Page 75 - Handbook of Gold Exploration and Evaluation

56 Handbook of gold exploration and evaluation

`The vestiges of the rupture reveal themselves, if someone brings forward a map
of the world and considers carefully the coasts of the three [continents]'. Francis
Bacon (1561±1626) who noted the parallelism of shores facing one another
across the Atlantic also remarked upon this. Similar ideas of the assembly and
disassembly of continents, as originally argued by Suess and Snider-Pellegrini in
the 19th century, were also rejected.
Orogenic processes were believed to be reactions of the Earth's crust to cool-
ing and contraction. Geosynclinal concepts of crustal development had emerged
as first attempts to explain the formation of fold mountains and the genesis of
gold ore bodies. These theories left many important questions unresolved and it
was not until 1912 that the movement of continents was seriously considered.

Plate tectonics
In 1915 a meteorologist, Alfred Wegener (1912), published the concept of
continental drift and of a supercontinent comprising all of the world's continents
merged into a single mass, which he called Pangaea. His proposition was that
Pangaea had since split apart, the continents moving into their present locations.
His theory was based upon evidence of similarities of rock structures and
palaeoclimates when the continents are put together as he proposed. Lack of
knowledge of the seafloor was a major constraint. It was only during the Second
World War that underwater mapping and sounding techniques (developed for
submarine warfare) began to uncover some of its secrets.
Recognition of the economic potential of the seabed led to expansion of
wartime techniques for more general scientific purposes, including minerals and
oil exploration. Observations and measurements were made of rifts in mid-
ocean ridges, thicknesses and age of sediments on the ocean floor, palaeo-
magnetic reversals and the localisation of seismic activity and volcanism to
specific crustal areas. Efforts to understand the processes that might have
operated inside and at the surface of the Earth during its formation were
intensified. In 1963, Hess wrote a paper called Essay in Geopoetry (summarised
in Tarbuck, 1984) in which he proposed the theory that new crust is created at
mid-ocean ridges and, due to seafloor spreading, is returned to the mantle at
deep-sea trenches. From re-examination of echo-sounding data compiled during
his experience as Commander of a ship during the Second World War, he later
suggested that movement of convection currents in the Earth's mantle could be
the cause of seafloor spreading. His evidence related largely to the age of the
seafloor rocks, no part of the present seafloor being older than about 200,000
years. Seismic measurements of the speed of earthquakes, which showed that a
low velocity zone (asthenosphere) exists at a depth of about 100±200 km, was
recognised by Hess as being due to partial melting of the solid crust. It was
deduced that the asthenosphere could constitute a lubricated layer over which
the upper solid part of the lithosphere could travel.