Plate tectonics and Continental Drift
1915: Continental Drift
Introduced by Alfred Wegener
Suggested that all the continents were together at one point in time and have moved to their current location
Gave lots of evidence to support this hypothesis
Wegener’s Evidence
Fit of the continents
Fossils
Mesosaurous
Glossopteris
Rock types and structures
Paleoclimates & glacial deposits
Great Evidence but what was missing?
Glomar Challenger (1960’s and 70’s)
Deep sea drilling project
Multibeam sonar, remote sensing and submersibles
Volcanic mountain chains in the middle of the oceans
Paleomagnitism
Additionally:
Magnetic signatures of rocks didn’t match where they were
Polar wandering or continents moving
Age of the crust
Leads to theory of Plate Tectonics
Earth’s crust is broken into several plates
Plates interact with each other in different ways
Collide
Slide by
Move away
Divergent Boundaries
Plates move away from each other
Volcanic mountain chain creates new ocean crust (basalt)
Known as a ridge or rift and has a valley in the center
Usually located in the middle of the oceans…exceptions?
What does this imply?
Continental break up can occur
Pangaea?
How does this happen
Stages of continental break up
Upwarping of continent due to heat
Rift valley forms as crust thins
East African Rift Zone
A small linear sea forms
Red Sea and
An ocean and Mid-ocean ridge form
Atlantic
Transform Boundaries
Plates slide by each other
Associated with divergent boundaries
Off set divergent boundaries
Convergent boundary
Plates come together
3 different outcomes depending on type of crust involved
REVIEW:
Continental Crust vs Oceanic crust…which is the thickest? Which is the most dense?
Continental to Oceanic Convergence
Continental crust is lighter and thicker so therefore forces oceanic crust underneath
Subduction zone is formed
Volcanic mountain chain forms on the over riding plate (continental volcanic arc)
Accretionary wedge forms
Oceanic to Oceanic convergence
Older crust is more dense so it is subducted under younger crust
Volcanic chain forms on over riding crust (island arc)
Continental to Continental convergence
Neither plate subducts….VERY large mountains are created
Why is there no really old oceanic crust?
Driving Mechanisms for Plate Tectonics
Examine the uppermost region of the asthenosphere to understand what is happening
Melt zone represents “weak” area and this zone allows crust to “detach” from the asthenosphere and move around on the surface
How did motion begin?
Heat generated inside of earth began the motion
Radioactive decay
Primordial heat
Convection cells in the asthenosphere
Primary mechanism is slab pull
Due to weight of crust as it is “pulled” into earth at subduction zones
Look at rate of motion in Pac
and
Lesser Mechanism is ridge push
Gravitational “push” of material piling up at ridges
How do Plate boundaries relate to earthquakes and Volcanoes
How do we know the rate and direction of plate motion?
Hot spots/ Mantle Plumes
The Wilson Cycle