December 3, 2001
New details of Earth's internal structure emerge from seismic data
By Tim Stephens
About 1,800 miles beneath the surface, Earth's internal structure changes abruptly
where the solid rock of the mantle meets the swirling molten iron of the outer core.
But the boundary between the core and the mantle may not be as sharply defined as
scientists once thought.
By analyzing earthquake waves that bounce off the core-mantle boundary, UCSC researchers
have found evidence of a thin zone where the outermost core is more solid than fluid.
|In this diagram of a cross-section through the core-mantle boundary, a core-rigidity
zone lies on the fluid core side of the boundary and an ultra-low velocity zone lies
on the solid mantle side. Image: Sebastian Rost and Justin
The existence of such "core-rigidity zones"--small patches of rigid material
within the fluid outer core--has been suggested before, but this report marks the
first time scientists have detected one.
Postdoctoral researcher Sebastian Rost and associate professor of Earth sciences
Justin Revenaugh published their findings in the November 30 issue of the journal
The nature of the core-mantle boundary is important because researchers now think
it influences phenomena ranging from the behavior of Earth's magnetic field to the
massive plumes of hot rock that rise through the mantle and erupt on the surface
at volcanic hot spots such as Hawaii. The interaction of core-rigidity zones with
the magnetic field, for example, may help explain the slow wobbling of Earth's rotation
axis, called nutation, Revenaugh said.
"Studies of Earth's nutation provided one line of evidence that got people thinking
there might be these little patches of rigid material in the outer core," he
said. "So previous evidence was consistent with that idea, but now we have evidence
that cannot be explained any other way."
The picture of the core-mantle boundary has grown increasingly complicated in recent
years with advances in seismic tomography, which uses seismic waves from earthquakes
to probe the internal structure of the Earth. As seismic waves radiate outward from
the epicenter of an earthquake, their speed and other properties are affected by
the different materials they pass through.
In the 1990s, seismic tomography showed the existence of "ultra-low velocity
zones" at the base of the mantle, which some scientists interpret as evidence
of partial melting of the mantle. Rost said an ultra-low velocity zone overlaps the
area where he detected a core-rigidity zone, but that doesn't necessarily mean there
is a connection between the two. He said the structure of the core-mantle boundary
may turn out to be as complex as Earth's surface layer.
"I think what we have down there is just as complicated as the crust,"
Rost said. "I have a dataset that shows a very sharp core-mantle boundary just
a little north of where we detected a core-rigidity zone. As we look at smaller scales,
I think we will see more and more variation."
Rost and Revenaugh studied seismic shear waves, which cannot travel through a fluid
and reflect off the core-mantle boundary. They looked at waves generated by earthquakes
near the islands of Tonga and Fiji in the South Pacific and recorded by an array
of instruments in Australia.
According to Rost, the high quality of the seismic data collected by this array was
essential for detecting the rigid zone, which is only a few miles across and about
150 meters (about 500 feet) thick. "It's very thin and about the size of Santa
Cruz," Revenaugh said.
There are two schools of thought about how this rigid material could occur in the
molten metal of the outer core. One idea is that the core and the mantle react with
one another to produce a material with intermediate density. But this process seems
unlikely to produce a layer more than a few meters thick, Rost said.
The other idea relates to the growth of the solid inner core. As the Earth cools
and heat flows out of the core, iron from the molten outer core solidifies onto the
inner core. This increases the concentration of lighter elements in the outer core,
and if those elements are near the saturation point they will also solidify out.
But because they are lighter than iron, they will float to the top of the core and
collect at the core-mantle boundary.
"You can think of it as an upside-down puddle formed by material rising up to
the top of the core," Revenaugh said.
Whereas puddles of water form at low points on the land, "puddles" of solidified
light elements from the core would form at high points in the core-mantle boundary.
The seismic evidence suggests the rigid zone consists of a solid matrix with some
molten iron in it, Rost said.
"It fits with the idea of an area where solid material has collected within
the liquid outer core," he said.
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