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Continental Drift. Who? When? And what did they learn?. Early Explorers obtained new information about the continents and their coastlines 400 years ago.
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Who? When? And what did they learn? • Early Explorers obtained new information about the continents and their coastlines 400 years ago. • When the explorers studied new world maps they noticed that the continents looked as though they could fit together like parts of a giant jigsaw puzzle.
Wegener’s Hypothesis • Continental drift is a hypothesis that German scientist Alfred Wegener proposed. • Wegener hypothesized that the continents formed part of a single land mass, or supercontinent. • Wegener believed that small continents began forming 250 million years ago. • Wegener speculated that over millions of years these small continents drifted to their present locations.
More of Wegener’s Hypothesis • Wegener hypothesized that the crumpling of the crust in places, produced mountain ranges such as the Andes. • If the continents had once been joined, fossils of the same plants and animals should be found in areas that were once connected. • The fossils from the extinct land reptile called Mesosaurus were found in South America and western Africa. • It was unlikely that the reptiles had swum across the Atlantic and there was no evidence that land bridges had once connected the continents.
Supporting Wegener’s Hypothesis • Although the areas where the rocks came from were widely separated, the ages and types of rocks matched closely. • Mountain chains that ended at coastline of one continent seemed to continue on other continents across the ocean. • An example of a mountain chain that seems to continue from one continent to other continents across the ocean, are the Appalachian Mountains. They extend northward along the east coast of North America; mountains of similar age and structure are found in Greenland, Scotland, and northern Europe. • The layers of debris from ancient glaciers in southern Africa and South America indicates to geologists that the continents have not always been located where they are now.
Supporting Wegener Some More • Fossil evidence, such as plant fossils is evidence that tropical or subtropical swamps used to cover areas that now have colder climates. • If the continents were once joined and positioned differently, there would have been climate differences. • Wegener thought that the continents plowed through the rock of the ocean floor. • Scientist disagreed with Wegener’s theory of how the continents moved, because of the geologic evidence. • Wegener’s theory was not proven in his lifetime because he died before he identified a plausible mechanism for the movement of continents.
What are mid-ocean ridges? • Undersea mountain ranges with steep, narrow valleys in the center. • Compared to sediment found farther from a ridge, sea-floor sediment closer to a ridge is thinner. • Compared to rocks farther from a ridge, rock closer to a ridge are younger. • The oldest ocean rocks are 175 million years old.
Sea-floor Spreading • Magma is molten rock • Paleomagnetism is the study of the magnetic properties of rocks. • Rift is a crack in the Earth’s crust. • Sea-floor spreading is the mechanism that causes the continents to move. • The process of sea-floor spreading is that magma rises from a rift in the ocean floor and fills it. As the ocean floor moves away, the magma cools and solidifies to form new rock that replaces the ocean floor.
The Giant Magnet • Earth is like a giant magnet because it has north and south geomagnetic poles. • The compass needle aligns with the field of magnetic force that extends from one of Earth’s poles to the other. • As magma turns to rock, iron-rich minerals align with Earth’s magnetic field. The magnetic orientation becomes permanent when the rock hardens. • Scientists have discovered rocks whose magnetic orientations point opposite to Earth’s current magnetic field.
Rocks with Magnetic Fields • Rocks with magnetic fields that point north have normal polarity. • Rocks with magnetic fields that point south have reversed polarity. • When scientists placed rocks into chronological periods they discovered alternating normal and reversed polarity. • The pattern of normal and reverse polarity in rocks enabled scientists to create the geomagnetic reversal time scale.
Magnetic Patterns • Scientists found that the striped magnetic pattern on one side of a mid-ocean ridge is a mirror image of the striped pattern on the other side of the ridge. • Alternating bands of normal and reversed polarity matching the geomagnetic reversal time scale. • Scientists think that as new sea floor forms, it records reversals in Earth’s magnetic field. • Scientists matched the magnetic patterns on each side of a mid-ocean ridge to the geomagnetic reversal time scale in order to assign ages to sea-floor rocks.
Rocks on the Sea Floor • The youngest rocks on the sea floor were at the center of the ridge. • The older rocks on the sea floor were farther away on either side of the ridge. • New rock forms on the sea floor at the rift in a mid-ocean ridge. • New rock forms at the center of a ridge and then moves away from the center in opposite directions. • The symmetry of magnetic patterns and the symmetry of ages of sea-floor rocks supports Hess’s theory of sea-floor spreading.
Wegener Redeemed • Scientists have found evidence of reversal patterns in rocks on the ocean floor and on land. • Continents move over Earth’s surface by the widening sea floor, which acts as a conveyor belt. • The mechanism that verifies Wegener’s hypothesis of continental drift is sea-floor spreading.
