Turbulent mantle nudges Earth's crust up and down
How the thickness of the sea floor changed over time has shown geophysicists that the planet is more dynamic that we thought. Belinda Smith reports.
Earth's surface has had a bumpier ride over history than expected, all thanks to pressures rising from the underlying mantle – a 3,000-kilometre-thick layer of hot rock sandwiched between core and crust.
Three researchers from the University of Cambridge in the UK mapped the ocean bed thickness to "see" what was going on under the crust. They found the wave-like motions of the mantle are ticking along 10 times faster than predicted, and over a million years or so, that wavy flow can push and pull the surface by hundreds of metres.
The work, published in Nature Geoscience, also has ramifications for ocean circulation and climate change, both past and present.
"Although we're talking about timescales that seem incredibly long to you or me, in geological terms, the Earth's surface bobs up and down like a yo-yo," lead author Mark Hoggard says.
For a century, geologists have postulated that the Earth's surface is deformed by stresses and pressures from the dynamic mantle below. German geophysicist Alfred Wegener, in 1912, put forward the concept of "continental drift" – that vast landmasses on Earth "drift" around.
British geologist and champion of continental drift Arthur Holmes proposed in 1928 that radioactivity can create the heat and convection currents needed to churn hot rock to the surface. In 1935, Lithuanian geophysicist Chaim Perekis showed those convection currents are, indeed, capable of pushing and pulling on the overlying surface.
Today, ascertaining what happens below Earth's crust relies mostly on computer simulations and modelling, or measurements taken from the surface. Drilling can only get so far before the rock is so hot it becomes soupy.
So Hoggard, along with Nicky White and David Al-Attar, tracked bulges in the Earth's skin by using oceanic seismic surveys, which send shockwaves into the crust and measures its thickness and density from the reflections. They were also able to date various crustal layers by checking magnetic patterns locked in the rock.
From 2,120 measurements taken from the ocean floor across the globe, they calculated the crust – and the upper mantle layers – oscillated faster than predicted.
Instead of molten mantle waves around 10,000 kilometres long, the trio calculated them at 1,000 kilometres.
Shorter mantle wavelengths meant the Earth's surface bulged and flopped at a faster rate.
"We've never been able to accurately measure these movements before – geologists have essentially had to guess what they look like," Hoggard says.
"Over the past three decades, scientists had predicted that the movements caused continental-scale features which moved very slowly, but that's not the case."
The work can be used by earth and atmospheric scientists to track how the ocean's currents have changed throughout history, as the rise and fall of the sea floor blocks or redirects them. And given that the ocean floor does move faster than expected, Hoggard says, "it could also affect things like the stability of the ice caps and help us understand past climate change".