The tectonic plates making up Earth’s rigid outer shell are in constant
motion, grinding past their neighbours at transform boundaries or diving beneath
one another in regions called subduction zones. A rarer and shorter-lived event
is where a new boundary is born and another dies. Geologists know, for example,
that California’s famous San Andreas Fault formed 20 million years ago when a
plate cracked as it was being gobbled up beneath the continent of North
America.

Kristen Rohr of the Geological Survey of Canada and Kevin Furlong of
Pennsylvania State University in University Park had an early hint that
something similar might be happening at the Explorer plate because a surprising
number of quakes had been mapped recently within it. “This is Canada’s most
seismically active region but it shouldn’t be,” says Rohr. In a subduction zone,
he explain, the earthquakes should occur under the continental edge, rather than
offshore within the oceanic plate.

So in a study last year, Rohr and Furlong used seismic reflection profiles,
made by sending pulses of sound into the ocean floor, to try to see where the
plate was deforming as a result of the quakes. They found a large field of
faults, not at its supposed border, but nearer its middle. The researchers
estimate the plate is about 90 per cent transformed to a plate boundary. The
whole transition takes about a million years, the merest moment on the
geological timescale.

Dave Sanderson, a geologist at the University of Southampton says that if
Explorer is still splitting, it provides the only known contemporary “snapshot”
of this important type of event. “Boundary formation is where a lot of
interesting chemistry happens and yet most gets obliterated with time,” he says.
“This would be a real chance to study these fundamental processes.”

BIG earthquakes are most likely to occur along the boundary between two
tectonic plates at the spot which has been free of quakes for longest. Or so
most geologists thought. But now two researchers have found that some of these
points, known as seismic gaps, may remain earthquake-free forever.

Wang-Ping Chen at the University of Illinois and Honn Kao, his former student
now at Academia Sinica in Taipei, Taiwan, studied patterns of faults and
earthquakes along subduction zone boundaries, where one plate plunges into the
mantle beneath another. They have identified 10 seismic gaps which they claim
are permanent. A large earthquake in southern Peru in November adds weight to
their theory because it took place near the edge of one of these permanent gaps
but did not enter it. Their work is published in the December issue of the
Journal of Geophysical Research.

At most subduction zones, the oceanic plate bends as it sinks beneath the
continental plate, building up stress. Earthquakes occur at the plate boundary
when the subducting plate tries to bend back. However, at a permanent seismic
gap, says Chen, the oceanic plate bends earlier, faster and more easily, and by
the time it reaches the plate boundary it has been deformed beyond the point at
which it can resist. After this any deformation is permanent, and does not
involve the build-up of stress in the plate.

It is unclear why some sections of oceanic plates behave in this way,
although one theory suggests that the older sections are more likely to do so.
However, seismic gaps are often associated with earthquakes out on the oceanic
plate above where the plate begins to bend, a region known as the outer rise.
Such events could relieve the stress in the plate before it reaches the
subduction trench. Large earthquakes have been recorded near the outer
rise—rather than above the subduction trench—at each of the 10
permanent gaps identified by Chen and Kao, which include areas in central
Mexico, Japan, Java, Sumatra and the Philippines.

The subduction zone in southern Peru is a good test for the model, says Chen,
because “the big earthquake has repeated itself but it never enters the gap”.
November’s earthquake broke the subduction zone near the town of Nazca,
rupturing a region to the south of the seismic gap. And in 1974, an earthquake
and its subsequent aftershocks occurred all around the gap but did not enter it,
leading to a widely publicised prediction by two geophysicists that this gap
would soon be filled. “They really stuck their necks out,” says Chen. But, 23
years later, the gap still exists.

If Chen and Kao are right, the theory of seismic gaps will have to be
drastically modified. “It needs confirmation, but the idea is interesting,” says
Stephen Kirby, a geophysicist with the US Geological Survey in Menlo Park,
California.

