The Theory of Plate Tectonics and Earth’s Interior

Alfred Wegener HOW do the plates move? Wegener hypothesized that the continents were moving. He called it continental drift. Sea-floor spreading validated his hypothesis BUT… HOW do the plates move?

Earth is like an oven…a really nice convection oven. Convection current – the continuous flow that occurs in a fluid because of differences in density Heat rises, cold sinks

Continental Drift Wegener argued that the continents had once been a supercontinent called Pangea. He supported his hypothesis with the following evidence: Fossils – Identical fossils of the Mesosaurus had be found in both South America and West Africa. Rock Formations – the ages and types of rocks in the coastal regions, such as S. America and W. Africa, closely matched. Climate – Geologists discovered layers of debris from ancient glaciers in S. Africa and S. America. These are now tropical climates.

Mid-Ocean Ridges Wegener did not have a mechanism by which to explain HOW the plates were moving. Nearly 20 years after his death, it was discovered how the plates moved. Scientists discovered that sediment closer to the mid-ocean ridge is younger than the sediment far away. Therefore, the sea-floor was spreading

Mid-Ocean Ridge and Sea-Floor Spreading

Convection So, heat fuels the movement of the plates, but where does all this heat come from? RADIOACTIVE DECAY!

Earth’s Interior Crust – what we live on! The thin, solid, and outermost layer of the Earth above the mantle Moho – the lower boundary of the crust Mantle Lithosphere – the solid, outer layer of the earth Asthenosphere – the solid, plastic layer of the mantle beneath the lithosphere. This layer flows slowly which allows tectonic plates to move on top of it. Mesosphere – “the middle sphere”; the strong, lower part of the mantle between the asthenosphere and the outer core.

The CORE The central part of the Earth Outer Core – Liquid Inner Core - Solid

Earth’s Interior

How do they know that? Scientists have used seismic waves to learn more about the interior of the Earth. Seismic waves are vibrations that travel through the Earth, and they are the result of earthquakes and explosions Changes in the speeds and direction of seismic waves were used to determine the locations and properties of the Earth’s interior.

The other processes at work Ridge Push Slab Pull Freshly formed rock at a mid-ocean ridge is warm and less dense than the older, more dense rock around it. So it is elevated, while the older more dense rock slopes downward away from the ridge. As the cooling rock sinks the asthenosphere below it exerts force on the rest of the plate As the lithosphere cools it becomes more dense. This density leads to subduction, as as the edge of the plate sinks, the rest of the plate is pulled behind it.

The Force(s) Tectonic plates are moved by THREE processes Convection cells Ridge Push Slab Pull

How do you figure? Distance = Rate x Time! In km, how far would a tectonic plate move in 2 millions years if it moved 4 cm/year. Distance = Rate x Time!