* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Introduction to Plate Tectonics - EHS
Survey
Document related concepts
Post-glacial rebound wikipedia , lookup
Ocean acidification wikipedia , lookup
Age of the Earth wikipedia , lookup
Anoxic event wikipedia , lookup
History of navigation wikipedia , lookup
Tectonic–climatic interaction wikipedia , lookup
Earth's magnetic field wikipedia , lookup
Physical oceanography wikipedia , lookup
Geochemistry wikipedia , lookup
History of geology wikipedia , lookup
Geological history of Earth wikipedia , lookup
Large igneous province wikipedia , lookup
Abyssal plain wikipedia , lookup
Geomagnetic reversal wikipedia , lookup
Transcript
Plate Tectonics – New Evidence Alfred Wegener & Continental Drift • No mechanism to move the continents from one place to another • Scientists did not know much about the ocean floor • dismissed as being eccentric, preposterous, and improbable Early Studies • 1855, U.S. Navy Lieutenant Matthew Maury – a bathymetric chart – revealed the first evidence of underwater mountains in the central Atlantic – This was later confirmed by survey ships laying the trans-Atlantic telegraph cable. • World War I (1914-18) – echo-sounding devices (primitive sonar systems) measure ocean depth by recording the time it took for a sound signal from the ship to bounce off the ocean floor and return. – revealed that the ocean floor was much more rugged than previously thought. – demonstrated the continuity and roughness of the submarine mountain chain in the central Atlantic World War II Sea-Floor Topograpy • Ships and Submarines used SONAR to map the ocean-floor topography – Submarines used topography to hide from ships and had to avoid under-sea topography – Ships used SONAR to find submarines • 1950s -- oceanographic surveys – a great mountain range on the ocean floor, the global mid-ocean ridge, encircled the Earth. • more than 50,000 kilometers (km) long and up to 1000 km across • zig-zags between the continents • Rising about 4,500 meters(m) above the sea floor, – Taller than all mountains in the US except for Mount McKinley (Denali) at 6,194 m • The most prominent topographic feature on the surface of our planet. – Deep Sea trenches -- incredibly deep Age of the Ocean Floor • 1947 -- seismologists on the U.S. research ship Atlantis – Sediment layer on the floor of the Atlantic was much thinner than thought. – Presumed age of the oceans = 4 billion years: the sediment layer should have been very thick. Youngest rocks at Ridge Oldest furthest away from Ridge • 1968 -- a research vessel, the Glomar Challenger, designed specifically for marine geology studies – Deep-Sea Drilling Project -samples from ocean floor. – Criss-crossed the Mid-Atlantic Ridge between South America and Africa – the ages of the samples • paleontologic • isotopic dating studies Earth’s Magnetic Field & Polar Reversals • Early 1900’s – Paleomagnetists (people who study the Earth's ancient magnetic field) – Bernard Brunhes in France (in 1906) – Motonari Matuyama in Japan (in the 1920s) – rocks generally belong to two groups according to their magnetic properties • Normal: the magnetic minerals in the rock having the same polarity as that of the Earth's present magnetic field (North is North) • Reversed: the magnetic minerals in the rock having the opposite polarity as that of the Earth's present magnetic field (North is South) How can this happen? • Currents in the liquid outer core cause the Earth’s magnetic field • • • Convection currents of liquid iron and nickel generate a “dynamo” , an electrical current. The Earth is a giant electro-magnet! Grains of magnetite behave like little magnets • Align with the orientation of the Earth's magnetic field. • Magma cools to form solid rock • Alignment of the magnetite grains is "locked in" • The grains “record” the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling. How did they figure this out? • World War II – magnetometers used to detect submarines • 1950s, scientists used magnetometers to study the ocean floor – Odd magnetic variations across the ocean floor – Ocean floor rock, basalt, contains magnetite (a magnetic iron mineral) – Magnetite can distort compass readings – The magnetic variations are not random or isolated! • Magnetic striping = a zebra-like pattern – alternating stripes of normal & reversed rock – laid out in rows on either side of the mid-ocean ridge New oceanic crust forms continuously at the crest of the mid-ocean ridge • As you move away from the ridge – Crust is cooler – Crust is older • Crust moves away from the ridge crest with seafloor spreading – a. the spreading ridge about 5 million years ago – b. about 2 to 3 million years ago – c. now Sea-Floor Spreading: Putting it all together • Evidence – Sea-Floor Topography – Young crust at ridges, old crust far from ridges – Magnetic Striping • 1961 – New Questions: – Are mid-ocean ridges structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest? – Could new magma from deep within the Earth rises easily through these weak zones and erupts along the crest of the ridges to create new oceanic crust? Sea-Floor Spreading: Putting it all together • Harry H. Hess & Robert S. Dietz = seafloor spreading. – Crust is added at the ridges • new oceanic crust continuously spread away from the ridges in a conveyor belt-like motion – Crust is destroyed at the trenches • After millions of years, oceanic crust descends into very deep, narrow canyons along the rim of the Pacific Ocean basin. – the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. – The ocean basins were perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously Sea-Floor Spreading: Putting it all together • Thus, Hess' ideas neatly explained – why the Earth does not get bigger with sea floor spreading – why there is so little sediment accumulation on the ocean floor – why oceanic rocks are much younger than continental rocks. • The oceanic crust -– a natural "tape recording" of the history of the reversals in the Earth's magnetic field. The Theory of Plate Tectonics • The outer layer of the Earth is broken up into rigid plates • These plates are made of the lithosphere – Crust – Upper most part of the mantle “stuck” to the bottom of the crust • Plates move around on the hot, weak, mobile asthenosphere – Weak zone in the mantle below the lithosphere The Plates • Rigid - movement doesn’t happen within a plate • Plates move relative to each other resulting in – Earthquakes – Volcanoes – Mountains • The intensity of earthquakes, types of volcanoes, and types of mounains are dependent on how the plates interact – 3 different plate boundaries Divergent Boundaries • Plates move away from each other • Mostly found at midocean ridges • Crust is made as plates move away from each other and magma oozes up at the ridge • Earthquakes are common but shallow • Volcanic Activity is prevalent • Examples: Mid-Atlantic Rise and the Red Sea Transform Boundary • Plates move past each other • Accommodate the different types of movement on a sphere • Earthquakes very common and can be severe • Volcanoes absent • Examples: San Andreas Fault and Mendicino Boundary Convergent Boundary • Plates move toward each other • Three different types – Ocean-Continent – Ocean-Ocean – Continent-Continent Ocean-Continent Convergent • The more dense oceanic lithosphere is subducted under the less dense continental lithosphere forming a trench • Continental Volcanic Arcs result as the water in the subducted oceanic lithosphere is released and melting occurs in the mantle below the continent. • Earthquakes are severe and common. • Examples: Cascade and Andes Mountains Ocean-Ocean Convergence • The older, colder more dense oceanic lithosphere is subducted beneath the younger, warmer less dense segment. • Deep trenches form at edge of subduction zone • Volcanic Island Arcs form as water is released into the mantle causing melting • Earthquakes are common and severe. Often result in Tsunamis. Continent-Continent Convergence • Continental lithosphere cannot be subducted because it is not dense enough. • As the continents are forced together, they form huge mountains • Earthquakes are severe and common. • Volcanism is rare. • Examples: Himalaya, Alps, and Appalachia What Drives the Plates? • http://www.visionle arning.com/library/f lash_viewer.php?oi d=1683&mid=65