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FITZ GORO / LIFE TIME INC.
September 24, 1979
Geologist Harry Hess *31 saw that convection currents from deep within the earth provided the driving force behind continental drift.
HESS’S GEOLO GICAL REVOLU T ION
How an “essay in geopoetry” led to the new science of plate tectonics
By J. I. Merritt ’66
O
n the evening of March 26, 1957, faculty and students of Princeton’s Geology Department gathered
in 220 Guyot Hall for a lecture on the latest in a
series of discoveries that were soon to revolutionize the earth
sciences. Bruce Heezen, a respected geologist from
Columbia’s Lamont Geological Observatory, had been invited to speak on recent evidence from ocean soundings that
a continuous “rift” or valley ran along the crest of an underwater mountain chain known as the Mid-Atlantic Ridge.
Heezen proposed that this rift and others like it, by extruding magma, or molten lava, had been creating ocean floor
for the last 200-300 million years. Before then, he said, no
oceans had existed—a single great continent had covered
the planet, and the earth had been only about half of its
present size.
While many in the audience were willing to accept that
rifts indeed existed along the world’s ocean ridges, the idea
that the earth had somehow expanded like a balloon to accommodate its oceans seemed patently absurd. When Heezen
finished there was polite applause, followed by silence. Then
Harry Hess *31, the department chairman and Heezen’s host
for the evening, stood up in the back of the room. Accounts
vary, but in essence this is what he said: “Thank you, Bruce,
for a lecture that shakes geology to its very foundations.”
Hess’s colleagues, who shared his orthodox views of earth
history, puzzled over these remarks—could he possibly mean
what he said? Three years later they would recall his words
when he circulated a paper he had written incorporating
Heezen’s concept that the oceans were young and growing.
Instead of accepting the theory of an expanding earth, however, Hess suggested the following:
• The new crustal material created at the mid-ocean ridges
was eventually consumed in what he called the “jaw crusher”
of deep ocean trenches like those lying along much of the
western Pacific basin.
• It was convection—the slow circulation of hot,
semimolten rock from the earth’s interior—that powered this
crustal movement.
• The continents, which were once welded together but
had been split apart by this great convection engine, were fixed
in the earth’s spreading crust and rode along like rocks imbedded in moving glacial ice.
Thus Hess’s grand scheme made sense out of the neat fit of
land masses such as South America and Africa, long noticed
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by geologists and laymen alike but never satisfactorily explained trenches to outer space. Hess championed a project, called
“Mohole,” to drill four miles deep into the earth, and he was
until then.
The Hess paper—innocuously titled “History of Ocean Ba- among the select group of geologists to examine the first moon
sins” and described by him as “an essay in geopoetry”—was rocks brought back by Apollo astronauts. And his sense of
eventually published in 1962 as part of a volume honoring justice compelled him, virtually alone among scientists of statPrinceton geologist Arthur F. Buddington *16. In the interim, ure, to speak publicly on behalf of the controversial Immanuel
a government geologist named Robert Dietz developed, inde- Velikovsky—not because Hess believed Velikovsky’s crackpendently of Hess, a similar theory and applied the term “sea- pot theories about a near collision between Earth and Venus
floor spreading” to the earth’s crustal dynamics. Others soon (he didn’t) but because, as he told him, “You deserve a fair
began building on the framework constructed by Hess and hearing.”
Although primarily a mineralogist whose reputation beDietz, including two young Princeton geologists, Frederick Vine
and Jason Morgan *64, who would make significant contribu- fore 1960 rested on his studies of silicate rocks, Hess from
tions in their own right to the new science that by 1967 was the beginning of his career maintained a related interest in
the ocean floor. After graduating in 1927 from Yale (where
known as plate tectonics.
