Increasing world knowledge and science led Alfred Wegener (the son-in-law of
the climate classifier Vladimir Ko ̈ppen) to publish papers and a 1915 book sug-
gesting disparate evidence proved the existence of continental drift. He stated that
fossilized remains strongly pointed to the existence of a supercontinent from
which all the present continents evolved. Wegener named this supercontinent
“Pangaea” and sought to prove its existence through the rest of his career. He died
in a 1930 Greenland blizzard attempting to amass more evidence. Wegener is
credited as the modern scientific father of continental drift, but he did no serious
work on establishing the causal mechanisms and was harshly criticized by the
majority of the scientific community. Through the 1940s, Arthur Holmes proposed
that the causal mechanism was giant convectional currents within the Earth but he
had no means to prove it.
Continental drift proponents were in the small majority until the early 1960s,
when Harry Hess postulated that continental drift is a result of sea floor spreading.
He came to this conclusion because increased knowledge of ocean bottoms
showed complex underwater ridge structures that trailed around the planet. Hess
had no direct proof of the spreading but that was quickly solved by Fred Vine
and Drummond Matthews (1963) after they read Hess’s work. They realized that
their magnetic research along the Indian Ocean ridge was proof of seafloor spread.
The Indian Ridge is composed of volcanic rocks, and volcanic rocks contain evi-
dence of paleomagnetism. Iron in magma is aligned with the magnetic polarity256 Plate Tectonics
Continental Shelf
A continental shelf is the undersea extension of a continent, geologically related to the rocks
of the continent and much shallower than most of theocean. The shelf extends out to sea
with a gentle slope toward the open ocean. Around the world, the outer edge of the
continental shelf is 100 m–200 m (330 ft–660 ft), where the ocean bottom plummets along
a steep incline called the continental slope. Geographically, continental shelves average
80 km (50 mi) and can be nonexistent along a coastline (e.g., the subduction zone off the Chil-
ean coast) or as much as 1,500 km (930 mi) (e.g., the Siberian Shelf in the Arctic Ocean). Typ-
ically, the oldest rocks of the submarine continental shelf structure are overlain by sediments
and sedimentary rocks eroded from the above-ocean parts of the continents. Continental
shelves have become economically important because they are laden in places with oil and
gas reserves and other minerals. Additionally, the world’s richest fisheries tend to be on
continental shelves. The United NationsLaw of the Sea(1982) grants nations exclusive
mineral rights for 200 nautical miles (370 km) away from their coasts. This works well in most
places but nations arranged around gulfs and seas have had contending claims because of
overlapping “exclusive” zones.