Ch. 7 Less. 1 Cont.

Clues supporting Wegener’s Continental Drift Theory cont. Volcanic Rock Rock Clues 1. Volcanic rock that is identical in chemistry and age has been found on both the western coast of Africa and the eastern coast of South America.

2. The Caledonia mountain range in northern Europe and the Appalachian Mountains in eastern North America are similar in age, structure, and rock type. If you put the puzzle pieces together, it appears that both mountain ranges formed one long mountain range.

What was missing? Although Wegener had a lot of evidence, scientists were still skeptical. Why? He lacked data. At the time, he could not measure how fast the continents moved. He could not explain the forces that made the continents move. The crust and lithosphere are solid rock. What kind of force is strong enough to make them move?

The evidence to prove continental drift would come much later, after Wegener’s death.

Review

What term did Wegener use to describe the constant motion of continents on the surface of Earth? A. Pangaea B. continental drag C. continental movement D. continental drift

What term did Wegener use to describe the constant motion of continents on the surface of Earth? A. Pangaea B. continental drag C. continental movement D. continental drift

In which of these did Wegener observe similarities that suggested continents might fit together like the pieces of a puzzle? A. fossils B. ice sheets C. plates D. continental coastlines

In which of these did Wegener observe similarities that suggested continents might fit together like the pieces of a puzzle? A. fossils B. ice sheets C. plates D. continental coastlines

Wegener proposed that 250 million years ago, South America, Africa, India, and Australia were located closer to what? A. Equator B. South Pole C. North Pole D. Pacific Ocean

Wegener proposed that 250 million years ago, South America, Africa, India, and Australia were located closer to what? A. Equator B. South Pole C. North Pole D. Pacific Ocean

Which of these could be evidence that distant continents were once connected? A. similar fossils found on them B. ice sheets on some but not on others C. plates are made of crust and upper mantle D. each continent has its own distinct climate pattern

A. similar fossils found on them Which of these could be evidence that distant continents were once connected? A. similar fossils found on them B. ice sheets on some but not on others C. plates are made of crust and upper mantle D. each continent has its own distinct climate pattern

Lesson 2 Development of a Theory The evidence of seafloor spreading would prove the theory of continental drift.

Mapping the Ocean Floor During World War II, the navy used a device called an echo sounder (sonar) to locate enemy ships. After the war, scientists began using this to map the seafloor.

An Echo Sounder produces sound waves that travel from the ship to the seafloor. The waves bounce off the seafloor and back to the ship. The deeper the ocean, the longer it takes to do this. So scientists can determine depth and create topographic maps of the seafloor.

The topographic maps shows vast mountain ranges on the seafloor, longer than any mountain range on land. Mid-ocean ridges are the mountain ranges in the middle of the oceans.

Seafloor Spreading In the 1960s, scientists discovered that the seafloor spreads along these ridges. The mantle below melts, forming magma. The magma is less dense than solid mantle rock, so it rises through cracks in the crust along the ridges. It cools really fast in the cool water, forming “pillow lava” made of the volcanic rock basalt. Magma is called lava when it erupts.

As the seafloor continues to spread, the older crust moves away from the ridge. The crust is youngest near the ridge.

Topography of the Seafloor Rugged mountains form on the seafloor. How? Large amounts of lava builds up along the ridge. As the lava cools and forms new crust, it cracks. Rocks move up and down along the cracks, forming jagged mountains.

Over time, sediment accumulates on top of the oceanic crust farther away from the ridge. As it build up, it makes the seafloor smoother – an abyssal plain. Close to the ridge, there is almost no sediment.

Evidence for Seafloor Spreading The theory of seafloor spreading sounds good, but we need evidence of that too! Basalt on the seafloor contains iron-rich minerals that are magnetic. Magnetic minerals align with the Earth’s magnetic field. When the erupted lava cools and crystallizes, it lines up with the Earth’s magnetic field.

The direction of the Earth’s magnetic field is not constant The direction of the Earth’s magnetic field is not constant. Sometimes, it reverses direction. It changes over time, but it takes a very long time. Normal Polarity – magnetized objects orient north – a compass points north. Today’s magnetic field is Normal.

Magnetic Reversal – magnetic field reverses direction Reversed Polarity – magnetized objects orient south – a compass points south

Scientists have found parallel patterns in the magnetic structure of the rocks on either side of a ridge. When the volcanic basalt came out of the ridge, its magnetic minerals aligned with the Earth’s magnetic orientation at the time.

The pattern of the stripes confirm the theory because each pair on either side of the ridge has similar composition, age, and magnetic character. The left side of the ridge matches the right side. The older stripes are farther from the ridge.

More Evidence of Seafloor Spreading 2. Temperature readings below the ocean’s surface - made by drilling holes in the oceanic crust and taking its temperature – show that there is more thermal energy near the ridges than on the abyssal plains. Thermal energy = heat energy Hot Not Hot

3. Sediment farther from the ridge is thicker and older than near the ridge. Thick sediment No sediment

The Proof Now we have the proof for continental drift. Continents do not move through the solid rock of the mantle. Instead, they move as the seafloor spreads along a mid-ocean ridge.

Review

As the seafloor continues to spread apart, the older oceanic crust moves in which direction with respect to the mid-ocean ridge? A. toward it B. above it C. away from it D. under it

As the seafloor continues to spread apart, the older oceanic crust moves in which direction with respect to the mid-ocean ridge? A. toward it B. above it C. away from it D. under it

On the seafloor, the sediment is thicker ________ the mid-ocean ridge? A. toward B. above C. away from D. under

On the seafloor, the sediment is thicker ________ the mid-ocean ridge? A. toward B. above C. away from D. under

On the seafloor, the thermal energy is highest _________ the mid-ocean ridge. A. toward B. above C. away from

On the seafloor, the thermal energy is highest _________ the mid-ocean ridge. A. toward B. above C. away from

Today’s magnetic field is described as having which of these? A. reversed polarity B. normal polarity C. magnetic reversal D. no polarity

Today’s magnetic field is described as having which of these? A. reversed polarity B. normal polarity C. magnetic reversal D. no polarity

In which state do magnetized objects reverse themselves to point south? A. reversed polarity B. normal polarity C. seafloor spreading D. magnetic polarity

In which state do magnetized objects reverse themselves to point south? A. reversed polarity B. normal polarity C. seafloor spreading D. magnetic polarity

Which statement is true? A. Continents move through the solid rock of the mantle. B. Continents move as a result of seafloor spreading. C. Seafloor spreading does not support the Continental Drift Theory.

Which statement is true? A. Continents move through the solid rock of the mantle. B. Continents move as a result of seafloor spreading. C. Seafloor spreading does not support the Continental Drift Theory.