The model as fleshed out showed that the earth’s crust was he claimed to have failed his first mineralogy course and was
told he had no future in that field), he
divided into approximately 20 sections,
took his doctorate at Princeton and in
or plates, whose interactions could help
1931 joined an undersea expedition
to explain many long-standing questions
with the eminent Dutch geophysicist
about mountain building, earthquakes,
Felix Vening Meinesz. During this and
ore concentrations, and the similarity of
subsequent investigations of the ocean
fossils between widely separated contibottom, Hess was encouraged by his
nents. A scientific revolution—owing in
mentor on the Princeton faculty, the
no small measure to Hess’s insight and
A trace of the Pacific floor, obtained by Hess
flamboyant Richard M. “Dicky” Field,
leadership—was underway.
with
a
depth
recorder
aboard
the
navy
ship
he
an early proponent of deep-sea exploYears after his death, Harry Hamcommanded in World War II, showing one of
ration. While no theoretician, the outmond Hess remains a larger-than-life
the flat-topped seamounts he named guyots,
going Field was a catalyst and organizer
figure. Anecdotes about him abound in
after Princeton geologist Arnold Guyot.
who brought together many of the
Guyot Hall, the crenellated, turnof-the-century home of the university’s Department of Geo- people, including Hess and Meinesz, who would play critical
logical and Geophysical Sciences, where he was a fixture for 40 roles in the development of plate tectonics.
Meinesz, a huge man who could barely squeeze into the tiny
years. A quiet, unpretentious man with a small mustache and
a constantly lit cigarette, he worked out of an office of leg- submarines of his day, was fascinated by the so-called “gravity
endary clutter, whose every surface was piled high with pa- anomalies” that existed along the trenches of island arcs in the
pers and hydrologic charts. He had tremendous powers of East and West Indies. Using a highly sensitive pendulum
concentration, and his wife, Annette, recalled his ability to gravimeter, he charted the gravitational pull along the ocean
think exclusively about geology “from the time he woke up in floor and found, contrary to a fundamental geophysical law,
the morning until he went to bed.” The one vacation she could that it was remarkably weaker over the trenches. These anomaremember away from geology was their honeymoon on Nan- lies would puzzle Hess and others for three decades but would
tucket: “The island has only one rock, and that was brought finally be explained by the new theory of plate tectonics: the
weakness of gravity over the trenches resulted from the conin as a monument. He used to look at it longingly.”
Hess had a wonderfully unpredictable sense of humor. At a vective force that was pulling the edge of the sea floor down
banquet celebrating the end of a field trip to Russia by Ameri- into the earth.
To carry out his work on U.S. submarines Hess joined the
can geologists in 1937, vodka was flowing freely. Suddenly
Hess leaped onto the table and proposed a toast: “Here’s to Navy Reserve, a seemingly routine act but one fraught with
the Revolution! . . . The Hercynian Revolution!”—a reference later consequences. On the day after the Japanese attack on
Pearl Harbor he put on the one Navy uniform he owned and
to the geological event that had thrust up the Urals.
His professional interests ranged from the deepest ocean took the 7:42 a.m. train to New York to volunteer for active
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duty. He was soon in charge of estimating the daily positions
of German submarines in the Atlantic. Later in the war he
transferred to sea duty, taking part (eventually as captain of
the assault transport U.S.S. Cape Johnson) in landings in the
Marianas, Leyte, Linguayan Gulf, and Iwo Jima.
During his time at sea Hess made a remarkable discovery.
For research purposes he had a special deep-sea fathometer
installed on his ship and ordered that it be kept on all the time
during his frequent criss-crossings of the Pacific. He thus accumulated some 250,000 miles of soundings and found that
the Pacific bottom was studded with at least 160 flat-topped
seamounts rising to within 3,000 feet of the surface. He called
these structures “drowned ancient islands” and named them
guyots, after Arnold Guyot, the Swiss geologist who founded
Princeton’s department in the mid-19th century.
T
hese guyots, which were clearly volcanic in origin and
had once risen above sea level, would figure prominently
15 years later when Hess announced his theory of sea-floor
spreading and the subduction of the ocean bottom into the
deep marine trenches. Research in the 1950s showed that many
guyots in the mid-Pacific were strikingly young in geologic
terms, only 100 million years old, but that those closest to the
ocean trenches were older and also stood at a slight angle. Hess
deduced that guyots and atolls (submerged mountains with coral
growth) were created at the crest of a now extinct mid-Pacific
ridge and were carried away from it by the moving ocean floor.
