1. Plate Tectonics
Seafloor Spreading

2. Technology Advances (WWII & Post WWII)
Echo sounding
Sonar → for ocean depth measurements
Magnomometer
Detects small changes in magnetic fields
Ships towing magnomometer record magnetic fiels strengths of ocean floor rocks

3. How did this technology revive Continental Drift?
Sonar -allowed ocean floor maps to be made
Mountain chains found
Deep sea trenches found

6. How did this technology revive Continental Drift?
Ocean rock & sediment samples
Rock ages vary along ocean floor
Youngest rock closest to ridges
Oldest rock farthest from ridges
Ocean rock much younger (on average)than continent rock

8. How did this technology revive Continental Drift?
Sediments
Thinnest near ridges
Thickest farthest from ridges
Ocean sediments, in general, much thinner than expected
Since continental crust erodes & much is carried to the oceans, the ocean sediments should be extremely thick

9. How did this technology revive Continental Drift?
Magnetism
Ocean floor rock
Basalt
Mafic
High in iron
Iron minerals align parallel to Earth’s magnetic field
Like a compass needle
Minerals freeze that way
Become a record of the magnetic field
Normal polarity
minerals aligned same as today, causing strong reading
Reverse polarity
minerals aligned opposite today, causing weak reading (+, - cancel each other)

10. How did this technology revive Continental Drift?
Study of ocean floor magnetic polarity shows a stripe pattern on the ocean floor
The patterns (*age & polarities) are symmetrical on the two sides of the Mid-Atlantic Ridge
*Age of reversals were compared with age of magnetic record on land

14. New Theory: Seafloor Spreading
Harry Hess – American scientist
States that new ocean crust is formed at ocean ridges & destroyed at deep sea trenches.

15. How is Crust Formed?
Magma (less dense than solid rock) moves up to fill gap left as seafloor spreads apart.
Magma cools & hardens, adding to ocean floor between continents

16. Missing Link
Hess’s Seafloor Spreading explains how the continents move → they are pushed apart as new seafloor is added.
Hess’s Seafloor Spreading + Wegener’s Continental Drift = current Theory: Plate Tectonics