105-15 9-28-2011 Geologic time

Of rocks and geologic events through geologic studies

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Unformatted text preview: ive ages of rocks and geologic events through geologic studies … Correlation of strata from one region to another — fossils (remains of past life) are most useful in doing this, since strata of the same age in different regions can be quite different in rock type and appearance. BUT, strata of the same age in different regions can have the same or very similar fossil assemblages. Paleontology tells us that once an organism becomes extinct, it does not reappear in the younger rock record. Life evolves. Physical correlation of rock layers • Correlation — matching strata of similar age in different regions based on rock similarities and the fossils preserved in them. http://www.uwsp.edu/geo/faculty/ozsvath/images/stratigraphy.jpg Figure 9.14 leaf, dinosaur no trilobite trilobite, no leaf no dinosaur Fossils allow correlation in spite of unconformities Geologic area A Geologic area B lava — 450 m.y. granite — 480 m.y. These two stratigraphic sections are hundreds of kilometers apart and quite different, but the pre-450 m.y. fossils in each allow them to be correlated. Serendipity! —a limestone section between two basalt lava flows can be dated accurately. Basalt Lava flow 2 200 mya We can bracket this limestone’s age between 209 and 200 mya Lava flow 1 209 mya Geologic time scale EONS —>ERAS —>PERIODS–> EPOCHS • Structure of the geologic time scale was originally based on the fossil record • Era – subdivision of an eon • Eras of the Phanerozoic (visible life”) eon – Cenozoic (“recent life”) begins ~ 65 mya – Mesozoic (“middle life”) begins ~ 250 mya – Paleozoic (“ancient life”) begins ~ 540 mya (the start of the Cambrian period) THE GEOLOGIC TIME SCALE EONS,ERAS Phanerozoic Hadrian 4600-3800 my Archean 3800-2500 my Proterozoic 2500-540 my Paleozoic 540-250 my Mesozoic 250-65 my Cenozoic 65 my to now Geologic time scale • Precambrian time (Hadrian + Archean + Proterozoic) • Nearly 4 billion years prior to the Cambrian period • Lenthy time units because the events of Precambrian history are not know in detail – few fossils, most rocks of this age have been modified or … lost • The Precambrian spans an immense space of Earth age — about 88% of Earth’s history Determining Geological Ages Relative ages – it is often possible to establish the relative ages of rocks and geologic events throug...
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