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Steps in the refinement of interpretations of seafloor geology came rapidly at the end of the
1950s and into the early 1960s. Here, for example, rapid-fire improvements in the crossdisciplinary connections being made between seismological observations and the
geography and topography of mid-oceanic rifts. Earthquakes are no longer thought to be
confined to the edges of continents the way they were 40 and 50 years earlier. But midocean rifting is somehow a focal point for seismicity. Why?
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Points of especial interest have become, and remain, mid-oceanic ridges, trenches,
subduction zones, accrectionary prisms and supposed hot spots over which oceanic crust
can move, and be bombarded from beneath with rising magma.
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1.
This graphic is a faint reflection, or vastly incomplete sample of major syntheses
contributing to the development of Global Tectonic Theory by the late 1960s.
Research and synthesis attempts during the 20th century brought about associations of
people and ideas in patterns that only make sense in hindsight.
1. U.S. resistance to Continental Drift was based on methodological, conceptual, and
philosophical preferences among U.S. earth scientists;
2. Big names in 20th century earth science tended to be European, familiar with American
investigations, but to have learned even more from European war-related endeavors;
3. U.S. institutions, nevertheless played pivotal roles in theoretical developments that took
place at an accelerating rate, notably at Cambridge, Princeton, Lamont, and Scripps.
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It sometimes happened that one group of investigators made a series of observations, shared
them with others, and then had someone else notice a pattern that had escaped everyone’s
notice. This one composite figure of seafloor mapping along the Pacific coast of North
America actually contains two examples of insights that came about through this
roundabout pathway. {You may need reminding that much of the funding for this oceanic
exploration originated from U.S. Navy concerns about where enemy submarines could
possibly hide to avoid detection in the event of a conflict.}
1) the original discoverers of the Mendocino submarine fault and others, could not figure
out a coherent explanation for the patterns of topography that these apparent faults
represented, so other scientists were able to use comparative observations in other
geographic areas to come up with an interpretation eventually that there was a new type of
fault, known as transform fault.
2) The alternating light and dark stripes depicted here off the mouth of the Columbia River
represent magnetic polarity reversals found beneath the layers of sediments. A pair of
investigators who had no idea of the topography of the seafloor were able to see that these
zebra patterns could be seen to be symmetrical, mirror images on either side of old
spreading centers in former mid-oceanic ridge systems.
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One of the most vexing problems late in the process of Global Tectonic Theory’s
acceptance was the realization that there was a type of fault previously unrecognized by
earth scientists, transform faults, that transected roughly perpendicularly mid ocean ridges.
A couple of papers by Tuzo Wilson, and one each by Heezen and Goode, all in 1965, finally
unified all the theoretical bases for how continents could be “rafting around” continuously
atop denser components of the lithosphere. The scoresheet now seemed pretty complete.
Some observers think it was more than a coincidence that the Great (Good Friday)
Earthquake of 1964 in Alaska receded these syntheses by just a year. Some of us believe
that Alaska’s Great Earthquake provided an essential catalyst for the final acceptance of
Tectonics as a paradigm. It was a huge event, it occurred along the Pacific Rim of Fire, and
it confirmed the overwhelming importance and stature of subduction in accounting for the
recycling of lithospheric structures in the Earth’s mantle. Moreover, the Alaska Good Friday
Earthquake drew heavily on experts from all over the contiguous states, and from all
subdisciplines in earth sciences. While heavily engaged in months of field work in Alaska
in 1964 and 1965, seismologists, geophysicists, theoretical types and practical field hands
all had a rare chance to TALK to one another, and to grasp the details of other geologists’
work.
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Mason and Raff, in this case, had missed the symmetry of polarity-shift bands in
magnetism, but Wilson and Vine solved the geometric puzzle deftly some 12 years after it
had been shared. J. Tuzo Wilson, pictured here, was a Canadian geologist whose career
included a number of such insights into the significance of widely disparate observations by
others. The symmetry of zebra stripes combined with solving the transform fault puzzle
guaranteed Wilson’s place in the pantheon of geological sciences.
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Dead-ends & goose chases: Project Mohole: Phase One was executed in spring 1961.
