Encyclopedia of Physical Science and Technology - Earth Sciences

265 Pages·2001·4.33 MB·English

by  Robert A. Meyers (Editor-in-Chief)

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Earth Sciences, History of
Naomi Oreskes
University of California, San Diego
I. Earth History and Geological Time
II. Extinction, Evolution, and the Age of the Earth
III. Continental Drift and Plate Tectonics
IV. Oceanography, Meteorology, Seismology,
and Planetary Sciences
V. Historiographic Reflections
GLOSSARY and coastal islands of Scandinavia were rising. In the
early twentieth century, this uplift was recognized as a
Asthenosphere A region of the earth’s upper mantle at a regional phenomena encompassing Norway, Sweden,
depth of approximately 75–150 km, characterized by and Finland, resulting from landscape adjustment to
low mechanical strength, attenuation of seismic shear post glacial conditions. During the Pleistocene period,
waves, and partial melting. The term was coined in the land had sunk under the weight of glacial ice. When
the 1910s by geologist Joseph Barrell to describe the the ice melted, the land began to rebound in a gradual
zone in which isostatic adjustment occurs and basaltic process, still continuing today.
magmas are generated. With the development of plate Geognosy A term introduced in the eighteenth cen-
tectonics in the 1960s, the asthenosphere is now un- tury by German mineralogist Abraham Gotlob Werner
derstood as the plastic zone over which the rigid plates to denote factual knowledge of rocks, minerals, and
move. their spatial relations, without reference to theoretical
Bathymetry Measurement of ocean depths. interpretation.
Benioff zones Zones of intermediate and deep-focus Geosynclines Regions bordering the continental margins
earthquakes, dipping 30◦–45◦ from the ocean toward where thick sequences of shallow-water sedimentary
the continents, described by seismologist Hugo Benioff strata accumulated, and subsequently were compressed
in the 1950s. While Benioff interpreted them as fault and folded into mountain belts. The concept was de-
planes along continental margins, these zones are now veloped in the nineteenth century by geologists James
understood to mark the locations where oceanic crust Dana and James Hall, Jr., to account for the features of
is subducted beneath continental margins at convergent the American Appalachians, and in the United States
plate margins. it became the generally accepted explanation for the
Fennoscandian rebound In the early nineteenth century, origins of mountains prior to plate tectonics. However,
farmers and fishermen observed that the shorelines it was never widely accepted elsewhere.
761
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762 Earth Sciences, History of
Isostasy The theory that the earth’s crust floats in hy- (1452–1519) multitude of interests was a fascination with
drostatic equilibrium within a denser, fluid or plastic earth materials. As an architect and engineer supervising
substrate. excavations, he recognized the similarity of fossils with
Orogenesis The theory or study of mountains and their living forms and interpreted rock strata as evidence of a
origins. progressive earth history in which terrestrial regions were
Uniformitarianism The principle, articulated by nine- previously inundated. Da Vinci also advanced one of the
teenth century British geologist Sir Charles Lyell, earliest recorded estimates of the duration of a geological
that geological history can be explained by reference process: 200,000 years for the Po River to lay down its
to presently observable processes. In his now-classic alluvial plain.
work, Principles of Geology (1830), Lyell argued that Leonardo’s interest in strata exposed in building sites
past geological processes were the same in rate, in- was paralleled by Georgius Agricola’s (1494–1555) inter-
tensity, and kind as those currently in operation, and est in the rocks of the mining districts of Saxony. Best
earth history was not progressive (substantive uni- known for De Re Metallica, his influential treatise on
formitarianism). Many geologists were unconvinced ore deposits, mining techniques, and mineral processing,
by Lyell’s arguments for constant rates and inten- Agricola also wrote extensively on physical geology, sub-
sity, but enthusiastically adopted uniformitarianism as surface fluid flow, mineralogy, fossils, and the causes of
a methodological program of interpreting the geo- geological features. He proposed that mineral veins were
logical record by analogy with modern environments precipitated from circulating fluids, and suggested that se-
and events (methodological uniformitarianism). In the quences of geological events could be interpreted from the
twentienth century, uniformitarianism was often said structural relations of rocks and minerals.
to be the fundamental guiding principle of modern The realization that materials could be introduced into
geology. rocks after their formation advanced the study of ore de-
posits but confounded the study of fossils. Renaissance
naturalists used the term fossil to refer to any object dug
ALTHOUGH THE TERM “geology” did not come into up from rocks, and there was considerable dispute over
widespread use until the nineteenth century, interest in whether they were the remains of past life or had grown in
geological phenomena dates back at least to the ancient situ. Much of the debate hinged on the degree of similarity
Greeks. Historian Mott T. Greene has suggested that the between fossil and living forms. Conrad Gesner’s (1516–
battle between Zeus and Kronos in Hesiod’s Theogony 1565) On Fossil Objects was the first text to include sys-
is a description of the eruption of the volcano at Thera tematic woodcuts explicitly comparing fossils with living
in the fifteenth century BC. No doubt the inhabitants of organisms. In the late sixteenth century, the advent of cop-
the ancient world were fearful of the volcanoes and earth- per engraving enabled naturalists to produce illustrations
quakes that intermittently threatened their existence. No of great clarity and detail. Together with the development
doubt also they paid heed to the earth materials used for of museum collections and private “cabinets” of speci-
the construction of their cities, art, pottery, and weaponry. mens, this innovation facilitated communication among
Explicit speculations on the causes of earthquakes, vol- specialists, who increasingly acknowledged the similarity
canoes, lightning, and floods are found in the writings between some fossil and living forms. The organic origins
of Ionian philosophers such as Thales, Anaximander, and of fossils remained disputed, however, in part for lack of
Xenophanes (c. sixth century BC). Earthquakes and floods an account of how the fossils got inside the rock.
