A huge slab of folded Earth that scientists think used to be part
of the ocean floor has been detected near the planet's core.
The discovery supports the theory that Earth's
crust is constantly recycled deep into the planet as molten
material from below simultaneously pushes up to refresh the
surface.
The structure is about 125 miles deep and at
least 125 miles wide and 370 miles in the north-south direction.
In consistency, it is more like a
giant, folding mush of taffy, researchers said today.
"If you imagine cold honey pouring onto a plate,
you would see ripples and folds as it piles up and spreads out,
and that's what we think we are seeing at the base of the mantle,"
said Alex Hutko, a graduate student University of California,
Santa Cruz and lead author of a paper describing the discovery in
the May 18 issue of the journal Nature.
Giant recycling machine
The slab began its plunge toward the
center of the Earth about 50 million years ago. It is denser
than surrounding material, which is why it sinks. Its lower
reaches are near the core, about 1,740 miles down. Yet it is still
attached to the surface, much like a conveyor belt.
"It's like a carpet sliding off the dining room
table," said study team member Edward Garnero of Arizona State
University. "If it is more than half way off, it just goes taking
everything with it."
Earth is divided into three main layers: the
core, mantle and crust. The crust, a thin surface layer, is
divided into more than a dozen major plates. In the middle of the
Pacific Ocean, plates spread apart and fresh material from the
mantle wells up.
Along the west coast of North America, crust
beneath the ocean
dives under a continental plate, creating
earthquakes and
volcanoes. Geologists have long speculated that when crust is
folded into the planet, it sinks to the bottom of the mantle,
where it displaces the material down there and forces some of it
up.
"Since there is a conservation of mass in the
mantle, something must return as the slab sinks into the Earth,"
Garnero explained. "This return flow can include plumes of hot
material that gives rise to volcanism."
Seeing sound
If the scientists have correctly interpreted
their data, the folding slab is the first hard evidence that
sinking crust drives the upwelling of material so deep inside the
planet.
"It's the first evidence from direct imaging to
support the idea that ancient seafloor makes its way down to the
bottom of the mantle," Hutko said.
The slab was found by monitoring seismic
waves—generated by earthquakes in South America—reflecting from
deep inside the mantle and recorded in the United States.
The diving crust is made of essentially the same
material as the lower mantle, the researchers said, but it is much
cooler, by about 1,260 degrees Fahrenheit. The lower mantle is
roughly 4,500 degrees.
Seismic waves are altered as they move through
the hot and cooler regions, which allowed computer programs to
generate the picture of the slab. It is possible, Garnero told
LiveScience, that they are just seeing a formation of rock
from the mantle that has different chemical components, but the
temperature difference is best explained by crustal material that
has been compressed, he said.
The sound-imaging technique also revealed plumes
of hot material at the lower edges of the slab.
"We think there is a kind of pushing and
bulldozing away of a hot basal layer of the mantle, giving rise to
small plumes at the edges," Hutko said.
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What's Down There
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 The
Earth’s radius is about 4,000 miles (6,400
kilometers). The main layers of its interior are in
descending order: crust, mantle and core.
The
crust thickness averages about 18 miles (30
kilometers) under the continents, but is only about 3
miles (5 kilometers) under the oceans. It is light and
brittle and can break. In fact it's fractured into
more than a dozen major plates and several minor ones.
It is where most earthquakes originate.
The
mantle is more flexible – it flows instead of
fractures. It extends down to about 1,800 miles (2,900
kilometers) below the surface.
The
core consists of a solid inner core and a fluid outer
core. The fluid contains iron, which, as it moves,
generates the Earth’s magnetic field. The crust and
upper mantle form the lithosphere, which is broken up
into several plates that float on top of the hot
molten mantle below.
SOURCE: LiveScience reporting |
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