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continental drift: theory & definition
tectonic plates of the earth. original image
continental drift was a theory that explained how continents shift position on earth's surface. set forth in 1912 by alfred wegener, a geophysicist and meteorologist, continental drift also explained why look-alike animal and plant fossils, and similar rock formations, are found on different continents.
the theory of continental drift
wegener thought all the continents were once joined together in an "urkontinent" before breaking up and drifting to their current positions. but geologists soundly denounced wegener's theory of continental drift after he published the details in a 1915 book called "the origin of continents and oceans." part of the opposition was because wegener didn't have a good model to explain how the continents moved apart.
when wegener proposed continental drift, many geologists were contractionists. they thought earth's incredible mountains were created because our planet was cooling and shrinking since its formation, frankel said. and to account for the identical fossils discovered on continents such as south america and africa, scientists invoked ancient land bridges, now vanished beneath the sea.
researchers argued over the
before the constriction theory, many thought that the world's formations were caused by a worldwide flood. this theory is called catastrophism, according to the usgs.
plate tectonics is now the widely accepted theory that earth's crust is fractured into rigid, moving plates. in the 1960s, scientists discovered the plate edges through magnetic surveys of the ocean floor and through the seismic listening networks built to monitor nuclear testing, according to encyclopedia britannica. alternating patterns of magnetic anomalies on the ocean floor indicated seafloor spreading, where new plate material is born. magnetic minerals aligned in ancient rocks on continents also showed that the continents have shifted relative to one another. [related: what is plate tectonics? ]
the theory of continental drift reconciled similar fossil plants and animals now found on widely separated continents. gondwana is shown here. original image
evidence for continental drift
a map of the continents inspired wegener's quest to explain earth's geologic history. trained as a meteorologist, he was intrigued by the interlocking fit of africa's and south america's shorelines. wegener then assembled an impressive amount of evidence to show that earth's continents were once connected in a single supercontinent.
evidence for sea-floor spreading
several types of evidence supported hess’s theory of sea-floor spreading: eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves. this evidence led scientists to look again at wegener’s hypothesis of continental drift.
evidence from molten material
in the 1960s, scientists found evidence that new material is indeed erupting along mid-ocean ridges. the scientists dived to the ocean floor in alvin, a small submarine built to withstand the crushing pressures four kilometers down in the ocean. in a ridge’s central valley, alvin’s crew found strange rocks shaped like pillows or like toothpaste squeezed from a tube. such rocks form only when molten material hardens quickly after erupting under water. these rocks showed that molten material has erupted again and again along the mid-ocean ridge.
evidence from magnetic stripes
when scientists studied patterns in the rocks of the ocean floor, they found more support for sea-floor spreading. you read earlier that earth behaves like a giant magnet, with a north pole and a south pole. surprisingly, earth’s magnetic poles have reversed themselves many times during earth’s history. the last reversal happened 780,000 years ago. if the magnetic poles suddenly reversed themselves today, you would find that your compass needle points south.
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