History of Oceanography
Click on the arrows below to learn more
Drift and Seafloor Spreading
Keys to Modern Earth and Oceanographic Sciences
Until only recently, geologists had thought that Earths surface hadnt
changed much since the planet formed 4.6 billion years ago. They believed that
the oceans and continents were always where they are now.
But less than 100 years ago, a German scientist named Alfred Wegener took notice
of some interesting findings. Similar plant and animal fossils were found in
both Africa and South America and on other continents separated by oceans. Similar
rock formations were also found on distant continents. This suggested that the
formations were once whole and later divided.
Alfred Wegener was born in Berlin, Germany on November 1, 1880. He spent a
great deal of time in Greenland as part of several exploration and research
expeditions. His book The Origin of Continents and Oceans was
originally published in Germany in 1929 and was instrumental in the development
of the theory of seafloor spreading. Wegener died on the ice in Greenland
in Nov. 1930.
Wegener also noticed that if you could shove western Europe and Africa together
with North and South America, their coastlines would fit together very neatly.
All this evidence led Wegener to believe that the continents were once connected
but had separated and drifted apart.
1915, Wegener proposed his continental drift theory. He said that
the continents floated atop the mantle-a heavier, denser layer
of rocks deep within the earth. Wegener predicted that heat rising
within the hot mantle created currents of partially melted rocks
that could move the continents around the earths surface.
Like many revolutionary theories, Wegeners was not initially accepted
by scientists. The good fit of the continents and the fossil
and rock evidence did not provide enough proof. For decades afterward, scientists
still did not understand how massive continents could be transported across
the face of the Earth, and they had no evidence of any process that could cause
continents to move.
In the 1950s and 1960s, marine geologists such as Bruce Heezen, Marie Tharp,
and Henry Menard used data from echo sounders to map ocean ridges in the North
Atlantic and the Pacific. They noticed first that these ridges stretched on
for thousands of kilometers in long, continuous mountain chains that wound
around the Earths surface, almost like the stitches on a baseball. The
scientists also observed that the crest of the ridges had a topography that
closely resembled volcanic rift zones on land. At their crests, they had V-shaped
central valleys with steep faults on either side. This evidence led early marine
geologists to deduce that the mid-ocean ridges were formed by seafloor volcanoes.
Reconstruction of the
map of the world according to drift theory for three epochs, the
upper one is the upper Carboniferous, the middle one is for the
Eocene, and the lower one is for the Lower Quaternary. The dotted
areas on the continents are areas where shallow seas existed during
those time periods. This figure appears as Figure 4 in Wegeners
When these volcanoes erupted, they spewed out lava
that cooled and solidified to become new seafloor. It was soon discovered
that when this lava cooled, magnetic particles within it aligned
with Earths magnetic field. After World War
II, when magnetometers began to be used to survey the seafloors magnetic
properties, scientists were surprised to learn that Earths magnetic
field had flip-flopped many times over its history, with the north and south
poles exchanging places! So depending on when seafloor rocks were formed,
their particles are aligned in either one direction or the other, and they
are said to have either positive or negative magnetic anomalies.
the late 1960s, magnetometer data revealed an alternating striped pattern
of seafloor rocks. Rocks that formed when Earths magnetic field
was in one position alternated with rocks that formed when the field
was reversed. The stripes ran parallel to the mid-ocean ridges and
extended out hundreds of miles on either side of them. The seafloors
permanent magnetic signatures showed that new ocean crust was created
at the ridge crests and then spread outward in both directions.
This seafloor spreading hypothesis had been proposed a few years earlier by Harry
Hess, a petrologist at Princeton University, and Robert Dietz, an oceanographer
in the US Coast and Geodetic Survey (the federal department that made maps of
the oceans and US coastlines). Hess went on to say that as the ocean crust spreads
and cools over millions of years, it becomes denser and eventually sinks down
into oceanic trenches, or subduction zones, a long way from where it forms at
the mid-ocean ridge crest. As ocean crust descends toward the hot mantle, it
melts and becomes recycled into the mantle.
Volcanoes and earthquakes are common in subduction zones, which often occur at
the edges, or margins, of continents. The Rim of Fire, which is named for its
volcanoes and earthquakes, is created by a series of subduction zones along the
coastlines surrounding the Pacific Ocean-from western South and Central America
to the Aleutian Islands in Alaska, down the western Pacific, from Japan and the
Philippines, all the way to Indonesia and New Zealand.
In 1965, a Canadian geophysicist, J. Tuzo Wilson, combined the continental
drift and seafloor spreading hypotheses to propose the theory of plate tectonics.
Tuzo said that Earths crust, or lithosphere, was divided into large, rigid
pieces called plates. These plates float atop an underlying rock
layer called the asthenosphere. In the asthenosphere, rocks are under such tremendous
heat and pressure that they behave like a viscous liquid (like very thick honey).
The term continental drift was no longer fully accurate, because
the plates are made up of continental and oceanic crust, which both drift over
Tuzo Wilson predicted three types of boundaries between plates: mid-ocean ridges
(where ocean crust is created), trenches (where the ocean plates are subducted)
and large fractures in the seafloor called transform faults, where the plates
slip by each other. Plate tectonics has provided a unifying theory that explains
the fundamental processes that shape the face of the Earth.