Geoscience Reference
In-Depth Information
diligent search for Pu 244 in the boundary clay came up empty
however, so the scientists had to abandon the supernova theory.
A Berkeley colleague, Chris McKee, suggested that a large aster-
oid could have provided the iridium. This made sense, for iridium is
present at high levels in lunar soils, where it has presumably been
emplaced by impacting meteorites. For many months, however, the
Alvarez team was unable to figure out how the impact of an aster-
oid at one spot on the earth's surface could have caused a mass
extinction everywhere. How did the effects get spread around the
globe? Luis later recalled that he had invented a new scheme a
week and shot each down in turn.
The critical clue arose in a way that further illustrates the
strong scientific ties within the Alvarez family. In 1883, the island
volcano Krakatoa, in the Sunda Strait between Java and Sumatra,
blasted itself to pieces in one of the most violent eruptions of mod-
ern times, scattering debris as far as Madagascar. People 5,000 km
away heard the explosion. Walter Alvarez, Sr., the physician father
of Luis, had given his son a volume describing the Krakatoa event
published by the Royal Society of London in 1888; Luis in turn had
passed it on to the younger Walter. Now Luis asked for the volume
back so that he could study the consequences of a dust-laden
atmosphere. The Royal Society volume estimated that the Krakatoa
explosion had blasted 18 km
3
of volcanic material into the atmo-
sphere, of which about 4 km
3
reached the stratosphere, where it
stayed for more than two years, producing some of the most re-
markable sunsets ever witnessed. (In comparison, the eruption of
Mount St. Helens in 1980 is estimated to have released about
2.7 km
3
of volcanic rock; the eruption that formed the giant Yellow-
stone crater about 3,000 km
3
.)
Krakatoa caught Luis's attention, and he proposed by analogy
that 65 million years ago a large meteorite struck the earth and sent
up such a dense cloud of mixed meteoritic and terrestrial debris
that it blocked the sun. This successively caused world temperature
to drop, halted photosynthesis, choked the food chain, and led to
the great K-T mass extinction and the death of the dinosaurs. Luis
and his Berkeley gang phoned Walter, then in Italy, to announce
their exciting conclusion and to propose that the idea be presented
at an upcoming meeting on the K-T boundary in Copenhagen,
which both Alvarezes could attend. Although Luis was anxious to
explain to paleontologists the cause of dinosaur extinction, Walter
knew better and urged him to stay home.
9
While the physicist and
his chemist colleagues did remain in Berkeley, Walter journeyed to
