Scientist, 177 (2385), p 32. 8th March 2003
island chains to towering mountains, geologists reckon
they can explain how any volcano forms. But what if
a crucial part of their theory is wrong, asks Nicola
WELCOME to Hawaii,
a beautiful chain of volcanic islands that stretches
for thousands of kilometres across the Pacific. According
to most geologists, Hawaii is a classic example of a
"hot-spot trail", created by a giant plume
of magma that rises from the core of the Earth to the
surface. As the Pacific plate passes over the plume,
the fountain of hot rock leaves behind a trail of burnt
spots as it scorches the surface. Such mantle plumes
are deeply entrenched in the geological "standard
model", going hand-in-hand with plate tectonics.
But a few renegade geologists have
been saying for years that plume theory is fundamentally
flawed, and have come up with alternative explanations
for volcanoes and island chains such as Hawaii. And
now others are starting to agree.
Researchers at last year's American
Geophysical Union meeting in December presented
swathes of such evidence, and more are expected to join
ranks with these dissenters at a meeting in Iceland
this August. Most aren't convinced that mantle plumes
should be dumped entirely. But they're willing to admit
that the geological community is standing on the brink
of a radical shift in thinking that could completely
change our ideas about the inner workings of the Earth.
One of the renegades is Don
Anderson. Over the past decade, his work at the
Institute of Technology has made him famous for
his belief that mantle plumes simply don't exist. Forget
fountains from the centre of the Earth, he says: volcano
chains are due to cracks or rents in tectonic plates.
After all, plates stretching 10,000 kilometres are unlikely
to remain perfectly unbroken. Break the skin and it
seems reasonable that molten rock would flood up to
fill the gap. Bingo, you've got a volcano - without
a mantle plume.
on the planet form along the edges of tectonic plates,
where slabs of ocean floor squeeze under continents
and cause the rock to melt and well upwards through
faults in the crust. But some volcanoes, such as those
in Hawaii, seem to appear from nowhere, right in the
middle of a plate. Not only that, but the Hawaiian islands
get progressively older towards the north-west, leaving
a trail of dead underwater volcanoes. Other strings
of ancient volcanoes in the Pacific, such as the Tuamotu
Archipelago and the Austral Islands, are lined up almost
parallel to the Hawaiian chain and they, along with
dozens of other volcanic features around the world,
are a long way from the nearest plate boundary. Back
in the 1960s, anomalies like these got geophysicist
Tuzo Wilson from the University of Toronto thinking.
Maybe, Wilson supposed, these volcanoes are caused by
sources of intense heat lurking beneath the surface
- "hot spots" that remain still while the
plates wander about above them.
What could these heat sources be? In
the early 1970s Jason Morgan, a physicist at Princeton
University in New Jersey, suggested they were fountains
of magma rising vertically from the edge of the Earth's
core, 2900 kilometres below. It certainly made sense.
The plumes would help set up convection currents that
would drive tectonic plates around and play an important
role in transporting heat from the core of the planet.
Their existence could explain why these hot spots seem
to stay still relative to each other, and why the magma
at these volcanoes appears to have a mix of minerals
and gases unlike those from volcanoes at the edges of
plates. Mantle plumes could even provide us with a direct
glimpse into the centre of the planet. Morgan proposed
that plumes existed at around 20 spots, including Hawaii,
Yellowstone and Iceland. The idea was seized upon with
great enthusiasm and has been entrenched in textbooks
ever since. Today, some researchers say there are hundreds
of volcanic hot spots caused by plumes.
Yet by the 1990s experts had found
plenty of good reasons to question the idea. In 1996,
Foulger, a seismologist at the University
of Durham in the north of England, set out for Iceland
with a team of researchers to look for plumes. Although
Iceland is on the mid-Atlantic ridge, where American
and European plates are spreading apart and new ocean
floor is being born, the fact that volcanic activity
is concentrated in a relatively small area around Iceland
suggested to many geologists that a plume must be responsible.
So Foulger and her team planned to use a network of
seismometers laid across Iceland to measure the seismic
waves that bounce through the Earth following an earthquake.
Hot, molten rock has a lower density than cool rock,
and this reduces the speed at which seismic waves travel.
Time the arrival of seismic waves at different points
and you build up a map of the rock beneath.
However, the data they collected seemed
to show that rather than a long narrow plume of magma
coming from deep below Iceland, there was actually a
broad reservoir of molten rock less than 400 kilometres
down. That left Foulger puzzled - until she realised
it would be easy to find an explanation if she dropped
the plume theory altogether.
