LESSON 2: RESTLESS CONTINEN TS Chapter 4: Plate Tectonics

Lesson Objective: Describe Wegener’s hypothesis of continental drift. Explain how sea-floor spreading provides a way for continents to move. Describe how new oceanic lithosphere forms at mid-ocean ridges. Explain how magnetic reversals provide evidence for sea-floor spreading.

SCIENTIFIC METHOD

Continental Drift: Wegener’s hypothesis all continents had once been joined together in a single landmass and have since drifted apart Pangaea – “all lands” – 300 million years ago How did other scientists react to this hypothesis?

5 types of Data support the Continental Drift Theory of A. Wegener

Continental Drift – the evidence Continents Glaciers Climate Fossils Rocks and Mountains

1. Fit of the continents Scholars and cartographers as early as the 1500s saw that Africa appeared to tuck in neatly against the coasts of North and South America In fact, all of the continents can be arranged to join with nearly no overlap or gaps. Coincidence? Wegener thought not. What evidence did Wegener have to support his hypothesis?

What other evidence did Wegener have to support his hypothesis? 2. Location of glaciations The passage of a glacier causes deep grooves to be carved into underlying rocks, called STRIATIONS. Glaciers also leave behind a distinctive sediment type, called TILL.

What other evidence did Wegener have to support his hypothesis? 2. Location of glaciations Using the distribution of till and striations that were created ~300 million years ago, Wegener saw that South America, southern Africa, India, Australia and Antarctica All experienced a glaciation!!! Today, these places are widely separated and tropical. The striations pointed to a common center of the glacial ice cap.

What other evidence did Wegener have to support his hypothesis? 3. Distribution of tropical climates Rock types such as coals, salts, desert sand dunes and reefs built by coral- like organisms indicate a tropical climate. These rock types are found exactly within the belt predicted by Wegener’s hypothesis.

What other evidence did Wegener have to support his Hypothesis? Today, certain animals live only on certain continents. Many fossil animals and plants exhibit a similarly restricted geographic range. If continents were all joined, then this would have provided an opportunity for plants and animals to disperse among joined continents. 4. Distribution of fossils

What other evidence did Wegener have to support his Hypothesis? 4. Distribution of fossils Once continents drifted apart, Similar species separated by large expanses of ocean.

What other evidence did Wegener have to support his Hypothesis? 5. Matching rock units & Mountain Belts Rocks that were created all at the same place and at the same time.

What other evidence did Wegener have to support his Hypothesis? 5. Matching rock units and Mountain Belts Similar rock, BUT separated by ocean basins. #1 –South American & Africa #2 – in a belt along the east coast North America, & Greenland West coast of Africa, Great Britain and Scandinavia

Sea-Floor Spreading supports the continental drift hypothesis Sea-Floor spreading process by which new oceanic lithosphere forms as magma rises toward the surface and solidifies.

Sea-Floor Spreading Mid-Ocean Ridges Mid-ocean ridges are underwater mountain chains that run through Earth’s ocean basins. Sea-Floor Spreading Mid-ocean ridges are located where seafloor spreading takes place.

Sea-Floor Spreading Moving Sea-floor collides into continental crust Subduction occurs when a denser (oceanic) crust moves under a less dense (continental) crust Subduction at “convergent” boundary

Sea-Floor Spreading Subduction form “trenches” Trench hemispheric-long, narrow depressions of the sea floor. deepest parts of the ocean floor! Some not measurable most important natural boundaries formed between two lithospheric plates.

Subduction form “trenches” A trench marks the position at which the denser slab begins to descend beneath another lithospheric slab (less dense). 3 to 4 km (1.9 to 2.5 mi) below the level of the surrounding oceanic floor. The deepest ocean depth to be sounded is Mariana Trench at a depth of 10,911 m (35,798 ft) below sea level.

Sea-Floor Spreading Sea-Floor Spreading Who How Where When

MAGNETIC FIELDS A field of attraction or repulsion to another substance based on its composition

MAGNETIC FIELDS Outer core of the Earth The molten iron flows creates an electric current (flow of electrons) & a magnetic field that surrounds the planet. The magnetic field created in the center of the planet makes your compass point north Protects the Earth from solar winds.

MAGNETIC FIELDS Who How Where When

MAGNETIC FIELDS

Magnetic Strips along the Mid-Ocean Ridge The older crust moves outward. Dates are confirmed by the Magnetic Reversal of the crust.

Earth Today Earth Today

Earth tomorrow Earth tomorrow

RESOURCES projects/project_scientific_method.shtml projects/project_scientific_method.shtml rench.htm rench.htm Continental_Drift.pptx Continental_Drift.pptx