feature in Plato’s Timaeus in the account of the disap-
pearance of Atlantis; in Meteorologica Aristotle (384–
322 BC) discusses the processes responsible for changing
I. EARTH HISTORY AND
the earth’s surface. In the Roman world, the natural his-
GEOLOGICAL TIME
torian Pliny the Elder was killed in the 79 AD eruption of
Vesuvius that destroyed Pompeii. In the medieval period,
A. From Stratigraphy to Earth History
interest in rocks and minerals is recorded in the lapidaries
of European and Arabic scholars. The first scientific account of sedimentation and stratigra-
Interest in geological matters flourished in the late phy is credited to Niels Stensen, better known as Nicholas
fifteenth and early sixteenth centuries, when the grow- Steno (1638–1686). As physician to the Grand Duke of
ing commercial and intellectual activity of Renaissance Tuscany, Ferdinand II, Steno dissected a shark that fisher-
Europe stimulated demand for minerals and building ma- man brought ashore in 1666 and was struck by the similar-
terials and curiosity about natural objects encountered ity between the shark’s teeth and the ”tongue-stones” of
through travel and trade. Among Leonardo da Vinci’s naturalists’ discussions. He concluded that tongue-stones
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Earth Sciences, History of 763
were petrified teeth, preserved in soft mud, later hard- the start of geological time, perhaps one million years ago,
ened into rock. From this insight he developed the law the earth was entirely fluid. The first three units formed
of superposition: that stratified rocks are precipitated in by progressive precipitation, with the least soluble materi-
horizontal layers, oldest at the bottom; tilted rocks have als forming the primitive rocks, the most soluble forming
been disrupted by later events. Therefore, a sequence of the flo¨tz (which included such obviously soluble materi-
historical events could be inferred from the structural re- als as salt and gypsum). The fossil content became more
lations of rocks. In England, Steno’s work was furthered abundant and complex with time, suggesting that plant
by Robert Hooke (1635–1703). In one of the first uses of and animal life began to flourish as the waters gradually
microscopy in paleontology, Micrographia (1665), Hooke receded. Structural dislocations were attributed to periods
had compared the fine structure of fossil and living wood of stormy conditions, and lithological variations were at-
as demonstration of the organic origins of fossils. Drawing tributed to spatial and temporal heterogeneity in the ocean
on Steno, he argued persuasively for the preservation of from which the materials precipitated. Above the three
organic remains by sedimentary processes. But how long aqueous formations were the alluvial materials, formed
did such processes take? by erosion and redeposition of the earlier sequences, and
Traditionally, scholars had assumed that earth history the volcanic rocks, pumice and lava, formed from burning
and human history were coextensive, and texts were re- coal beds in the flo¨tz. Not all rocks were given a chemical
liable sources of evidence about natural historical events interpretation, but all were given superficial origins.
such as the Biblical flood. Most saw little reason to doubt Neptunism was subsequently discredited in the debate
the work of Archbishop James Ussher, who placed the over the origins of basalt, leading later writers to caricature
origins of the world at 4004 BC on the basis of textual Werner’s theory and dismiss his contributions. This is a
evidence, or Thomas Burnet (1635–1715) whose Sacred mistake. By enabling correlation between distant locales,
Theory of the Earth (1680) integrated natural and super- Werner’s formation concept made it possible to talk about
natural explanation to account for the physical features earth history in a global manner. Formations became the
of the earth’s surface and the literary record provided by organizing principle of stratigraphy, inspiring scores of
scripture. This view was challenged by George Buffon geologists in the early nineteenth century to take up sys-
(1707–1788), who argued for human history as only the tematic field mapping. One example is William Maclure,
last of seven epochs preserved in the stratigraphic record. who produced the first geological map of the United States
Buffon argued that the study of human history had to be in 1809.
decoupled from earth history, and rocks rather than texts The proof of the igneous origins of basalt is prop-
were the relevant source of information about the latter. erly credited to the French geologist, Nicolas Desmarest
Inspired by Leibniz, he experimented with cooling globes (1725–1815), who, in explaining the basalts of Auvergne,
to obtain an estimate of the age of the earth of several tens articulated the principle of uniformity generally associ-
of thousands of years. ated with his Scottish contemporary, James Hutton (1726–
The eighteenth century scholar who most advanced nat- 1797). In fact, many French and German geologists in the
uralistic study of earth history was the German miner- late eighteenth century interpreted geological patterns by
alogist Abraham Gottlob Werner (1749–1817). Hailing reference to observable processes of sedimentation, ero-
from a family associated with ironworks, Werner was ed- sion, and volcanism; by late century the habit of examining
ucated at the Bergakademie Freiberg, where he then built modern environments for interpretive analogues was es-
his career. Students came from across Europe to hear his tablished. Hutton, however, elevated this practice into a
famed lectures (including later luminaries Leopold von unifying methodological precept, linked to a steady-state
Buch, Alexander von Humboldt, and Friedrich Mohs), and theory of earth history.
