1. Ch. 17 Sec. 2
Sea Floor Spreading

2. Technology
Until the mid 1900’s, scientists believe that the ocean floor was flat
Sonar helped change the ideas and map the ocean floor
Sound waves bouncing back from the floor
Magnetometer detects small changes in magnetic fields

3. Ocean floor Topography
Maps made from sonar & Magnetometer really stunned scientists
Noticed many trenches and volcanoes
Common along ocean ridges
Mariana Trench: 11 km deep

4. Ocean Rocks Scientists collected samples of the deep-sea sediments
1st: Ages of rocks vary from place to place
Rocks near ocean ridges are younger that those taken by trenches
2nd: Thickness of ocean-floor sediments is less than predicted
Continental Crust is extremely thick, 20 km or so
Thickness of the sediments increased further from ridges

5. Magnetism Oceanic rock contains iron-bearing materials
Paleomagnetism: study of magnetic record
Basalt provides an accurate record of ancient magnetism
As Basalt cools, materials become parallel with the ridges
Magnetic Reversal: change in earth’s magnetic field
As the sea floor spreads, so too the magnetic field is reversed
Long change in earth’s magnetic field is Epochs

6. Magnetic Symmetry Noticed a series of + and – strips
Help to create Isochron Maps
Line on a map that connects points that have the same age

7. Sea-Floor spreading Harry Hess proposed the theory
New ocean crust formed at the ocean ridges and destroyed and deep trenches
Magma forced up, hotter & less dense than mantle material, fills the gap created at the ocean ridges
Sea-floor spreading was the missing link for Wegener’s Continental Drift theory
Answers the how question, but not the why