THE next “big one” to hit the San Francisco Bay area could be more
devastating than anyone has so far predicted, say two researchers at Stanford
University. They have shown that the whole of the Hayward fault, which
runs down
the east side of the bay, could rupture at once. If it did, it could cut a
swathe of destruction through an urban corridor stretching more than 75
kilometres.

The Hayward fault is considered one of the main seismic risks in the bay
area. Until now, scientists looked on the fault as two separate segments that
were unlikely to rupture at the same time. But Ellen Yu and Paul Segall have
overturned this theory with their analysis of the disturbance caused by a
magnitude 7 earthquake in 1868.

The 1868 quake was thought to have hit only the southern segment, running
from the town of Fremont to a kink in the fault just south of Oakland that is
thought to mark the end of the segment (see
Map). But Yu and Segall say that it
also ripped along a portion of the northern segment, passing through what are
now the densely populated cities of Oakland and Berkeley.FIG-20410901.gif

The first hint that the Hayward fault could rupture along both segments at
once came in 1994, when James Lienkaemper of the US Geological Survey in Menlo
Park found evidence in the soil layers in a trench north of Oakland that a
small
amount of vertical movement had taken place there in 1868.

But the tension building up on the Hayward fault is mostly
horizontal—the west side is pushing northwards and the east side
southwards. So Yu and Segall reanalysed triangulation data collected between
1853 and 1860 and again between 1876 and 1891. This revealed that the hills
surrounding the bay had moved relative to one another in the period between the
two surveys.

In the July issue of the Journal of Geophysical Research, Yu and
Segall conclude that in the 1868 quake, the Hayward fault slipped by 2 metres
over at least 50 kilometres of its length, including portions of both segments.
“This study should serve as a reminder that large quakes do not always stop at
the supposed boundaries between fault segments,” says Segall.

David Oppenheimer, a seismologist with the USGS, says that Yu and
Segall have
shown that current assessments of earthquake risks on the Hayward fault are not
good enough. “We need to consider other rupture patterns,” he says.

Gregory Zielinski and colleagues at the University of New Hampshire in Durham
discovered the correlation after studying an ice core from Greenland. The core,
obtained by a US drilling project completed in 1993, contains a record of
environmental conditions in the northern hemisphere dating back 110 000
years.

Zielinski’s team measured the amounts of volcanic sulphates present in the
core to get a picture of past rates of volcanic eruptions (Quaternary
Research, vol 45, p 109). Violent eruptions, such as that of Mount Pinatubo
in the Philippines in 1991, often send huge plumes of sulphuric acid into the
atmosphere. These can block sunlight and cool the climate.

As expected, the researchers found that the global cooling that led into the
last glacial period, between 35 000 and 22 000 years ago, coincided with more
active volcanism. But to their surprise, there were also high rates of eruptions
during periods of climate warming. The longest period of enhanced volcanism
recorded in the ice core, between 13 000 years ago and 7000 years ago,
corresponded to the end of the last ice age, when the glaciers were melting.

Zielinski says the correlation between volcanism and both warming and cooling
is probably due to the build-up and melting of ice during the cycles of
glaciation. During glaciation, the Earth’s crust is pushed down by about
one-third the thickness of the ice that accumulates on the surface. When the ice
later melts, the crust rebounds. This movement would cause changes in the amount
of pressure on the magma chambers that power volcanoes. The chambers would then
erupt violently—just like an uncorked champagne bottle.

The idea that rapid climate change could increase the rate of eruptions was
first suggested about two decades ago by Michael Rampino, a glaciologist at New
York University, and his colleagues. Rampino says that Zielinski’s records
provide the first convincing evidence for the idea. “This is the best dated
record of climate and volcanism together,” says Rampino.

He says that the new results hint that the interaction between the solid
Earth and the climate is much more significant than any one realises. “Everybody
talks about the ocean and atmosphere being linked,” he says. “But why not the
ocean, the atmosphere and the solid Earth as well? It is, after all, a single
˛ő˛â˛őłŮ±đłľ.”

