Chapter 4, Section 1 Objectives:

1

• Chapter 4, Section 1 Objectives
• 1. Explain Wegeners hypothesis of continental
  drift.
• 2. List evidence for Wegeners hypothesis of
  continental drift.
• 3. Describe seafloor spreading.

2

• Continental Drift (1912) the hypothesis
  proposed by Alfred Wegener, which stated that the
  continents had moved.

3

• Pangaea (all lands) a single land mass formed by
  all of the continents.
• Began to break up into smaller continents about
  200 million years ago.
• Panthalassa (all seas) the huge ocean that
  surrounded Pangaea.

4

• Pangaea (all lands) a single land mass formed by
  all of the continents.
• Began to break up into smaller continents about
  200 million years ago.
• Panthalassa (all seas) the huge ocean that
  surrounded Pangaea.

5

• Pangaea (all lands) a single land mass formed by
  all of the continents.
• Began to break up into smaller continents about
  200 million years ago.
• Panthalassa (all seas) the huge ocean that
  surrounded Pangaea.

6

• Evidence of Continental Drift
• 1. Fossil links between Africa and South
  America

7

• Evidence of Continental Drift
• 2. Geological
• Age and type of rocks match
• Mountain ranges fit together

8

• Evidence of Continental Drift
• 3. Climatic
• -Matching layers of debris from glaciers in areas
  that are warmer today
• Fossil evidence (coal deposits) indicate a
  matching tropical or subtropical swamps in the
  northern hemisphere
• 4. Seafloor spreading
• 5. Paleomagnetism
• Despite evidence, most scientists disbelieved the
  hypothesis!

9

• Seafloor Spreading (1947) Magma or molten rock
  welling up through rift in center of mid-ocean
  ridge, as ocean floor moved away from both sides
  of ridges.

10

• Mid-Atlantic Ridge an undersea mountain range
  with a steep, narrow valley running down its
  center.
• Mid-Ocean Ridges 65,000 km long system of
  undersea mountain ranges that wind around the
  Earth.

11

• Evidence of seafloor spreading
• Age of rocks
• Oldest oceanic rocks no more than 150 million
  years old
• Oldest continental rocks are about 4 billion
  years old

12

• Paleomagnetism of the ocean floor (1960s)
• Magma cools and iron-bearing minerals become
  magnetized
• Orientation becomes permanent pointing north

Normal magnetic polarity Points north
13

• Paleomagnetism of the ocean floor (1960s)
• Scientists have found bands pointing south
• Earths magnetic field must reverse itself
• Verified by dating of rocks

Reverse magnetic polarity Points south