Earth Science Notes

Seafloor Spreading

PT and Seafloor Spreading

• The PT Theory was not accepted at first because there was no explanation for how the continents moved.– Scientist did not know yet about convection

cells or the information we discussed in our last lecture.

• After a while, evidence began to mount for seafloor spreading. This provided an explanation for what moved the continents.

Evidence for Seafloor Spreading:Magnetic Reversals

• During World War II, sensitive instruments called magnetometers were developed to help detect steel-hulled submarines. When research scientists used magnetometers to study the ocean floor, they discovered a surprising pattern. Measurements of magnetic variations showed that, in many areas, alternating bands of rocks recording normal and reversed polarity were arranged symmetrically about mid-ocean ridges

Evidence for Seafloor Spreading:Magnetic Reversals

• Magnetic Reversals– The Earth has a magnetic north pole and a

magnetic south pole.– These poles have flipped periodically in

Earth’s past.• This means the north pole has become the south

pole and vise versa.

– The record of the reversal of the magnetic field is preserved in magnetic rocks which lie along the ocean floor.

Evidence for Seafloor Spreading:Magnetic Reversals

• Magnetism in the Earth is created by – Magnetic material – Currents

• A bar of iron can be made into a magnet by wrapping it with wires and running a current through the wires.

Evidence for Seafloor Spreading

Evidence for Seafloor Spreading

Evidence for Seafloor Spreading: Age of Rock

• Rocks samples taken near the Mid-Ocean ridges show that rock to be the youngest on Earth.

• Rocks increase in age as the get farther away from the mid-ocean ridges.

Evidence for Seafloor Spreading: Age of Rock

Evidence for Seafloor Spreading

Summary

Lets talk about the main points of what we just discussed.