Inside the Earth
For many centuries, the bowels of the Earth were a matter of intense conjecture with little evidence against which to assess the worth of rival ideas. For some time after the Renaissance, most scholars accepted the system proposed by Empedocles and elaborated by Aristotle, which maintained that there were four elements - air, earth, fire, and water. They believed that the Earth was a solid, spherical body composed of assorted metals, rocks, and earth, within which were underground regions of water, air, and fire (see Kelly 1969, 217). By the time of the Restoration in England, cosmogonists speculated on the formation of the Earth. Thomas Burnet (1635-1715) opined that the Earth started as a chaotic mixture of earth, water, oil, and air that gradually consolidated to form a sphere (Burnet 1691, see also Burnet 1965). As time passed, the rocky ingredients separated out from the chaotic fluid. The heaviest material in the liquid fell and collected at the Earth's centre where it formed a spherical core. The next heaviest portions of the chaotic fluid then became the terrestrial fluids, while the least heavy portions became the atmosphere. The terrestrial fluids further separated, oily, fatty, and light fluids rising to the surface to float on underlying water. Further separation also took place in the atmosphere, which was then thick and dark owing to the suspension of terrestrial particles. Slowly, the terrestrial particles settled out and mixed with the fatty and oily materials floating on the water to form a hard, congealed skin lying on the surface of the terrestrial fluids and completely sealing them in a watery abyss. When humans sinned, God released water from the abyss, so engineering Noah's Flood. John Woodward (1665-1728) proposed a variation on the Burnetian theme, in which the Floodwaters dissolved all the Earth, the watery materials then sinking down in an ordered sequence according to their specific gravity (Woodward 1695; see also Whitehurst 1778).
During first part of the nineteenth century, geologists proposed several models for the interior of the Earth, many thinking it had a very thin crust, about 25 to 50 miles thick, lying on a large and molten core (see Brush 1979). Such a structure, they postulated, would explain volcanoes, earthquakes, and mountain formation. Astronomers and physicists were against this idea, arguing that the crust must be at least 800 miles thick (and could well be solid throughout) to explain the planet's high rigidity, which they inferred from astronomical and tidal arguments. By the end of the nineteenth century, many geologists accepted a completely solid Earth containing isolated 'lakes' of liquid rock.
In the early twentieth century, seismic waves from earthquakes helped to reveal the basic structure of the Earth's interior - core, mantle, crust - that still has currency. Nevertheless, that finding did not end speculation about the dynamics of the geosphere or about the detailed structure of Earth's interior. The story of continental drift versus fixed continents has entered into geological folklore, with the theory of plate tectonics reigning supreme since the early 1960s (see Hallam  for an excellent account). However, the plate tectonic paradigm is beginning to crack around the edges (indeed, some would say at the core). Some researchers question the existence of a 'subduction recycling factory' that brings subducted rocks and sediment back to the surface as basalt at mid-ocean ridges. Others question the ability of plate tectonic mechanisms to restock the continental lithosphere after loss by erosion.
Soon after 1971, the plume hypothesis shared the ruling theory role with plate tectonics, the two combining to explain many features of global tectonics. However, the plume hypothesis is now the subject of strident criticism in some quarters and its antithesis, the plate hypothesis, is gaining support.
Ideas that are more radical centre on the question of the changing size of the Earth. The conventional view is that the Earth has a constant radius, but Earth contraction was once a popular idea and Earth expansion, always regarded as a strong possibility by a very small band of geologists, is enjoying a measure of new support.
Comets were particularly noticeable in the last quarter of the seventeenth century, with a bright comet seen in 1680 and another in 1682. William Whiston (1666-1753) was perhaps the first to argue that comets might have played a role in Earth history, speculating that a comet approaching close by the Earth in the year 2349 BC had led to widespread flooding and wholesale extinction of animals, plants, and humans (Whiston 1696). Edmund Halley (1656-1742), in a paper read to the Royal Society in 1694, proposed that a collision between the Earth and comet had been God's instrument for unleashing a cataclysm as enormous and powerful as Noah's (Halley 1724-5). At the conclusion of his classic paper on comets, Halley (1705) noted that the comet of 1680 had come close to the Earth and was prompted to write: 'But what might be the consequences of so near an appulse; or of a contact; or lastly, a shock of the celestial bodies, (which is by no means impossible to come to pass) I leave to be discussed by the studious of physical matters'. In 1755, Thomas Wright of Durham (1711-1786) noted that it was 'not at all to be doubted from their vast magnitude and firey substance, that comets are capable of distroy-ing such worlds as may chance to fall in their way' (quoted in Clube and Napier 1986a, 261). Pierre Simon, Marquis de Laplace (1749-1827), in his Exposition du Systeme du Monde of 1796, elaborated this view, asserting that a comet encountering the Earth would cause cataclysmic events to occur. He wrote of a change in the rotation axis and the direction of rotation imparting violent tremors to the globe, and causing the seas to abandon their basins and to precipitate themselves towards the new Equator. He envisioned a universal flood and massive earthquakes in which a great proportion of humans and animals would drown, entire species would be wiped out, and all the monuments of human endeavour would be destroyed. However, these catastrophic prognostications were not widely accepted by the scientific intelligentsia of the Enlightenment, many of whom regarded the notion of celestial missiles as agents of catastrophism as a drawing-room joke (Clube and Napier 1986a, 261). Cosmic catastrophism thus became regarded as improbable, a view which has persisted, and indeed was reinforced, for much of the twentieth century (Bailey et al. 1986, 91).
