PLATE TECTONICS
2. Sea-Floor Spreading In 1948, the Mid-Atlantic Ridge was discovered. This is a huge mountain range that runs roughly north–south along the floor of the Atlantic Ocean. The Mid-Atlantic Ridge consists of volcanic rock that emerged from the earth’s mantle. Similar mid-ocean ridges were discovered on the floors of the Pacific and other oceans. In the 1960s, a geologist named Harry Hess
studied the ages of rocks on the seabed of the Atlantic Ocean. He discovered that rocks were youngest in the Mid-Atlantic Ridge and that they became gradually older as one moved away from the ridge towards the shores of America or Europe. This discovery was very important. It suggested that, as new volcanic rock emerged to form the Mid-Atlantic Ridge, it pushed the older existing rocks apart. This caused the sea floor to get wider and wider. It also caused the continents of North and South America to drift slowly away from the continents of Europe and Africa (see Figure 4). Harry Hess’s theory of sea-floor spreading
supported the earlier theory of continental drift. These two theories together form the basis of the theory of plate tectonics, which scientists now accept as fact.
Mid-Atlantic Ridge (New rock being formed) older rock Atlantic Ocean America North American Plate
convection currents of magma
Atlantic Ocean Eurasian Plate
convection currents of magma
Europe older rock
3 The Mid-Atlantic Ridge. (a) Identify the Mid-Atlantic Ridge in this picture. (b) What do you know already about the Mid-Atlantic Ridge?
Did you know?
The Mid-Atlantic Ridge is the world’s longest physical feature.
mantle
4 How sea-floor spreading is making the Atlantic Ocean wider. Scientists calculate that sea-floor spreading is pushing America and Europe further apart by about 2.5 centimetres per year. That is the approximate speed that our fingernails grow at.
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