A chemical ghost from a lost world

In the remote, ice-scoured terrain of southwestern Greenland lies the Isua Supracrustal Belt (also called the Isua Greenstone Belt), a formation containing some of the oldest preserved rocks on Earth. These rocks, dating back 3.7 to 3.8 billion years, hold a chemical signature that changes our understanding of Earth's formation. They contain a peculiar excess of an isotope called tungsten-182 (¹⁸²W). This anomaly shows the planet's violent beginnings, a time before the Earth we know today had fully formed.

The story of this anomaly begins with another element, hafnium-182 (¹⁸²Hf). This radioactive isotope, forged in supernovae before our solar system existed, decays into tungsten-182 with a half-life of only about 9 million years. This is extremely short in geological terms, meaning all primordial ¹⁸²Hf decayed into ¹⁸²W within the first 60 million years of the solar system's history. During Earth's formation, hafnium preferred to stay in the rocky, silicate mantle, while tungsten was drawn to the planet's iron core as it separated. The timing of this core formation relative to the decay of ¹⁸²Hf is what makes the tungsten isotope ratio a powerful planetary clock.

Echoes of a planetary collision

About 4.5 billion years ago, a Mars-sized planetoid named Theia is thought to have slammed into the proto-Earth. This cataclysmic event, known as the Giant-Impact Hypothesis, melted a vast portion of the mantle into a churning magma ocean and ejected the debris that would coalesce to form the Moon. This intense melting and mixing should have homogenized the mantle's chemistry, giving all subsequent rocks a uniform ¹⁸²W signature.

The Isua rocks tell a different story. They show a positive ¹⁸²W anomaly, meaning they have more of this isotope compared to modern mantle rocks—an excess of about 13 parts per million. This discovery implies that the part of the mantle from which these rocks originated was never fully mixed back into the rest of the planet after the Moon-forming impact. This ancient, isolated reservoir preserved chemical evidence from a time before the giant collision, a piece of a primordial Earth that has survived for billions of years. The existence of such unmixed domains proves that our planet's mantle is not a well-stirred pot but a complex body with deep, persistent heterogeneities that date back to the very dawn of Earth.

Practical Information

Opening Hours

Open 24/7. This is a remote, unfenced wilderness area, only accessible when free of snow, typically in the summer months.

Admission

Free, though access requires a privately chartered expedition.

Accessibility

The site is extremely remote and inaccessible by public transport. Reaching it requires a chartered helicopter or boat from Nuuk, followed by extensive trekking over rugged, pathless, and challenging terrain.

Earth's Unmixed 3.8-Billion-Year-Old Mantle

A chemical ghost from a lost world

Echoes of a planetary collision

💡Fun Facts

References

There's more in the app

Practical Information

Earth's Unmixed 3.8-Billion-Year-Old Mantle

A chemical ghost from a lost world

Echoes of a planetary collision

💡Fun Facts

References

There's more in the app

Practical Information

Nearby curiosities

Nearby curiosities