Seafloor Spreading Chapter 10:2 (pg 304-306) SPI 0707.7.6

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

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?

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

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)

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

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

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?