The following short assignment, set in a science foundation course, seemed a fairly simple task: the title, 'The structure of the earth', indicated a description shape for the writing, and the student assumed that his main job was to collect, select and organize the information. He used three encyclopaedias. After he had drafted the piece on a computer, he printed it out and talked it over with a tutor and together they worked out how it could be organized a little differently to make the sequence clearer. The changes he made are indicated by numbers on the text:
The Structure of the Earth
The following essay will give a brief explanation as to the formation of
the earth and its position in our solar systejjij| It will also give information on the. earth' e surf ace, Crust (outer layer), Mantle (layer below the crust) and Core (the centre), including measurements and si2es.
The earth is the 3rd largest planet from the sun and the fifth largest in our solar system. It was formed approximately five billion years ago. It is believed to have formed from swirling 9loads of dust and gases. The inside of the earth is in constant activity, evidence of this is shown on the surface by way of earthquakes, volcanoes, land shifts and subsidence.
The earth is flatter at the poles, north and south, than at the equator. It revolves around the sun at 18.5 million miles per second and takes roughly 365 days to complete a circuit. It also spins on its axis at 1000 miles per hour and ¿akes about 24 hours to complete a rotation.
The earth's surface covers about 197 million sq. Miles. Of this 140 million sq. miles is water and 57 sq miles is land - about 30%. It has an equational circumference of 24, 902 miles , an equatorial radius of 3, 963d miles and a polar radius of 3, 950 miles. It has a mass of 5.976 x 1027 grams and a means density of 5.517 grams per cubic cm. (see chart)
The core consists of an outer and inner part. The outer core starts at about 1800 miles below the crust and is 1400 miles thick. It behaves like a liquid and consists of molten iron. The inner core is 860 miles thick and forms the centre of the earth. Evidence suggests that this inner core consists almost entirely of extremely dense iron and nickel (Collier 1995 p. 478).
The mantle also consists of inner and outer parts. Combined, the mantle extends 1800 miles below the crust and surrounds the core. The upper mantle is 600 ailes thick and the lower mantle 1200 miles thick. The mantle is composed of dense magnesium iron silicates. This in turn forms rocks such as peridotite, dunite, and eclogite. During natural mountain forming, pieces of crust are pushed down through to the upper mantle. Due to the heat, this is melted and eventually sent back to the crust surface via volcanic eruptions, the lava sometimes contains rock material from the mantle produced by the iron silicates.
The crust is basically the skin of the earth. It only accounts for 5 percent of the earth's thickness. It comes in two forms continental and oceanic. The continental crust is rich in silica and alumina minerals. The oceanic crust is of a silica magnesia composition. Due to the processes of nature some of the continental crust is becoming oceanic and vice versa. The crust's thinnest points can be found under the oceans, between three and five miles thick. The oceans form v fro roughly two thirds of the earth's surface, of which there are three main sections: the pacific, the atlantic and the Indian. The continents form the other third of the earth's surface and are divided into seven parts. Host of these continents are found in the northern hemisphere.
In fact there is still a great deal to learn about the earth's formation and its compositions. Technology has not yet been developed to explore the earth' s core. At present we have only been able to examine the outer part of the mantle. Also, because we know very little about the earth's beginning, we cannot get any pointers as to the exact materials that made the planet what it is today. This is in total contrast to what we know about the earth surface. This is obviously because we are in contact with the earth' s surface daily.
He decided to omit the first sentence because it suggested that the essay was about the formation and position of the earth (1). This was background but not central information. The central point of the essay was the earth's structure. However, he kept the following paragraph (3) as background and context. He turned the second sentence into an introductory sentence and added mention of the surface (2). He decided to omit the third paragraph as irrelevant for his topic (4). Then he rearranged the rest of his information about each part of the earth that he was discussing, to put it in the same order as in his introduction: the earth's crust (6) comes after the surface (5), then the earth's mantle (7) and finally the core (8). This is a logical progression from the earth's surface inwards.
As a result of his reorganization the student can now finish his assignment with a conclusion to sum up what is still unknown, and here he makes a general contrast between knowledge about the earth's surface and the rest of its structure, really bringing together the different parts of his piece to make his general point about why this is so: 'We are in contact with the earth's surface daily'.
Although the above example is obviously a very simple and short piece of written work - in fact, it was an introductory course assignment - it should help you to see how you can move things about to change the emphasis of ideas in your writing. We hope that overall this chapter has made you think about both the different ways in which you can approach editing and redrafting your work and the flexibility available to you as a writer when using introductions and conclusions.
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