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Answers to the midterm questions

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University of Toronto Mississauga
Earth Science
Henry Halls

Q1. If you travelled from the surface to the centre of the Earth, describe the four major layers you would encounter in terms of their physical and chemical properties. Suggest how each layer influences (or may influence) our life on the Earths surface. THE HISTORY OF EARTH The Earth, as we know it today, is thought to have formed approximately 4.6 billion years ago. With the passing of hundreds of millions of years, the early Earth was created by accretion. It continued to transform due to asteroidal barrage by a special kind of meteorite known as a Carbonaceous Chondrite (a carbon-rich stony meteorite). These chondrites in particular have much more of the element Iron (Fe) than typical mantle rocks and are very similar in composition to the Sun (except for the absence of a few gaseous elements). They are known to have an age of roughly 4550 million years which is early in the evolution of the solar system. Therefore, the Earth was built up by the successive impacts of these carbonaceous chondrite asteroids drawn into it as a result of gravity. Eventually, the interior of the Earth would begin to heat up under gravitational pressure, heat of asteroidal impacts as well as the enormous heat generated by the decay of naturally occurring radioactive elements such as Potassium and Uranium. The hot interior of the Earth serves the role of a blast furnace where liquid iron is freed from the interior of carbonaceous chondrite as carbon draws off iron from iron-rich ores. This liquid iron then gradually falls to the Earths centre due to its high density where it forms the Core. So, deep in the Earth while the iron is separating out and sinking, water vapor, gases and other light compounds are expelled and move upwards towards the surface where they form the Earths Crust, Oceans and Atmosphere. The material that is left behind has a composition and density very close to the Earths Mantle. This entire process by which the heating of a uniform starting material produces different compositional fractions that separate out according to their density is known as differentiation. Therefore, as we go down to the centre of the Earth, the following four layers can be distinguished based upon their physical and chemical composition: THE CRUST The Crust is the outermost layer of the Earth made up of silicate rock materials. The crust makes up only about 0.1% to 1% of the Earths radius and is the thinnest of all the layers. Geologists distinguish between two different types of crust oceanic crust which underlies the sea floor, and continental crust which underlies continents. The crust is not simply cooled mantle, but rather consists of a variety of rocks that differ in chemical composition from mantle rock. The thickness of the crust varies on the land and on the ocean. For example, oceanic crust is only 7 to 10 km thick and consists of fairly uniform layers. The top layer consists of a blanket of sediments,
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