Freiberg became the most celebrated school of geology in A polymathic founding member of the Royal Society
the world. In his systematic mineralogy, Werner laid the of Edinburgh and avid Newtonian, Hutton believed that
foundations of the system of identifying minerals by their rocks recorded the “natural history” of the earth: as mon-
external characteristics still used by field geologists today. uments recorded human history and their functions could
He also introduced the concepts of geognosy—knowledge be understood by comparison with contemporary edifices,
of rock masses, their mineral contents, and their spatial so ancient rocks could be understood by comparison with
relations—and formations. Challenging taxonomic tra- processes presently operating on the earth’s surface. Sed-
dition, he argued that rocks should be grouped by their imentary rocks formed by erosion and redeposition of ter-
mode and time of formation, even if they looked different, restrial materials, but this implied that an earlier generation
thereby providing a basis for regional correlation. of rocks had been exposed at the surface. How? Hutton’s
Werner divided the rock record into five basic forma- answer, presented in his Theory of the Earth (1795), was
tions: primitive, transition, flo¨tz, alluvial, and volcanic. At a geological cycle of deposition, burial, heating, melting,
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764 Earth Sciences, History of
expansion, uplift, erosion, and deposition again. But ero- Geology (1830), he argued the view later dubbed unifor-
sion and deposition were scarcely detectable; their rates mitarianism: that the accumulation of incremental change,
of operation were nearly vanishingly slow. So Hutton con- operating in the past at the same rate and intensity as
cluded that geological time must be “indefinite,” prompt- in the present, was sufficient to explain the transforma-
ing his famous description of earth history as bearing “no tions of the geological record. It was not merely that the
vestige of a beginning, no prospect of end.” laws of nature were temporally and spatially uniform—
Hutton and Werner have commonly been cast as antag- any Newtonian would have accepted that—but also that
onists, in part because their theories diverged on the ques- the processes themselves were uniform. Study of modern
tion of whether earth history was progressive or steady analogues was therefore both necessary and sufficient for
state. But there was an important commonality: a shared understanding past geological process. While the details of
emphasis on the primacy of fieldwork as the method- Lyell’s position were hotly argued by his contemporaries,
ological tool to unravel earth history through the struc- particularly the assertion of constant rates of change, the
tural relations of rocks. In the nineteenth century geolog- uniformitarian framework was widely accepted. Many
ical field work came to prominence, culminating in the would later call it the central methodological principle of
establishment of the geological time scale. geology.
The uniformitarian reading of geological time was far
greater than any literal or even semiliteral interpretation
B. The Geological Time Scale
of the Old Testament might allow, thus contributing to a
In 1835, the British Geological Survey was founded un- more flexible interpretation of religious doctrine. Geologi-
der Henry Thomas De la Beche (1796–1855), reflecting cal knowledge also facilitated the exploitation of earth ma-
an increased governmental interest in the materials fu- terials, warranting the expansion and institutionalization
eling the industrial revolution: coal, limestone, iron, tin, of the science. By the late nineteenth century, academic
copper, and lead. The Survey was preceded and com- departments and government surveys had been established
plemented by the work of private land surveyors, such across Europe and the United States.
as William Smith (1769–1839), who pioneered the use
of fossil assemblages to distinguish lithologically similar
units, and produced the first geological map of England
II. EXTINCTION, EVOLUTION,
in 1815. Complementing economic motivation, academic
AND THE AGE OF THE EARTH
natural theologians such as William Buckland (1784–
1856) and Adam Sedgwick (1785–1873) pursued field
A. The Fact of Extinction
geology as a means to know God through his works,
while gentlemen-scientists such as Roderick Murchison With the establishment of geological time, historian Paolo
(1792–1871) adopted natural history an appropriate vo- Rossi notes, fossils “become the clocks that can measure
cation for men of independent means. The task of ge- the long time periods of natural history.” The newly es-
ological reconstruction was facilitated by the British tablished geological time periods were partly defined by
Isles’ remarkably complete and largely undeformed se- their distinctive fossil assemblages. Most notably, the ends
quences. Field mapping flourished; by midcentury ge- of geological eras—Paleozoic, Mesozoic, Cenozoic—as
ologists had reconstructed the sequence of rock units well as the ends of the periods—Cambrian, Permian, Cre-
across Great Britain, Europe, and portions of North taceous, etc.—were marked by the disappearance of one
America, and defined the geological time scale in use or more major species.
today. Extinction had often been addressed in the context of di-
Geological mapping established that earth history was vine providence: Why would God destroy his own work?
marked by distinctive rocks with characteristic fossil as- Why would he create imperfect forms? Initially, many
semblages. However, in any one place only fragments of paleontologists thought that seemingly missing creatures
the whole record were found, and gaps were commonly might still be lurking in the depths of oceans and the inte-
associated with structural dislocations where rocks were riors of uncharted terrains. However, as increasing num-
tilted or folded like putty. It seemed logical, even obvious, bers of fossils were discovered and their relations to living
to interpret these gaps and dislocations as the result of cat- forms scrutinized, this explanation became increasingly
aclysmic events such as the Lisbon earthquake of 1756, the untenable. In his 1796 paper “On the Species of Living and
Etna eruption of 1669, or the Biblical flood. Challenging Fossil Elephants,” the French paleontologist and compara-
this view, Charles Lyell (1797–1875) revived Hutton’s in- tive anatomist George Cuvier (1769–1832) demonstrated
terpretive reliance on observable processes and promoted that the fossil “elephants” (mammoths) of Siberia were
it as the basis for geological explanation. In Principles of distinct from living forms. The fact of the extinction of
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Earth Sciences, History of 765
mammoths and many other species, from belemnites to C. Millions or Billions?
ammonites, was soon accepted.
