Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
History of geomagnetism wikipedia , lookup
Anoxic event wikipedia , lookup
Post-glacial rebound wikipedia , lookup
Geomagnetic reversal wikipedia , lookup
History of geology wikipedia , lookup
Abyssal plain wikipedia , lookup
Oceanic trench wikipedia , lookup
Plate Tectonics The Continental Drift Hypothesis Proposed by Alfred Wegener in 1915. Supercontinent Pangaea (meaning “all land”) started to break up about 200 million years ago. Continents "drifted" to their present positions. Continents "plowed" through the ocean crust. Earth ~200 million years ago Wegener’s Evidence for Continental Drift Geographic fit of South America and Africa (Wegener argued that the coastlines on opposite sides of the South Atlantic Ocean have similarities and look like they fit together). Rebuttal from opponents: Erosion continually changes shorelines over time. Didn’t necessarily look the same millions of years ago. Continental Drift: Evidence Tight fit of the continents, especially using continental shelves. More Evidence from Wegener Fossils match across oceans. Including Mesosauraus, an aquatic reptile whose fossil remains are limited to eastern South America and Africa. Other scientists had argued that land bridges could account for this (during an ice age, the sea level lowered to allow animals to cross) – no evidence supports that one existed between South America and Africa. Continental Drift: Evidence Fossil critters and plants Yet More Evidence – Rock Types and Structures Several mountain belts end at one coastline and reappear on a landmass across the ocean. The Appalachian belt ends off the coast of Newfoundland and we find similar rocks and structures in the British Isles and Scandinavia. Continental Drift: Evidence Correlation of mountains with nearly identical rocks and structures The Evidence of Ancient Climates Wegener was a meteorologist and he found glacial deposits showing that between 220-300 million years ago, ice sheets covered large areas of the Southern Hemisphere. He thought that if the landmasses did exist as a supercontinent, with South Africa centered over the South Pole, that this would explain the conditions necessary to form large areas of glacial ice over much of the Southern Hemisphere (it would also place the northern landmasses nearer the tropics, which would account for their vast coals deposits). Continental Drift: Evidence Glacial features of the same age restore to a tight polar distribution. Continental Drift: Reactions Received well in Europe and southern hemisphere. Rejected in U.S., where scientists staunchly preferred induction (incremental progress built on observation) over what they perceived as speculative deduction. Lack of a suitable mechanism crippled continental drift’s widespread acceptance. Conflict remained unresolved because seafloors were almost completely unexplored. No Mechanism Wegener proposed that the tidal influence of the Moon was strong enough to give the continents a westward motion – physicists showed that physically incompatible with what we know of the tides and the Earth’s rotation. Wegener also proposed that the continents broke through oceanic crust, but no evidence existed to indicate that the ocean floor was that weak. The Rise of Plate Tectonics WW II and the Cold War: Military Spending U.S. Navy mapped seafloor with echo sounding (sonar) to find and hide submarines. Generalized maps showed: oceanic ridges—submerged mountain ranges fracture zones—cracks perpendicular to ridges trenches—narrow, deep gashes abyssal plains—vast flat areas seamounts—drowned undersea islands Dredged rocks of the seafloor included only basalt, gabbro, and serpentinite—no continental materials. The Rise of Plate Tectonics Marine geologists found that seafloor magnetism has a striped pattern completely unlike patterns on land. Mason & Raff, 1961 Black: normal polarity White: reversed polarity Both: very magnetic The Rise of Plate Tectonics Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s magnetic field. To test this hypothesis, scientists determined the eruptive ages AND the polarity of young basalts using the newly developed technique of K-Ar radiometric dating. The study validated the reversal hypothesis... The Rise of Plate Tectonics And then (1962-1963) geologists realized that the patterns are SYMMETRICAL across oceanic ridges. The Rise of Plate Tectonics Meanwhile, U.S. military developed new, advanced seismometers to monitor Soviet nuclear tests. By the late 1950s, seismometers had been deployed in over 40 allied countries and was recording 24 hrs/day, 365 days/year. Besides the occasional nuclear test, it recorded every moderate to large earthquake on the planet. With these high-precision