by John Carey
Shortly after the full moon of the months from March to August the grunion appear in the surf on the beaches of California. The tide reaches flood stage, slackens, hesitates, and begins to ebb. Now on these waves of the ebbing tide the fish begin to come in. Their bodies shimmer in the light of the moon as they are borne up the beach on the crest of a wave, they lie glittering on the wet sand for a perceptible moment of time, then fling themselves into the wash of the next wave and are carried back to sea. For about an hour after the turn of the tide this continues, thousands upon thousands of grunion coming up onto the beach, leaving the water, returning to it. This is the spawning act of the species.
During the brief interval between successive waves, the male and female have come together in the wet sand, the one to shed her eggs, the other to fertilize them. When the parent fish return to the water, they have left behind a mass of eggs buried in the sand. Succeeding waves on that night do not wash out the eggs because the tide is already ebbing. The waves of the next high tide will not reach them, because for a time after the full of the moon each tide will halt its advance a little lower on the beach than the preceding one. The eggs, then, will be undisturbed for at least a fortnight. In the warm, damp, incubating sand they undergo their development. Within two weeks the magic change from fertilized egg to larval fishlet is completed, the perfectly formed little grunion still confined within the membranes of the egg, still buried in the sand waiting for release. With the tides of the new moon it comes. Their waves wash over the places where the little masses of the grunion eggs were buried, the swirl and rush of the surf stirring the sand deeply. As the sand is washed away, and the eggs feel the touch of the cool sea water, the membranes rupture, the fishlets hatch, and the waves that released them bear them away to the sea.
But the link between tide and living creature I like best to remember is that of a very small worm, flat of body, with no distinction of appearance, but with one unforgettable quality. The name of this worm is Convoluta roscoffensis, and it lives on the sandy beaches of northern Brittany and the Channel Islands. Convoluta has entered into a remarkable partnership with a green alga, whose cells inhabit the body of the worm and lend to its tissues their own green colour. The worm lives entirely on the starchy products manufactured by its plant guest, having become so completely dependent upon this means of nutrition that its digestive organs have degenerated. In order that the algal cells may carry on their function of photosynthesis (which is dependent upon sunlight) Convoluta rises from the damp sands of the intertidal zone as soon as the tide has ebbed, the sand becoming spotted with large green patches composed of thousands of the worms. For several hours while the tide is out, the worms lie thus in the sun, and the plants manufacture their starches and sugars; but when the tide returns, the worms must again sink into the sand to avoid being washed away, out into deep water. So the whole lifetime of the worm is a succession of movements conditioned by the stages of the tide – upward into sunshine on the ebb, downward on the flood.
What I find most unforgettable about Convoluta is this: sometimes it happens that a marine biologist, wishing to study some related problem, will transfer a whole colony of the worms into the laboratory, there to establish them in an aquarium, where there are no tides. But twice each day Convoluta rises out of the sand on the bottom of the aquarium, into the light of the sun. And twice each day it sinks again into the sand. Without a brain, or what we would call a memory, or even any very clear perception, Convoluta continues to live out its life in this alien place, remembering, in every fibre of its small green body, the tidal rhythm of the distant sea.
Source: Rachel Carson, The Sea Around Us, 1951, reprinted in Rachel Carson, The Sea, with an Introduction by Brian Vesey-Fitzgerald, London, Toronto, Sydney, New York, Hart Davis, MacGibbon, Granada Publishing, 1976.
The Hot, Mobile Earth
Since the 1960s the study of plate tectonics has transformed the way scientists think about the earth. Charles Officer, Research Professor in Earth Sciences at Dartmouth College, and Jake Page, former editor of Natural History and Smithsonian magazines, give a vivid account of this development in their book Tales of the Earth: Paroxysms and Perturbations of the Blue Planet (1993).
Only in the last thirty years have we discarded the prevailing wisdom that the continents and ocean basins have always been in the same geographical locations. This was a comforting model, in a sense; it just happened to be entirely wrong. The competing hypothesis of ‘continental drift’ – or ‘plate tectonics’, as it is now known – has permitted us a far greater understanding of the nature of such things as volcanoes. Understanding them does nothing to tame them, of course, but it does serve to make the Earth appear a bit less whimsical in its outbursts.
In plate tectonic theory, the Earth’s outer surface, or crust, is considered to be divided into a number of plates, which move horizontally at rates of a fraction of an inch to a few inches per year. New plate material is formed at their originating ends and old plate material is subducted back into the Earth at their trailing ends. The new plate material consists of molten magma which has been brought up from depth, particularly along the mid-ocean volcanic ridge system. The old plate material is carried back down into the mantle of the Earth, principally along the major earthquake zones surrounding the Pacific Ocean. The plates themselves move as rigid slabs over the viscous and underlying mantle and are considered to be driven by thermal convection currents in the mantle.