Iceland sits smack at a geological
crossroads where the mid-Atlantic ridge crosses an ancient
fault line - a "suture" where Europe and North
America collided 400 million years ago. This geometry
and the presence of volcanic activity on Iceland has
to be ignored as a coincidence if you believe a classic
mantle plume feeds the island. But Foulger realised
that remnants of the crust left behind in the mantle
at this fault line have a lower melting point than the
surrounding mantle. Where this ancient crustal rock
crosses the ridge it should melt more than the rock
around it, creating the shallow reservoir of magma that
feeds Iceland's volcanoes. She didn't need a mantle
But when Foulger first presented her
ideas she met outright hostility and had difficulty
getting them published. So when she talked to Anderson
in 1999 and realised there were other geologists out
there with similar doubts about the existence of mantle
plumes, it came as a huge relief.
The problem with
mantle plume theory, says Anderson, is that it requires
some crucial assumptions. Back when Morgan was developing
his theory, the upper mantle was thought to be fairly
uniform and not very hot. To explain hot spots in the
middle of tectonic plates, geologists needed to find
some mechanism to pull magma from deep below.
These days, however, geologists know
the upper mantle is highly variable, both in geological
make-up and temperature. Besides, the ages of some islands
in chains that geologists first thought to be formed
by plumes - the Canaries, for example - turn out to
vary almost randomly from one island to the next. Even
the theory's central tenet - that all the world's hot
spots stay still relative to each other while the plates
wander about - seems to be wrong. Geologists have discovered
that the positions of the top 50 candidates for mantle
plumes move relative to each other at the rate of a
few centimetres a year, about the same speed as tectonic
Even the unusual composition of magma
from hot-spot volcanoes does little to back mantle plume
theory. Samples of magma from Hawaii are flooded with
helium-3, an isotope thought to be left over from the
big bang. Researchers had assumed that such high abundances
of this primordial helium could only have survived in
a reservoir deep inside the Earth. This seemed to confirm
that the magma came from near the core. But geochemists
have grown worried by this. It turns out that helium-3
doesn't necessarily come from deep underground; it can
be trapped and released by any number of rock crystals,
such as olivine, a pale green mineral with a high melting
point that is common in the upper mantle. And researchers
have found that while the magma at some volcanic hot
spots releases a flood of helium-3, others show a dearth.
Anderson argues that the mantle is simply highly variable
in its composition. Sometimes a volcanic hot spot in
a plate coincides with high levels of the gas. Other
times it doesn't. "At some point you have to say
enough is enough," says Anderson. "Maybe there's
a better idea." Geochemist Anders
Meibom from Stanford University in California agrees.
"From a geochemical point of view, the plume hypothesis
is way overused. Whenever geologists see an anomalous
trace, they yell 'plume!'. It's an easy way out."
Besides, Anderson has a more fundamental
objection to the plume theory. He claims that the pressure
at our planet's core is simply too high to allow narrow
plumes to form in the first place. Like a blob of warm
oil in a lava lamp, each bulge of hot magma would have
to attain a certain critical mass before breaking away.
Anderson's calculations show that at the temperatures
and pressures down near the core, these blobs would
have to be between 500 and 1,000 kilometres in length
and 5,000 kilometres across - a huge chunk of the Earth's
interior - before they could begin to rise. If one managed
to break through the Earth's surface, it would leave
a massive flood plain of lava 10 times as big as anything
geologists have seen. And even if a narrow plume of
magma just 10 or a 100 kilometres across were created,
Anderson argues that the magma itself wouldn't be able
to punch through a layer near the lower mantle about
800 kilometres down where there is a chemical change
in the rock. He calculates that the pressure and temperature
changes at those depths aren't large enough to make
the lower mantle rocks less dense than the rock above.
They physically shouldn't be able to rise.
So how do you
explain the progression in age seen in chains of volcanoes
such as those at Hawaii, the trait that led to the birth
of mantle plume theory in the first place? The answer
might be found in Yellowstone, once an archetype of
a plume-fed hot spot. Although there is a chain of progressively
older volcanoes marching from Yellowstone towards Idaho,
a variety of seismic studies from a number of plume-hunters
have shown no signs of hot magma below a depth of 200
kilometres and no disturbance in the mantle below that.
"Yellowstone was supposed to be the grandaddy of
all continental plumes. It's a huge volcanic centre
but its status as a plume has evaporated," says
Presnall, a petrologist with the University
of Texas at Dallas.
Anderson's idea seems to fit instead,
says Foulger, now working with the Volcano Hazards Team
at the US Geological Survey in Menlo Park, California.
She has worked with colleagues in Yellowstone and says
the region's geology could actually be the result of
a crack slowly propagating in a north-east direction.
Imagine someone ripping a map of the US by pulling the
two bottom corners apart. If there were enough hot magma
just below the crust, then the freshest volcanoes would
follow the tip of that tear up the page. This could
explain Yellowstone, says Anderson, and Hawaii too.
