212
PLATE TECTONICS
IS NOT ALL HAVOC
AND DESTRUCTION
MOVING CONTINENTS AND EVOLUTION
T
he surface of Earth is
constantly moving, very
slowly, and has been doing
so for more than three billion years.
The lithosphere (Earth’s crust and
upper mantle) is divided into seven
large sections and many smaller
ones, called tectonic plates. Where
plates meet, the type of movement
determines the nature of the
boundary. Where plates push
against each other, new mountains
are created. If plates pull apart, new
crust forms on the ocean floor.
The first inkling that the
continents may not have always
been in their current positions came
in the late 16th century. European
explorers sailing to the Americas
saw from their newly created maps
that the coastlines on each side
of the Atlantic Ocean mirrored
each other. Later, geologists found
strong structural and geological
similarities between the Caledonian-
era mountains of Northern Europe
and the Appalachian Mountains of
North America.Lookalike fossils
There are various examples of fossil
finds straddling different continents
that can only be explained by
continental movement—since theanimals or plants concerned would
have been unable to cross the ocean
divide. These include Cynognathus
crateronotus, a mammal-like reptile
that lived over 200 million years ago
in southern Africa and eastern South
America. Glossopteris, a genus of
woody trees, grew in South America,
South Africa, Australia, India, and
Antarctica, but nowhere else, around
300 million years ago.
To German geophysicist Alfred
Wegener, such fossil patterns
indicated that these continents had
once been joined together. In 1915,
he published his theory that all the
continents were once a single land
mass, “Pangaea,” which has since
broken up and drifted apart.IN CONTEXT
KEY FIGURE
Alfred Wegener (1880 –1930)BEFORE
1596 Abraham Ortelius, a
Dutch scholar, is one of several
geographers who observe that
the two sides of the Atlantic
Ocean seem to “fit” each other.AFTER
1929 British geologist
Arthur Holmes proposes
that convection in Earth’s
mantle drives continental drift.1943 George Gaylord Simpson
dismisses fossil evidence for
continental drift and argues
for “stable continents.”1962 American geologist
Harry Hess explains how the
seafloor spreads, by molten
magma rising from below.2015 A group of Australian
scientists propose that periods
of rapid evolution in the oceans
were triggered by collisions
between tectonic plates.This fossilized head of the extinct
reptile Cynognathus crateronotus was
found in southern Africa. The same
fossils occur in South America: evidence
that the two continents were once one.US_212-213_Moving_continents.indd 212 12/11/18 6:25 PM