Advances in Conception about Earthquakes - Earthquake Engineering for Structural Design - page 109

About 65 million years ago, the Atlantic and Indian oceans were formed, the
Eurasia continent began to have a clear contour, India continued its travel towards
Eurasia, and Australia began to separate from Antarctica. About 50 million years
ago, India, then an island continent, collided and subducted Asia, giving birth to
the Himalayan Mountains. The last image of Figure 4.6 shows the present
continental configuration.
Wegener's theory is based on the old observations that the Earth's rocky crust is
broken into massive slabs named Tectonic plates, floating over the interior molten
basalt and the actual conformation of continents is the result of the movement of
these plates. This theory failed to explain the nature of the forces propelling the
plates, because the simple interpretation that continents float through the ocean
floor is not enough. Because Wegener was basically a meteorologist, many of his
arguments related to the solid Earth are erroneous from the perspective of present-
day knowledge (Uyeda, 2002).
4.1.4ModernPeriod
The classical geologists contested the Theory of Continental Drift for a long time.
But the new hypothesis of the early 1960s explained several puzzling sets of
observations. The theory was confirmed after the examination of deep-sea
expeditions to the mid-ocean ridges performed by Maurice Ewing . In 1961 Hess
and Dietz developed the Theory of Ocean Ground Expansion. The ocean ridges,
where magma is constantly extruded into the ocean floor, push the plates apart.
Therefore, the mantle convection drives the plate motions. The zone where this
process takes place is referred to as a mid-oceanic ridge; the most known is the
Mid-Atlantic Ridge, where Eurasian and North American continental plates are
drifting apart at a rate of around 2.5 centimeters each year. In 1967 Jason Morgan
proposed that the Earth's surface consists of 12 rigid plates that move relative to
each other. In the same year, Xavier Le Pichon published a synthesis showing the
location and type of plate boundaries as well as their direction of movement. These
discoveries in the period 1966-1970 led to developing the scientific Theory of
Tectonic Plates , which was the basic hypothesis during more than 50 years.
Numerous methods tested this theory and now almost all seismologists accept it.
But the history of hypotheses about the formation and migration of continents
continues nowadays, when new theories are developed. The Earth Expansion
Theory , which considers that the dimensions of the Earth increase, the oceanic rifts
are simply cracks in the skin of the planet produced by its growing and, therefore,
the distances between the continents are increasing. Another theory is the Shock
Dynamic Theory , which discovered that all the continents move by keeping one
unique central point, located in the North part of Madagascar. This theory proposes
the hypothesis that the Earth was catastrophically struck by a giant meteorite 180
million years ago, creating a giant crater and producing a dynamical shock. The
meteorite impact was like a hammer blow for the brittle crust, which cracked as a
glass globe.