Reports of stones falling from the sky also promoted speculation about cosmic objects striking the Earth. However, the scientific establishment did not take the notion of bombardment seriously until trustworthy witnesses actually observed a large fall of meteorites. During the early part of the twentieth century, the discovery of asteroids on a potential collision course with the Earth led to the suggestion that some craters at the Earth's surface might have an impact origin, and several astronomers, following their illustrious predecessors, conjectured about the consequences of a large bolide strike. Even so, the geological community remained unconvinced. As Ursula Marvin put it:
In the minds of geologists, the idea [of bombardment] aroused, and continues to arouse, an uneasy sense of insult to one's professional heritage, to one's well-studied structures, and to the Earth itself. Into the orderly, steadily ticking uniformitarian world, where changes always have been perceived as taking place grain-by-grain and millimeter-by-millimeter over eons, hurtles a projectile from space! Instantaneously, the collision excavates a crater, melts and shock-metamorphoses the floor materials, cracks and tilts the country rock, and blankets the surroundings with ejecta. The process is sudden, random, unpredictable, and not to be contemplated while there remains any possible endogenous alternative.
(Marvin 1990, 152)
The almost complete acceptance of the bombardment hypothesis had to wait until the early 1960s, when astronomers found unmistakable signatures of hypervelocity impacts. The hunt for impact craters then began in earnest. When in 1980 Walter Alvarez and his colleagues reported evidence for a huge impact at the close of the Cretaceous, geologists embraced the bombardment hypothesis without question, although many remained sceptical about its significance for the history of the biosphere and geosphere. In July 1994, Comet Shoemaker-Levy 9 hit Jupiter. The spectacle of 21 short sharp strokes vindicated the long-held and much-ridiculed belief in cosmic catastrophism and gave a huge boost to the bombardment hypothesis. Over 170 impacts craters are now identified based on secure impact signatures, and crater form and distribution are better understood. A debate surrounds the history of impacts events - are they random, one-off occurrences or do they occur periodically in clusters? The evidence for periodicity in the cratering record is slim, but a period is suggested. Several putative mechanisms are proposed to explain the periodic change in asteroid and comets flux. As well as the original cosmic catastrophism, involving essential random or perhaps periodic strikes by stray celestial bodies ('stochastic catastrophism'), two rival brands of cosmic catastrophism, both controversial, have emerged -coherent catastrophism and coordinated catastrophism.
The Earth is currently in an interglacial stage of an ice age. Large ice sheets disappeared from wide tracts of North America and Eurasia as recently as 12,000 years ago. Ice sheets, ice caps, and glaciers still exist today, so it is not too difficult to recognize landforms and sediments that betray the action of ice and glacial meltwater. Similar landforms and sediments preserved from earlier periods of Earth history point to ice ages before the Quaternary.
Since the promulgation of the glacial theory in 1840, scientists have energetically debated the nature and causes of ice ages. Proposed causes of the Quaternary Ice Age include the disposition of land and sea, true polar wander, hot and cold regions in space, variable output of the Sun, and volcanoes. Variations in Earth's orbital parameters (ellip-ticity, precession, and obliquity of the ecliptic) became a popular explanation, starting with the pioneering work of James Croll (1875) and following through with the time-consuming calculations of Milutin Milankovitch (1920, 1930, 1938). However, the scientific community did not generally accept the astronomical theory of ice ages until the late 1960s and early 1970s when proxy temperature data from deep-sea cores and loess sequences from central Europe showed that Quaternary climate had changed in sympathy with orbital forcing in the Croll-Milankovitch frequency band. The ice ages now had a pacemaker (Hays et al. 1976). The scientific community then went 'orbital forcing crazy', as more and more work seemed to confirm the hypothesis. Frustratingly, having accepted the astronomical theory of ice ages, some researchers started to discover problems with it, which are still under discussion. They include the '100,000-year problem', the '400,000-year problem', the variable length of ice ages, and the presence of climatic cycles unrelated to orbital forcing.
The cause of the Quaternary ice ages has always been a debatable issue. Global climatic cooling, probably associated with decreased levels of carbon dioxide in the atmosphere, seems a prerequisite. Then again, ice sheets grow only if winter snow (which requires moisture to form) can survive the summer heat. Something other than orbital forcing presumably triggered the onset of the Quaternary ice ages in the Northern Hemisphere because orbital forcing was seemingly powerless to cause ice ages during the Palaeogene and Neogene.
