From the New York Times News Service by Walter Sullivan, New York Times Science Editor Art
From the New York Times News Service
by Walter Sullivan, New York Times Science Editor
Article appeared in _The Oregonian_, Wednesday, July 14, 1993
Abstract: Scientists find that giant plumes of hot rock emerge to
form strings of oceanic islands or to blanket continents with huge
sheets of lava.
Exploring Earth's Great Heat Engine
Geophysicists have recently made striking progress in understanding
the mechanism of the great heat engine within the Earth.
Over the last several months they have reached a rough consensus
that the vast plates that comprise the ocean floor sink deep into
the Earth's mantle, only to be recycled into giant ascending plumes
of hot rock.
The plumes, driven, some believe, by heat from the Earth's core,
emerge to form strings of oceanic islands such as the Hawaiian chain
or to blanket continents with huge sheets of lava.
The discoveries, said Michael E. Wysession, a professor at Washington
University in St. Louis who is an authority on the Earth's
interior, constitute "a second revolution."
The revolution is comparable in scientific terms to the theory of
plate tectonics, which came to be accepted three decades ago, showed
that the continents had been in constant motion over the course of
geological time, carried along by great plates.
The new theory explains a lot of what is happening beneath the great
plates. It reveals "the other half of the picure," Wysession said.
The plate tectonic theory recognized that the slowly moving slabs of
ocean floor eventually cool and plunge into the Earth's mantle,
disappearing in deep trenches like those that lie along the west
coast of South America and the southern arc of the Aleutian Islands.
But the plates' fate thereafter was uncertain.
The new theory holds that the plates, carrying a burden of oceanic
sediment built up over many millions of years, get recycled within
the mantle into a system of giant plumes of hot rock that burst to
the surface millions of years later.
Sixty plumes have been identified, rising under ocean islands and
volcanoes in the hearts of continents. The depth from which those
plumes originated remains uncertain, but it is being argued that
they contain material from the slabs of ocean floor -- carbon and
oxygen once associated with life on the surface.
Analysis of oxygen in volcanic glasses that erupted from a plume in
the South Pacific "leads to the virtually inescapable conclusion
that they contain a component that was once at the surface of the
Earth," according to a commentary in a recent issue of Nature by
William M. White, a geophysicist at Cornell Universtiy.
The glasses, dredged by the German research vessel Sonne from
seamounts off Pitcairn Island, were analyzed by a team of
Australian, British and German scientists.
Although a plume had presumably carried the glasses up from deep in
the Earth, it contained oxygen that showed contact long ago with
the surface, the authors said. In other words, some of it had once
been part of either the seafloor or a continent.
White argued in his commentary that since it appears that the plumes
carry carbon-rich remnants of ancient life, the resulting delivery
of carbon dioxide to the atmosphere could affect levels of that
"Deep mantle recycling and mantle plumes," he said, "may play a role
in long-term climate regulation."
When the hot plumes reach the surface, they may create strings of
oceanic islands. On land a plume may produce a succession of
volcanoes, such as the one now semidormant under Yellowstone
National Park that 600,000 years ago blanketed North America with
350 cubic miles of ash.
As the continent has drifted west, this plume has left a series of
volcanic remnants across southern Idaho.
Other plumes may blanket a vast region with lava, as occurred 15
million years ago when much of Washington and Oregon were covered,
in some instances in little more than a single day. The plumes are
slowly remolding the Earth's surface at the dictates of events in
the mantle far beneath.
At a recent meeting of the American Geophysical Union in Baltimore,
scientists debated the details of the slabs' journeys as they
plunged through the mantle and rose in plumes.
Many participants argued that the slabs dropped through the entire
thickness of the mantle, reaching down to the turbulent layer that
separates the mantle form the Earth's moten core. Hints from
earthquake recordings suggest that this layer is more uneven than
the most mountainous part of the Earth's surface.
Earthquake data make clear that the mantle is divided into an upper
section and a lower one, although the difference between them is a
matter of debate.
Some scientists argue that the lower mantle is chemically different
from the upper part and that the slabs of seafloor material only
sink 400 miles to the boundary between those regions instead of
1,800 miles to the bottom. The plumes would then originate at that
Many geophysicists now believe that the only difference between the
two sections is that the lower mantle is denser and that many, if
not most, slabs sink all the way to the bottom.
Widely discussed at the meeting was an intermediate theory according
to which the sinking ocean slabs pile up at the bottom of the upper
mantle until they are squeezed into a denser state and finally break
through. Only then do the slabs sink all the way to the bottom of
Wysession proposes that the same process may affect rock rising in a
plume. It presses against the bottom of the upper mantle until
eventually penetrating it. The plume breaks through not as a
continuous stream but as separate giant bubbles of hot rock.
When the bubbles reach the ocean floor at the top of the mantle,
thousands of years apart, each one creates a continental volcano or
oceanic island. Since the seafloor may be drifting gradually over
the hot plume, the bubbles' successive arrivals may be marked by a
chain of islands.
The ascent of the plumes can lift an entire region, such as the
3,000-mile-wide section of South Pacific floor where several "hot
spots" have erupted, forming chains of Polynesian islands. Similar
swells occur under Iceland, the Hawaiian chain and the Kerguelen
Islands in the southern Indian Ocean.
Besides seismic waves from earthquakes, other evidence about
processes in the mantle comes from magnetic measurements and
comparison of rocks tested under deep-earth pressures with those
collected on midocean islands and ridges.
The region between the mantle and the liquid core, where many
believe the plumes originate, is described in a recent issue of
Scientific American as "the most dramatic structure of the earth."
The authors were Raymond Jeanloz of the University of California at
Berkeley and Thorne Lay of the university's Santa Cruz campus.
The region, they added, may be the planet's "most geologically
active zone." The layer, mapped by dense clusters of earthquake
detectors in Norway and elsewhere, has proved highly diverse, with
"lumps" only a few miles wide and ranging in thickness from little
more than a mile to 200 miles.
The plate-plume recycling theory allows geophysicists to offer a
complete theory of volcano formation, for which there now seem to be
three sources. Plumes breaking through the Earth's crust or ocean
floor are one.
The compression of the slabs as they descend is another. This
mechanism is believed to have created the chain of volcanoes in the
Cascade Range in the Pacific Northwest and along western South
The third source of volcanism is midocean ridges and other places
where plates of the Earth's surface are pulling apart. Lava wells
up to fill the gaps between parting slabs of ocean floor. The
lava's composition shows that it originated in the upper part of the
mantle and, contrary to earlier belief, may not be part of the deep
circulation driving the plates.
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