The Dutch cartographer, Abraham Ortelius, first suggested in 1596 that
the Americas were "torn away from Europe and Africa"; but there was
little evidence to support his hypothesis. In England in 1620, Francis
Bacon also noted that the similarity of many of the edges of various
continents suggested that they once might have fit together like puzzle
pieces. Evidence mounted gradually over the course of the next few
centuries that continents were once joined: fossils of similar plant and
animal species found on widely separated continents, long and linear
zones of deformed rocks occurring at the edges of continents, and certain
geologic and glacial features shared across different continents.
German meteorologist Alfred Wegener proposed in 1912 that the
continents were all joined in a common landmass he named ―Pangaea‖,
which began breaking up approximately 200 million years ago. In fact,
precursors of this theory existed in maps depicting the joined continents,
which had, it may be noted, been drawn almost a century earlier, but it
was Wegener who was the first to combine the accumulating evidence for
continental drift into a common framework—to weave seemingly
dissimilar, unrelated facts into a theory. His proposal was not well
received, however; it remained unclear how the continents actually
moved, and science had not developed accurate radiometry to date the
fossils or the linear belts of rock at the edges of continents. Geologist
Arthur Holmes proposed in 1929 that the hot and melted rocks that made
up the mantle of the Earth, the layer just beneath the Earth‘s thin crust,
flowed upward, downward, and laterally, pushing apart regions of ocean
floor or allowing nearby regions to collide and overrun each other; but
again little evidence existed to support the idea. In the following decades,
magnetic studies of the ocean floor, showing that the orientation of rocks
had changed over the course of recent geologic time, helped confirm
Holmes‘ ideas that ocean plates were the cause of the rifts and valleys on
the ocean floor, as well as of the larger movement of landmasses.
By the early-1960s, a wealth of new evidence (much of it from studies
of the ocean floor) formed a picture of what caused continents to drift.
The sedimentary rocks of an oceanic origin were different from predial
samples previously found, and geologists reasoned from this that
continents were not simply upwellings of ocean floor. Continents are built
of blocks of crust varying in age, size, rock composition, structure, and
fossil assemblage (fauna and flora), with relatively stable, older interiors
(the oldest rocks of which are more than 3 billion years old); the sea
floors are significantly younger. The theory of mantle convection currents
and sea-floor spreading became the prevailing explanation of how large
plates of the Earth‘s crust continually move upward, downward, and to
the side, allowing the separation of and collision of landmasses well
above the moving ocean plates. In 1994, however, Seiya Uyeda
concluded that subduction (the gravity-controlled sinking of a cold,
denser oceanic slab into the subduction zone) ―plays a more fundamental
role than seafloor spreading in shaping the earth's surface features" and
"running the plate tectonic machinery." Current analysis of seismic waves
and other geophysical studies continue to vastly expand our
understanding of the Earth‘s interior and the components of plate
tectonics theory.