Ch. 17 Sec. 2 Sea Floor Spreading

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

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

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

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

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

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