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Transcript
Evidence for Continental Movement Evidence of Continental Drift - The shape of coastlines match Philosopher Sir Francis Bacon remarked about this in 1620 Evidence of Continental Drift - The plants and animals match Fossil records a. Mesosaurus (aquatic reptile) b. Glossopteris flora (plant fossils) Late Paleozoic seed ferns Gondwanaland (India, Africa, Australia, S. America, Antarctica) Evidence of Continental Drift Map showing distribution of fossils on the southern continents. These distributions caused Wegener to join the continents as shown in the map. Evidence of Continental Drift - The rocks match Geologic similarities between S. America and Africa. Same stratigraphic sequence (i.e. same sequence of layered sedimentary rocks Mountain belts and folded rocks line up { Stratigraphic sequence S. America Africa Evidence of Continental Drift - The rocks match Evidence of Continental Drift - Match in glacial landform Continental ice sheet covered parts of South America, southern Africa, India, and southern Australia about 300 million years ago Resultant glacial landforms exist in parts of the world today that are not covered by ice – like U-shaped valleys History in Geography (...the interesting bits!) Developing the Continental Drift Theory… Poor Wegener… • Wegener proposed Continental Drift Theory (CDT) and published his book The Origins of the Continents and the Oceans in 1915 • Wegener's book translated into English, French, Spanish, and Russian in 1924 ridiculed! • Data based on continents, but the oceans cover 70% of the earth's surface However… • Major technical and scientific developments allowed scientists to: ▫ Map the ocean floor ▫ Detect paleomagnetic reversals in the rocks on the ocean floor Before 1920s… • Ships did not have sonar • Crust below seas thought to be flat and featureless After 1920s… • Sonar maps showed very exciting seafloor • Tremendously deep valleys, mountain chains and vast plains • A long ridge was found to run down the middle of the Atlantic Ocean ▫ Rising 1 to 2 km above the surrounding ocean floor and paralleling the continental coasts on both sides • Similar mid-ocean ridges were mapped in the eastern Pacific Ocean and the western Indian Ocean 1962 • Harry Hess, geologist (History of Ocean Basins) ▫ Proposed that the mid-ocean ridges marked regions where hot magma rose close to the surface ▫ This extrusion pushed the ocean floor away from the ridges like a conveyor belt ▫ In deep trenches (e.g. off the coast of South America and Japan) spreading ocean floor forced down below the thick continents into subduction zones. ▫ Hess's theory of “seafloor spreading” offered a driving mechanism for Wegener's CDT but needed more proof A not very hairy Hess 1962 • 1962: U.S. Navy published a report that summarized findings concerning seafloor magnetism • During WWII, ships dragging devices (magnetometers) to measure magnetism to locate submarines ▫ Found bands of alternating strong and weak magnetism in the rocks of the seafloor Irregular in width Paralleled the mid-ocean ridges Patterned symmetrically about the crest of these ridges. (Just to note) • Magnetism caused by presence of magnetic minerals in the rocks • Scientists were not surprised to learn that seafloor rocks contain the magnetic mineral magnetite ▫ When magma from the ridges rises and cools, it crystallizes, locking the magnetite crystals into alignment with the earth's magnetic field like the needle of a compass • Existence of the earth's magnetic field had been known since ancient times, but only after WWII did scientists realize that the magnetic field is not constant – ▫ Fluctuates in intensity ▫ Reverses itself • Today we consider the field to have “normal” polarity, and our compass needles point to the north 1963 • Fred Vine and Drummond Matthews (British geologists) joined the map of the Mid-Atlantic Ridge with the symmetric bands of magnetism on the seafloor • Strongly patterned paleomagnetic reversals recorded on the seafloor provided the necessary proof of Hess's seafloor spreading ▫ Proved that new crust was continuously being generated at the mid-ocean ridges, where magma cooled and magnetite crystals “locked in” according to the orientation of the earth's magnetic field at that time ▫ Continents no longer had to “drift” to their present locations-they were driven by the slow and steady magma “conveyor belts” at the mid-ocean ridges The work of Hess, Vine, and Matthews resulted in a new map of the earth, one that included plate boundaries in addition to coastlines. Boundaries were drawn at mid-oceanic ridges and subduction zones. Blue = mid-ocean ridges, Red = subduction zones. Today… • Satellite technology • Global positioning system (GPS) and other satellite-based data collection techniques ▫ Scientists can directly measure the movement and speed of plates on the surface of the earth ▫ Speeds range from 10 to 100 mm per year, confirming the long-held belief that plates move at a slow but constant rate Before Wegener, few had conceived of such a world. His continental drift theory was the first step in the development of plate tectonic theory, the foundation upon which modern geology is built. The work goes on, the cause endures, the hope still lives and the dreams shall never die. - Edward Kennedy