That the continents may have drifted about on the Earth’s surface is an idea often attributed to Francis Bacon, essayist, lord chancellor to James I, candidate for those who don’t believe Shakespeare wrote his own plays, and later subject of a bribery conviction. While computer analyses and other studies have shown that the author of Shakespeare’s plays was almost certainly a man named William Shakespeare, a consultation of Bacon’s own writings shows that while he did note in 1620 the obvious similarities of the continental outlines on either side of the Atlantic Ocean, he did not suggest that at one time they might have formed a unified land mass. That possibility was espoused in The Origin of Continents and Oceans, an elegant book by German meteorologist Alfred Wegener, first published in German in 1912 and translated into English in 1915. The idea was dismissed by most Earth scientists as inept and unscientific. After all, it challenged the very fundament of geologic thinking and, if accepted, would have called for a wholesale rethinking of how the Earth works.
Wegener was present at a meeting in 1926 of the American Association of Petroleum Geologists during which a symposium was held on the subject. Or perhaps it might be more aptly called an ambush. A professor from the University of Chicago commented that geologists might well ask if theirs could still be regarded as a science when it is ‘possible for such a theory as this to run wild’. Another from Johns Hopkins University commented on Wegener’s methodology: ‘It is not scientific but takes the familiar course of an initial idea, a selective search through the literature for corroborative evidence, ignoring most of the facts that are opposed to the idea, and ending in a state of auto-intoxication in which the subjective idea comes to be considered as an objective fact.’ In these words, not only Wegener’s hypothesis but Wegener himself was under attack. Contrary to a common perception of scientists as dignified, objective investigators, they often play hardball with subjective zeal – especially when their basic premises are challenged.
Widely considered a pseudoscientific notion, the matter of continental drift rested until the 1960s. A sudden change in attitude toward the matter is generally attributed to the publication in 1963 of a scientific article by Fred Vine and Drum Matthews of Cambridge University. It was well known by then that the Earth acts like a great magnet that switches its magnetic polarity through geologic time. Sometimes, in essence, the North Pole becomes the South Pole, and vice versa. And as molten rock cools and hardens, magnetic particles in the lava are ‘frozen’ like little compass needles in the hard rock, their direction depending on the state
of the Earth’s magnetic polarity at the time. Vine and Matthews took note of the alternating positive and negative magnetic ‘stripes’ in the rock, which parallel the great ridges that occur on the mid-Atlantic sea floor, and suggested that they could best be explained if the sea floor itself were spreading out – moving away from the ridges. The particular magnetic signature would be picked up as the molten lava cooled, the signatures alternating in stripes as the Earth’s polarity switched back and forth through the geologic ages. Thus, however dimly perceived, there was now a mechanism by which the continents might indeed have spread. Evidence in favor of this hypothesis began to cascade, and, in spite of a few serious contrarians and the waggish carping of a group that called itself the Stop Continental Drift Society, continental drift is now accepted as the explanation for the present configuration of the continents and is considered a major feature of the Earth’s continuing metabolism …
Volcanism is directly associated with the mid-ocean ridges, where molten material fills the gaps that occur as the sea floor spreads. Such spreading occurs as two ‘plates’ move in opposite directions from the ridge. The ridges are thought to be fed, either directly or indirectly, with molten material that comes up as giant plumes from a great depth. Called mantle plumes‚ they are presumed to originate near the boundary between the lower mantle and the liquid core of the Earth, about halfway to the center of the Earth. The rising molten material in these plumes is basic in composition (as opposed to acidic), dominated by heavy minerals, and enriched in sulfur dioxide, which, along with the contained carbon dioxide, chlorine, and water, is vaporized when the molten magma erupts at the Earth’s surface. These latter components of the magma are called its volatile constituents.
It stands to reason that if two plates are moving away from each other in one place, they will be crashing against something else at the other, leading end, and this is what happens. In many cases, where two plates collide, there is a subduction zone, where one or both of them descend back into the mantle. When the edge of a plate is subducted to a sufficient depth, its material reaches temperatures high enough to bring about at least partial melting, which in turn produces chambers of magma that tend to rise up. Volcanism then occurs at the Earth’s surface. Volcanoes that occur in subduction zones (such as Indonesia) typically spew out more acid debris composed of lighter materials from the subsumed and overlying crustal materials. One such group of volcanoes circumscribing the Pacific Ocean is known as the Ring of Fire.
One of the mantle plumes that feeds the mid-Atlantic ridge rises in the North Atlantic directly under the island of Iceland, which can be thought of as the child of this plume, and is one of the several visible parts of the ridge, others being the Azores, Saint Peter and Saint Paul Rocks, and Tristan da Cunha. On Iceland, a volcano called Laki erupted with a gigantic lava flow in June 1783, and the eruption continued for eight months, a dramatic example of volcanic pollution. An enormous amount of sulfur dioxide was ejected into the atmosphere, returning to Earth as an estimated 100 million tons of sulfuric acid rain. This is about the same amount of acid rain attributable to human causes that today falls on the Earth in an entire year.