Geologists’ conviction in a prodigiously old earth was rad-
This led to a new scientific question: Why did species
ically challenged in the late nineteenth century by British
go extinct? And where did new species come from? It was
physicist William Thomson, later Lord Kelvin (1824–
increasingly clear that new species appeared at intervals
1907). Kelvin revived an idea earlier proposed by Buffon
in the fossil record as surely as some old ones had dis-
that the earth’s age could be deduced on the principle of
appeared, and these changes provided a metric by which
a cooling sphere. Drawing on the Kant–LaPlace nebular
to measure geological time. Cuvier recognized three ex-
hypothesis, that the solar system had formed by condensa-
planatory options: destruction, modification, or migration.
tion from a gaseous cloud, and applying Fourier’s theory
Seeing no direct evidence of modification in the fossil
of heat transfer, Kelvin calculated the time required for
record, he opted for destruction followed by migration:
the earth to cool to its present temperature at no more than
rapid marine transgressions had destroyed the faunal as-
20–40 million years. If uniformitarianism implied other-
semblages of earlier periods, after which species migrated
wise, then uniformitarianism must be wrong, and geologi-
in from elsewhere. Drawing on the political events of his
cal practice based on a fallacy. Moreover, Darwin’s theory
own time, he called these periods revolutions. These nat-
of the origin of species by natural selection must also be
ural revolutions were not necessarily sudden or violent,
wrong, resting as it did on the premise of eons of time.
but they were major transformations in which old species
This was a profound challenge. Some geologists tried
vanished, new ones appeared.
to adjust themselves to Kelvin’s numbers; some sought
a compromise or defended the geological view. Darwin
B. From Revolution to Evolution suggested that the rate of evolution might be faster than
he had supposed. The debate was resolved in the early
The question of modification of species was taken up by
twentieth century on the geological side by the discov-
Jean-Baptiste Lamarck (1744–1829) and later more suc-
ery of radiogenic heat, which mooted Kelvin’s starting
cessfully by Charles Darwin (1809–1882). A follower and
premises. (Today it is further mooted by the theory of cold
later a friend of Lyell, Darwin was deeply influenced by
accretion of the terrestrial planets.) By 1913, British geol-
his argument for the efficacy of small changes accumu-
ogist Arthur Holmes (1890–1965) had used U–Pb ratios
lated over geological time. This became the foundation
to demonstrate that Precambrian rocks were at least 2 bil-
for his theory of the origin of species by natural selection.
lion years old. In the 1930s, the development of the mass
By analogy with a presently observable process—artificial
spectrometer by Alfred Nier (1911–1994) made it pos-
selection by breeders—Darwin conceptualized a mecha-
sible to differentiate lead isotopes, pushing calculations
nism of natural evolutionary change. Uniformitarianism
upward toward 3 billion. In 1953, American geochemist
provided theoretical justification for a gradualist interpre-
Clair Patterson (b. 1922) used iron–nickel meteorites to
tation of faunal change in the absence of direct evidence.
determine primeval lead ratios, producing the presently
Darwin famously attributed the “missing links” to gaps
accepted earth age of 4.55 billion years.
in the fossil record, and most geologists concurred. By
the end of the nineteenth century, the fact of evolution (if
not the mechanism of it) was widely accepted. This fact
III. CONTINENTAL DRIFT
hinged in turn on a very old earth.
AND PLATE TECTONICS
But how old was old? Most geologists spoke in qualita-
tive terms. Werner had described the great duration of ge-
A. The Origin of Mountains:
ological time “in contrast to which written history in only
Thermal Contraction
a point.” Hutton had called geological time “indefinite”;
others spoke of its “limitless stores.” Uniformitarianism Geologists in the eighteenth and nineteenth century made
inspired attempts at quantification: Darwin used modern great advances in documenting earth history and explain-
sedimentation rates to estimate the time for the deposition ing the origins of sedimentary rocks; they were less suc-
of the Weald (a unit of the lower Cretaceous) at 300 mil- cessful in accounting for the origins of mountains, earth-
lion years—implying an earth age of hundreds of billions. quakes, and igneous processes. In Europe, mountains had
Irish geologist John Joly (1857–1933 ) used the salt con- often been viewed as frightening and dangerous places,
centration of modern rivers to calculate how long it would but they also contained many valuable mineral deposits.
take to salinize the world’s oceans. His result: 90 million. In the early nineteenth century, they attracted increasing
While the rigor of Joly’s calculations appealed to some, artistic attention as sites of great natural beauty, cultural
many geologists intuitively felt it was too low. Most pre- attention as potential sites of human conquest, and polit-
ferred billions to millions. ical attention as a barriers to expansion. They attracted
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766 Earth Sciences, History of
scientific attention as well. How were they formed? What sediments to accumulate, causing more subsidence, until
caused their spectacular structural features? What made finally the weight of the pile caused the sediments to be
the earth move? heated, lithified, and by some mechanism not elucidated,
Most nineteenth-century theories of orogenesis invoked uplifted into mountains. Dana modified Hall’s view by
terrestrial contraction as a causal force. In Europe, Aus- arguing that thick sedimentary piles were not the cause
trian geologist Edward Suess (1831–1914) popularized of subsidence but the result of it. Either way the theory
the image of the earth as a drying apple. Like Kelvin, Suess provided a concise explanation of how thick sequences of
built on the premise of secular cooling. Drawing on the shallow-water rocks could form.
