An invisible force is creating giant ripples in the Earth’s crust – in a geological blink of an eye
BRYAN LOVELL likes to show his fellow geologists an image of a network of river valleys. “I ask them where they think this might be on Earth,” he says. It is, as you probably guessed, a trick question. The river valleys are in the North Sea, north of Scotland, beneath a kilometre of water and a further 2 kilometres of sediment.
The sea floor here has been slowly sinking ever since it formed. Yet 55 million years ago, something very strange happened. In a geological blink of an eye the sea floor was thrust upwards nearly a kilometre, until it was high and dry above the waves. It remained above the sea for about a million years, long enough for rainwater to carve deep valleys. Then it sank down again. It all happened astonishingly fast.
The big question is why. None of the usual explanations apply. The collision of tectonic plates can lift vast
regions many kilometres up into the air, but there are no colliding plates under the North Sea. Sea level can fall when huge ice sheets form, but not by a kilometre and not in only one area. Volcanic hotspots, where hot molten rock rises up from deep within the Earth, can also produce uplift, but the nearest hotspot is hundreds of kilometres to the west, where it has lifted up Iceland. What’s more, hotspots produce slow uplift over tens of millions of years, not a relatively sudden rise and fall.
The North Sea is not the only example of a geological jack-in-the-box. Something similar seems to have happened in several regions around the world, including the coast of Angola and the Arabian peninsula. In fact, there are many long-standing geological mysteries of this nature.
Now, at long last, there may be an answer. A team from the University of Cambridge reckons that the likely culprits are pulses of particularly hot rock or, to give them their highly technical name, “hot blobs”. These blobs are less dense than the cooler material around them, so when they pop up under the crust they temporarily lift large areas up in the air. In some cases, they appear to spread out in a gradually subsiding circular ripple. (Check out our animated graphic to see how it works). What’s more, in many places there may have been a series of pulses, lifting and dropping the land over and over again.
And while hot blobs are most likely to occur at or near major hotspots, the Cambridge team says, they could pop up almost anywhere. If the idea is correct, it could solve a lot of irritating riddles in geology. Maybe even one of the biggest of all: what triggered one of the hottest periods in Earth’s history. “It’s exciting because you can see ancient problems through new eyes,” says Lovell, a member of the team and president of the UK’s Geological Society. “We are re-examining observations we’ve never understood because we didn’t have this hypothesis to hand.”
That idea that hot blobs exist in the mantle, the region between Earth’s crust and its core, is not new. It has long been known that some parts of the Earth’s mantle are hotter than others. In fact, one of the few things known for sure about the mantle is that its temperature, viscosity and density vary widely. This keeps the mantle constantly churning, as hotter and less dense rock rises towards the crust while cooler and denser slabs sink towards the core. This mantle convection is what powers the movement in plate tectonics, and brings heat to volcanoes and to hotspots such as the plumes beneath Hawaii and Iceland.