Happily for modern scientists, there is a record of this and other such events, a record as precise as the annual growth rings in trees but going further back than any living tree. In the more northerly latitudes of Greenland, snow and ice deposition increases in layers year by year, and the effects of a great variety of unusual atmospheric events become trapped in these layers, accessible by means of ice cores. Peaks of high acidity from the sulfur dioxide aerosols that have settled back to Earth in the ice cores have now been correlated with all known volcanic eruptions, and the Laki eruption in 1783 created higher-acidity peaks than any other volcanic eruption in the past thousand years….
While it has been determined that so far as earthquake (and tornado) damage are concerned, the safest place to live in the United States is near a tiny town called Crossroads in south-eastern New Mexico, earthquakes can occur virtually anywhere. Their geographic distribution is generally categorized in the terms of plate tectonics. Thus we have either interplate or intraplate earthquakes.
Most quakes are of the interplate variety, occurring along the boundaries of the Earth’s great plates where they grind against each other. The mechanisms of such quakes are fairly well understood in a general way. As one plate moves slowly past its neighbor, enormous strain builds up, not unlike the way in which strain builds up when you try to open a firmly closed jar. Eventually, the strain placed on the lid is enough, and it opens with a pop. Similarly, the strain built up by the plates eventually results in its quick release in the form of earthquake movement, and the plates return to a relatively unstrained state.
The process is more complicated than that, of course, depending on the type of boundary between plates, of which there are three kinds. At mid-ocean ridges, the plates form with the up-welling of magmatic material, and their lateral movement is away from the ridge in opposite directions. This is what is happening at the mid-Atlantic ridge, for example. On the other hand, there are places where two adjoining plates move horizontally relative to each other, usually at different velocities, along what are called transform faults (as is the case with the San Andreas fault and others in California). The third boundary type is when one plate is subducted under its neighbor and back into the deep interior of the Earth. Quakes along the mid-ocean ridges are relatively few in number and are usually small. Transform-fault quakes can be either small or large in magnitude. The subduction-zone quakes are the most numerous and are often among the largest; their focal zones (that is, where the energy is released) can extend to depths greater than 400 miles. In contrast, the focal zones at the other two boundary types are typically shallow.
The least-understood earthquakes are those that occur within a plate – the intraplate quakes. Though far less common than quakes that occur along the boundaries of plates, the intraplate quakes account for about half of the high-magnitude, shallow-depth earthquakes that rattle into human consciousness and wreak havoc on humankind’s works. The Haicheng quake, the only one to have been so successfully predicted that the community could be evacuated, was an intraplate earthquake.
That the first such prediction took place in China seems fitting, since seismology as a science had its origins there with the development in AD 132 of the first instrument to record ground motion from earthquakes. Chinese concern with earthquakes is understandable, even at so early a time in history. These violent upheavals are common there and have taken a tremendous toll on the Middle Kingdom’s huge population. The State Seismological Bureau of the People’s Republic reported that in the thirty-seven years from 1949 to 1976, some 27 million people died and 76 million more were injured as a result of 100 earthquakes. If these figures are correct, the toll is nearly unimaginable. By comparison, the total number of Americans killed in the American Civil War has been estimated at 364,000; those in World War II, at 407,000. The greatest hazard to life in the United States, it is generally agreed, is the automobile, which accounts for 50,000 deaths each year (or in a 37-year period, by way of comparison 1,850,000) – not even a tenth of China’s earthquake toll.
Source: Charles Officer and Jake Page, Tales of the Earth: Paroxysms and Perturbations of the Blue Planet, Oxford University Press, New York, 1993.
The Poet and the Surgeon
This was the title-poem of James Kirkup’s first collection (1952). He has since become widely known as a poet, translator and travel-writer. In 1977 his poem about the love of a Roman centurion for Christ had the distinction of being the subject of the first prosecution for blasphemous libel for over 50 years. Philip Allison was, at the time of his death in 1972, Nuffield Professor of Surgery at Oxford. The operation Kirkup describes was a new development in the early 1950s. Designed to remedy the narrowing of the mitral valve of the heart, it entailed the surgeon putting his finger into the valve in order to split it.
A Correct Compassion<
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To Mr Philip Allison, after watching him perform a Mitral Stenosis Valvulotomy in the General Infirmary at Leeds.
Cleanly, sir, you went to the core of the matter.
using the purest kind of wit, a balance of belief and art,
You with a curious nervous elegance laid bare
The root of life, and put your finger on its beating heart.
The glistening theatre swarms with eyes, and hands, and eyes.
On green-clothed tables, ranks of instruments transmit a sterile gleam.
The masks are on, and no unnecessary smile betrays
A certain tension, true concomitant of calm.
Here we communicate by looks, though words,
Too, are used, as in continuous historic present
You describe our observations and your deeds.
All gesture is reduced to its result, an instrument.
She who does not know she is a patient lies
Within a tent of green, and sleeps without a sound
Beneath the lamps, and the reflectors that devise
Illuminations probing the profoundest wound.
A calligraphic master, improvising, you invent
The first incision, and no poet’s hesitation
Before his snow-blank page mars your intent:
The flowing stroke is drawn like an uncalculated inspiration.
A garland of flowers unfurls across the painted flesh.
With quick precision the arterial forceps click.
Yellow threads are knotted with a simple flourish.