Chapter8 - Chapter 8 The Dynamic Planet Geologic Time Scale...

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Chapter 8 The Dynamic Planet Geologic Time Scale Geologic time scale – a summary timeline of Earth’s history Earth 4.6 billion years old Eons, eras, periods,epochs Based on relative position of rock strata Younger rocks on top of older rocks -- superposition Uniformitarianism is guiding principle of Earth Science Proposed by James Hutton in 1795 The physical processes that are operating on Earth today are the same ones that have been operating through geologic time Go over time scale, p. 251 in book Sequence in scale based on relative position of rock strata Æ relative time Absolute ages on scale reflect absolute time Radiometric dating Some elements in the Earth’s crust and interior have atoms with unstable nuclei Protons and neutrons break away and radiation is emitted Æ atom decays into a different element Decay rates, known as half lives can be measured – the time required for one-half of the unstable atoms in a sample to decay Amount of original isotope is compared with amount of decayed end product Earth’s Structure and Internal Energy Earth in Cross Section As Earth solidified, heavier elements gravitated towards center, lighter elements welled upward to surface Earth’s interior is arranged in concentric layers We know this through indirect evidence (we have never sampled deeper than a few miles) – physical properties such as seismic wave transmission Earth in Cross Section From surface to center of Earth is 6370 km (3981 miles), ca. 4000 miles Inner core, outer core, lower mantle, upper mantle, athenosphere, lithosphere (includes crust) Inner core solid iron at temperature of 5400 o F to 12,000 o F Above melting temperature of iron, but remains solid due to pressure Outer core is molten, metallic iron Outer core generates at least 90% of Earth’s magnetic field and magnetosphere (protects surface from solar wind) Circulation patterns in outer core influenced by Earth’s rotation Æ generate electric
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currents which induce the magnetic field Geomagnetic reversals have occurred many times during the Earth’s history
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Chapter8 - Chapter 8 The Dynamic Planet Geologic Time Scale...

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