Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 25

The ultimate causes of the origin of mountains has been one of the great enigmas of
science. Even as late as the 1950s, geologists subscribed to at least half a dozen major
theories. One of these was that mountains were created by the rise of batholiths (Greek
for “deep rock”). Batholiths are masses of molten magma that intrude into rocks near
the Earth's surface, where they cool, crystallize, and solidify. Because batholiths occur
at the core of most mountain ranges, it was thought that they were also responsible
for the uplift and deformation of the surface. Gravitational sliding of rock strata down
mountain slopes was also recognized as important in folding, faulting, and distortion of
overlying rocks. A major early theory was that Earth was contracting because it was
thought to have been molten material ejected from the Sun and now gradually cooling.
As the Earth cooled and contracted, the outer skin shriveled and wrinkled like a drying
apple. An alternative theory was that Earth was actually expanding, which would cause
rift valleys and tensional fault zones, as well as continental drift (Carey 1976). A still
important early idea envisioned giant convection currents within the Earth; mountains
develop where rising currents from two opposing convection systems converge, result-
ing in great compressional forces, which cause folding and deformation of the surface
(Holmes 1931). Yet another theory held that mountain building took place as a result
of continental drift, as the leading edge of a continent encountered resistance from the
material through which it was moving and buckled under pressure.
Many of these theories are not mutually exclusive, and elements of each are still re-
tained in one form or another, but no one believes they adequately explain primary initi-
ating mechanisms for creating mountains. Revolutionary discoveries of the 20th century
replaced them with the unifying concept of plate tectonics. This concept has proved to
be one of the most significant developments in the history of science. Its immediate pre-
decessor, continental drift, was not so well received.
Although the idea had been around in various forms for some time, the full theory
of continental drift was proposed in 1910 by Alfred Wegener, a German meteorologist
who noticed the complementarity of the continental outlines of the coasts of Europe and
Africa with the Americas. It seemed to him, as it had to others in prior centuries, that if
these continents were pushed together they would fit like the pieces of a jigsaw puzzle
(Fig. 2.5). Wegener was the first to make this observation into a theory. He uncovered
evidence of identical plant and animal fossils from coastal areas in Brazil and Africa.
This strengthened his conviction that there had been land connections, and from that
point on he devoted his life to searching for evidence to support ideas of continental
drift. Similarly in the southern hemisphere, South African Alexander du Toit compiled
evidence of a former supercontinent that later broke into the separate units that now
exist as individual continents (du Toit 1937).
Wegener's and du Toit's evidence for former land connections included matching
fossils and unusual rock types, alignment of fault zones, matching former areas of gla-
ciation, and the presence of similar mountain types in Europe and North America. Most
geologists, especially in the northern hemisphere, rejected the theory of continental
drift as too fantastic. The similarities across oceans were all explained by other means.
The major objection, however, was the mechanism Wegener offered for the movement
of entire continents—the tidal attractions of the sun and moon. In succeeding years, in-
terest in continental drift waxed and waned as new theories were proposed and sympo-
sia were held, but by and large the scientific community remained unconvinced. Discov-