Geologists have known of ancient ice ages for a long time. One of the most puzzling of these is the Neoproterozoic glaciations where ice occurs in tropical latitudes. A controversial theory arose - the snowball Earth hypothesis - that argued that the world had frozen over entirely. This hypothesis has generated much discussion, a deal of fieldwork, and a flurry of simulations with general circulations models. Some researchers now argue that the world was only part ice-covered, with some oceans remaining ice-free. This is called the 'slushball hypothesis' and contrasts with the 'hard snowball hypothesis', which demands global refrigeration.
Geological climates seem to have alternated between 'icehouse' and 'greenhouse' states over a roughly 300-million-year cycle, at least during the Phanerozoic. The causes of these very long-term climatic changes are unclear, but a link with cosmic processes seems credible. A questionable possibility is very long-term variations in the cosmic ray flux, resulting from the solar system's moving through galactic spiral arms every 140 million years or thereabouts.
Before the glacial theory of 1840, the extensive deposits of diluvium blanketing large tracts of North America and northern Eurasia were thought to be vestiges of a grand flood, possibly corresponding to Noah's Flood. When these deposits were reinterpreted as glacial till (boulder clay), the notion of cataclysmic floods waned, although slow transgressions of the sea were accepted.
During the 1920s, J Harlen Bretz (1923) found field evidence of a cataclysmic flood, though not as large as the floods envisaged by the old diluvialists, in northwestern North America. By assiduous field observation and mapping, Bretz revealed 'a pattern of abandoned erosional waterways, many of them streamless canyons (coulees) with former cataract cliffs and plunge basins, potholes and deep rock basins, all eroded in the underlying basalt of the gently southwestward dipping slope of that part of the Columbia Plateau', which he was to call the Channeled Scablands (Bretz 1978, 1). He attributed these features to a huge debacle, which he later christened the Spokane Flood. This brief but immense outburst of water had filled normal valleys to the brim, and had then spilled over the former divides, eroding the summits to complete the network of drainage ways. He argued that the water had come from the sudden release of a large glacial lake. This suggestion generated a flood of high-handed criticism almost as big as the Spokane Flood itself. Here is how Bretz recalled the episode in later years:
Catastrophism had virtually vanished from geological thinking when Hutton's concept of 'the Present is the key to the Past' was accepted and Uniformitarianism was born. Was not this debacle that had been deduced from the Channeled Scabland simply a return, a retreat to catastrophism, to the dark ages of geology? It could not, it must not be tolerated.
This, the writer of the 1923 article learned when, in 1927, he was invited to lecture on his finding and thinkings before the Geological Society of Washington, D.C. an organization heavily manned by the staff of the United States Geological Survey. A discussion followed the lecture, and six elders spoke their prepared rebuttals. They demanded, in effect, a return to sanity and Uniformitarianism.
But Bretz stood by his guns and doggedly pursued his research into this enormous debacle. He painstakingly brought to light more and more detail of the flood and its effects. He managed to trace the flood down the Columbia river as far as Portland, Oregon, adding a 200 square mile delta in the Willamette Valley. In 1930, he reported his prize discovery - Glacial Lake Missoula, the source of the voluminous floodwaters. Bretz had to wait many years until his outrageous hypothesis, for so it was regarded, was vindicated. It was not until 1956, with the publication of a report on a further set of field investigations, that the sharp knives of the critics were finally turned. In a field study made in the summer of 1952, Bretz, approaching 70 years of age, discovered a criterion of undeniable validity for the occurrence of a flood:
Hidden largely by sagebrush were numerous occurrences of current ripple marks. They were discovered because the U.S. Bureau of Reclamation had taken aerial photographs of the area to be irrigated with Grand Coulee water. Then it became clear that some gravel surfaces, curiously humpy, were covered with giant current ripples. An investigator, standing between two humps, could not see over either one. Indeed, the size of these ripple ridges made them really small hills. Finally came the discovery of giant current ripples in parts of Lake Missoula where, in a catastrophic emptying, strong currents were formed.
In 1973, Victor Baker, by measuring records for depths of water and water-surface gradients in channels with proper cross sections, was able to estimate the discharge of water during the Spokane Flood. The flood discharge reached 21.3 million m3/sec, and in some channels, the flood flow velocity touched 30 m/s; but even at that phenomenal discharge, it would take a day to empty the lake of its 2.0 x 1012 m3 of water (Baker 1973). Over the past few decades, researchers have found evidence for other huge outburst floods in the Pacific Northwest and in Russia.
A perhaps rare kind of megaflood occurs when a rising ocean overtops a sill, behind which there is a basin of dry land or lakes lying below sea level. This probably happened about 5.3 million years ago when the Strait of Gibraltar opened and water from the Atlantic Ocean poured into the then empty Mediterranean Basin as a gigantic waterfall. It also seems to have happened when the Mediterranean Sea filled the nearly empty Black Sea basin. Less spectacular, but probably more devastating to human life, are the tsunamis triggered by earthquakes and underwater landslides. But the tsunamis experienced in historical times would be nothing compared with the superwaves generated by a hyper-velocity bolide landing in an ocean.
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