As the guyots approached a trench they rode down toward it,
tilted on its steeper slope. The guyots were moving away from
the old ridge at a rate of about an inch every five years.
Hess also pointed to other evidence, such as the surprisingly
thin layer of sediment on most sea bottom, to show that while
the continents and the ocean’s water were old, the ocean floor
was geologically young, which indicated that it was continually being created and destroyed.
The model of a spreading sea floor was a radical departure
from conventional geological thinking. Although a German
meteorologist named Alfred Wegener had plumped for a
theory of “continental drift” earlier in the century, no satisfactory mechanism for explaining the drift had been found.
Both Hess and Dietz proposed convection—the circular
movement of a heated substance, like hot air rising in a room—
as the driving force. Elaborating on an idea put forth years
earlier by his old friend Meinesz, Hess suggested that the
earth’s mantle (the hot, dense layer of rock lying beneath the
crust) had a certain plasticity that under extreme heat and
pressure allowed it to move.
The mantle was actually composed of a series of convection
cells whose boundaries were marked on the surface by ocean
ridges and trenches. In an endless cycle, the heated rock in the
cells rose toward the surface, then cooled and descended to a
depth where it took on more heat and began to ascend again.
“The mid-ocean ridges could represent the traces of rising limbs
of convection cells,” Hess wrote, while the belt of trenches and
mountains ringing the Pacific would represent descending
limbs. The continents “ride passively on mantle material as it
comes to the surface at the crest of the ridge and then move
laterally away from it.”
While new evidence and theories have emerged since Hess’s
paper to show that the driving mechanism is far more complicated than the simple model he outlined, the basic concept of
convection continues to offer the best explanation for continental drift.
A worldwide network of ultrasensitive seismographs, set up
in 1960 to monitor the nuclear test-ban treaty, soon offered
preliminary evidence in support of Hess’s theory. The seismographs recorded that earthquakes along the Pacific
trench-and-mountain ring occurred on a downward sloping
plane to a depth of 400 miles or more beneath the surface—
clearly, some terrible gnashing and grinding was going on down
there. The most startling evidence, however, would come from
work by a 24-year-old Cambridge graduate student named
Frederick Vine and his thesis adviser, Drummond Matthews.
On the basis of some preliminary recordings in the Indian
Ocean, Vine and Matthews suggested in 1963 that a permanent record of continental drift might be found on the ocean
floor in the form of magnetic striping. For unknown reasons
the earth has periodically reversed its magnetic polarity. When
lava cools and solidifies, it “locks in” the magnetic lines of force
in effect at the time. Magma extruded along mid-ocean ridges,
therefore, should record the earth’s polar flip-flops like a moving film strip.
Such magnetic striping was confirmed the following year
with the publication of data from airborne magnetometers off
the coast of British Columbia. By 1968, about half the world’s
ocean floor had been magnetically mapped, and a clear picture
had emerged of the rate and direction of sea-floor spreading
during the last 150 million years.
In 1965 Hess took a year’s sabbatical at Cambridge, where
he worked closely with Vine, Matthews, and other “drifters”—
as proponents of moving continents were known to their
still-skeptical colleagues. The following year Hess brought Vine
back to Princeton as an instructor. Vine (who in 1971 elected
for family reasons to return to Britain despite a tenure offer
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of the earth’s surface as the Afar Triangle in East Africa. At
this and similar areas, according to the theory, an upwelling
plume forms a bulge in the crust. More pressure on the bulge
causes it to crack, frequently along three lines radiating from
a central point and approximately 120 degrees apart—thus
the convergence near Ethiopia and Saudi Arabia of the Afriorgan provided a further clue to understanding the can Rift Valley, the Red Sea, and Gulf of Aden, all of which
earth’s surface dynamics by applying a mathematical form the border of diverging plates.
