The Mechanism for Continental Drift Seafloor Spreading The Mechanism for Continental Drift

Learning Targets: Explain how mantle convection, ridge push, and slab pull drive the movement of lithospheric plates.

Magnetic Evidence Rocks record the magnetic field of the earth from when they first cool What is cooling of magma called? The mineral Magnetite forms. Magnetic minerals = compasses Align with the poles in molten rock Locked in place once rock crystallizes

Magnetic Evidence Rocks less than 800,000 years old point toward North Pole Rocks 800,000 to 2,500,000 years old point in opposite direction Normal and reversed polarity

Magnetic Reversals

Magnetic Evidence Rocks from different continents did not agree on location of the poles Either the poles moved, the continents moved, or both Only the continents moved while the poles remained stationary

Magnetic Evidence Continents seemed to move over history Wegener’s theory began to become popular Still no mechanism for the movement or explanation about how continents could push through oceans

Seafloor Spreading Mechanism for continental drift Continental Drift + Seafloor Spreading = Plate Tectonics Discovered due to WWII research Echo locating submarines Bathymetric map of the seafloor Magma that is brought to the surface of Earth at mid-ocean ridges and pushes older oceanic crust out of the way results in seafloor spreading.

Seafloor Spreading Seafloor is not all flat Underwater features Abyssal plain - flat parts Underwater features Mountain chains: Mid-ocean ridges Valleys: Trenches Volcanoes: Seamounts This is what we found out

Mid-Ocean Ridge System

Seafloor Magnetism Magnetometers Search for submarines in war Discovered magnetic properties of seafloor All ocean basins have stripes of magnetic reversals Magnetic stripes are formed symmetrically on either side of mid-ocean ridges as a result of a reversal of the Earth’s magnetic poles.

Magnetic Striping Same pattern on both sides of the mid- ocean ridge All ridges have current polarity No striping on the continents Rocks farther from ridge have more sediment What does this prove? Proves rocks are older

Ages of the seafloor Samples prove age of rocks Oldest crust on earth Close to ridge: young rocks Far from ridge: old rocks Oldest crust on earth Oceanic: 180,000,000 years old Continental: 4,000,000,000 years old Oceanic crust is youngest at mid-ocean ridges.

Seafloor Spreading ~1960 seafloor spreading hypothesis Buoyant magma comes to the surface at mid-ocean ridges Cools to form new seafloor Seafloor must expand to make room How does this explain magnetic striping and the age of the seafloor?

Seafloor Spreading Seafloor acts like giant conveyor system that moves the plates This is the mechanism Wegener never discovered Continents don’t drift through the oceans; oceans push the continents around

Seafloor Spreading New crust is being made, but the earth is not getting bigger What does this mean? Seafloor eventually gets pushed under the continents or other seafloor Subduction