1. Continental Drift and Seafloor Spreading Notes
Part 2

2. Sea Floor Spreading
Hess combined his observations with the earlier ideas of Wegener into the concept of sea floor spreading, which lead to plate tectonics.

3. Can you explain this diagram!?
Animation of sea floor spreading
Can you explain this diagram!?

4. Find the Mid-Ocean Ridges

5. Evidence of Sea-Floor Spreading
1. Evidence from molten material:
Scientists used Alvin (a small submarine) to explore the mid-ocean ridges, 4km deep.
They found rocks shaped like pillows, giving evidence of quickly hardened molten material.

6. Evidence of Sea-Floor Spreading Animation: http://www. wwnorton
2. Evidence from magnetic stripes:
Magnetic stripes can be found in the ocean floor.
These stripes show a change in the Earth’s magnetic poles.
Iron bits lined up in the direction of Earth’s poles and when the rock cooled, it locked the iron bits in place.
Paleomagnetism – Study of Earth’s magnetic field as it existed in the past

7. What do we mean by a magnetic reversal of the Earth?
The Earth has a magnetic field, as can be seen by using a magnetic compass. We can say that there are currently two poles observed on the surface of the Earth, one in the Northern hemisphere and one in the Southern hemisphere.
By magnetic reversal, or 'flip', we mean the process by which the North pole is transformed into a South pole and the South pole becomes a North pole.
How often do reversals occur?
The Earth's magnetic field has undergone numerous reversals of polarity. We can see this in the magnetic patterns found in volcanic rocks, especially those from the ocean floors. In the last 10 million years, there have been, on average, 4 or 5 reversals per million years. Reversals are not predictable.
Is the Earth's magnetic field reversing now?
Measurements have been made of the Earth's magnetic field more or less continuously since about Some people believe the field may be in the early stages of a reversal.
reversal.html#lesson