonalite-trondhjemite-granodiorite, or TTG, which makes up a major part of Earth’s early continental crust.

For decades, scientists have debated how these ancient continental rocks formed. While it is widely accepted that TTGs were produced by the partial melting of water-rich mafic rocks, the origin of those mafic source rocks has remained uncertain. Were they formed deep inside the Earth, or had they once existed near the surface and been altered by seawater before being buried and melted?

To answer this question, the team analysed sulfur and silicon isotopes in the ancient rocks. These isotopes act like geological fingerprints. Sulfur isotopes can preserve signals from Earth’s early atmosphere and surface sulfur cycle, while silicon isotopes can reveal whether rocks had been altered by seawater or silica-rich surface processes.

The researchers found both non-zero sulfur isotope signatures and enriched silicon isotope values in the ancient granitoids. Together, these signals indicate that the rocks were derived from “supracrustal” sources — materials that had once been exposed at the surface or near-surface environments, such as the seafloor, before being transported into deeper parts of the Earth and melted to form continental crust.

In simple terms, the findings suggest that ancient seafloor rocks interacted with seawater, were later buried deep within the Earth, and eventually melted, helping to form some of the earliest continental crust.

The discovery has broader implications for understanding early Earth. It suggests that surface-to-interior recycling — the movement of materials from Earth’s surface into its deep interior — was already active in the Archean Eon, more than 2.5 billion years ago. This recycling may have played an important role in building stable continents and shaping Earth into a long-term habitable planet.

This article is based on the research paper “Coupled sulfur-silicon isotopes reveal supracrustal origin of Archean continents”, published in Nature Communications. DEPS PhD student Kun SHANG is the first author, with Professors Jian ZHANG and Guochun ZHAO as corresponding authors. The study was conducted in collaboration with the University of Science and Technology of China.