F13 quiz 1 condensed notes

After one halflife by definition onehalf of the

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Unformatted text preview: roscopic crystals of muscovite aligned in a single direction. Alignment of needle ­like or sheet ­like crystals in metamorphic rocks perpendicular to the direction of rock stresses is a very common feature known as metamorphic foliation. More intense metamorphism creates schist in which the mica crystals are large enough to be easily visible to the unaided eye. A high ­grade, layered metamorphic rock called gneiss could have been developed from almost any sort of parent rock. In dynamic metamorphism, intense shearing [item (iii) above] predominates, transforming the rock into a mass of tiny crystals. In contact metamorphism, high temperature [item (i) above] predominates as a mass of hot magma invades and bakes the host rock. The effects of contact metamorphism are restricted to the near vicinity of the invading magma, along its contact with the host rock. Regional metamorphism involves all of factors (i) through (iv) in a vast terrain deep beneath a major mountain range under construction. Regional metamorphism is probably in progress today deep beneath the Himalaya Mountains. LECTURE 7: ISOTOPIC AGES James Hutton, the Scottish naturalist, is credited with first fully grasping the the notion deep time, an assumption that is the foundation of geological science, evolutionary biology, and astronomy and which has profound implications within human philosophy, culture and religion. During the 1700’s and 1800’s, early geologists tried to decipher the lengths of time required to produce various aspects of the Earth as we observe it. For example, Scottish geological pioneer Charles Lyell counted ancient lake varves (annual sediment deposits) in a rock formation in the Massif Central in France, and found that it took over 300,000 years to form, much longer than the accepted paradigm at that time seemed to permit. Once the idea of geologic deep time became established, various "hourglass" methods were devised to tell ages of geologic events. One example is the salinity of the ocean, as studied by Irish physicist John Joly in 1899: by taking the amount of salt in the oceans today (grams), and dividing it by the rate at which salt is delivered to them by rivers (grams/year), he determined that it would take about 90 million years for an originally fresh ­water ocean to become as salty as it is now. However, Joly’s calculation assumes that no salt ever left the ocean. We now know of various mechanisms by which salt can be lost (organic activity, albitization, etc.), though because they occur far beneath the ocean they are very hard to quantif...
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