Video 9: What Drives Plate Tectonics

what forces drive plate motion?

1. slab pull 2. slab suction 3. ridge push Slab pull and its associated force, slab suction and ridge push are forces put into motion by convection in the mantle

Evidence to support sea-floor spreading and plate tectonics

All evidence for continental drift + parallel magnetic anomalies record changes in earth's magnetic polarity as sea floor created Age of ocean floor increases away from crest of mid-ocean ridge (--> leads to thickness of seafloor deposit increases as time for accumulation of the sediments to be deposited increases) Earthquake and volcano distribution --> heat flow and volcanic activity tends to decrease with increasing distance away from ridge Earthquake focal mechanisms Oceans younger than continents --> Oldest ocean floor - 200 million years old

Age of ocean floor increases away from crest of mid-ocean ridge

If sea floor spreading does indeed occur, then the youngest seafloor rocks would be atop the mid-ocean ridge, and the ages of rocks would increase on either side of the ridge in a symmetric pattern. Ocean floor is the youngest along the mid-ocean ridge, where new ocean floor is created, and the age of rocks increases with increasing distance in either direction away from the axis of the ridge. The symmetric pattern of ocean floor ages confirms that the process of seafloor spreading must indeed be occurring.

The mantle is convecting

That means there is a very slow creeping motion of earth's solid sillicate mantle caused by convection currents carrying heat from the earth's core to the surface -Convection in the mantle drives plate motion. -The Earth is cooling slowly over time, and the heat loss results in convection in the mantle, the rise of heat and the sinking of cooler material. This is driven by numerous forces, but strong factors are believed to be slab pull and slab/mantle suction—processes that essentially suck down the oceanic lithosphere. As the oceanic lithosphere is sucked down at the trenches, it is pulled apart at ridges, and the pressure is relieved below the ridge, which allows magma to rise to the surface. As this material rises, it forms new crust. The crust is continually pulled and pushed, and as it is, the lithospheric plates shift accordingly

What causes abrupt shifts in plate tectonics?

The abrupt shifts in plate tectonics likely occur when subducting slabs disconnect from their plate because of thick crustal plugs and weakened mineral grains. The thick crustal plugs and weakened mineral grains give a mechanism for why the subducting slabs disconnect and then accelerate change in plate motion. Thick continental/oceanic crust (crustal plugs) gets stuck in the subduction zone, plugging it up. Mineral grains in this plug shrink, which causes it to rapidly weaken. The stuck slab then breaks off and subducts, allowing the rest of the tectonic plate to abruptly shift horizontally

slab suction

Which is created as a subducting plate drags against the viscous mantle and causes the mantle to flow in toward the subduction zone, Thereby sucking in nearby plates much in the same way pulling a plug from a full bathtub draws floating objects toward its drain.

slab pull

Which is generated by the pull of the weight of a plate as it sinks underneath an overlying plate, pulling the rest of the plate behind it in a similar fashion to how a heavy comforter often slides off a bed onto the floor,

Oceans younger than continents

new ocean floor is created at the mid-ocean ridge by sea floor spreading and moves off the ridge to eventually be subducted and re-melted in the mantle. In this way, the ocean floor keeps regenerating itself. The floor beneath the oceans today is not the same one that existed beneath the oceans 4 billion years ago.