The Ecology Book

221

volcanic activity, changes in the

composition of the atmosphere and

oceans, climate change, sea level

rises and falls, tectonic movement of

the continents, and meteor impacts

are all likely causes. Some scientists

suggest we have now entered a

sixth mass extinction, this time the

result of human activity.

End of the dinosaurs

The mass extinction that scientists

understand best is also the most

recent, around 65 million years ago.

Geologists refer to it as the K-Pg

extinction event because it occurred

at the end of the Cretaceous and

start of the Palaeogene periods.

Although an extraterrestrial origin

was first suggested for the event in

the 1950s, this was not taken

seriously until two discoveries, in

Europe and North America.

In 1980 a team of scientists

working in Italy, including physicist

Luis Alvarez and his geologist son,

Walter Alvarez, discovered a clay

layer between Cretaceous and

Paleogene deposits. Examination

of the clay revealed that it contained

the mineral iridium, which is rare

on Earth but common in asteroids.

The discovery led to the Alvarez

Hypothesis, which proposed that

the extinction at the end of the

Cretaceous period was caused

by a catastrophic meteor strike.

The location of the impact was still

a mystery, until 11 years later, when

a massive crater 106 m i le s (170 km)

across on Mexico’s Yucatan

Peninsula was found to date from

the time of the extinction.

The scientific consensus is that

a massive comet or asteroid struck

Earth, producing a blast of radiation

and a destructive megatsunami

more than 328 f t (10 0 m) high. The

radiation would have killed animals

nearby, and the megatsunami

would have obliterated coastal

regions around the Gulf of Mexico.

The main damage, however, would

be more gradual. A vast cloud of

soot and dust would have spread

through the atmosphere, blocking

out sunlight for several years. Plants

died because they could no longer

photosynthesize, and algae in coral

reefs also succumbed, disrupting

food chains worldwide. The ❯❯

See also: Ancient ice ages 198–199 ■ Moving continents and evolution 212–213 ■

The Gaia hypothesis 214–217 ■ Ocean acidification 281

Luis Alvarez

Considered one of the greatest

physicists of the 20th century,

Luis Alvarez was born in

San Francisco in 1911. He

graduated from the University

of Chicago in 1936 and went

on to work at the Radiation

Laboratory in the University

of California, Berkeley. There

he helped develop nuclear

reactors and, during World

War II, nuclear weapons. He

witnessed the atomic bombing

of Hiroshima and helped build

a plutonium bomb.

After the war, Alvarez

developed the liquid hydrogen

bubble chamber, used to

discover new subatomic

particles. For this, in 1968 he

was awarded the Nobel Prize

for Physics. Later he provided

the calculations to back up the

Alvarez Hypothesis of mass

extinction caused by a meteor

strike. He died in 1988.

THE LIVING EARTH

Key works

1980 “Extraterrestrial Cause

for the Cretaceous–Tertiary

Extinction,” Science

1985 “The Hydrogen Bubble

Chamber and the Strange

Resonances”

1987 Alvarez: Adventures

of a Physicist

We have very strong physical

and chemical evidence for

a large impact ... the

extinction coincides with

the impact to a precision

of a centimetre or better.

Walter Alvarez

All geologic history is full

of the beginnings and the

ends of species–of their

first and last days.

Hugh Miller

Scottish geologist

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