Download Chapter 33 Plate Tectonics

Plate Tectonics
Evolution of the Earth
and Our Restless
Planet: Chapter 33
Review: Interior
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
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?
Theory
of Plate Tectonics
Finally Wagner's’ Ideas became the:
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 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
Our first evidence of tectonic motion is based on similar fossils and
rock types on opposing sides of the ocean
Tectonic Plates
Today plate boundaries are determined by examining
the location of volcanoes and earthquakes.
Volcanoes result from the friction (heat) of the plates
motion.
Earthquakes occur where plate rub against one another
Tectonic Plates
Volcanoes
Tectonic Plates
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 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 Boundaries
• Convergent – plates move toward
one another
• Divergent – plates move away from
each other
• Transform-Fault – plate moves
sideways from each other
Plate Boundaries
Convergent Plates
Convergent Plates
Ocean-Continental
Convergence
Ocean-Ocean
Convergence
Continental-Continental
Convergence
Divergent Plates
Spreading centers on the ocean floor and on land
(create rifts)
These create the beginning of a new ocean basin
Transform Plates
Transform Plates
San Andreas
Fault
Mid-Plate Hotspots
Hawaii
Pangaea
Pangaea
Pangaea
The break up
of Pangaea
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