# notes - Notes for GeoSciences 105: Continents Adrift These...

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1 Notes for GeoSciences 105: Continents Adrift These notes complement the class text book: Earth: Portrait of a Planet . The first four chapters of the 3 rd edition of Earth: Portrait of a Planet is available as a custom print at the bookstore for around \$30. You can find older editions of the entire textbook or the book Understanding Earth for roughly the same price at online bookstores like Amazon.com. Both books have the same contents, albeit that the chapters are in a different order. The lectures are an integral part of the course. You will benefit from these notes when preparing for the exam, but these notes are not a substitute for attending the lectures. The text in bold indicates facts/concepts that will definitely appear on the Exam.

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2 1. Earth’s place in the Universe. (Sections 1.3 1.7 are not required reading). 1.1 The size of Earth Eratosthenes , an Egyptian librarian, made a clever and accurate measurement of Earth’s radius and, hence, circumference). The story goes that he was told the interesting observation that at noon on June 21, the bottom of a deep well in the city of Syene was illuminated by sunlight. As Syene lies on the Tropics of Cancer, the Sun is directly overhead at Summer solstice. In Eratosthenes’ home city of Alexandria, 800 km north o f Syene, the Sun does not appear directly overhead due to, as Eratosthenes hypothesized, Earth’s spherical shape. Eratosthenes realized that he could measure the circumference of the Earth using the position of the Sun. He put up a long pole in his backyard and, at noon on June 21, measured the length of the shadow behind the pole. Eratosthenes determined that the sun rays make a 7 o angle with Earth’s surface. A year later he conducted the same measurement in Syene. As expected, the shadow behind the pole was absent. With simple algebra (see below) Eratosthenes calculated that Earth circumference C is 40,320 km, within 1% of the Earth’s true circumference. Earth (average) radius = 6371 km Earth’s radius (R) is 6371 km. Due to its rotation, Earth is not a perfect sphere but rather an ellipsoid. The North Pole and South Pole are approximately 20 km closer to Earth ’s center than locations along the equator. Earth’s volume is V = 4/3 π R 3 1.08 10 21 m 3 . (Do this calculation!) Earth’s mass M = 5.97 10 24 kg. (We know Earth’s mass from Earth’s gravity field) The average density of Earth is = M/V = 5.5 10 3 kg/m 3 or 5.5 g/cm 3 . This is 5.5 times larger than the density of water. Rocks at Earth’s surface have densities between 2.6 3.0 g/cm 3 . Thus the average density of Earth ’s interior is higher than the density of any rock on Earth’s surface.
3 There are two reasons for the high average density of Earth. First, the core, Earth’s inner shell with a radius of 3400 km, is composed of mainly iron with an average density of 11 g/cm 3 . Second, hydrostatic pressure P =

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## This note was uploaded on 02/17/2011 for the course GEOSCI 105 taught by Professor Ritsema during the Winter '11 term at University of Michigan.

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notes - Notes for GeoSciences 105: Continents Adrift These...

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