By Michael Marshall


Because of the past position of the continents, the Pacific side of our planet is releasing more heat from Earth’s interior than the African side

Volodymyr Goinyk/Alamy

Our planet is a bit lopsided. One half of Earth is losing heat from the planet’s interior faster than the other, and has been for much of the past 400 million years.

The uneven heat loss is probably a relic of past supercontinents, when all the land masses were joined together on one side of the planet.

“We see that the Pacific has lost more heat,” says Krister Karlsen at the University of Oslo in Norway. “That is in large part due to the distribution of the continental land masses.”

Karlsen and his colleagues reconstructed the rates of heat loss from Earth’s interior over the past 400 million years by combining two existing data sets.

The first concerns the amount of heat from Earth’s interior that flows up through the crust. This data set shows that oceans aren’t as good at trapping heat inside Earth as the continents are, says Karlsen. That is partly down to the thickness of the rock: continental crust is often many kilometres thicker than oceanic crust, so it is a better insulator.

The second data set relates to the movement of the continents deep in prehistory. Some continental rocks carry telltale traces of Earth’s magnetic field, which varies around the globe.

Data from these rocks can be used to show that Earth has, on several occasions, been home to a supercontinent – and it can help establish some of those supercontinents’ approximate position. The most recent supercontinent was Pangaea, which existed from around 335 to 175 million years ago, and was centred roughly where Africa lies today.

Read more: The oceans are very slowly draining into the rock below Earth’s crust

When Karlsen and his colleagues reconstructed the pattern of heat loss through the past 400 million years, they found that more heat had been lost from the Pacific hemisphere of the planet than from the opposite African hemisphere, where Pangaea once lay. The Pacific side of our planet was – and still is – dominated by ocean.

Another factor is the geological activity in the Pacific Ocean. At mid-ocean ridges – long chains of volcanically active mountains on the sea floor – magma cools to form new oceanic crust. Crucially, the mid-ocean ridges in the Pacific create new crust faster than those in the Atlantic Ocean.

“The fast-spreading ridges produce lots of young oceanic crust that can transport heat out quickly,” says Karlsen.

The team also found that rates of heat loss were higher over most of the past 400 million years than they are today.

That is because Earth currently has an unusually large amount of old oceanic crust, according to Karlsen. “Older oceanic crust is thicker and doesn’t allow as much heat to escape,” he says. “The present-day Earth might not be very representative for Earth history.”

“They’re making the case that right now is not typical,” says Louis Moresi at Australian National University in Canberra. “I think that’s right.”

Earth has had several supercontinents in its 4.5-billion-year history, of which Pangaea is just the most recent. The cycle of supercontinent formation and destruction is intimately linked to the heat of Earth’s interior, says Moresi. “The supercontinents insulate the Earth,” he says, so heat accumulates underneath them.

Some of that heat escapes on the supercontinent-free side of the planet, creating the hemispheric imbalance that Karlsen’s team observed. But the heat build-up under the supercontinent may also be what destroys them. “When all the continents come together, they’re pushed together by the plates, so they heat up and everything moves faster and it breaks everything up,” says Moresi.

Journal reference: Geophysical Research Letters, DOI: 10.1029/2020GL092119