Page 30 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 25

religious beliefs or intuition, the hypothesis of continental drift was based on hard
evidence, and attracted sufficient attention that in 1928 Wegener’s proposals were
the subject of a symposium attended by many of the most prominent geologists
of the day. Wegener’s proposals were slammed verbally and in print, and his
credentials were questioned. However, with the persistence of a bear smelling a hot
apple pie, Wegener carried his book through three more editions.

Wegener died at the age of 50 while attempting to save a fellow Arctic explorer
on the Greenland icecap. Had he lived into his eighties, he would have witnessed
the most remarkable turnaround in scientific opinion since the Earth stopped
being flat. Wegener’s concepts were confirmed by drilling, sampling, and radio-
active dating of rocks from the sea floor. Contrary to what might be assumed,
the youngest rocks in the Atlantic Ocean basin are not close to the continents but
are out in the middle in what is called a rift zone, where the ocean floor is pulling
apart and pushing the continents away from each other. The rift zone also is
the site of extensive volcanic activity such as in Iceland. The major rift zone in
the Atlantic Ocean is called the Mid-Atlantic Ridge.

Wegener’s ideas thus sparked a revolution in geological thinking that rivals
an earlier revolution in biological thinking by Charles Darwin, but the matter has
been much less controversial because people do not take it personally.

Wegener got his ideas from observing a geometrical ‘‘fit’’ of the eastern edge of
the Americas to the western edges of Europe and Africa, and noting that fossils
are identical up until the age of the reptiles and then diversified.

The concept of sea-floor spreading is analogous to two opposing conveyor
belts that are carrying the continents apart. Many moving fragments have been
discovered that are bounded by faults, and because the drifting plates are not
limited to continents the continental drift hypothesis has progressed to a more
general theory called plate tectonics.

2.4.2 Paleomagnetism a Digital Switch

Not only does magnetic north move about, the magnetic poles periodically
switch, north for south and south for north. This rather astonishing conclusion
was verified by magnetometer measurements and radioactive dating of the sea
floor at different distances from the Mid-Atlantic Ridge, which show a regular
progression of flip-flops in magnetism with time, with the youngest rocks
closest to the Ridge.

As minerals in an igneous rock cool below a temperature called the Curie point,
they record and retain a record of Earth magnetism at the time of cooling.
Measurements of magnetism of the sea floor indicate a consistent symmetrical
pairing of magnetic patterns on both sides of rifts such as the Mid-Atlantic Ridge.
These patterns have helped to locate many more plate separations, and the

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