A 3.95-billion-year-old rock from northern China suggests Earth’s plates may have begun diving beneath one another almost 4 billion years ago

Geologists have found a 3.95-billion-year-old rock in northern China whose chemistry suggests a piece of seafloor was pushed deep into the Earth and melted at that time, an early sign of subduction, the process in which one of Earth’s plates sinks beneath another, according to a study published Nov. 11 in the journal Nature.

If the interpretation holds, it pushes evidence of that process back to Earth’s earliest days.  How and when plate tectonics began is one of geology’s long-running debates, the authors wrote.  Plate tectonics recycles Earth’s crust and causes earthquakes, including along California’s San Andreas fault zone, where two plates slide past each other, according to the U.S. Geological Survey.

What they found

• The rock is a gneiss, a banded rock formed under heat and pressure, from eastern Hebei province in the North China Craton, an ancient core of the continent.  Tiny zircon crystals in it date to 3.95 billion years.
• The only other rocks confirmed to be older than 3.9 billion years are in the Acasta Gneiss Complex in northern Canada, the paper says.
• Oxygen in the zircons points to a source of ocean-floor rock that had been altered by water near Earth’s surface.  Hafnium, another element, shows the rock’s source came from a mantle that had already been chemically depleted during the Hadean, Earth’s earliest eon, more than 4 billion years ago.
• Modeling indicates the rock’s parent magma formed when that crust partly melted at more than 1.6 gigapascals, roughly 16,000 times the air pressure at sea level, and 740°C to 820°C (about 1,360°F to 1,510°F).

That combination, surface rock melted at high pressure but fairly low temperature, means the crust was buried deep and fast, the authors wrote.  They propose the most likely cause was a brief, or transient, subduction event.  A Canadian rock of about the same age appears to have formed at low pressure, so the two may record different processes.

Limits

The conclusion comes from one rock unit and from chemistry and computer modeling, not from direct observation of an ancient plate boundary.  The authors describe a transient event, which is not the same as the continuous, worldwide plate tectonics seen today.  The study was led by the Beijing SHRIMP Center at the Chinese Academy of Geological Sciences, with Jilin University, Nanjing University and Curtin University in Australia.

Sources:  Xie et al., “Hadean mantle depletion and 3.95-Gyr subduction implied by North China rocks,” Nature, Nov. 11, 2026; U.S. Geological Survey, “Understanding plate motions”.