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Plate Tectonics
Chapter 17
The Earth’s Drifting
Continents
German scientist Alfred
Wegener, 1900’s
proposed the Theory of
Continental Drift
 It was widely disputed,
30 years after his
death, enough
evidence was collected
to support his theory

Evidence
Glossopteris, located in 250 myo rocks,
were found in South Africa, Australia,
India, and Antarctica
 Antarctica once had a warmer climate
 Rock formations and deposits lined up
on several continents
 Evidence of glaciers in now warm
climates

The Evidence
The Real Issue………..
How could the continents move through
the solid rock bottoms of the oceans?
 In the 1950’s, scientists using better
instruments found underwater mountain
chains with rift valleys in their centers

Midocean ridge
Earth’s Spreading Ocean Floor
Midocean ridges form the single largest
mountain range in the world
 80,000 km long and 3 km high
 Lava erupts to form new sea floor and
spread
 As it spreads it takes continents with it
 This explained the mechanism for
continental drift!

Magnetic stripes on the ocean floor were
further evidence that the sea floor was
spreading
 The patterns are identical on each side of
the ridge
 Closer examination shows that the magnetic
poles have reversed themselves nine times
in the past 3.5 million years


Iceland, "the land of fire and ice", is being split
by an oceanic ridge that surfaces to create an
oceanic island in the North Atlantic Ocean. Red
triangles show Iceland's active volcanoes, including
Krafla. Reykjavik is Iceland's capital.
If the ocean floor is being
created, is the Earth getting
bigger?



Rock in the ocean is relatively young
Ocean floor is being destroyed in trenches in a
process called subduction
The denser ocean crust is pushed downward into
the mantle and melts
The Earth’s Moving Plates

The Theory of Plate
Tectonics, which links
continental drift and
seafloor spreading,
explains how the Earth
has evolved over time.
It helps to explain the
formation, movement,
collisions, and
destruction of the
Earth’s crust
Lithospheric Plates
There are seven major plates, Pacific,
North American, South American,
Eurasian, African, Indo-Australian, and
Antarctic
 There are many small plates,
Caribbean, Arabian are examples
 Each plate moves at a different speed
and direction
 Some are oceanic and some are
continental

Plate Boundaries

Divergent boundaryplates moving apart

Convergent boundaryplates that are moving
together

Transform boundary,
slip-strike, lateral faultsplates slide past one
another
Tectonic Humor
More About Our Moving Earth
Plate Motion

The power of the convection currents in the
mantle is thought to be the force that moves
the plates
Collision of Continental
and Oceanic Plates
Collision of Continental
Plates
Collision of Oceanic Plates
Will the Earth Continue to
Change?
The Earth’s Changing Surface



Stress is what causes the surface of the Earth to
change
As the rocks undergo stress, they slowly change
shape and volume
They also move up and down or sideways
Types of Stresses



Compression
squeezes the rock
Tension causes the
rocks to stretch out
over a larger area
Shearing of the rock
causes it to twist or
tear
Faulting


A break or a crack
along which rocks move
Earthquakes are
sometimes along these
faults
Faulted Mountains and
Valleys



Mountains formed
when there are
many normal faults
in one area
Blocks of rock are
uplifted
Valleys may form
when the block of
land between 2
normal faults slides
downward
Folded Mountains


When stress is applied to the rock formation
and it bends but doesn’t break
Forms an anticline which is an upward fold and
a syncline which is a downward fold
Volcanic Mountains

Single peak
mountains formed
from volcanoes
The Floating Crust

Isostasy is the balance between the downward
force of the crust and the upward force of the
mantle
Peaceful changes!!!!!!