Post on 28-Dec-2015
Plate TectonicsPlate TectonicsEvolution of the Earth
and Our Restless Planet: Chapter 33
Review: InteriorReview: Interior
33.1 The Theory of Continental Drift
33.1 The Theory of Continental Drift
• Scientists in the early 20th century believed– Oceans and continents were fixed in place– The surface is a skin spread over a cooling
interior– Cooling caused contraction of the skin=
wrinkles and bumps aka mountains and valleys
• Scientists in the early 20th century believed– Oceans and continents were fixed in place– The surface is a skin spread over a cooling
interior– Cooling caused contraction of the skin=
wrinkles and bumps aka mountains and valleys
Alfred Wagner Thought Differently!
• He believed:– The continents are slowly moving– All the continents has once been attached in a
supercontinent and this was called Pangaea “all land”
– Pangaea split and the continents moved apart– The boundary of each continent is not at the
shoreline but the continental shelf
Alfred Wegener cont.
No one believed Wagner…..at least not yet!
Scientific Revolution
• Discovery of the Earth’s Magnetic Field gave a window into the past
• Paleomagnetism (magnetism from the geologic past…shows 3 major things)– The position of the magnetic north versus the
geographic north pole– Direction to the magnetic pole at the time the rock was
formed– Magnetic latitude of the rock’s location at the time it was
formed– The movement of the magnetic pole over time(500
million years) suggests that either the pole moves around or the continents move
• The Earths North and South pole have flipped many times
• These leaves magnetic ‘stripes’ in rock containing iron minerals
Seafloor Spreading: H. H. Hess
• 1950’s had detailed mapping of the ocean floor
• Figured out – Some really deep parts are next to the
continents and some shallower parts are out in the middle due to the mountains under the water
– Why?
Finally Wagner's’ Ideas became the: Theory of Plate Tectonics
Finally Wagner's’ Ideas became the: Theory of Plate Tectonics
The earth’s outer shell, the lithosphere, is divided into 8 large plates and a number of smaller ones
Most earthquakes, volcanoes, and mountains occur along the borders
All Based on MotionAll Based on Motion
• It turns out that the upper section of the mantle is stuck to the underside side of the crust to form what we call tectonic plates
Evidence of Tectonic Plates
Evidence of Tectonic Plates
Our first evidence of tectonic motion is based on similar fossils and rock types on opposing sides of the ocean
Tectonic PlatesTectonic Plates
Today plate boundaries are determined by examining the location of volcanoes and earthquakes.
Volcanoes result from the friction (heat) of the platesmotion.
Earthquakes occur where plate rub against one another
Tectonic PlatesTectonic PlatesVolcanoes
Tectonic PlatesTectonic Plates
Why do the Plates Move?Why do the Plates Move?• No single idea explains everything but we can
identify several forces that contribute to the movement of the plates.– Slab pull
• The sinking of the cooled dense oceanic plates pulls on the rest of the plate
– Ridge rises• The material deposited on the top of the ridge slides downs
from the rise pushing on the plate
– Convection• Movement within the mantle could be part of the driving force
behind the motion of the plates.
Why do the Plates Move?Why do the Plates Move?
Why are they spreading?
• Why is the Atlantic still getting wider
• The plates are pulled apart by convection currents in the mantle below
• Caused by heat released from natural radioactive processes
• At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock
Plate BoundariesPlate Boundaries• Convergent – plates move toward
one another
• Divergent – plates move away from each other
• Transform-Fault – plate moves sideways from each other
Plate BoundariesPlate Boundaries
Convergent PlatesConvergent Plates
Convergent PlatesConvergent Plates
Ocean-Ocean Convergence
Ocean-Continental Convergence
Continental-Continental Convergence
Divergent PlatesDivergent Plates
Spreading centers on the ocean floor and on land (create rifts)These create the beginning of a new ocean basin
Transform PlatesTransform Plates
Transform PlatesTransform Plates
San Andreas Fault
Mid-Plate HotspotsMid-Plate Hotspots
Hawaii
PangaeaPangaea
PangaeaPangaea
PangaeaPangaea
The break up of Pangaea
Where are we going?Where are we going?
We appear to be headed for another super continent as North America,
South America, Asia and Australia converge in the ever shrinking Pacific Ocean