Lecture 21 - Extinction and the Fossil Record

Lecture 21 - Extinction and the Fossil Record - 1 Lecture...

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1 Lecture 20 - Extinction and the Fossil Record I. Dating fossils - Some geological fundamentals: 1. Rock formation - rocks at Earth's surface originate as magma extruded from within the earth, some from volcanoes but most as new crust forms, called igneous rock. Sedimentary rock formed by deposition and solidification of sediments, either by breakdown of older rocks or precipitation of minerals from water. Most fossils found in sedimentary rocks. 2. Plate tectonics - lithosphere is the solid outer crust of the earth, consist of eight major and some minor plates that move over the denser, more plastic asthenosphere. Heat from the earth's core causes convection currents in the asthenosphere. In certain places magma from the asthenosphere rises to the surface, cools, and spreads out to form new crust, pushing existing plates to the side. Where two plates come together, one may slide under the other, rejoining the asthenosphere. Pressure of plate collisions is the major cause of mountain formation. When a plate moves over a "hot spot" where magma is rising from the asthenosphere volcanoes may form, or a continent may be rifted apart. Figures 1,2 A. Dating rocks - radioisotope dating Isotopes of certain elements will "decay", i.e., lose (or gain) either an electron or positron (beta decay) or an alpha particle (~ a He nucleus) over time. The rate of decay is a constant exponential decay. That is, a constant proportion of the material decays per unit time. For example, 87 Rb decays into 87 Sr at a rate of 1.42 x 10 -11 /yr; the rate of decay is called the "decay constant", λ . How to use the rate of isotopic decay to determine the age of an igneous rock. Recall that igneous rocks are formed by crystallization of liquid magma into solid rock. Need two measurements. The measurements are: 1. The isotope ratio of the rock now. This is easily measured (M1). 2. The isotope ratio of the rock when it was formed (M2). Elements have characteristic "closure temperatures", the temperature at which a system has cooled so that there is no longer any exchange of parent or daughter isotopes with the external environment. Thus an igneous or metamorphic rock, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the blocking temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
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2 - example: Potassium/Argon dating. 40 K decays into 40 Ca and 40 Ar. Argon is a gas that is volatilized out of lava. Initially the volcanic rock contains only 40 K and no 40 Ar. 40 K then decays into 40 Ar and 40 Ca. 3. Assumption is that all the change in the isotope ratio is due to radioactive decay. This assumption is sometimes valid and sometimes not.
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This note was uploaded on 06/08/2011 for the course PCB 4674 taught by Professor Baer during the Fall '08 term at University of Florida.

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Lecture 21 - Extinction and the Fossil Record - 1 Lecture...

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