Five holes were drilled off the coast of Guadalupe Island, Mexico, the deepest to 601 ft
(183 m) below the sea floor in 11,700 ft (3,600 m) of water. This was unprecedented: not in
the hole's depth but because of the depth of the ocean and because it was drilled from an
untethered platform. Also, the core sample proved to be valuable; penetrating through
Miocene-age sediments for the first time to reveal the lowest 13 m (44 ft) consisting of
basalt.
Project Mohole contracted with Global Marine of Los Angeles for the use of its oil drillship
CUSS I. A consortium of Continental, Union, Superior and Shell Oil Companies, CUSS had
originally developed it in 1956 as a technological test bed for the nascent offshore oil
industry. CUSS I was one of the first vessels in the world capable of drilling in water depth
up to 11,700 ft (3,600 m), while maintaining a position within a radius of 600 ft (180 m).
Project Mohole expanded its operational range by inventing what is now known as dynamic
positioning.[1]
Phase One proved that both the technology and expertise were available to drill into the
Earth's mantle. It was intended as the experimental phase of the project, and did succeed in
drilling to a depth of 601 feet below the sea floor.[2][3] However, deeper drilling never took
place. An attempted shift of operational control to the National Science Foundation proved
unsatisfactory, and in short order the American Miscellaneous Society dissolved itself,
phase two of the project was abandoned and the entire project was discontinued by
Congress, which objected to increasing costs in 1966.[2][3]
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Roger Ravelle, like many of his colleagues in the early 1960s, was eager to drill into the
“Mojo” at some thin spot in the marine crust of the Earth. This meant in the ocean,
somewhere lateral—but not too far away—to a mid-ocean ridge. Geologists couldn’t stand
not knowing how processes worked at the Mohorovic discontinuity between cool dense
overlying crust, and the underlying asthenosphere. The Mohole Project was conceived as a
joint project between the petroleum industry and the academic fields associated with marine
geology. Here Roger is pictured surrounded by geologists in the messhall on CUSS I.
Notice the galley crew, beyond the geologists.
As the preceding image and notes showed, Project Mohole died at the hands of Congress,
for want of funding, at about the same time as (inset) resistance to the cost of war in
VietNam escalated.
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On the left, a young Teddy Bullard at geophysical fieldwork in Africa in the 1930s. At
right, Roger Ravelle and Sir Edward (“Teddy”) Bullard in 1973 (12 years after the startup of
ill-fated Project Mohole), when one of them is being awarded the Albatross Prize at Scripps
Institute of Oceanography.
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Why the emphasis on the Pacific Ocean? I like this view of the world: the North American
Plate is not cut in two at the International Date Line by this depiction.
There is nothing in western Eurasian Plate or eastern North American Plate modern
experience to rival the dynamism expressed around the Pacific Plate: consider the “ring of
fire” that stretches from southern South America, round the northern Pacific through Japan,
Indonesia, and to New Zealand.
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This is an even better view of the world because the relative motions of major crustal plates
are shown by red arrows along plate boundaries.
Note that along the mid-Atlantic ridge only spreading is taking place. The only convergence
zone being experienced by the edges of the Eurasian Plate is where the Indian Plate is
jamming into its southern edge, where the Himalayan upthrust is taking place. Major
convergences where marine crust is subducted beneath continental crust are confined to the
Pacific Ocean and its rim, such as in Alaska, Kamtchatka, along the coasts of Central and
South America. In short, all actions of convergence are overwhelmingly concentrated on the
Pacific Plate and are therefore best interpreted by observations in the Pacific Ocean, and
parallel or around the “Ring of Fire.”
Notice also that the only region where deep ocean trench and subduction of marine crust
occurs in the Atlantic is really at the edge of the Pacific Ocean, where the Carribean Plate is
wedged against the Cocos Plate.
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Another way to appreciate the “significance” of the Pacific to the development of Global
Tectonic Theory is to list the major trenches (10, here numbered 1-10) and the major
fracture zones (10, here numbered 11-20) identified in the course of 20th century
explorations by often mystified marine geologists.
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