Earth Essay

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CHAPTER 1 AN OVERVIEW OF OUR PLANETARY ENVIRONMENT Important Concepts 1. Most astronomers believe that the universe originated in a “Big Bang” about 15 to 20 billion years ago. 2. The formation of the solar system from a rotating cloud of gas and dust was eventually complete about 4.5 billion years ago. 3. The planets of the solar system differ in their compositions as a function of their distance from the sun. The composition of the earth is quite different from the composition of most of the planets, and so far it is the only planet known to contain life. 4. The early earth, which lacked oceans and an atmosphere, was heated by two main processes: (a) impact of colliding meteorites and dust particles; and (b) decay of radioactive elements within the earth. 5. The melting and subsequent cooling of the earth resulted in the formation of three main compositional zones: (a) the high-density, central core, composed mainly of iron and nickel; (b) the surrounding mantle, composed mainly of iron, magnesium, silicon, and oxygen; and (c) a low-density, thin crust, composed mainly of oxygen and silicon. 6. The heating and subsequent differentiation of the early earth led to the formation of the atmosphere and the oceans. The early atmosphere lacked free oxygen and probably consisted of nitrogen, carbon dioxide, methane, ammonia, and various sulfur gases. The later accumulation of free oxygen in the atmosphere is attributed to the photosynthetic activity of primitive plantlike organisms (single-celled blue-green algae). 7. The first multicellular, oxygen-breathing organisms appeared on earth about one billion years ago; the first modern humans (Homo sapiens) appeared about half a million years ago. 8. Geology is an interdisciplinary science that seeks to explain geologic

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