From: Andy Soos, ENN
Published March 11, 2013 09:01 AM
Convection at the Center of the Earth
The inner core of the Earth, its innermost part, is a primarily solid
ball with a radius of about 760 miles, according to seismological
studies. It is believed to consist primarily of an iron—nickel alloy,
and to be about the same temperature as the surface of the Sun. A new
study, by an international team of researchers from Leeds, London and
California, states that rocks could be circulating in the inner core
which may explain the unusual behavior of seismic waves passing through
it.
All models that invoke convection to explain the observed seismic
variations in Earth's inner core require unstable inner core
stratification. Previous work has assumed that chemical effects are
stabilizing and focused on thermal convection, but recent calculations
indicate that the thermal conductivity at core temperatures and
pressures is so large that the inner core must cool entirely by
conduction.
Natural convection is a mechanism, or type of heat transport, in which
the fluid motion is not generated by any external source (like a pump,
fan, suction device, etc.) but only by density differences in the fluid
occurring due to temperature gradients. In natural convection, fluid
surrounding a heat source receives heat, becomes less dense and rises.
The surrounding, cooler fluid then moves to replace it. This cooler
fluid is then heated and the process continues, forming a convection
current; this process transfers heat energy from the bottom of the
convection cell to top.
"Slow cooling of the whole Earth is causing the liquid outer core to
solidify from the bottom up, adding to the edges of the solid inner
core. That material, at the top of the inner core, is denser than the
material below," says Dr Chris Davies, one of the authors of the study,
from the University of Leeds, "When you have dense material overlying
light material the light material wants to rise and the dense wants to
sink - making it unstable. It is this instability which causes
convection to occur."
Some researchers assumed the inner core was hotter in the center and
that this change in temperature — from the center to the edge — could
also cause convection as the cool material at the edge wants to sink.
But Davies says the convection is due to heavier, not cooler, material.
"We show the driving force of the convection is different to previous
assumptions. Instead of the driving force being a difference in
temperature, it's now a difference in composition."
"Whilst previous work put a dampener on the convection argument, and it
seemed convection wasn't enough to prove these seismic anomalies, we
show that convection could be behind the striking inner core complexity
we observe." says Davies. "We show that this mechanism is possible, in
principle."
For further information see
Convection.
Core image via Wikipedia.
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