earlier theories of Le´once Elie de Beaumont (1798–1874)
and Henry De la Beche, he proposed that the earth con-
B. Continental Drift as Alternative
tracted as it cooled, and mountains resulted from crustal
to Contraction Theory
wrinkling in response. Initially, the crust was continuous,
but it broke apart as the earth’s interior shrunk, and the In the early twentieth century, contraction theory was re-
collapsed portions formed the ocean basins. With further futed by three independent lines of evidence. First, field
cooling, the remaining elevated portions (the continents) mapping in the Swiss Alps and the North American Ap-
became unstable and collapsed to form the next generation palachians demonstrated hundreds of miles of shortening
of ocean floor; what had formerly been ocean now became of strata, which would require impossibly huge amounts
dry land. This explained the presence of marine deposits of terrestrial contraction to explain. Second, geodesists
on land (which had puzzled da Vinci), and the interleav- studying the problem of surface gravitational effects
ing of marine and terrestrial materials in the stratigraphic showed that the surface mass associated with mountains
record. Suess’s theory also explained the widely known was counterbalanced by a mass deficit within or beneath
similarities of fossil assemblages in parts of India, Africa, them. Mountains were held aloft not by their internal
and South America by attributing them to an early period strength, but by floating—a concept called isostasy. Conti-
when these continents were still contiguous. He called this nents and oceans were not interchangeable, because conti-
ancient supercontinent Gonawanaland. nents could not sink to form ocean basins. Third, physicists
In North America, a different version of contraction the- discovered radiogenic heat, which refuted the premise of
ory was developed by James Dwight Dana (1813–1895), secular cooling. With contraction no longer axiomatic,
famous at the age of 24 for his System of Mineralogy, earth scientists were motivated to search for other driving
which comprehensively organized minerals according to forces of deformation. Many did; Alfred Wegener (1880–
their chemical affiliations. (First published in 1837, a ver- 1930) is the most famous.
sion is still in print today.) Drawing on his understanding Primarily known as a meteorologist and author of a
of the chemical properties of minerals, Dana suggested pioneering textbook on the thermodynamics of the atmo-
that the earth’s continents had formed first, when miner- sphere (1911), Wegener realized that paleoclimate change
als with relatively low fusion temperatures such as quartz could be explained if continents had migrated across cli-
and feldspar had solidified. Then the globe continued to mate zones, and the changing configurations of continents
cool and contract, until the high temperature minerals such and oceans periodically altered climate patterns. However,
as olivine and pyroxene finally solidified: on the moon, to continental drift was more than a theory of paleoclimate
form the lunar craters, on earth, to form the ocean basins. change. It was an attempt at unification of disparate ele-
Continued contraction after solidification caused surface ments of earth science: on one hand, the paleontological
deformation. The greatest pressure was experienced at the evidence that the continents had once been connected; on
boundaries between the oceanic and continental blocks, the other, the geodetic evidence that they could not be con-
explaining the localization of mountains (particularly the nected in the way Suess had imagined. Wegener’s answer
Appalachians). Because continents and oceans were un- was to reconnect the continents by moving them laterally.
derstood as permanent features of the globe, Dana’s ver- Wegener’s theory was widely discussed in the 1920s
sion of contraction came to be known as permanence and early 1930s. It was also hotly rejected, particularly
theory. by Americans who labeled it bad science. The standard
In North America, permanence was linked to the the- explanation for the rejection of Wegener’s theory is its
ory of geosynclines, developed by Dana and James Hall lack of a causal mechanism. But this explanation is false.
(1811–1889), State Paleontologist of New York and the There was a spirited and rigorous international debate over
first President of the Geological Society of America the possible mechanisms of continental migration. Much
(1889). Hall suggested that materials eroded off the of it centered on the implications of isostasy: if conti-
continents accumulated in the adjacent marginal basins, nents floated in a denser substrate, then this substrate had
causing the basin to subside. Subsidence allowed more to be plastic or fluid, and continents should be able to
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Earth Sciences, History of 767
move through it. The Fennoscandian rebound—the pro- mitted to the method of multiple working hypotheses,
gressive uplift of central Scandinavia since the melting of and Wegener’s work was interpreted as violating it. For
Pleistocene glacial ice—provided empirical evidence that Americans, right scientific method was empirical, induc-
they did, at least in the vertical direction and at least in tive, and involved weighing observational evidence in
the Pleistocene. But here the cause of motion was known: light of alternative explanatory possibilities. Good the-
the weight of glacial ice and the pressure release upon its ory was also modest, holding close to the objects of study.
removal. What force would cause horizontal movement? Most closely associated with the University of Chicago
And would the substrate respond comparably to horizon- geologist T. C. Chamberlin (1843–1928), who named
tal as to vertical movement? Debate over the mechanisms it, the method of multiple working hypotheses reflected
of drift therefore concentrated on the long-term behavior American ideals expressed since the eighteenth century
of the substrate, and the forces that could cause conti- linking good science to good government: Good science
nents to move laterally. Various proposals emphasized the was antiauthoritarian, like democracy. Good science was
earth’s layered structure, which allowed for decoupling of pluralistic, like a free society. If good science provided an
the continental layer from the one beneath. exemplar for good government, then bad science threat-
John Joly linked the problem of continental drift to ened it. To American eyes Wegener’s work was bad sci-
the discoveries in radioactivity. He had demonstrated that ence. It put the theory first, and then sought evidence for it.
pleochroic haloes in mica were caused by radiation dam- It settled too quickly on a single interpretive framework.
age from tiny inclusions of U- and Th-bearing miner- It was too large, too unifying, too ambitious. Features that
als, proving that radioactive elements were ubiquitous in were seen as virtues of plate tectonics were attacked as
rocks. Then radiogenic heat was also ubiquitous, and as it flaws of continental drift.
