The
inner core of the Earth is simultaneously melting and freezing due to
circulation of heat in the overlying rocky mantle, according to new
research from the University of Leeds, UC San Diego and the Indian
Institute of Technology.
The findings, published today in Nature,
could help us understand how the inner core formed and how the outer
core acts as a ‘geodynamo’, which generates the planet’s magnetic field.
“The
origins of Earth’s magnetic field remain a mystery to scientists,” said
study co-author Dr Jon Mound from the University of Leeds. “We can’t go
and collect samples from the centre of the Earth, so we have to rely on
surface measurements and computer models to tell us what’s happening in
the core.”
“Our
new model provides a fairly simple explanation to some of the
measurements that have puzzled scientists for years. It suggests that
the whole dynamics of the Earth’s core are in some way linked to plate
tectonics, which isn’t at all obvious from surface observations.
“If
our model is verified it’s a big step towards understanding how the
inner core formed, which in turn helps us understand how the core
generates the Earth’s magnetic field.”
The
Earth’s inner core is a ball of solid iron about the size of our moon.
This ball is surrounded by a highly dynamic outer core of a liquid
iron-nickel alloy (and some other, lighter elements), a highly viscous
mantle and a solid crust that forms the surface where we live.
Over
billions of years, the Earth has cooled from the inside out causing the
molten iron core to partly freeze and solidify. The inner core has
subsequently been growing at the rate of around 1mm a year as iron
crystals freeze and form a solid mass.
The
heat given off as the core cools flows from the core to the mantle to
the Earth’s crust through a process known as convection. Like a pan of
water boiling on a stove, convection currents move warm mantle to the
surface and send cool mantle back to the core. This escaping heat powers
the geodynamo and coupled with the spinning of the Earth generates the
magnetic field.
Scientists
have recently begun to realise that the inner core may be melting as
well as freezing, but there has been much debate about how this is
possible when overall the deep Earth is cooling. Now the research team
believes they have solved the mystery.
Using
a computer model of convection in the outer core, together with
seismology data, they show that heat flow at the core-mantle boundary
varies depending on the structure of the overlying mantle. In some
regions, this variation is large enough to force heat from the mantle
back into the core, causing localised melting.
The
model shows that beneath the seismically active regions around the
Pacific ‘Ring of Fire’, where tectonic plates are undergoing subduction,
the cold remnants of oceanic plates at the bottom of the mantle draw a
lot of heat from the core. This extra mantle cooling generates
down-streams of cold material that cross the outer core and freeze onto
the inner core.
Conversely,
in two large regions under Africa and the Pacific where the lowermost
mantle is hotter than average, less heat flows out from the core. The
outer core below these regions can become warm enough that it will start
melting back the solid inner core.
Co-author
Dr Binod Sreenivasan from the Indian Institute of Technology said: “If
Earth’s inner core is melting in places, it can make the dynamics near
the inner core-outer core boundary more complex than previously thought.
“On
the one hand, we have blobs of light material being constantly released
from the boundary where pure iron crystallizes. On the other hand,
melting would produce a layer of dense liquid above the boundary.
Therefore, the blobs of light elements will rise through this layer
before they stir the overlying outer core.
“Interestingly,
not all dynamo models produce heat going into the inner core. So the
possibility of inner core melting can also place a powerful constraint
on the regime in which the Earth’s dynamo operates.”
Co-author
Dr Sebastian Rost from the University of Leeds added: “The standard
view has been that the inner core is freezing all over and growing out
progressively, but it appears that there are regions where the core is
actually melting. The net flow of heat from core to mantle ensures that
there’s still overall freezing of outer core material and it’s still
growing over time, but by no means is this a uniform process.
“Our
model allows us to explain some seismic measurements which have shown
that there is a dense layer of liquid surrounding the inner core. The
localised melting theory could also explain other seismic observations,
for example why seismic waves from earthquakes travel faster through
some parts of the core than others.”
