Review participation point: The evidence for a fluid outer core is:

Transcription

1 DDA1 Continental Drift to Plate Tectonics PS 100 Chapter 28 Review participation point: The evidence for a fluid outer core is: A. Average density of the earth is greater than the density of the crust. B. S-wave shadow zone C. P-wave shadow zone D. P and S waves slow in asthenosphere There are sharp changes in the composition of the earth with increasing depth Why will S waves not travel through the outer core? Shadow zone for S waves? Why do P waves slow down and refract in the outer core? The evidence for a sharp boundary between the mantle and core is: A. Average density of earth is higher than the density of the crust B. P-waves do not travel in fluids C. S-waves refract in fluids D. P-wave shadow zone E. P and S waves slow in asthenosphere Earthquake wave speed with depth: s-waves and p-waves Continents and Ocean Basins (solid) (mushy) (solid) (liquid) (solid) 1

2 Slide 1 DDA1 based on skinner 08 R Skinner David Allred, 11/17/2009

3 Shape and fit of the continents was the initial evidence Antonio Snider-Pelligrini (1858) Catastrophism Noah s Flood Not popular Fit is enhanced by fitting continental Shelf at the break, and not at the shore line Frank Taylor (1908) Ortelius Continental Drift The Meteorologist Who Started a Revolution Wegner ( ) Pangea The Origin of the Continents and Oceans (1915) The predecessor to modern plate tectonics Similar shapes and fossils Continental drift maps by Wegner (1915) not widely accepted Evidence for Continental Drift Paleontological Similar fossils on opposite sides of the Atlantic Ocean Plants and animals Glossopteris on all southern continents No mechanism to transport across ocean Ancient Mesosaurus Habitat 2

4 Evidence for Continental Drift Rock type & structures Distinct rocks and geologic structures on both sides of the Atlantic Ocean Only occur in rocks > 145 mya Evidence for Continental Drift Glaciation Late Paleozoic glaciation (300 mya) Covered large portions of the southern continents Evidence for Continental Drift Paleoclimate Evidence of extreme changes in climate as compared to the present Continents Plow through ocean crust, driven by forces related to Earth s rotation Not widely accepted! Development of Plate Tectonic Theory Original evidence for continental drift was from the continents Technological advances in the 1950 s and 1960 s allowed investigation of the sea floor Geophysics & paleomagnetism provided new data 3

5 Topography of the ocean basins Basins are divided by a large ridge system Ridge system is continuous around the entire globe Physical properties Composed of basalt Younger and thinner than most continental rocks No evidence of crustal deformation folded mountains Central rift valley within the ridge Seafloor spreading proposed (Hess-1960) Considered new data on ocean floor Paleomagnetism Fe rich rocks are weakly magnetized Orientation of magnetic field is preserved Magnetic reversals Earth s magnetic field polarity has reversed through time Normal polarity N magnetic = N geographic Reversed polarity - N magnetic = S geographic At least 12 reversals in last 4 my Vine & Matthews (1963) tested Hess s hypothesis using magnetism Magnetic polarity reversals recorded in ocean floor Magma cools forming new crust Polarity at time of cooling preserved Old crust pushed aside 4

6 Then geologists began to measure the magnetic character of the ocean floor by towing magnetometers behind ships. Magnetic polarity stripes in ocean crust parallel ridges Symmetrical on either side of the ridge Polarity chrons give age of seafloor Increases away from ridge Rates of plate motion may be calculated Seafloor sediments support plate tectonic theory Youngest sediments resting directly on basalt near the ridge Sediment gets older moving away from the ridge Accumulation rates of ~3 mm/ 1000 yr Kurile trench EURASIA Emperor Seamount chain PACIFIC OCEAN Aleutian Trench Suiko seamount (59.6) Koko guyot (48.1) Midway (27.2) Kaua i (5.1) Oahu (3.7) Hawai ian ridge Aleutian arc Maui (0.8) Direction of plate Hawai i movement (hot spot) NORTH AMERICA Trench Guyot approaches subduction zone Extinct volcanic island erodes, falls below sea level as oceananic lithosphere cools and subsides Volcanic island becomes extinct as it moves off hot spot Volcanic island Submarine volcano Hot spot Plate Margins Lithosphere is divided into plates Boundaries based on structural features, not land and ocean Plates outlined by ridges, trenches and young mountain belts Plates are not permanent 5

7 Benioff Zones Subduction Named for U.S. geophysicist Hugo Benioff He observed a pattern of earthquake focal depths near trenches, which he concluded confirmed the idea of recycling of the oceanic crust and its convection. What was this pattern? The best way to model the Earths interior is with? a) Meteors b) Drill holes c) Seismic Waves d) Underground mines e) Calculating the Earths mass ma Cambrian Late Middle Silurian Permian Triassic ma Early 105 Triassic/Jurassic Mississippian Cretaceous 65 ma Proterozoic Ordovician Cambrian Devonian 1.5 Cretaceous/Tertiary 35 ma Quaternary Today Tertiary Wegner did not have what source of data? a) Fossils b) Sea Floor c) Continental Structure d) Continental Shape e) Glacial Deposits 6