Seafloor Spreading

Discovering the Mid Ocean Ridge• In 1925, Germany outfitted a boat and set

out for two years to systematically and scientifically look at the oceans -- and to show the flag, since the Versailles Treaty had banned German navy boats from foreign ports.

• This expedition was the first use of closely spaced echo sounders to map deep sea topography and the first to reveal the extent of the sea floor's rugged terrain. The expedition also found that a continuous mountain-like ridge runs through the Atlantic to the southwest of Africa.

• The expedition also found that a continuous mountain-like ridge runs through the Atlantic to the southwest of Africa.

• That ridge, it was later discovered, extended through the major oceans of the world. It is now called the Mid-Ocean Ridge.

• In some places the canyon, called the Great Global Rift, came very close to land. The rift appeared to be breaks in the earth's crust, but perfectly fitted breaks, like joints made by a carpenter. The rift outlined chunks of the earth's crust, which were named tectonic (from a Greek word for "carpenter") plates.

Seafloor Spreading• Harry Hess was a geologist and Navy

submarine commander during World War II.• Part of his mission had been to study the

deepest parts of the ocean floor. In 1946 he had discovered that hundreds of flat-topped mountains shape the Pacific floor.

• The discovery of the Great Global Rift in the 1950s inspired him to look back at his data from years before.

• After much thought, he proposed in 1960 that the movement of the continents was a result of sea-floor spreading. In 1962, he added a geologic mechanism (seafloor spreading) to account for Wegener's moving continents.

Seafloor Spreading• It was possible, he said, that molten

magma from beneath the earth's crust could ooze up between the plates in the Rift Valley.

• As this hot magma cooled in the ocean water, it would expand and push the plates on either side of it -- North and South America to the west and Eurasia and Africa to the east.

• This way, the Atlantic Ocean would get wider but the coastlines of the landmasses would not change.

                                                                                

(Fig 1.11) Sea floor Spreading.

Seafloor Spreading Video Clip

•The weaknesses between the diverging plates fill with molten rock from below. Sea water cools the molten rock, which quickly turns to rock, and pushes the older rock to the outside. 

•This continuous process builds a chain of volcanoes and rift valleys called a Mid Ocean Ridge.

Seafloor Spreading

Subduction: process in which one lithospheric plate collides with and is forced down under another plate and moves back into the Earth's mantle.

• If new oceanic lithosphere is created at mid-ocean ridges, where does it go?

• The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere

• Thus, oceanic lithosphere is created at mid-ocean ridges and consumed at subduction zones, areas where the lithosphere sinks into the asthenosphere

Subduction• Earthquakes are

generated in the rigid plate as it is subducted into the mantle

• Magma generated along the top of the sinking slab rises to the surface to form volcanoes.

Paleomagnetism• lava gets erupted at the

mid-ocean ridge axis it cools and turns into hard rock

• As it cools it becomes permanently magnetized in the direction of the Earth’s magnetic field

• Earth’s magnetic field has reversed its polarity (direction) hundreds of times during the past several hundred million years

Paleomagnetism • A polarity reversal means that the magnetic

North flips to where we know the South Pole is

• At the mid-ocean ridge spreading axis, these flips in the direction of the Earth’s magnetic field are recorded in the magnetization of the lava

• This creates a symmetrical pattern of magnetic stripes of opposite polarity on either side of mid-ocean ridges

Polar Reversal Video Clip

Paleomagnetism

Review of Seafloor Spreading

New Rock is created by rising magma at the mid-ocean ridge

Old rock gets pushed away and sinks back into mantle