* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Nance Chapter 02 Lecture PPT
Survey
Document related concepts
Geochemistry wikipedia , lookup
Physical oceanography wikipedia , lookup
Earth's magnetic field wikipedia , lookup
Abyssal plain wikipedia , lookup
Magnetotellurics wikipedia , lookup
Oceanic trench wikipedia , lookup
History of geology wikipedia , lookup
History of geomagnetism wikipedia , lookup
Geological history of Earth wikipedia , lookup
Geomagnetic reversal wikipedia , lookup
Transcript
Chapter 2: Plate Tectonics Fig. 2.26 OBJECTIVES • Outline Wegener’s idea of drifting continents and describe the evidence he cited to support his hypothesis as well as the primary objection to his claims. • Explain how paleomagnetism provides support for continental drift by providing evidence for the concepts of apparent polar wander and magnetic polarity reversal. • Summarize the process of seafloor spreading and the evidence that indicates that new ocean floor is continuously being created at mid-ocean ridges. OBJECTIVES • Summarize the evidence that old ocean floor is continuously being destroyed beneath deep ocean trenches by a process called subduction. • Explain how the theory of plate tectonics provides an explanation for continental drift and describe the three types of plate boundaries. • Describe how volcanic hotspots are attributed to plumes of hot material rising from deep within Earth’s interior. Plate Tectonics: An Overview • Earth’s rigid outer layer is made up of slabs of rock known as tectonic plates. • Plate movement is driven by heat flow from Earth’s interior. • Plate movement causes • continental drift • earthquakes • formation of mountains ranges • some volcanoes Continental Drift • Hypothesis of continental drift proposed by Alfred Wegener in 1912 • Wegener’s evidence for continental drift • Jigsaw fit of the continents • Continuity of ancient mountain belts • Continuity of major faults or major crustal fractures • Patterns of ancient climates (as interpreted in rocks) • Distribution of certain fossil species Fig. 2.1 Evidence for Continental Drift Fossils Glacial deposits Figs. 2.5, 2.6 Continental Drift • Wegener claimed that 300 million years ago, the continents formed a single landmass called Pangea (“all lands”). • Continents have drifted slowly to their current positions. Fig. 2.2 Continental Drift to Plate Tectonics • Continental drift hypothesis was not generally accepted. • No reasonable mechanism proposed • Additional evidence for movement of the continents was gathered between the 1920s and 1960s. • Paleomagnetism • Age of the seafloor • Earthquake locations and depths • By the 1960s, the hypothesis of continental drift had evolved into the theory of plate tectonics. Evidence: Paleomagnetism • Earth’s magnetic field resembles that of a simple bar magnet. • Iron-rich minerals become magnetized as they crystallize. • Orientation of the magnetic field and apparent location of the magnetic poles over time can be inferred from the rock record. Fig. 2.7 Apparent Polar Wander • Differences in the apparent locations of the north magnetic pole over time are evidence for continental drift. Fig. 2.10 Paleomagnetic Reversals • Earth’s magnetic field periodically reverses (magnetic polarity reversal). Fig. 2.11 In normal magnetic polarity, the north magnetic pole is near the north geographic pole. In reversed magnetic polarity, the north magnetic pole is near the south geographic pole. Magnetic Reversals on the Seafloor • Magnetometers reveal magnetic anomalies on the seafloor: alternating strips of high-intensity and lowintensity magnetism. • High Intensity: rocks formed under normal polarity • Low Intensity: rocks formed under reverse polarity • Magnetic anomalies are symmetrical about the mid-ocean ridges. • Interpretation: seafloor spreading Fig. 2.12 Seafloor Spreading • Partial melting beneath the mid-ocean ridge causes volcanism. • Earth’s magnetic polarity is recorded as lava crystallizes. • Rock moves to either side and new magma is emplaced along the ridge. • The seafloor moves symmetrically away from mid-ocean ridge, moving the ocean floor and continents. • Basalts increase in age with distance from the ridge (confirmed by drilling and analysis of seafloor rocks). Fig. 2.13 Subduction • Roughly the same volume of ocean crust created at mid-ocean ridges is destroyed in deep ocean trenches through the process of subduction. • Old, dense ocean crust dives beneath less dense ocean or continental crust • Subduction zones characterized by • Ocean trenches • Shallow to deep earthquakes • Volcanic arcs • Evidence • Inclined zone of earthquakes • Chemistry of arc lavas Fig. 2.17 Subduction Zones Subduction of oceanic lithosphere beneath (a) oceanic lithosphere or (b) continental lithosphere both result in seismic activity and volcanism. Fig 2.20 Moving Plates • Earth’s surface is broken into huge, moving, tectonic plates. • Tectonic plates are made of rocky and brittle lithosphere. • Crust plus uppermost mantle • Plates move over asthenosphere. • Weak zone in Earth’s mantle • Plates may be thousands of kilometers across but are only 50-150 km (30-100 miles) thick. • Plates are in continuous motion. Fig. 2.22 Major Plates and Relative Motion Plate Boundaries • Earthquake zones outline plate boundaries. • Three types of plate boundaries: • Divergent (or Constructive): plates move apart • Continents split • Oceans widen • Convergent (or Destructive): plates move toward each other • Subduction of oceanic lithosphere • Continental collision • Transform: plates slide past each other along transform faults Plate Boundaries Fig. 2.23 Hotspots: A Plate Tectonics Enigma • Geologic activity is focused around plate boundaries. • Hotspots (isolated areas of long-lived volcanism) also occur far from plate boundaries. • Appear to be stationary beneath moving plates • Widely attributed to mantle plumes Fig. 2.27 Fig. 2.28 Hawaiian Islands: Movement of the Pacific plate over the fixed Hawaiian hotspot has led to the formation of the Hawaiian island chain. SUMMARY • Theory of plate tectonics is the unifying theory of Earth science. • Earth’s surface is broken into plates that move slowly relative to each other. • Divergence of plates at ocean spreading centers and continental rifts • Convergence of plates at subduction zones and continental collision zones • Plates sliding past each other at transform boundaries • Explains patterns of earthquakes, volcanism, mountain belts, and outlines of continents. • Theory evolved from the hypothesis of continental drift proposed in the early 1900s. • Continental drift was initially rejected, in part because no viable mechanism was proposed. • Evidence accumulated over the next 50 years. SUMMARY • The theory of plate tectonics is supported by • Jigsaw fit of the continents (in features such as mountain belts, major fault lines, and distinctive rock formations) • Matching climate and fossil patterns on continents separated by major oceans • Apparent polar wander, reflecting the movement of the continents • Magnetic stripes on the seafloor • Deep ocean trenches • Patterns of earthquakes • Shallow along transforms and divergent boundaries • Inclined zones along convergent boundaries (subduction zones) • Hotspot tracks