Stress fractures in the Pacific are already known to
form perpendicular to mid-ocean ridges such as the East
Pacific Rise that runs roughly parallel to the coast
of South America, about 4000 kilometres out in the Pacific.
And island chains such as Hawaii and Tuamotu lie pretty
much where you would expect stress fractures to form.
One of the more damning pieces of evidence
against mantle plume theory is that regions of the crust
above suspected mantle plumes don't actually appear
to be hot - despite the fact that huge fountains of
magma from the hot core should be rising directly beneath.
Stein, a husband-and-wife team at Northwestern
University at Evanston and the University
of Illinois at Chicago, have been charting heat
flow from the ocean floor for decades, in part to see
if they can spot these plumes. In Hawaii, however, they
found the temperature below the sea floor to be much
the same as everywhere else - there is no anomalous
Some geologists argue that the heat
from a plume takes tens of millions of years to leak
up through the crust, and in that time the plates would
have moved so there would be no extra heat trace on
any single region of rock. The Steins decided to visit
Iceland, where the crust is thinner and should let heat
leak through about 10 times as fast. Still they saw
nothing. In fact, they saw the opposite of what plume
theory predicted - a slightly hotter place to the east
of Iceland, where the plume supposedly hasn't yet arrived.
"You just have to keep making up excuses and modifications
to make plume theory work," says Foulger.
Despite the promise of stress cracks,
some say they can't explain all volcanic features. Take
the island of Réunion in the Indian Ocean, for
example. This lies at the end of a volcanic trail stretching
back towards India, which passes right over a mid-ocean
ridge and spans two separate tectonic plates. It is
difficult to see how a stress crack can propagate over
a ridge like that and continue from one plate to its
neighbour. It's like ripping one piece of paper and
expecting a second piece beside it to somehow rip too.
If Anderson is right and plumes don't
exist, then plate tectonics alone must explain everything,
including rogue volcanoes such as Hawaii. In fact, volcanism
and plate motion become one and the same. It's almost
a "unified theory" for geologists, jokes Foulger.
That simplifies a few things, but it raises a mass of
new questions. Morgan envisioned plumes as something
that would set up giant convection currents in the mantle,
driving plates around like the heat of a stove churning
water in a pan. Few people now think things are so simple.
Instead, Anderson says that currents of cooler rock
do the stirring - but forget simple convection currents:
there's a muddle of circulating rock with many layers.
And there are no samples of the inner Earth being spat
out of the Hawaiian volcanoes as we once thought. Everything
is up for grabs.
"We'll have to acknowledge we
know far less about the centre of the Earth than we
thought we did," says Foulger. But that's not a
disaster. It's exciting.
Taking a closer look
Nolet and Tony
Dahlen at Princeton
University in New Jersey wanted to search for
mantle plumes they knew they couldn't dig a hole to
the centre of the Earth, so they did the next best
thing - develop a better pair of seismic binoculars
to peer into it with.
The problem with most seismic studies, says Nolet,
is that they don't see much detail of the planet's
innards. To improve the resolution, the pair went
back to their computer simulations. It is well known
that seismic waves slow as they pass through hot rocks
inside the Earth, but their simulations revealed things
weren't as simple as everyone assumed. Seismic waves
passing by the edge of a patch of hot rock are actually
slowed more than those passing right through the middle,
so you get a complex pattern of seismic wave speeds
by the time they reach the surface. Some of them actually
even speed up. That means all previous interpretations
of the seismic data were blurred by an incorrect assumption.
By working out a mathematical correction for this
blurring effect, Nolet and Dahlen have been able to
"refocus" pictures from deep underground.
They say their preliminary results reveal at least
seven plumes that travel continuously from the core
right up to the surface.
Many at last December's American Geophysical Union
meeting in San Francisco were impressed when this
analysis was presented. Even Gillian Foulger of the
University of Durham in the north of England thought
the analysis was remarkable - but she remains unconvinced.
The seismic waves could have been picking out a number
of things other than a hot plume, she says, including
water or carbon dioxide. And Don Anderson of the California
Institute of Technology notes that these preliminary
studies could just show a statistical mirage - a result
of several regions of hot rock one above the other,
Even though the data supports the existence of deep
mantle plumes, Nolet admits it also raises new questions.
If plumes do exist, it is assumed that they must come
from one of two special zones - where the core meets
the mantle 2900 kilometres down, as Morgan suggested,
or at the transition zone about 650 kilometres down.
At both of these layers it is thought the temperature
changes rapidly with depth, which should make it easier
for a hot blob to rise - displacing it just a tiny
bit upwards would make it significantly hotter and
less dense than its surroundings. Dahlen sees plumes
coming from both of these spots, but also from another
layer around 1400 kilometres down. How and why would
a plume start there? Nolet confesses he just doesn't
know. By trying to bury one controversy, he says,
they may have dug up another.