built up it would melt the substrate. During these molten Continental drift was also incompatible with the ver-
periods the continents could move under the influence of sion of isostasy to which Americans subscribed. In the
small forces that would otherwise be ineffectual. late nineteenth century, two accounts of isostatic compen-
Joly’s theory responded to a geophysical complaint sation had been proposed: John Henry Pratt (1809–1871)
against a plastic substrate: that the propagation of seis- attributed it to density variations, George Biddell Airy
mic waves indicated a fully solid and rigid earth. More (1801–1892) attributed it to differences in crustal thick-
widely credited was the suggestion of Arthur Holmes that ness. Until the early twentieth century, there had been no
the substrate was partially molten or glassy. Underscoring empirical confirmation of the concept beyond the original
arguments made by Wegener, Holmes emphasized that the evidence that had inspired it, nor any means to differenti-
substrate need not be liquid, only plastic. Furthermore, ate the two explanations. Then American geodesists John
it might be rigid under high strain rates (during seismic Hayford (1868–1925) and William Bowie (1872–1940)
events) yet be ductile under the low strain rates prevail- used the Pratt model to demonstrate that isostatic com-
ing under most geological conditions. If it were plastic in pensation was a general feature of the crust. By making
response to long-term stress, then continents could move the assumption of a uniform depth of compensation, they
within it. Holmes’s driving force was convection currents were able to predict the surface effects of isostasy to a
in the mantle. He argued that the midocean ridges were the high degree of precision throughout the United States.
sites of upwelling convection currents, where continents At first, their work was hailed as proof of isostasy in gen-
had split, and the ocean deeps (geosynclines) were the eral, but in time, it was viewed as confirmation of the Pratt
sites of downwelling currents, where continents were de- model in particular. However, if continental drift were true,
formed as the substrate descended. Between the ridges and then the large compressive forces involved would squeeze
the trenches, continents were dragged along in conveyor- the crust, generating thickness differentials. Continental
like fashion. drift seemed to refute Pratt isostasy, which had worked for
Americans so well, and because it had worked, they had
come to believe was true. Rather then reject Pratt isostasy,
C. The Rejection of Continental Drift
they rejected continental drift.
Arthur Holmes’s papers were widely read and cited; many Finally, Americans rejected continental drift because
thought he had solved the mechanism problem. However, of the legacy of uniformitarianism. By the early twen-
opposition to drift was nonetheless for that, particularly tieth century, the methodological principle of using the
in the United States, where reaction to Wegener’s theory present to interpret the past was deeply entrenched in the
was harshly negative, even vitriolic. Evidently more was practice of historical geology. Many believed this to be
at stake than a matter of scientific fact. the only way to interpret the past, and that uniformitari-
Three factors contributed to the American animosity anism made geology a science, for without it what proof
to continental drift. One, Americans were widely com- was there that God had not made the Earth in seven days,
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768 Earth Sciences, History of
fossils and all? Historical geologists routinely used faunal convection drives the crust apart at midocean ridges and
assemblages to make inferences about climate zones, but downward at ocean trenches, forcing the continental mi-
on drift-based reconstructions, continents placed in the grations in their wake. Hess interpreted the oceanic crust
tropics did not necessarily have tropical faunas, because as a hydration rind on serpentinized mantle; Dietz mod-
the reconfiguration of continents and oceans might change ified this to generate oceanic crust by submarine basalt
things altogether. Wegener’s theory raised the specter that eruptions. Dietz’s interpretation was later confirmed by
the present was not the key to the past—it was just a mo- direct examination of the sea floor.
ment in earth history, no more or less important than any Meanwhile American researchers, Richard Doell
other. This was not an idea Americans were willing to (b. 1923), Brent Dalrymple (b. 1937) and Allan Cox
accept. (1923–1987) were studying a different aspect of rock mag-
In North America, the debate over continental drift was netism: the record of reversals in the earth’s magnetic field.
quelled by an ad hoc explanation of the faunal evidence. Detailed field studies of basaltic lava flows convinced them
In 1933, geologists Charles Schuchert (1858–1942) and that reversals were not an artifact of cooling or labora-
Bailey Willis (1857–1949) proposed that the continents tory procedures. In fact, one could construct a chronology
had been intermittently connected by isthmian links, of paleomagnetic reversals—a geomagnetic time scale.
just as the isthmus of Panama connects North and South Magnetic reversals plus seafloor spreading added up to a
America and the Bering Land Bridge once connected testable hypothesis, proposed independently by Canadian
North America to Asia. The isthmuses had been raised Lawrence Morley and British geophysicists Frederick
up by orogenic forces, subsided under the influence of Vine (b. 1939) and Drummond Matthews (1931–1997):
isostasy. This explanation was patently ad hoc—there was If the seafloor spreads while the earth’s magnetic field re-
no evidence of isthmian links other than the paleontolog- verses, then the basalts forming the ocean floor will record
ical data they were designed to explain. Nevertheless, the these events in the form of a series of parallel “stripes” of
idea was widely accepted, and a major line of evidence normal and reversely magnetized rocks. Since World War
of continental drift undercut. In 1937, South African II, the United States Office of Naval Research had sup-
geologist Alexander du Toit (1878–1948) published Our ported seafloor studies for military purposes, and large
Wandering Continents, a comprehensive synthesis of the volumes of magnetic data had been collected. American
geological evidence of continental drift, but it had little and British scientists quickly set to work examining these
impact in North America. data, and by 1966 the Vine and Matthews hypothesis had
been confirmed.
At this point, many workers turned to the problem.
D. Plate Tectonics
Among the most important were J. Tuzo Wilson (1908–
In the late 1930s, Dutch geodesist Felix Vening Meinesz 1993), who thought of another test of the theory. The
(1887–1966) and American Harry Hess (1906–1969) ap- midocean ridges were repeatedly offset by faults; the slip
plied the idea of convection currents to explain down- direction on these faults would be one direction if seafloor
warpings of the oceanic crust associated with gravity spreading were taking place, the opposite if it were not.
anomalies in the Caribbean and the Dutch East Indies. Wilson called the latter transform faults, as they trans-
However, this work was cut short by World War II. In formed one segment of a spreading ridge into another.
the 1950s, continental drift was revived by British geo- Seismic data analyzed by Lynn Sykes (b. 1937) at the
physicists working on rock magnetism as a means to in- Lamont–Doherty Geological Observatory confirmed the
vestigate the earth’s magnetic field, one group at Imperial existence of transform faults.
