The Faber Book of Science (45 page)

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.

To
Mr
Philip
Allison,
after
watching
him
perform
a
Mitral
Stenosis
Valvulotomy
in
the
General
Infirmary
at
Leeds.

(A true incident)

Only
a
local
anaesthetic
was
given
because
of
the
blood
pressure
problem.
The
patient,
thus,
was
fully
awake
throughout
the
operation.
But
in
those
days
–
in
1918,
in
Cardiff,
when
I
was
Lambert
Rogers
’
dresser
–
they
could
not
locate
a
brain
tumour
with
precision.
Too 
much
normal
brain
tissue
was
destroyed
as
the
surgeon
crudely
searched
for
it,
before
he
felt
the
resistance
of
it
…
all
somewhat
hit
and
miss.
One
operation
I
shall
never
forget
…

Dr Wilfred Abse