Imagine a shift in the Earth so profound that it could force our
entire planet to spin on its side after a few million years, tilting it
so far that Alaska would sit at the equator. Princeton scientists have
now provided the first compelling evidence that this kind of major
shift may have happened in our world's distant past.
By analyzing the magnetic composition of ancient sediments found in the
remote Norwegian archipelago of Svalbard, Princeton University's Adam
Maloof has lent credence to a 140-year-old theory regarding the way the
Earth might restore its own balance if an unequal distribution of
weight ever developed in its interior or on its surface.
The theory, known as true polar wander, postulates that if an object of
sufficient weight -- such as a supersized volcano -- ever formed far
from the equator, the force of the planet's rotation would gradually
pull the heavy object away from the axis the Earth spins around. If the
volcanoes, land and other masses that exist within the spinning Earth
ever became sufficiently imbalanced, the planet would tilt and rotate
itself until this extra weight was relocated to a point along the
equator.
"The sediments we have recovered from Norway offer the first good
evidence that a true polar wander event happened about 800 million
years ago," said Maloof, an assistant professor of geosciences. "If we
can find good corroborating evidence from other parts of the world as
well, we will have a very good idea that our planet is capable of this
sort of dramatic change."
Maloof's team, which includes researchers from Harvard University, the
California Institute of Technology and the Massachusetts Institute of
Technology as well as Princeton, will publish their findings in the
Geological Society of America Bulletin on Friday, Aug. 25.
True polar wander is different from the more familiar idea of
"continental drift," which is the inchwise movement of individual
continents relative to one another across the Earth's surface. Polar
wander can tip the entire planet on its side at a rate of perhaps
several meters per year, about 10 to 100 times as fast as the
continents drift due to plate tectonics. Though the poles themselves
would still point in the same direction with respect to the solar
system, the process could conceivably shift entire continents from the
tropics to the Arctic, or vice versa, within a relatively brief
geological time span.
While the idea that the continents are slowly moving in relation to one
another is a well-known concept, the less familiar theory of true polar
wander has been around since the mid-19th century, several decades
before continental drift was ever proposed. But when the continents
were proven to be moving under the influence of plate tectonics in the
1960s, it explained so many dynamic processes in the Earth's surface so
well that true polar wander became an obscure subject.
"Planetary scientists still talk about polar wander for other worlds,
such as Mars, where a massive buildup of volcanic rock called Tharsis
sits at the Martian equator," Maloof said. "But because Earth's surface
is constantly changing as the continents move and ocean crustal plates
slide over and under one another, it's more difficult to find evidence
of our planet twisting hundreds of millions of years ago, as Mars
likely did while it was still geologically active."
However, the sediments that the team studied in Svalbard from 1999 to
2005 may have provided just such long-sought evidence. It is well known
that when rock particles are sinking to the ocean floor to form layers
of new sediment, tiny magnetic grains within the particles align
themselves with the magnetic lines of the Earth. Once this rock
hardens, it becomes a reliable record of the direction the Earth's
magnetic field was pointing at the time of the rock's formation. So, if
a rock has been spun around by a dramatic geological event, its
magnetic field will have an apparently anomalous orientation that
geophysicists like those on Maloof's team seek to explain.
"We found just such anomalies in the Svalbard sediments," Maloof said.
"We made every effort to find another reason for the anomalies, such as
a rapid rotation of the individual crustal plate the islands rest upon,
but none of the alternatives makes as much sense as a true polar wander
event when taken in the context of geochemical and sea level data from
the same rocks."
The findings, he said, could possibly explain odd changes in ocean
chemistry that occurred about 800 million years ago. Other similar
changes in the ocean have cropped up in ancient times, Maloof said, but
at these other times scientists know that an ice age was to blame.
"Scientists have found no evidence for an ice age occurring 800 million
years ago, and the change in the ocean at this juncture remains one of
the great mysteries in the ancient history of our planet," he said.
"But if all the continents were suddenly flipped around and their
rivers began carrying water and nutrients into the tropics instead of
the Arctic, for example, it could produce the mysterious geochemical
changes science has been trying to explain."
Because the team obtained all its data from the islands of Svalbard,
Maloof said their next priority would be to seek corroborating evidence
within sediments of similar age from elsewhere on the planet. This is
difficult, Maloof said, because most 800-million-year-old rocks have
long since disappeared. Because the Earth's crustal plates slide under
one another over time, they take most of geological history back into
the planet's deep interior. However, Maloof said, a site his team has
located in Australia looks promising.
"We cannot be certain of these findings until we find similar patterns
in rock chemistry and magnetics on other continents," Maloof said.
"Rocks of the same age are preserved in the Australian interior, so
we'll be visiting the site over the next two years to look for
additional evidence. If we find some, we'll be far more confident about
this theory's validity."
Maloof said that true polar wander was most likely to occur when the
Earth's landmasses were fused together to form a single supercontinent,
something that has happened at least twice in the distant past. But he
said we should not worry about the planet going through a major shift
again any time soon.
"If a true polar wander event has occurred in our planet's history,
it's likely been when the continents formed a single mass on one side
of the Earth," he said. "We don't expect there to be another event in
the foreseeable future, though. The Earth's surface is pretty well
balanced today."
Maloof's research was sponsored in part by the National Science Foundation. The five-year field work in Svalbard was led by Maloof and Harvard University's Galen Halverson.
Abstract
Combined paleomagnetic, isotopic, and stratigraphic evidence for true
polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard,
Norway
By Adam C. Maloof, Galen P. Halverson, Joseph L. Kirschvink, Daniel P. Schrag, Benjamin P. Weiss, and Paul F. Hoffman
We present new paleomagnetic data from three Middle Neoproterozoic
carbonate units of East Svalbard, Norway. The paleomagnetic record is
gleaned from 50 to 650 m of continuous, platformal carbonate sediment,
is reproduced at three locations distributed over >100 km on a
single craton, and scores a 5–6 (out of 7) on the Van der Voo (1990)
reliability scale. Two >50° shifts in paleomagnetic direction are
coincident with equally abrupt shifts in delta-Carbon 13 and transient
changes in relative sea level. We explore four possible explanations
for these coincidental changes: rapid plate tectonic rotation during
depositional hiatus, magnetic excursions, nongeocentric axial-dipole
fields, and true polar wander. We conclude that the observations are
explained most readily by rapid shifts in paleogeography associated
with a pair of true polar wander events. Future work in sediments of
equivalent age from other basins can test directly the true polar
wander hypothesis because this type of event would affect every
continent in a predictable manner, depending on the continent’s
changing position relative to Earth’s spin axis.
Keywords: polar wandering, paleomagnetism, Svalbard, Neoproterozoic, carbon cycle, paleogeography.