College led by P. M. S. Blackett (1897–1974), and one at Sykes’s Lamont coworkers, Walter Pitman (b. 1931)
Cambridge (later at Newcastle) led by S. Keith Runcorn and James Heirtzler (b. 1925), used paleomagnetic data to
(1922–1995). refine the reversal time scale, and confirm that the patterns
Both groups found evidence that rocks had had moved on either side of the mid-Atlantic ridge were symmetrical.
relative to the earth’s magnetic poles, so either the conti- In 1967–1968, these various lines of evidence were inde-
nents or the poles had moved. Initially geophysicists were pendently synthesized by Daniel P. McKenzie (b. 1942)
more receptive to the idea of polar wandering, but by the and Robert L. Parker (b. 1942) working at the Scripps Insti-
late 1950s comparative evidence from India and Australia tution of Oceanography, and by Jason Morgan at Prince-
pointed in the direction of differentially moving conti- ton University. Both showed that existing data could be
nents. Inspired by these results, Harry Hess revisited con- used to analyze crustal motions as rigid body rotations
vection currents as driving force for continental motion, on a sphere. The result became known as plate tectonics,
and proposed the hypothesis Robert Dietz (1914–1995) which by the early 1970s had become the unifying theory
dubbed sea floor spreading. Hess suggested that mantle of the earth sciences.
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Earth Sciences, History of 769
IV. OCEANOGRAPHY, METEOROLOGY, this work at the U.S. Navy Hydrographic Office, but re-
SEISMOLOGY, AND PLANETARY signed from federal service during the American Civil
SCIENCES War.
The voyage of H. M. S. Challenger (1872–1876) was
A. Historical Background the first explicitly oceanographic expedition. Lasting three
and a half years and covering 110,000 km, the expedition
Oceanography, meteorology, seismology, and planetary
took thousands of soundings, measurements of temper-
science have only recently received sustained historical
ature and salinity, and biological and bottom sediment
attention. One reason is itself historical: these are young
samples. The results (published in 50 volumes) provided
sciences. The earth’s surface permitted scientific study
the first systematic description of the oceans. The data
sooner than the other parts. Anyone could pick up a rock
demonstrated the existence of the abyssal plains at ap-
or a fossil and begin to develop a collection, but it required
proximately 5 km depth, the constancy of the proportion
instruments, financing, and often national or international
of salts in seawater, and the variation of deep-water tem-
cooperation to study the oceans, atmosphere, and earth’s
perature. The detection of a systematic temperature dif-
interior. The distant planets were even less accessible.
ference between the eastern and western half of the At-
Prior to the late nineteenth century, oceanographic and
lantic Ocean was first suggestion of the existence of a
meteorological questions were commonly understood as
mid-Atlantic Ridge. Dredging the seafloor revealed the
issues of geography, and investigations were piggybacked
predominance of calcareous oozes and red clays, quite
on expeditions that were not primarily scientific. Seismol-
unlike terrestrial sediments, providing an early argument
ogy began to flourish around the same time in conjunction
against the interchangeability of continents and oceans.
with earthquake studies and the development of precise
Among the biological results, 3225 new species were dis-
seismographs. Early in the twentieth century, it grew sub-
covered, including many at great depths, disproving the
stantially, first though its application to petroleum explo-
idea that ocean depths were devoid of life.
ration, later more dramatically through its application to
Toward the end of the century, interest in oceanography
nuclear test ban verification. Planetary science similarly
received impetus from Arctic explorers, particularly Scan-
developed in a limited way in the early century with the
dinavians Fritjof Nansen (1861–1930) and Roald Amund-
construction of powerful reflecting telescopes, and then
sen (1872–1928?). In the Fram expedition (1893–1896),
much more fully in conjunction with the U.S. and Soviet
Nansen completed the first drift across the Arctic, dis-
space programs. ◦
covering that the ice drift diverged 45 from prevailing
wind directions. Suspecting the Coriolis force [articulated
by French natural philosopher Gaspard Coriolis (1792–
B. Oceanography
1843), but as yet unapplied to ocean dynamics], Nansen
Oceanography is the oldest of the ancillary earth sciences, proposed the problem to a young Swedish oceanography
with its links to navigation and exploration, colonialism student, Wagn Ekman (1874–1954), who demonstrated
and trade. In the seventeenth century, members of the mathematically that the drift was indeed consistent with
British Royal Society compiled sea temperatures; Isaac wind-driven circulation modified by the Coriolis effect—
Newton (1642–1727) and Edmund Halley (1656?–1743) dubbed the “Ekman spiral.” Amundsen repeated the Arctic
theorized the astronomical cause of tides. In the eigh- drift on the Maud (1918–1925) with improved instruments
teenth century, British sea captain James Cook (1728– permitting better measurement of wind and current direc-
1779) recorded tides and temperatures during his voyages. tions, and confirmed Ekman’s results.
