The typical textbook description of volcanoes erupting when magma gushes out as narrow jets from deep inside the Earth is wrong, according to a new study.
New seismology data are now confirming that such narrow jets don't actually exist, said Don Anderson, Professor at California Institute of Technology.
In fact, he added, basic physics doesn't support the presence of these jets, called mantle plumes, and the new results corroborate those fundamental ideas.
"Mantle plumes have never had a sound physical or logical basis. They are akin to Rudyard Kipling's 'Just So Stories' about how giraffes got their long necks," Anderson said.
According to current mantle-plume theory, Anderson explains, heat from Earth's core somehow generates narrow jets of hot magma that gush through the mantle and to the surface.
The jets act as pipes that transfer heat from the core, and how exactly they're created isn't clear, he said.
But they have been assumed to exist, originating near where the Earth's core meets the mantle, almost 3,000 kilometres underground - nearly halfway to the planet's centre.
The jets are theorised to be no more than about 300 kilometres wide, and when they reach the surface, they produce hot spots.
While the top of the mantle is a sort of fluid sludge, the uppermost layer is rigid rock, broken up into plates that float on the magma-bearing layers.
Magma from the mantle beneath the plates bursts through the plate to create volcanoes. As the plates drift across the hot spots, a chain of volcanoes forms.
"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," Anderson said.
To look for the hypothetical plumes, researchers analysed global seismic activity. Everything from big quakes to tiny tremors sends seismic waves echoing through Earth's interior.
By measuring those waves using hundreds of seismic stations installed on the surface, researchers can deduce whether there are narrow mantle plumes or whether volcanoes are simply created from magma that's absorbed in the sponge-like shallower mantle.
Thanks in part to more seismic stations spaced closer together and improved theory, analysis of the planet's seismology is good enough to confirm that there are no narrow mantle plumes, Anderson and James Natland, a professor at the University of Miami, said.
Instead, data reveal that there are large, slow, upward-moving chunks of mantle a thousand kilometres wide.
The study was published in the journal PNAS.
