1. Seafloor Spreading
Chapter 10:2 (pg )
SPI

2. Learning Objectives Explain seafloor spreading.
Recognize how age and magnetic clues support seafloor spreading.

3. Mapping the Ocean Floor
How was the ocean mapped before WWI?
After 1940s & 1950s sound waves were used
Longer it takes for waves to bounce off the ocean floor and back = greater the depth
Using this technology researchers discovered an underwater system of mountains & valleys like those found on continents
Some of these ridges are long rift valleys where volcanic eruptions and earthquakes occur
Pacific, Atlantic, & other oceans have mid-ocean ridges
How did they form?

4. The Seafloor Moves?!?!
Harry Hess (1960s – Princeton University) suggested seafloor spreading
Proposed the hot, less dense material below Earth’s crust rises towards the surface at the ridges
Flows sideways carrying seafloor away from ridge in both directions
Magma is forced upward and flows from the cracks
It becomes solid as it cools and forms new seafloor
As it moves away from the mid-ocean ridge, it cools, contracts, becomes denser & the cooler seafloor sinks to form the ridge

6. Evidence for Spreading
Glomar Challenger was a ship used to gather info about rocks on the seafloor
Drilled into floor to obtain rock samples
Younger rocks were found at ocean ridges
Older rocks were found in distances farther away from ridges
New seafloor features and life- forms also discovered
Heat & chemicals brought up with lava provide environment for exotic ocean life (giant clams, mussels, & tube worms)

7. Magnetic Clues Earth’s magnetic field has a north & south pole
Magnetic lines or directions of force leave Earth near the south pole and enter Earth near the North Pole
During a reversal the lines of magnetic force run the opposite way
Scientists have determined this has happened several times
Happens in intervals of thousands or even millions of years
Recorded in rocks forming along mid-ocean ridges

9. Magnetic Time Scale
Iron-bearing minerals (magnetite) found in seafloor rocks can record magnetic field direction when they form
When fields reverse minerals show change occurring in the reversal
Magnetometer shows this reversal in alignment over time in strips parallel to the ridges
Strong magnetic readings are recorded when polarity of rocks are the same as Earth’s magnetic field today
Normal polarities show up as large peaks
Reversals show new rock forming at ridges
Helped explain how crust could move – something the continental drift theory could not do

10. Review Questions
What properties of iron-bearing minerals on the seafloor support the theory of seafloor spreading?
How does the age of rocks on the ocean floor support the theory of seafloor spreading?
How did Hess’s hypothesis explain seafloor spreading?
Why does some partly molten material rise toward Earth’s surface?