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Forget everything you learned about volcanoes in your geography class.

The textbook explanation of magma being spewed out from narrow jets deep within Earth is wrong, according to a new study.

Researchers now believe that these narrow jets – called mantle plumes - don't exist, and they say basic physics can back up their findings.

According to current mantle-plume theory, heat from Earth's core somehow generates narrow jets of hot magma that gush through the mantle and to the surface. But scientists now believe this theory is incorrect

'Mantle plumes have never had a sound physical or logical basis,' said Don Anderson, a professor at the California Institute of Technology. 

According to current mantle-plume theory, heat from Earth's core somehow generates narrow jets of hot magma that gush through the mantle and to the surface. 

They have been assumed to exist, originating near where the Earth's core meets the mantle, almost 1,860 miles (3,000km) underground - nearly halfway towards the planet's centre.

While the top of the mantle is a type of fluid sludge, the uppermost layer is rigid rock, broken up into plates that float on the magma-bearing layers.

The textbook image (left) of magma being spewed out from narrow jets deep within Earth mantle (right)  is wrong, according to a new study.  Researchers now believe that those narrow jets don't exist

Magma from the mantle beneath the plates bursts through the plate to create volcanoes.

'Much of solid-Earth science for the past 20 years - and large amounts of money - have been spent looking for elusive narrow mantle plumes that wind their way upward through the mantle,' said Professor Anderson.

Now, thanks in part to more seismic stations spaced closer together, analysis of the planet's seismology is good enough to confirm that there are no narrow mantle plumes.

Instead, data reveals that there are large, slow, upward-moving chunks of mantle 620 miles (1,000km) wide.

In the mantle-plume theory, Professor Anderson explains, the heat that is transferred upward via jets is balanced by the slower downward motion of cooled, broad, uniform chunks of mantle.

The new measurements suggest that what is really happening is just the opposite: Instead of narrow jets, there are broad 'upwellings', which are balanced by narrow channels of sinking material called slabs.

What is driving this motion is not heat from the core, but cooling at Earth's surface.

In fact, Professor Anderson says, the behaviour is the regular mantle convection first proposed more than a century ago by Lord Kelvin.

When material in the planet's crust cools, it sinks, displacing material deeper in the mantle and forcing it upward.

'What's new is incredibly simple: upwellings in the mantle are thousands of kilometres across,' Professor Anderson said.

When material in the planet's crust cools, it sinks, displacing material deeper in the mantle and forcing it upward. Magma, which is less dense than the surrounding mantle, rises until it reaches the bottom of the plates or fissures that run through them, according to the theory

He calls this theory 'top-down tectonics,' based on Kelvin's initial principles of mantle convection.

Magma, which is less dense than the surrounding mantle, rises until it reaches the bottom of the plates or fissures that run through them.

Stresses in the plates, cracks, and other tectonic forces can squeeze the magma out, in a similar way to how water is squeezed out of a sponge.

That magma then erupts out of the surface as volcanoes. The magma comes from within the upper 124 miles (200km) of the mantle and not thousands of miles deep, as the mantle-plume theory suggests.

'This is a simple demonstration that volcanoes are the result of normal broad-scale convection and plate tectonics,' Professor Anderson says.