October 2012, Week 2


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Sun, 14 Oct 2012 20:50:45 -0400
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No Absolutes: How Shifting Plates Completely Remake 
the Earth
Tracking geologic hotspots shows a wobbly Earth 
in constant flux.
by John Timmer
Ars Technica
October 5 2012

Plate tectonics is one of the most successful theories
in the history of science. Beyond its scientific
successes, it's widely accepted by the public, since it
explains a lot about the world that we see around us.

But like other successful theories, it has its share of
awkward inconsistencies. A recent paper in the Journal
of Geophysical Research attempted to tackle one of these
inconsistencies-finding an absolute reference frame for
the movement of the plates-but failed so badly that its
authors advise other scientists not to even bother
trying. But as part of their failure, they came up with
a new measure of one of the more unexpected consequences
of plate tectonics.

All of plate tectonics is driven by density differences
in the material beneath our planet's solid surface.
These drive the shifting plates, power hot-spot
volcanoes, and recycle material to the planet's surface.
They also make sure that the mass of the Earth is never
evenly distributed. As that mass shifts internally, it
actually causes the Earth's spin to wobble around a bit.
As a result, even the Earth's axis of rotation doesn't
provide an absolute reference frame. In the process of
failing to find an absolute reference frame, though, the
authors have provided a detailed map of how the Earth's
true pole has wandered over millions of years. Making
the numbers add up

One of the larger successes of plate tectonics is that
it offers an explanation for island chains. Groups of
volcanic islands, like the Hawaiian islands, can trace
straight lines for thousands of miles if one considers
the largely submerged remains of former islands. Plate
tectonics offers an explanation: there are stationary
hot spots in the mantle that drive volcanic eruptions.
The plates simply slide over a hot spot, which builds
volcanoes that later become inactive and erode as they
slide past.

This process is so regular that it's one of the ways
that scientists have tracked past plate motion. For the
Hawaiian chain, it's even possible to see a sudden left
turn, as the Pacific plate changed its direction of
motion. These measurements tend to line up well with
others based on measurements made at the boundary of

So, stationary hot spots, moving plates. That would make
hot spots a great reference frame for plate movement.
Just pick absolute locations for the hot spots, then you
could track the entire planet's plates as they slid
across them. Just one small problem: it doesn't work.
"It was soon realized," the authors write, "that a
reference frame defined by fixed hot spots from the
Pacific Ocean could not adequately reproduce hot spot
tracks in the Indian and Atlantic Ocean."

In other words, although a hot spot appears to be a
fixed reference frame for a given plate and its
neighbors, our best data indicates that different hot
spots appear to be moving relative to each other.

But our best data is an ever-changing thing, and the
authors decided it was time for another try. They went
through the literature and pulled out any information
they could find about rates and directions of plate
motion, and integrated it all into a single model.
Despite several iterations that made for a progressively
better fit to the data from individual hotspots, there
was no way to get things to work out globally. "Our
attempts to define a global fixed hot spot reference
frame have failed to produce acceptable fits to the
segments of hot spot tracks formed from Late Cretaceous
to Paleogene time (80-50 Ma [million years])," the
authors concede.

Their conclusion? It's time to give up on the idea of
hot spots being fixed. If we're ever going to have an
absolute reference frame, it's not going to come from
hot spots. Shifting references on an unstable Earth

Based on the paper, though, it's hard to tell what else
might provide an absolute reference. The authors' work
provides further evidence that the entire surface of the
Earth (the lithosphere) is moving relative to its
interior, the mantle. In other words, the surface of the
Earth is not completely coupled to the core, something
that had previously been suggested to be the case for
Saturn's moon Titan. In the case of Earth, the so-called
"lithosphere rotation" involves a slow drift westward,
shifting about a tenth of a degree every million years
on average. However, the rate isn't even, and has been
nearly three times that at some point in the past,
apparently at the time when the Indian plate was
accelerating toward Asia.

It isn't just that we lack a fixed reference frame to
track the plates. It's that plate tectonics itself
shifts such enormous masses around that these skew the
reference frame.

As it turns out, this also wipes out another potential
reference frame, the axis of the Earth's rotation. If
the Earth were a uniform, solid sphere, its axis of
rotation would remain stable. But the whole idea behind
plate tectonics is that the mass isn't distributed
evenly. The hot spots at issue here push to the surface
of the crust precisely because they're hotter and thus
less dense than the surrounding material. On larger
scales, it's this density-driven convection that powers
the shifting of the continents themselves. As crust is
driven into the Earth's interior at subduction zones, it
places sheets of solid material deep under the crust
that take millions of years to come to equilibrium with
their new surroundings.

So, not only is the Earth not uniform, but its internal
differences are constantly shifting around. All of which
feeds back into the dynamics of its rotation, leading to
a phenomenon called "true polar wander"-its axis of
rotation hasn't always run through the sites of the
current North and South Poles. In fact, during the
period from 90 million to 40 million years ago, the
poles drifted nearly 10 degrees and then snapped back.
Technicalities and the big picture

The paper itself is long, dry, and very technical; it's
not the sort of thing that I'd recommend anyone read
unless they're actively working in this field. But the
ideas within it are important and compelling.

One important idea is that even our most successful
scientific theories are filled with enough discrepancies
and inconsistencies to keep scientists gainfully
employed for generations. But it's important to keep
these in perspective. Not knowing something, or even
getting it wrong, doesn't mean that we don't know
anything, or that every little inconsistency means we
should throw the entire structure out.

The story of plate tectonics itself highlights how
oddities on their own aren't enough to overthrow a
dominant idea. Instead, you have to come up with
something that explains not only the discrepancies, but
everything else that the dominant idea gets right. Even
then, it's not easy, something that is very clear from
the reception that plate tectonics got when it was first
suggested almost precisely 100 years ago.

One of the reasons that many people have a hard time
accepting some aspects of science is that these ideas
make them feel uncomfortable. Plate tectonics doesn't
have the same emotional impact as the Copernican
revolution, which told us that our place in the Universe
wasn't special. But it does tell us that our place
wasn't even a place for most of its history: continents
shift, islands grow and vanish, and there are apparently
no fixed frames of reference.

Without the energy brought to the Earth's surface by
plate tectonics, however, it's not even clear that life
itself would have been able to flourish, and it
certainly wouldn't have evolved the way it has without
the changing climates and landscapes.

Giving up a fixed frame of reference to have all that
seems like a worthwhile tradeoff.

Journal of Geophysical Research, 2012. DOI:
10.1029/2011JB009072, 2012


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