Chapter 10-1 Continental Drift

CONTINENTAL DRIFT THEORY- Wegener’s Hypothesis CONTINENTAL DRIFT THEORY- Hypothesis that a large single landmass broke up into smaller landmasses to form the continents, which then drifted to their present location.

Wegener’s Evidence Fossil evidence- Rock formation- Climatic evidence-

MID-OCEAN RIDGES Undersea mountain ranges through the center of which run steep, narrow valleys. Forms as magma rises from the asthenosphere and creates new lithosphere as tectonic plates move apart.

Mid-Ocean Ridge

Mid-Atlantic Ridge

About Mid-Ocean Ridges The sediment that covers the sea floor is thinner closer to the ridge than it is farther away from the ridge. Close to ridge – thin layer of sediment Farther from ridge- thicker layer of sediment

Closer to the ridge- younger sea floor And more…….. Closer to the ridge- younger sea floor Farther from the ridge- older sea floor Ocean floor rocks are less than 200 million years old!

Sea Floor Spreading Harry Hess’ s hypothesis At the center of the mid-ocean ridge is a RIFT. Molten rock rises to fill the crack. As the ocean floor moves away from the ridge, magma cools and solidifies creating new rock that replaces the ocean floor. Named Sea Floor Spreading by Robert Dietz

If the sea floor is spreading, is the Earth getting bigger? NO

In the mid 1960’s more evidence for sea floor spreading was found. PALEOMAGNETISM- THE STUDY OF THE ALIGNMENT OF THE MAGNETIC MINERALS IN ROCK, specifically as it relates to the reversal of the Earth’s magnetic poles; also the magnetic properties that rock acquires during rock formation.

Paleomagnetism Earth acts as a giant magnet Earth has north and south geomagnetic poles As magma solidifies to form rock, the iron-rich minerals in the magma align with Earth’s magnetic field, like a compass. When the rock hardens this magnetic orientation becomes permanent.

GEOMAGNETIC REVERSAL TIME SCALE Evidence shows Earth’s magnetic field has not always pointed north Rocks were discovered whose magnetic orientations point opposite to Earth’s current magnetic field. Rocks with magnetic fields pointing north have NORMAL POLARITY Rocks with magnetic fields pointing south have REVERSED POLARITY.

Scientists placed these periods of reversal in chronological order and discovered an alternating pattern

Scientists discovered reversals in rocks on land also. They matched the pattern of reversal with the ages, they were confident that magnetic patterns show changes over time. CONCLUSION: Sea floor spreading provides the mechanism for the continents to move over Earth’s surface.