Transcript Chapter 3
Chapter 3 Plate Tectonics Plate Tectonics In 1855, Antonio Snider published a sketch showing how the two continents could fit together, jigsaw-puzzle fashion In 1912, Alfred Wegener published the concept of continental drift Continental drift is just one aspect of a broader theory known as plate tectonics, which has evolved over the last several decades Tectonics is the study of large-scale movement and deformation of the earth’s outer layers Plate tectonics relates such deformation to the existence and movement of rigid “plates” over a weak or partly molten layer in the earth’s upper mantle Figure 3.6 Continental Drift and Plate Tectonics • Jigsaw-puzzle fit of continents observed a few centuries ago • Mechanism to describe how continental masses moved was not easily visualized for decades • Later half of 20th century the concept of continental drift was incorporated into a broader concept of Plate Tectonics – Mechanisms and processes of continent scale movement detailed – Evidence based on physics, chemistry, mathematics, and geology used to explain how rigid plates move relative to each other Figure 3.1 Figures 3.2 a and b Rock Response to Plate Tectonics • Stress –force applied on a rock – Compressive stress – squeeze or compress an object – Tensile stress – pull or stretch an object – Shearing stress – different parts of an object move in different directions or at different rates • Strain – results from stress; is the change in shape or size of an object because of the stress it experienced Strain • Temporary or permanent • Elastic deformation – temporary strain, object recovers original size and shape once the stress is removed – Elastic limit – strain that becomes permanent in an object once limit of recoverable strain has been exceeded – Plastic deformation occurs in materials once elastic limit has been exceeded – Brittle deformation occurs at the limit of strength of the material, a rupture or a break occurs Figure 3.3 Lithosphere and Asthenosphere • Earth’s crust and upper most mantle are solid and compose the lithosphere – Stresses cause brittle and elastic deformation • Beneath the lithosphere is a plastic layer called the asthenosphere • Lithospheric plates can move over this plastic layer; plate tectonics plausible • Boundaries of the plates are active with earthquake and some with volcanic activity Figure 3.4 Evidence for Plate Tectonics • Earthquakes and volcanoes • Sea Floor topography – Trenches – Ridges • Paleomagnetism • • • • Magnetic patterns imprinted on oceanic crust Curie temperature Magnetic reversals Magnetic polar wandering curves • Sea Floor Spreading • Age of the sea floor • Other evidence – Fit of continents, GPS data, and more … Figure 3.5 Figure 3.7 Figures 3.8 a and b Figure 3.9 Figure 3.10 Other Evidence for Plate Tectonics • Distribution of rocks representing ancient deserts, sea shores, tropical areas, glaciated areas, swamps, and equatorial regions • Location of fossils that were originally restricted in their distribution but now separated by oceans and on separate continents • Fit of continents reveal super continent of Pangaea • Recognition of plate boundaries Figure 3.13 Figure 3.14 Figure 3.6 Figures 3.15 a, b, and c Plate Boundaries • Divergent Plate Boundary – Lithospheric plates move apart; form oceanic ridges – Upwelling of asthenosphere injects magma forming oceanic ridges and new oceanic crust – Forces plates apart – Sea floor spreading occurs • Transform Boundaries – short segments of a ridge – Transform faults offset ridge – San Andreas Fault – transform fault under continental crust Figures 3.16 a and b Plate Boundaries • Convergent Plate Boundaries – Lithospheric plates move toward each other – Higher density oceanic crust overridden by low density continental crust – Subduction zone forms and produces a trench – Subduction of older oceanic crust balances the spreading seafloor equation – Subduction zones are active geologic places • Volcanism • Earthquakes • Island arc formation Figures 3.18 a, b, and c Figure 3.19 Tectonics • Convection cells operate in mantle • Upwelling of heat and magma occurs at divergent plate boundaries – New oceanic crust formed – Oceanic crust pushed away from spreading centers • Hot spots located independent of plate boundaries – High heat flow radiate from them – Volcanic activity associated with them • Hawaiian Islands • Yellowstone Figure 3.20 Figure 3.21 Figure 3.22 Figure 3.23