Transcript World Geography 3200/3202

Theory Of Continental Drift
• Alfred Wegener
• It is this German man to whom
we credit with the proposal of
the theory of Continental Drift.
• While pondering
the similarities
between the
coastlines of South
America and
Africa, Wegener
came up with an
idea:
• What if the
continents were
once all connected
and just drifted
over the years?
Wegner’s evidence for Continental drift
• Ferns and Reptiles in many different continents
• Glaciers that were then tropical forests
• Same types of rocks on continents close to each
other (yet separated by an ocean)
• The coastlines of Africa and South America
The Flaw in Wegner’s Theory
• He thought that each of the continents were a
separate plate--they were just drifting on a neverchanging ocean.
• Like styro-foam floating on a pool of water!!
Canadian Correction
• J. Tuzo Wilson
• the 1960’s Canadian scientist who
resurrected Wegner’s theory after
years of disbelief by the science
community.
• Today, we know that that's false,
thanks to the discovery of crustal
plates.
• The plates of the earth are not
composed of just land; they're
composed of ocean too.
Canadian Correction – cont’d
• In some cases, the plates are just
land, in others they're just ocean,
and, in still other cases, they consist
of land and ocean.
• They each have different boundaries
and move in all different directions.
Plates of the earth p. 12
Continental Drift
• refers to the movement of
the more than 20 plates
(9 major) due to
convergent, divergent,
and transform
boundaries.
• The continents drift at a
rate of 2 inches a year.
• Started 200 million years
ago
• Pangea (land) &
Panthalasa (sea)
More about Continental Drift
• our text p. 10
• watch it at:
• http://www.ucmp.berkeley.e
du/geology/tectonics.html
Plate Tectonics
• Tectonic plates move or float on top of the
upper mantle.
• However they do not float freely.
• The plates are forced in specific directions
by the flow of magma beneath.
Plate Tectonics – cont’d
• Plates move with the flow of magma.
• The magma closer to the core heats and then
rises towards the surface as its density decreases.
• Once the rising magma reaches the lithosphere it
moves in opposite directions.
• The magma forms convectional currents.
Plate Tectonics – Convection Currents
Divergent Boundaries
• Tensional Forces occur where two tectonic plates
are pushed apart. The tension is created as the
plates move away from each other.
• Ridge Zones sometimes occur where two plates
move apart. The magma rises between the plates
and forms a ridge.
• Again caused by convectional currents in the
magma
Tensional Forces – Ridge Zones
This diagram above shows “Sea Floor Spreading”
Where’s the TENSION?
Convergent Boundaries
• Compressional Forces occur where two tectonic
plates come together. They compress against each
other.
• Subduction Zones sometimes occur where
compressional forces result from two plates colliding
and one plate slips under the other.
• Again caused by convectional currents in the
magma
Compressional Forces – Subduction
Where’s the Subduction /
Compression?
More about Plate Tectonics
• our text p. 10 - 12
• watch it at:
• http://www.pbs.org/wgbh/aso/tryit/tecto
nics/intro.html