He made maps showing how the
American and African continents
may once have fit together, and
attributed their separation to the
biblical Flood. When fossils of
Glossopteris ferns were found in
South America, India, and Africa,
Austrian geologist Eduard Suess
argued that they must have evolved
on a single landmass. He suggested
that the southern continents were
once linked by land bridges across
the sea, forming a supercontinent
that he called Gondwanaland.
Wegener found more examples
of similar organisms separated by
oceans, but also similar mountain
ranges and glacial deposits. Instead
of earlier ideas that portions of a
supercontinent had sunk beneath
the waves, he thought perhaps it
had split apart. Between 1912 and
1929, he expanded on this theory.
His supercontinent—Pangaea—
connected Suess’s Gondwanaland
to the northern continents of North
America and Eurasia. Wegener
dated the fragmentation of this
single landmass to the end of the
Mesozoic era, 150 million years
ago, and pointed to Africa’s Great
Rift Valley as evidence of ongoing
continental breakup.Search for a mechanism
Wegener’s theory was criticized by
geophysicists for not explaining
how continents move. In the
1950s, however, new geophysical
techniques revealed a wealth of
new data. Studies of Earth’s past
magnetic field indicated that the
ancient continents lay in a different
position relative to the poles. Sonar
mapping of the seabed revealed
signs of more recent ocean-floor
formation. This was found to occur
at mid-ocean ridges, as molten rock
erupts through cracks in Earth’s
crust and spreads away from the
ridges as new rock erupts.
In 1960, Harry Hess realized
that seafloor spreading provided
the mechanism for continental drift,
and presented his theory of plate
tectonics. Earth’s crust is made up
of giant plates that continually shift
as convection currents in the
mantle below bring new rock to the
surface, and it is the formation anddestruction of ocean crust that
leads to the displacement of
continents. This theory not only
vindicated Wegener but is now
the bedrock of modern geology. ■223
See also: Francis Bacon 45 ■ Nicholas Steno 55 ■ James Hutton 96–101 ■ Louis Agassiz 128–29 ■ Charles Darwin 142–49
A PARADIGM SHIFT
Alfred Wegener Born in Berlin, Alfred Lothar
Wegener obtained a doctorate in
astronomy from the University of
Berlin in 1904, but soon became
more interested in earth science.
Between 1906 and 1930, he made
four trips to Greenland as part of
his pioneering meteorological
studies of Arctic air masses. He
used weather balloons to track air
circulation and took samples from
deep within the ice for evidence of
past climates.
In between these expeditions,
Wegener developed his theory
of continental drift in 1912, and
published it in a book in 1915. Heproduced revised and expanded
editions in 1920, 1922, and 1929,
but was frustrated by the lack of
recognition for his work.
In 1930, Wegener led a fourth
expedition to Greenland, hoping
to collect evidence in support of
the drift theory. On November 1,
his 50th birthday, he set out
across the ice to get badly
needed supplies, but he died
before reaching the main camp.Key work1915 The Origin of Continents
and OceansWegener’s supercontinent is just
one in a long series. Geologists think
the continents may be converging
again, to form another supercontinent
250 million years from now.Pangaea, 200 million years ago75 million years agoPresent day