https://www.nytimes.com/2018/12/18/science/plate-tectonics-continents-earth.html?em_pos=large&emc=edit_sc_20181218&nl=science-times&nlid=74046398edit_sc_20181218&ref=img&te=1
The Earth’s Shell Has Cracked, and We’re Drifting on the Pieces
Plate
tectonics helped make our planet stable and habitable. But slow
peregrination of continents is still a mysterious process.
The
theory of plate tectonics is one of the great scientific advances of
our age, right up there with Darwin’s theory of evolution and Einstein’s
theory of relativity.
The idea that
Earth’s outer shell is broken up into giant puzzle pieces, or plates,
all gliding atop a kind of conveyor belt of hot, weak rock — here rising
up from the underlying mantle, there plunging back into it — explains
much about the structure and behavior of our home planet: the mountains
and ocean canyons, the earthquakes and volcanoes, the very composition
of the air we breathe.
Yet success is
no guarantee against a midlife crisis, and so it is that half a century
after the basic mechanisms of plate tectonics were first elucidated,
geologists are confronting surprising gaps in their understanding of a
concept that is truly the bedrock of their profession.
They
are sparring over when, exactly, the whole movable plate system began.
Is it nearly as ancient as the planet itself — that is, roughly 4.5
billion years old — or a youthful one billion years, or somewhere in
between?
They
are asking what caused the shell to crack apart in the first place, and
how the industrious recycling of Earth’s crust began.
They
are comparing Earth with its sister planet, Venus. The two worlds are
roughly the same size and built of similar rocky material, yet Earth has
plate tectonics and Venus does not.
Scientists want to know why.
Image
Scientists want to know why.
Image
Researchers
also are exploring the link between plate tectonics and the evolution
of complex life. Fortuitously timed continental collisions and mountain
smackdowns may well have supplied crucial nutrients at key moments of
biological inventiveness, like the legendary Cambrian explosion of 500
million years ago, when the ancestors of modern life-forms appeared.
“The
connection between deep Earth processes and Earth surface biology
hasn’t been thought about too clearly in the past, but that’s changing
fast,” said Aubrey Zerkle, a geochemist at the University of St. Andrews
in Scotland.
It’s increasingly
obvious that “you need plate tectonics to sustain life,” Dr. Zerkle
added. “If there wasn’t a way of recycling material between mantle and
crust, all these elements that are crucial to life, like carbon,
nitrogen, phosphorus and oxygen, would get tied up in rocks and stay
there.”
The origin and implications of plate tectonics were the subject of a recent meeting and themed issue of Philosophical Transactions of the Royal Society.
Researchers
said that pinning down when and how Earth’s vivid geological
machinations arose will do more than flesh out our understanding of our
home base. The answers could well guide our search for life and
habitable planets beyond the solar system.
Robert
Stern, a geoscientist at the University of Texas at Dallas, argues that
if we’re looking for another planet to colonize, we want to avoid ones
with signs of plate tectonic activity. Those are the places where life
is likely to have evolved beyond the “single cell or worm stage, and we
don’t want to fight another technological civilization for their
planet.”
“A relatively benign way for the Earth to lose heat”
The
idea that continents are not fixed but rather peregrinate around the
globe dates back several centuries, when mapmakers began noticing the
complementarity of various land masses — for example, the way the
northeast bulge of South America looks as though it could fit snugly in
the cupped palm of the southwest coast of Africa.
But
it wasn’t until the mid-twentieth century that the generic notion of
“continental drift” was transformed into a full-bodied theory, complete
with evidence of a subterranean engine driving these continental
odysseys.
Geologists
determined that Earth’s outer layer is broken into eight or nine large
segments and five or six smaller ones, a mix of relatively thin, dense
oceanic plates riding low and thicker, lighter continental plates
bobbing high.
At large fissures on
the ocean floor, melting rock from the underlying mantle rises up,
adding to the oceanic plates. At other fracture points in the crust,
oceanic plates are diving back inside, or subducting, their mass
devoured in the mantle’s hot belly.
The
high-riding continental plates are likewise jostled by the magmatic
activity below, skating around at an average pace of one or two inches a
year, sometimes crashing together to form, say, the Himalayan mountain
chain, or pulling apart at Africa’s Great Rift Valley.
All
this convective bubbling up and recycling between crust and mantle,
this creative destruction and reconstruction of parts — “tectonic” comes
from the Greek word for build — is Earth’s way of following the second
law of thermodynamics. The movement shakes off into the frigidity of
space the vast internal heat that the planet has stored since its
violent formation.
And while
shifting, crumbling plates may seem inherently unreliable, a poor
foundation on which to raise a family, the end result is a surprising
degree of stability. “Plate tectonics is a relatively benign way for
Earth to lose heat,” said Peter Cawood, an Earth scientist at Monash
University in Australia.
“You get
what are catastrophic events in localized areas, in earthquakes and
tsunamis,” he added. “But the mechanism allows Earth to maintain a
stabler and more benign environment overall.”
