Extreme heat helped Earth’s continents last for billions of years

Earth’s continents became long-lasting when parts of the deep crust briefly heated above about 900°C (roughly 1,650°F). A new study argues that this intense heat allowed the crust to shed its internal heat producers, then cool and toughen for the long haul.

The work centers on the lower crust, about 20 to 25 miles down beneath stable continental interiors. It links a simple driver – heat – to a big outcome, continents that survive for billions of years.

When the crust melted

Lead researcher Andrew J. Smye from Pennsylvania State University (PSU) examined rocks that once sat deep in the crust and reached extreme temperatures.

His team compared hundreds of rock samples from Europe and the southwestern United States, sorting them by the peak temperatures they endured.

Under ultrahigh-temperature conditions, uranium and thorium were stripped from the lower crust and moved upward. That change reduced radiogenic heat, heat from radioactive decay inside rocks, below and let the deep crust cool and strengthen.

Many stable interiors are cratons, a very old, stable block of continental crust that resist breaking for geologic ages. The new mechanism helps explain why cratons stay rigid after early forging.

Heat rising through the crust

The key is ultrahigh-temperature metamorphism. When rocks deep in Earth’s crust heat beyond roughly 1,650°F (900°C), partially melting to form small amounts of silica-rich magma.

This melt rises, carrying uranium and thorium with it, while the leftover rock gradually cools over time.

At lower temperatures, the minerals that host those elements barely dissolve. At higher temperatures, monazite – a phosphate mineral that carries thorium and rare earths – begins to release its cargo into the melt. Zircon, a hardy mineral that often hosts uranium, does the same.

Geologists see this fingerprint in rocks that record these spikes in heat. The residues are depleted in uranium and thorium. The upper crust, by contrast, shows complementary enrichment – a pattern supported by a global review of heat production in crustal rocks.

According to the U.S. Geological Survey (USGS), continental crust averages 19 miles (31 kilometers) thick and extends up to 62 miles (100 kilometers) beneath mountain belts.

In many regions, temperatures rise with depth at roughly 72 to 87°F per mile (about 25 to 30°C per kilometer), based on long-term USGS gradients.

Mining clues in ancient heat

The same ultrahigh-temperature reactions that relocate heat-producing elements can also mobilize rare earth elements – 15 metallic elements used in magnets and lasers.

These elements often bond with phosphate minerals and granitic melts, so their paths are guided by the same thermal events.

Understanding when and where the deep crust hit those temperatures can narrow the search for new deposits. USGS has long noted that rare earths accumulate in minerals like monazite and bastnasite.

That link between heat pulses and mineral transport gives explorers a more physical map. It points to ancient belts where the deep crust was hot enough to move metals, then cooled enough to lock them in place.

Heat is still forming continents

Today, the same ultrahigh-temperature processes continue in rare pockets deep beneath continents.

Beneath mountain belts like the Himalayas and active arcs such as the Andes, temperatures can still exceed 1,650°F (900°C) in the lower crust. Those regions act as natural laboratories, revealing how ancient continents once hardened.

High-pressure zones where plates collide can briefly recreate those ultrahigh-temperature conditions. Beneath continents where the crust thickens and holds radioactive elements, new granitic melts rise, carrying heat and rare elements upward.

Cooling crust, stable continents

The study also reaches beyond Earth. Ultrahigh-temperature differentiation concentrates heat producers near the surface, allowing continents’ deep crust to cool, stiffen, and stabilize.

“Stable continents are a prerequisite for habitability, but in order for them to gain that stability, they have to cool down,” said Smye.

Planets that can reach these conditions in their crusts may keep continents intact for long periods. That, in turn, supports climate regulation through weathering and long-lived land environments.

Ancient heating of continents

Earlier in Earth’s history, more radioactive isotopes were still decaying, so deep crustal heating was stronger. The team’s results suggest ultrahigh-temperature conditions were easier to reach, and stable crust may have formed more readily.

Today’s crust produces less internal heat, so ultrahigh-temperature events are rarer. Where the lower crust is thickened or underplated, however, the right temperatures can still be achieved during mountain building and arc volcanism.

Those settings line up with places where fluid-absent melting, melting driven by heat without added water, is possible. They also align with lower crustal depths near the Moho, the boundary between crust and the mantle, beneath mountain belts and continental arcs.

The ultra-hot episodes need not last long to matter. Even small volumes of melt can drain upward, remove the heat makers, and leave a stronger, cooler base behind.

The study is published in the journal Nature Geoscience.

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