Page 77 - Handbook of Gold Exploration and Evaluation

58 Handbook of gold exploration and evaluation

For the first time it is known why oceans open and close, mountains form and
volcanoes erupt, and why new seafloor is created at seafloor spreading centres
while old seafloor disappears at subduction zones.

Exploring the universe

Information was still sparse about the formation and very early history of the
Earth prior to the concept of plate tectonics, which fundamentally changed
scientific views of how the world was formed and how it works. Concepts of
universal processes involved with the origin and general structure of the
universe now suggest that the Earth was born about 4.5 billion years ago as a
solidified mass of dust and ashes left over from the creation of the Sun. It was
thought to be relatively cool at first, perhaps about 2,000 ëF, the main ingredients
being iron and silicates with minor amounts of other elements including the
radioactive minerals uranium, thorium and potassium. One currently popular
`giant impact' hypothesis is consistent with the idea that toward the end of its
construction, a huge impactor hit a glancing blow to the Earth, heating it and
then spinning off into orbit to form the moon. Other scientists also agree with the
impact theory of planet formation although there are different views on the
extent of melting and lines of evidence used to deduce it (Taylor, 1972).
Gravitational energy, together with energy from meteor bombardment and
radioactive decay would have provided heat for melting, which then acted to
concentrate the densest materials near the centre and the less dense near to the
surface. A layered structure comprising core, mantle and crust developed as the
result of magmatic differentiation. Core formation is believed to have been
contemporaneous with the accretion of planetesimals at ±4.5 Ga, as the result of
temperature increase and melting caused by accretional energy and heat
generated by short-lived nuclides. It has been deduced that the innermost molten
Fe-Ni core with a radius of about 1,225 km had an average density of about
5.5 g/cc, with temperatures lying between 2,200 and 2,750 ëC at pressures 3 to 4
million times that of the atmosphere. Such highly siderophile elements as Au
and platinum group elements were effectively concentrated into the core.
In 1923 the American astronomer Edwin Powell Hubble succeeded in
observing Cepheid variables in the spiral nebula of Andromeda and other spiral
nebulae. He was able to determine the distance of these nebulae from the
apparent magnitudes of the variables, so discovering that they were much further
away from the Earth than even the Magellan clouds. In comparing the distances
of a number of galaxies with their Doppler shifts (red shifts) he discovered that
with increasing distance from the Earth the faster it moves away. In 1927 a
Belgian priest, Georges Lemaire, had proposed that the universe started by the
explosion of a `primeval atom', i.e., the concentration of all the mass of the
universe in an extremely small space. Hubble's constant `the ratio of the
distance between the local group of galaxies and a receding cluster of galaxies,