Bruce Marsh of Johns Hopkins University in Baltimore made the discovery while
studying thin layers of igneous rock called sills in Dry Valleys, Antarctica.
Sills are formed when hot magma is pushed up from the Earth’s mantle into the
crust and spreads horizontally between the older beds.

These particular sills are very similar in composition to bodies of rock
called layered intrusions, which can be up to 8 kilometres thick and are
believed to be frozen magma chambers. Some geologists have suggested that
multiple injections of magma from below could have created their exotic layered
effects, but no one had been able to produce physical evidence revealing the
volume or timing of individual injections.

Last month, a meeting of the American Geophysical Union in Baltimore heard
that Marsh has now found the elusive evidence. The unusual feature of the Dry
Valleys sills is that their structure as well as their composition closely
parallels that of a layered intrusion and this means, Marsh says, that their
origin is probably the same too. The Antarctic sills are much easier to study
than layered intrusions, because they are not obscured by vegetation and
debris.

Marsh focused on the crystals which he says are brought up in the molten
magma from the mantle. Each layer in the sills was distinguishable by the size
and number of crystals it contained, suggesting that each was the result of a
separate magma injection. Layers with large numbers of big crystals were formed
from more powerful injections, says Marsh, just as a fast-flowing river carries
more big particles than a slow one.

The sills are “almost like an eruptive record above ground”, says Marsh. From
an analysis of the borders between the individual layers in the sills, Marsh
found that the magma injections occurred every 500 to 1000 years. This is
similar to the eruptive history of many volcanoes, he says.

Stewart McCallum, a specialist in layered intrusions at the University of
Washington in Seattle says that Marsh has come up with important evidence. “He’s
added a dimension of time to these things that wasn’t available before.”

The crust of continental South America is relatively rigid in the east, but softer near its western edge. This is because heat and volcanism generated as the Nazca oceanic plate plunges into the Peru-Chile Trench, just off South America’s west coast, soften the neighbouring rocks. As the western edge is held still and the eastern edge is pushed towards it, the western side pops up in a buckled ridge, forming the Andes.

But what giant force pushes the Andes up? The main force driving the South American plate is thought to be “ridge push”, which stems from the effect of gravity on the mid-Atlantic ridge, a few thousand kilometres off the east coast of South America. This downwards force is transmitted horizontally across the seabed, pushing South America against the Nazca oceanic plate to the west.

Nevertheless, some scientists believe that ridge push alone cannot support the Andes. “Ridge push is too small by a factor of five,” claims Paul Silver of the Carnegie Institution of Washington, who has studied models of the forces affecting the South American plate devised by Paul Meijer and his colleagues of Utrecht University in the Netherlands. Silver and other researchers argue that ridge push must be backed up by a current in the underlying mantle pulling to the west at the base of the continental plate.

Randy Richardson and David Coblentz of the University of Arizona in Tucson now say Silver’s camp has overestimated the forces required to support the Andes. If a drag force exists, they say, it must be tiny compared to the ridge push. “My argument is that there is no large shear stress between the South American plate and the mantle that could drag the plate to the west,” says Richardson. “The continent is overriding the Nazca plate and ridge push is enough to make it override.”

The Arizona researchers remodelled the theoretical stresses within the South American plate, checking their calculations against almost 300 stress measurements compiled by Mary Lou Zoback of the US Geological Survey in Menlo Park, California, as part of a global effort called the World Stress Map Project. They conclude that the stresses holding up the Andes lie between 10 and 20 megapascals – similar to the previous calculations of the ridge push.

“Both the direction and relatively uniform magnitudes of the stresses across the South American plate are consistent with a ridge-push origin,” agrees Zoback, who adds that a drag force would cause a build-up of stresses running from east to west that does not show up in her data.

These findings are good news for geoscientists who believe that the Earth’s plates are separated from the underlying mantle by a low-viscosity layer called the asthenosphere, which should act like a lubricant. If so, it is difficult to see how the mantle could be dragging South America along.

Richardson and Coblentz will publish their work in a future issue of the Journal of Geophysical Research.