May 6, 2008
Blobs Inside Earth Like Peanut Butter
You know Earth's schematic: core, mantle, crust, right? Sorry, not so simple.
Like the gooey center of a chocolate morsel harboring peanut butter
and honey, inner Earth is far more nuanced than outward appearances
would suggest. A new model is proposed in the May 2 issue of the
Earth is made up of several layers, once thought to be pretty distinct.
The skin, or crust, goes down about 25 miles (40 km). Below that is the mantle area, which extends about halfway to the center of the planet. The mantle is a thick layer of silicate rock surrounding a dense, iron-nickel core,
and it is subdivided into the upper and lower mantle, extending to a
depth of about 1,800 miles (2,900 km). The outer core is beneath that
and extends to 3,200 miles (5,150 km) and the inner core to about 4,000
miles (6,400 km).
New data reveal the mantle consists of more varying material than
was thought. So convection - how heated material bubbles up - is now
thought to work differently.
"Imagine a pot of water boiling," explains researcher Allen McNamara
of Arizona State University. "That would be all one kind of
composition. Now dump a jar of honey into that pot of water. The honey
would be convecting on its own inside the water and that's a much more
One clue to the new thinking is that seismic waves traveling through
the planet have long been measured to travel at inexplicably different
speeds. Sharp speed changes suggest differing materials. On each side
of the planet there are two big, chemically distinct, dense piles or blobs of
material that are hundreds of kilometers thick - one beneath the
Pacific and the other below the Atlantic and Africa, the researchers
"You can picture these piles like peanut butter," McNamara said. "It
is solid rock, but rock under very high pressures and temperatures
becomes soft like peanut butter, so any stresses will cause it to flow."
How stuff moves within the piles should help scientists better understand how surface plates move around, causing earthquakes and building mountains.
"The piles dictate how the convective cycles happen, how the
currents circulate," McNamara said. "If you don't have piles then
convection will be completely different."