Currents were obviously important to navigation, and Ekman’s work became a cornerstone of the “dy-
most early workers thought they were driven by density namic oceanography” developed by Bjørn Helland-
differences—hence attention to surface temperature. In Hansen (1877–1957), Johan Sa¨ndstro¨m (1874–1947), and
the early nineteenth century, scientists began to consider Harald Sverdrup (1888–1957). Using temperature and
other factors. English surveyor James Rennell (1742– salinity data to determine a density field, one deduced
1830) suggested the role of winds in driving surface cur- a pressure field, and from this, ocean currents. However,
rents, while the German polymath, Alexander von Hum- mathematical treatment relied on accurate data; thus the
boldt (1769–1859), promoted the idea that sinking of cold development of dynamic oceanography also depended on
water in high latitudes could drive deep circulation. Many improved instrumentation. Chief among these was the
ship voyages therefore included some measurements of “Nansen bottle.” In response to problems encountered on
salinity, temperature, surface currents, and soundings. The the Fram, Nansen invented a self-sealing insulated bottle
founding of systematic bathymetry is generally credited to collect deep-water samples and measure their temper-
to Matthew Fontaine Maury (1806–1873), who pioneered atures in situ with a pressure-protected thermometer. The
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770 Earth Sciences, History of
Nansen bottle was widely used until the development of more temperature fluctuations than previously recognized
the bathythermograph for temperature measurement in the from lithological evidence. Cesare Emiliani (1922–1995)
1940s, and electronic techniques for salinity measurement linked these fluctuations to Milankovitch cycles—small
in the 1960s. variations in earth’s orbit on a time scale of 20–100,000
Oceanography also benefited from international co- years—providing the first widely accepted theoretical
operation. Alarmed by declining fish stocks, Swedish account of the cause of the ice ages.
oceanographer Otto Pettersson (1848–1941) spearheaded
the creation of The International Council for the Explo-
C. Meteorology
ration of the Seas (ICES) in 1902, in the hope of stemming
further losses through improved scientific understanding. Natural philosophers in the seventeenth and eighteenth
From Germany, the Meteor expedition (1925–1927) con- century sporadically investigated atmospheric phenom-
firmed the existence of deep-water circulation from north ena, but systematic meteorology developed in the nine-
to south in the northern hemisphere and middepth flow teenth century when forecasters began to organize weather
south to north in southern hemisphere, while Albert Defant data to predict storms. This advance hinged on the tele-
(1884–1974) published the first German textbook of the graph, which permitted forecasters to integrate geograph-
dynamic method, Dynamische Ozeanographie (1929) and ically dispersed information. It also hinged on the military
George Wust (1890–1977) demonstrated that calculations value of weather forecasts, which in the United States were
of the Gulf Stream based on dynamic method matched sent alongside military communications.
empirical measurements. In the late nineteenth century this empirical tradition
Dynamic oceanography was brought to the United was criticized by researchers who hoped to reduce mete-
States by Harald Sverdrup, who had sailed with Amund- orological systems to physics and hydrodynamics. Lead-
sen on the Maud. Together with Martin Johnson (1893– ing the effort was Norwegian physicist Vilhelm Bjerk-
1984) and Richard Fleming (1909–1990), Sverdrup au- nes (1862–1951), whose polar front concept gave phys-
thored the first comprehensive textbook of oceanography ical interpretation to the behavior of storm systems and
in English, The Oceans (1942). Perhaps the single most the interaction of air masses. Bjerknes was an inspired
influential book in the history of oceanography, it framed teacher; among his students he counted the pioneers of
the agenda for oceanographic research for several decades. dynamic oceanography, who applied Bjerknes’s princi-
Together with Walter Munk (b. 1917), Sverdrup developed ples and methods to the movement of water masses. As
the methods used to predict surf conditions for amphibious oceanography received support in part for its application
landings during World War II. to fisheries and navigation, so meteorology grew with the
After World War II, oceanography flourished in the development of the aeronautics industry.
United States as the U.S. Office of Naval Research (ONR) Bjerknes believed that the behavior of weather sys-
provided abundant funding for research relevant to sub- tems could be deterministically calculated, much as one
surface warfare and communication. Munk and Henry could compute advance positions of planets by knowing
Stommel (1920–1992) improved prediction of ocean cir- their orbits and initial conditions. This problem was taken
culation by adding friction and detailed wind data. This up in midcentury by British mathematician Lewis Fry
work explained how wind-driven circulation leads to in- Richardson (1881–1953), who pioneered the use of digi-
tensification of currents on the western sides of oceans tal computers in weather forecasting. Richardson achieved
in the northern hemisphere (eastern sides in the south- great improvements in short-term forecasting, but the hope
ern hemisphere), helping to explain phenomena such as of a deterministic science of meteorology was dashed
upwelling currents and El Nin˜o events. Stommel also pre- by the work of Edward Lorenz (b. 1917), who discov-
dicted the existence of abyssal circulation involving the ered that small changes in initial conditions could create
sinking of cold water at high latitudes, as suggested by very large perturbations in meteorological models. This
Humboldt, now thought to be of major importance in con- realization—dubbed the “butterfly effect”—was a key
trolling the earth’s climate. The ONR also supported in- element in the development of chaos theory.
vestigations of the structure of the seafloor and oceanic In the late twentieth century attention shifted from
crust, including the paleomagnetic studies that demon- weather to climate, as researchers developed General
strated seafloor spreading. Circulation Models to understand the effect of altered at-
Postwar advances in isotope geochemistry, headed by mospheric chemistry caused by burning of fossil fuels. Just
American chemist Harold Urey (1893–1981) and geo- as telegraph communication was critical to nineteenth-
chemist Harmon Craig (b. 1926), led to the recognition century empirical forecasting, the development of high-
18 16
that O /O ratios in fossil foraminifera could be used speed digital computers has been critical to twentieth-
to measure paleotemperatures. The results revealed many century climate modeling. However, given the complexity