Continental Drift

Continental Drift The theory of Continental Drift was first proposed by Alfred Wegener in 1912. Wegener noticed that the shorelines of the continents seemed to ‘fit together’ like the pieces of a giant jig saw puzzle. Wegener’s Hypothesis stated: The continents were once all together in one place forming a supercontinent, Pangaea. The continents broke apart and drifted to their present locations.

Pangaea

Clues to Pangaea Fossils

Mesosaurus a freshwater reptile found on both South America and Africa

Fossils of Glossopteris a tropical plant found in Antarctica, South America, Australia, and Africa

Rock clues Rock sequences (meaning he looked at the order of rock layers) in South America, Africa, India, Antarctica, and Australia show remarkable similarities. Wegener showed that the same three layers occur at each of these places.

Similar Rock layers

Mountain rock age evidence

Continental Drift: Rock Ages Even before geochronology, the relative framework of rock ages showed strong correlation across the Atlantic, as did mountain ranges of similar age.

Climate Changes

Wegener’s quest ends 1930

Sea Floor Spreading During WWII Capt. Harry Hess (a geologist in civilian life) used a new technology called “SONAR” an acronym for “Sound Navigation and Ranging” With sonar he discovered the ocean bottom is not flat, that it has many features on it like land. He discovered flat-topped submarine volcanoes he named Guyots, and ocean ridges that lead to the theory on seafloor spreading. After the war Harry Hess returned to Princeton University and continued his studies using sonar and in 1962 published his theory on Seafloor spreading supporting Wegener’s Theory on Continental Drift.

Evidence to support Seafloor spreading In 1960, Harry Hess studied Wegener’s theory. Hess proposed the radical idea that the ocean floors move like a conveyer belt, which in turn move the continents. This movement begins at the mid-ocean ridge, which forms along in a crack in the oceanic crust. At the mid-ocean ridge, molten materials rise from the mantle and erupts. The molten material spreads out, pushing older rock to both sides of the ridge.

Seafloor Spreading Convective Current is the movement of rising hot mantle material then it cools and sinks Creating a movement of mantle that pulls the crust and causes the plates to move.

Mid-Ocean Ridges

Seafloor spreading

Iceland a Mid-Ocean Ridge lab

Evidence for Seafloor spreading: Magnetic Reversals

Magnetic Reversal

Magnetic Reversal The Earth is like a giant magnet with a north and south pole. The Earth’s magnetic poles reversed themselves ever so often, last time was 780,000 years ago. Rocks on the ocean floor are in a pattern of magnetized stripes. These stripes show when the Earth reversed it’s magnetic field.

Detecting Magnetic reversal in the mid-ocean ridge with a magnetometer

Evidence from Drilling Samples The Glomar Challenger is a drilling ship that recovered drilling samples from the ocean floor. They studied the age of the rocks sampled. They found that the farther from the ridge, the older the rock. The youngest rocks were at the center of the ridge.

Glomar Challenger 1968-1983

Summary Wegener hypothesized that continents drift apart from one another and have done so in the past. Clues to support Continental Drift of Pangaea are fossils, climate change, similar rocks structures. Harry Hess supported Wegener’s hypothesis with his theory on seafloor spreading. Seafloor spreading creates new ocean floor, and the further you move away from an mid-ocean ridge the older the rock.