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Millions of years before present

FIGURE 2.12 ► The established record of main field polarity reversals that have been obtained from global rock samples.

activity techniques can be assigned an approximate formation time by matching their paleomagnetic direction pattern to the well-established field reversal pattern (Figure 2.12). This pattern-matching method is similar to tree-ring dating, in which concentric rings of annual growth show a unique spacing pattern.

In recent years, special crustal regions, often toward the middle of our major oceans, have been found to show long ridges where hot magma is slowly rising from deep within the Earth. As this material cools, it becomes magnetized by the local field at the time of its emergence. The cooled magma, with its field identification, spreads perpendicular to the ridge (called sea-floor spreading) ever so slowly along the ocean bottom, becoming an oceanic plate that holds a recording of the reversals in field direction over the millions of years of Earth formation (Figure 2.13). The process has been compared to an extremely slow-moving tape recorder. The field directions have now been recorded with instruments towed near the ocean bottom. The dating of the ocean-bottom field-reversal patterns has revealed an oceanic plate motion spreading perpendicular to the ridge line at about 2.5 to 25 centimeters (1 to 10.0 inches) a year.

Some of the moving oceanic plates collide with the continents before they are eventually pulled down by gravity into deep ocean troughs near continental margins. The material is then recycled into the hot magma interior of the Earth's mantle. The Earth's continents are moved as the oceanic plates push against and under the continental margins causing what is called a continental drift. The obvious coastal pattern fit in the South Atlantic between eastern South America and western Africa represents the continental drift of these two continents away from their common oceanic spreading ridge. In those two matched continental regions, similar geology, ancient flora, and paleomagnetic field directions have been identified. Antarctica, Australia, and India were also once connected, but are continuing to separate. India is pushing up against Asia, forming the Himalayan Mountains. Africa is pushing into

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FIGURE 2.13 ► Seafloor spreading and magnetic field reversals provide evidence of the oceanic plate motion that causes continental drift.

Europe, closing the Mediterranean Sea. Earth satellites can now accurately measure the slow drifts of these continents.

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