Seafloor Spreading

Harry Hess Professor of Geology at Princeton University While serving in the US Navy during WWII, he had the opportunity to use sonar technology to map the ocean floor across the North Pacific Ocean. In 1962 he published a theory that could explain how the continents could actually drift. This theory later became known as “Seafloor Spreading”. What he discovered – Oceans were shallower in the middle Identified that the presence of Mid Ocean Ridges raised above the flat seafloor by as much as 1.5 km Deepest parts of the ocean (trenches) were very close to the continental margins

Layers of the Crust and Mantle Lithosphere – the outermost, rigid layer of the Earth. Two parts: crust and rigid upper part of the mantle Divided into pieces called tectonic plates Asthenosphere – the slow-moving, layer of the mantle on which pieces of the lithosphere move. Made of solid rock that flows very slowly. The movement of the lithospheric plates and the convection currents will be covered more in-depth during the next essential question.

Sea Floor Spreading Theory Process by which new oceanic lithosphere forms as magma rises toward the surface and solidifies. As the tectonic plates move away from each other, the sea floor spreads apart and magma fills the gap As this new crust forms, the older crust gets pushed away from the mid-ocean ridge

Seafloor spreading and tectonic activity processes are the result of mantle convection Slow, churning motion of Earth’s mantle Convection currents carry heat from the lower mantle and core to the lithosphere Convection currents also “recycle” old lithosphere at subduction zones

Evidence that the ocean floor is spreading 1st Piece of Evidence – Rock Age The oldest ocean floor rocks ever found are 180 million years old. Earth is about 4.5 billion years old Proof that the ocean floor is recycled at subduction zones

2nd Piece of Evidence – Drilling and the Age of the Ocean Floor Underwater drilling during the 1960s and 1980s produced accurate data of seafloor age Rocks closest to the mid-ocean ridge are younger than the rocks found further from the ridge New rocks are formed at the ridges and push older rocks away from the ridge

3rd Piece of Evidence – Magnetic Striping Paleomagnetism – the study of magnetism in ancient rocks Magma contains iron, which is magnetic, so the atoms in the iron line up with Earth’s magnetic north pole Earth’s magnetic poles switched directions about 800,000 years ago Geologists discovered that the ocean floor was made of parallel bands of crust with alternating magnetic directions – north and south alternated Magnetic directions were symmetrical at the mid-ocean ridge

Magnetic Domain - https://youtu.be/hK_Yi5nxuKQ?t=90