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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
- 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?