Sulfuric
gas in the Afar Triple Junction in Ethiopia, at the top of the Great
Rift Valley. Three tectonic plates meet at this spot: the Arabian plate
and two African plates, Nubian and Somali.CreditMassimo Rumi/Barcroft Media, via Getty Images
Image
Image
Unfortunately
for geologists, the very nature of plate tectonics obscures its
biography. Oceanic crust, where the telltale mantle exchange zones are
located, is recycled through the upwelling and subducting pipeline every
200 million years or so, which means hard evidence of tectonic origins
was destroyed long ago.
Continental
crust is older, and rocks dating back more than 4 billion years have
been identified in places like Jack Hills, Australia. But continental
plates float above the subductive fray, revealing little of the system’s
origins.
Nevertheless, geoscientists
are doing their best with extant rocks, models and laboratory
experiments to sketch out possible tectonic timelines. Dr. Korenaga and
his colleagues have proposed that plate tectonics began very early,
right after Earth’s crust solidified from its initial magmatic state.
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“That
is when the conditions would have been easiest for plate tectonics to
get started,” he said. At that point, he said, most of the water on
Earth — delivered by comets — would still be on the surface, with little
of it having found its way into the mantle. The heat convecting up
through the mantle would exert a stronger force on dry rocks than on
rocks that were lubricated.
At the
same time, the surface water would make it easier for the hot, twisting
rocks beneath to crack the surface lid apart, rather as a sprinkling of
water from the faucet eases the task of popping ice cubes from a tray.
The cracking open of the surface lid, Dr. Korenaga said, is key to
getting the all-mighty subduction engine started. With subduction
established, water, like oceanic crust, would cycle between Earth’s
surface and mantle.
Water is constantly recycled between the mantle and crust
A
map of tectonic plates in the Indian Ocean based on data showing
seafloor gravity anomalies. The red areas show areas where gravity is
stronger, largely aligning with underwater ridges, seamounts and plate
edges.CreditJoshua Stevens, Sandwell, D. et al., NASA
Image
Image
On
the opposite end of the origins debate is Dr. Stern, who argues that
plate tectonics is a mere billion years old or less, and that Earth
spent its first 3.5 billion years with a simple “single lid” as its
outer shell: a crust riddled with volcanoes and other means of heat
ventilation, but no moving plates, no subduction, no recycling between
inside and out.
As evidence of the youthfulness of the plate regimen, Dr. Stern points to two classes of rocks: ophiolites and blueschist.
Ophiolites
are pieces of oceanic crust atop bits of underlying mantle that have
made their way onto land and thus have escaped the relentless recycling
of oceanic crust. Recent research has shown that ophiolites are not just
any slice of oceanic crust, Dr. Stern said, but rather were formed by
the forces of subduction.
Similarly,
blueschists are rocks that are fashioned under very high pressure but
low temperatures, and “the only place you can do that is in a subduction
zone,” Dr. Stern said.
Nearly all
ophiolites are less than a billion years old, he added, while the most
ancient blueschists, found in China, are just 800 million years old. No
ophiolites, no blueschists, no evidence of subduction or plate
tectonics.
Most geologists opt for a
middle ground. “Science is a democratic process,” said Michael Brown, a
geologist at the University of Maryland and an editor of the themed
issue, “and the prevailing view is that Earth started to exhibit
behaviors that look like plate tectonics 2.5 to 3 billion years ago.”
Significantly,
that chronology decouples plate tectonics from the origin of life on
Earth: evidence of the earliest single-celled organisms dates back more
than 3.6 billion years. Nevertheless, scientists view plate tectonics as
vital to the sustained evolution of that primordial life.
In
Iceland, a visible fault between the North American and Eurasian
plates, which are pulling away from each other at a rate of about an
inch a year.CreditUniversal History Archive/UIG, via
Plate
tectonic activity did not just help to stabilize Earth’s heat
management system. The movement kept a steady supply of water shuttling
between mantle and crust, rather than gradually evaporating from the
surface.
It blocked the dangerous
buildup of greenhouse gases in the atmosphere by sucking excess carbon
from the ocean and subducting it underground. It shook up mountains and
pulverized rocks, freeing up essential minerals and nutrients like
phosphorus, oxygen and nitrogen for use in the growing carnival of life.
Dr.
Zerkle discerns a link between geological and biological high drama:
“It’s been suggested that time periods of supercontinental cycles — when
small continents smash together to make large supercontinents, and
those supercontinents then rip apart into smaller continents again —
could have put large pulses of nutrients into the biosphere and allowed
organisms to really take off.”
Plate tectonics also built the right playing fields for Darwinian games.
“Think
about what drives evolution,” Dr. Stern said. “It’s isolation and
competition. You need to break continents and continental shelves apart,
and separate one ocean from another, for speciation to occur.”
Life is always falling apart, on the rocks — and a good thing, too.
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