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Chapter 2

Chapter 2 - Plate Tectonics.docx

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University of Toronto Scarborough
Environmental Science
Nick Eyles

Chapter 2: Plate Tectonics What is Plate Tectonics? - Plate Tectonics – Earth’s surface is divided into a few large, thick plates that move slowly and change in size - Continental Drift – continents move freely over the Earth’s surface, changing positions relative to one another - Sea-floor spreading – hypothesis that the sea floor forms at the crest of mid-oceanic ridges, then moves horizontally away from the ridge crest toward and oceanic trench, two sides of the ridge are moving in opposite directions like slow conveyer belts How did the plate tectonics theory Evolve? The Early Case for Continental Drift: - Paleoclimatology – the study of ancient climates Wegener studied climates, rocks and fossil, magnetic poles – supported the continental drift Renewed Interest in Continental Drift Study of the Sea Floor - Multibeam sonar – measures water depth and draws profiles of submarine topography - Sidescan sonar – measures the intensity of sound reflected from the ocean floor and provides detailed images and information about sediments and bedforms on the sea floor - sub-bottom profilers - examine the strata (layers) beneath the floor of the lake/ocean - seismic reflection profiler – louder noise, lower frequency, reflects from layers within sediment and rock, records water depth and reveals internal structure of sea floor (Ex: faults, folds) - Rock dredge – an open steel container dragged over the ocean bottom - Rock corer – steel pipe dropped vertically into mud and sand of ocean floor - Sea-floor drilling – drilling derrick - Submersibles – small research submarines, observe, photograph and sample rock and sediment Geophysical Research - Polar wandering – an apparent movement of the Earth’s plates Recent Evidence for Continental Drift - Rocks in Brazil and African country Gabon are very similar - GPS allows us to watch continents move in real time 2.1 Measuring Plate Movement in Real Time - Space geodesy – a space-based technique for taking very precise measurements of points on the earth’s surface (ex: very-long-baseline interferometry (VLBI), satellite laser ranging (SLR), GPS) - GPS is the most useful technique for studying earth’s movements What is Sea-Floor Spreading? Wegener – thought ocean floors remained stationary as continents moved Harry Hess 1962 at Princeton – suggest that sea floor might be moving too - Spreading axis (spreading center) – the ridge crest (MOR) with the sea floor moving away from it on either side - Subduction – the sliding of the sea floor beneath a continent or island arc - Convection – a very slow circulation of a substance driven by differences in temperature and density within that substance How Old is the Sea Floor? - Fairly young only 200 million years old What are plates and how do they move? - Plate – a large, mobile slab of rock making up part of the Earth’s surface - Lithosphere – the rigid outer shell of the Earth, 70 to 125 more kilometers thick, includes rocks of the crust and the uppermost mantle - Asthenosphere – a region of the Earth’s outer shell beneath the lithosphere, is of indeterminate thickness and behaves plastically - Divergent plate boundary – boundary separating two plates moving away from each other - Convergent boundary – a boundary between two plates that are moving toward each other - Transform plate boundary – boundary between two plates that are sliding past each other How do we know that plates move? Paleomagnetic Evidence - Magnetic reversals – a change in the Earth’s magnetic field between normal polarity and reversed polarity. - Normal polarity – compass points north, enters earth - Reversed polarity – periods when the lines of magnetic force run from the south pole to the north pole and compass needles point to the south, leaves earth - Paleomagnetism – a study of ancient magnetic fields - Magnetic polarity time scale – records the pattern of magnetic reversals over time - Anomaly – a deviation from average readings of magnetic strength - Magnetometer – an instrument that measures the strength of the Earth’s magnetic field Marine Magnet Anomalies The Morley-Vine-Matthews Hypothesis – pattern of magnetic anomalies on one side of the MOR was mirror image on the other side, magnetized dikes form parallel to rift valley = cause of anomalies - Predicts age of sea floor How fast do plates move? Predicting Sea-Floor Age - Similar to dating tree rings Another Test: Fracture Zones and Transform Faults - Transform fault (Tuzo Wilson) – the portion of a fracture zone between two offset portio
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