Geophysicists have also argued that a series of plumes suffilaw called Euler’s theorem, which governs the motion of rigid
units on a sphere, to long gashes in the ocean bottom. These ciently close together would form a single volcanic rift like the
gashes, known as fracture zones, run at right angles to one that splits the Mid-Atlantic Ridge. It is also believed that
mid-ocean ridges and are apparently connected to the phe- sometimes, when a plume cracks the surface, one of the three
nomenon of sea-floor spreading. Indeed, geologists had al- lines becomes inactive and forms the basin for a river. Eviready matched up the magnetic striping patterns on either dence exists to suggest that the Amazon, Mississippi, and Rhine
side of certain Pacific fracture zones and found they are off- rivers resulted this way.
Unfortunately, we still have little direct evidence concerning
set, indicating that sections of once-continuous ocean botwhat goes on deep within the earth, so
tom had been rent apart and separated
Morgan’s hypothesis remains just that.
in some cases by nearly a thousand
There may never be a way of actually
miles.
“proving” the existence of plumes, says
Morgan noticed that groups of fracMorgan. In sciences like chemistry or
ture zones might in fact be segments of
physics, he explains, discrete experi“small circles” (like lines of longitude on
ments can be designed to test hypotha globe), suggesting a common axis
eses, but “proof doesn’t work very well
through the center of the earth. This
in geology.” As a theory, however, the
turned out to be the case when he traced
plume model conforms to observable
perpendicular lines from a series of fracsurface phenomena without contradictture zones in the South Atlantic and
Geophysicist Jason Morgan *64 unraveled the
found they converged in accordance mystery of fracture zones on the ocean bottom. ing the few existing facts we have concerning the earth’s interior.
with Euler’s theorem. A Cambridge
A graduate of the Georgia Institute of Technology, Morgan
geophysicist named Dan McKenzie, using different data, arrived at a similar finding concurrently with Morgan. The most earned a doctorate in physics from Princeton and came to geimportant implication of all this was that the fracture zones in ology via geophysics (his dissertation concerned the earth’s roquestion resulted from the movement of a rigid unit, or plate. tation and gravitational field). He never took a geology course
Largely as a result of the work by Morgan and McKenzie, the and says he is “still learning” about the field. In this regard his
term plate tectonics (tectonics meaning movement) came into largely theoretical background could not have been more different from Hess’s lifelong grounding in mineralogy. During
widespread use at this time.
Next Morgan attacked the problem of explaining specifi- his first few years on the faculty, says Morgan, he was very
cally how convection drove the plates. Various calculations had much aware of the importance of what Hess was doing but
been made to show that the simple convection-cell model of had little to discuss with him professionally.
“I couldn’t understand him,” he recalls. “I was too close to
Hess and Dietz was inadequate. Expanding on work by Canadian geophysicist J. Tuzo Wilson *36, Morgan’s model depicted the physics side of the story. I was becoming aware of the rock
huge convection “plumes,” or columns, 100 miles or more in side, but I didn’t really know it at the time. Fred Vine shared
diameter, rising from the earth’s deep mantle and spreading the office with me, and in a sense I learned what Hess was
like giant thunderheads beneath the asthenosphere, the layer doing through him. Later on I got to know Hess and the rock
side better.”
just below the crustal plates.
Significant work in expanding Morgan’s theory has been done
First promulgated in 1971, the plume theory has since been
elaborated by Morgan and others to account for such features by another Princeton geologist, Kenneth Deffeyes *59, who
from Princeton) shared an office in Guyot Hall with Jason
Morgan, a young geophysicist who would soon make brilliant
additions of his own to the new theory that was indeed, as
Hess had prophesied nearly 10 years earlier, shaking geology
to its foundations.
JOHN W.H. SIMPSON ’66
M
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as a graduate student was present at the 1957 Heezen lecture
and has followed the development of plate tectonics ever since.
Deffeyes points out that, while other universities and several
oceanographic institutes collected much of the data that ultimately transformed our picture of the world, it was Princeton
and Cambridge that led in interpreting the raw findings and
turning them into coherent theory.