data, seismologists found that activity happens in narrow bands. Bands of seismicity—chiefly at trenches and oceanic ridges The Theory of Plate Tectonics Earth’s outer shell is broken into thin, curved plates that move laterally atop a weaker underlying layer. The plates are called tectonic plates or lithospheric plates (remember that the lithosphere is the crust and the uppermost part of the upper mantle). Tectonic Plates on Modern Earth Major Plates, Secondary Plates, & Tertiary Plates There are 7 major plates (African Plate, Antarctic Plate, Eurasian Plate, Indo-Australian Plate, North American Plate, Pacific Plate, and South American Plate). Secondary plates tend to be much smaller. There are 8 (the Arabian and Indian are the biggest). The others are: Caribbean Plate, Cocos Plate, Juan de Fuca Plate, Nazca Plate, Philippine Sea Plate, Scotia Plate. Tertiary Plates Tertiary plates are grouped with the major plate that they would otherwise be shown as part of on a major plate map. Mostly these are tiny microplates, although in the case of the Nubian-Somalian and AustralianCapricorn-Indian plates these are major plates that are rifting apart. Different maps and references will have different numbers of tertiary plates – it is difficult to show what comprises separate plates and there tends to be less consensus with the smaller plates. Movement & Events at Plate Boundaries Most earthquakes and volcanic eruptions happen at plate boundaries. Three types of relative motions between plates: divergent convergent transform Divergent boundaries: Chiefly at oceanic ridges (aka spreading centers) Seafloor Spreading Divergent boundaries also can rip apart (“rift”) continents How rifting of a continent could lead to formation of oceanic lithosphere. e.g., East Africa Rift e.g., Red Sea e.g., Atlantic Ocean Presumably, Pangea was ripped apart by such continental rifting & drifting. Subduction zones form at convergent boundaries if at least one side has oceanic Modern examples: Andes, (denser) material. Cascades Major features: trench, biggest EQs, explosive volcanoes Another subduction zone—this one with oceanic material on both sides. Modern example: Japan Collison zones form where both sides of a convergent boundary consist of continental (buoyant) material. Modern example: Himalayas This probably used to be a subduction zone, but all the oceanic material was subducted. Most transform boundaries are in the oceans. Some, like the one in California, cut continents. The PAC-NA plate boundary is MUCH more complex than this diagram shows. Evidence Supporting Plate Tectonics Earth’s magnetic field periodically reverses polarity (the north magnetic pole becomes the south magnetic pole, and vice versa). A rock solidifying during one of the periods of reverse polarity will be magnetized with the polarity opposite of rocks being formed today. Magnetometers have been towed across the ocean floor and have revealed alternating strips of high-intensity (normal polarity) magnetism and low-intensity (reverse polarity) magnetism. This strongly supports seafloor spreading. How magnetic reversals form at a spreading center Alternating Magnetism More Support – Earthquake Patterns There is a close link between deep-focus earthquakes and ocean trenches. No earthquakes have been recorded below 700km, which fits with the theory because the slab is predicted to be heated enough to soften (without the rigid rocks, there would be no conditions for earthquakes to occur). Earthquake depth indicates subduction zones More Support – Ocean Drilling Data from sea floor drilling (especially that conducted by The Deep Sea Drilling Project (1968-1983) revealed that the youngest oceanic crust is at the ridge crest and the oldest oceanic crust is at the continental margins. Hotspots, such as the one under Hawaii, have also validated plate tectonic theory. Hot Spots Melting of hot rock in the mantle creates a volcanic area (hot spot) near the surface. As the plate moves over the hot spot, successive volcanic mountains form. This is how the islands of Hawaii formed. Kauai is the oldest of the islands and its volcanoes are extinct. Why do the plates move? Several related ideas are widely accepted: Slab pull: Denser, colder plate sinks at subduction zone, pulls rest of plate behind it. Ridge-Push: Oceanic lithosphere slides down the sides of the oceanic ridge due to gravity. Mantle convection: Hotter mantle material rises beneath divergent boundaries, cooler material sinks at subduction zones. So: moving plates, EQs, & volcanic eruptions are due to Earth’s loss of internal heat. How does convection work? No one knows—but they aren’t afraid to propose models! Whole-mantle convection Two mantle convection cells Complex convection