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Unformatted text preview: Geology 111 – Discovering Planet Earth A1) Early History of the Earth The earth and the rest of the solar system were formed about 4.57 billion years ago from an enormous cloud of fragments of both icy and rocky material which was produced from the explosions ( super novae ) of one or more large stars - [see page 11] 1 . It is likely that the proportions of elements in this material were generally similar to those shown in the diagram below. Although most of the cloud was made of hydrogen and helium, the material that accumulated to form the earth also included a significant amount of the heavier elements, especially elements like carbon, oxygen, iron, aluminum, magnesium and silicon 2 . As the cloud started to contract, most of the mass accumulated towards the centre to become the sun. Once a critical mass had been reached the sun started to heat up through nuclear fusion of hydrogen into helium. In the region relatively close to the sun - within the orbit of what is now Mars - the heat was sufficient for most of the lighter elements to evaporate, and these were driven outward by the solar wind to the area of the orbits of Jupiter and the other gaseous planets. As a result, the four inner planets - Mercury, Venus, Earth and Mars are "rocky" in their composition, while the four major outer planets, Jupiter, Saturn, Neptune and Uranus are "gaseous". As the ball of fragments and dust that was to eventually become the earth grew, it began to heat up - firstly from the heat of colliding particles - but more importantly from the heat generated by radioactive decay (fission) of uranium, thorium, and potassium (figure below). Within a few hundred million years the temperature probably rose to several thousand degrees, hot enough to melt most things. This allowed the materials to be sorted out so that the heavier substances sank towards the centre, and the lighter substances floated towards the surface. 1 In these notes all references to page numbers, figure numbers and chapters in An Introduction to Physical Geology (Tarbuck et al.) are enclosed in [square brackets]. 2 Hydrogen (H) makes up approximately 90% of the universe and helium (He) 9%. All of the rest of the elements combined account for less than 1% of the content of the universe. Vancouver Island University • Geology 111 • Discovering Planet Earth • Steven Earle • 2010 2 To begin with, much of the iron and magnesium would have combined with silicon and oxygen to form heavy silicate minerals such as olivine : (Mg,Fe) 2 SiO 4 and pyroxene (Mg,Fe)SiO 3 . Most of the remaining iron (along with some nickel and sulphur) would have migrated towards the centre - forming a very heavy metallic core . Meanwhile, much of the aluminum, sodium and potassium would have combined with oxygen and silicon to form minerals such as quartz ( SiO 2) and feldspar ( NaAlSi 3 O 8 ) that would have floated towards the surface to form the crust (see figure below)....
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This note was uploaded on 01/13/2012 for the course GLG 111 taught by Professor Staff during the Fall '11 term at Virginia Intermont.
- Fall '11