“We’ve been criticized at times for not generating the primary data that you get from doing things like radioactive age
dating, spectrometer work, or running an oceanographic ship,”
he says. “Other places were collecting and examining the details, but Cambridge and Princeton were looking at the larger
picture.”
T
he vision of the earth offered by plate tectonics is the
most recent in a series of scientific revolutions, beginning
with evolution in the mid-19th century and including relativity and quantum mechanics in physics and the breaking of the
DNA code by molecular biologists in the 1950s. In The Structure of Scientific Revolutions, a former Princeton historian of
science, Thomas Kuhn, views science as progressing through
phases, each governed by a particular “paradigm” or model in
effect at the time. Basically, a paradigm is a theory that best
explains known data. When new facts come to light that do
not “fit” the prevailing paradigm, science enters a revolutionary period of confusion and reassessment. Eventually a new
paradigm emerges and another period of what Kuhn calls “normal science”—working within the framework of an accepted
model—begins.
In the case of geology, a massive amount of new data, much
of it a direct result of defense technology, was collected following World War II. This new information about the ocean
floor, magnetism, seismology, and the earth’s interior pointed
ineluctably to a dynamic model of a convection-driven earth
with a patchwork surface of tearing, crashing, and plunging
plates. The new paradigm of plate tectonics explained the
puzzling postwar facts and opened innumerable fresh lines of
inquiry. In Kuhn’s phrase, geologists today are “mopping up”—
elaborating on the model and filling in gaps, but without radically altering the great schema sketched by Hess in the 1960s.
(Kuhn, who came to Princeton from Berkeley the year following publication of The Structure of Scientific Revolutions—
a milestone in modern intellectual thought—sat on the faculty with Hess for five years. He was aware of Hess and his
accomplishments and knew him by sight, but they never met.)
The revolutionary synthesis achieved by Hess can be traced
to certain personal and professional qualities. “He was a very
able person at the right spot at the right time, and he had enough
confidence in himself to make the jump,” says Sheldon Judson
’40, the department chairman. There was also the breadth and
diversity of his experience, which was “unique—very different
from any other geologist’s at the time.” This included, in the
1930s, his submarine gravity work over ocean trenches and
land-based studies of peridodite, a volcanic rock that later turned
out to be a primary component of the ocean floor. Following
his wartime discovery of guyots, Hess continued in the Navy
Reserve (eventually making rear admiral) and directed naval
efforts at sea-floor mapping. “He had,” adds Judson, “the whole
U.S. Navy working for him as a data-collecting agency.”
Hess’s open-mindedness was critical to his ability to come
to terms with accumulating facts that, by the late 1950s, were
seriously undermining conventional ideas about the earth. “He
had a wonderfully relaxed attitude about being wrong,” remembers long-time associate Franklyn Van Houten. Despite
Wegener and a school of South African and Australian geologists who embraced continental drift, scientific orthodoxy
could not reconcile it with known physical laws. Like most of
his peers, Hess for years had dismissed the match between
land masses as coincidence—even, apparently, up to the eve
of writing his convention-shattering paper on ocean basins.
About this time Van Houten recalls joking with Hess about a
lecture on continental drift he was to give undergraduates:
Hess remarked, “Don’t tell them that—they won’t believe it.”
How and when did it click in Hess’s mind that the deep
ocean trenches solved the puzzle of sea-floor spreading?
“Heezen established that the crust was expanding, and Hess
used the trenches to get rid of the crust,” says Deffeyes. “But
what we were all dying to know was, did Hess give Heezen
two years to catch on to the truth, or did it take Hess two years
to catch on?”
The mystery over Hess’s flash of insight remains. On August 26, 1969, he died of a heart attack at the Woods Hole
Oceanographic Institute, in Massachusetts. It was a month after the Apollo moon landing, and he was chairing a government conference on the future objectives of lunar exploration.
Two of the many questions Hess hoped the Apollo missions
would answer were whether the moon had ever been volcanic
and washed by seas. ■