1. Seafloor Spreading

2. 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

3. 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.

5. 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

7. 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

8. 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

9. 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

10. 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

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