Transcript Continental Drift

Continental Drift
12.1
• The continents of the world have not always
been in the same place....
Continental Drift Theory
• Alfred Wegener...
• “The continents have
“drifted” to their
present locations over
millions of years”.
• They were once joined
as a “supercontinent”
(Pangaea)
Pan - gaea
“all” “Earth”
Putting the Pieces Together...
• The continents do not fit
“perfectly” like a jigsaw puzzle.
• A better fit is found by matching
the continental shelves (original
shorelines that are now
underwater)
Geological/ Rock Evidence
• Mountain ranges
of one continent,
end at coastline
 begin again at
another
continent
(similar folds, ages)
Geological/ Rock Evidence
• Similarities between rocks
of adjacent continents are
evidence for continental
drift.
• The rocks found in
Newfoundland are the
same type and age as
rocks found in Greenland,
Ireland, Scotland, Norway.
Fossil Evidence
• The fossils of some
ancient species suggests
evidence for ‘pangaea’.
• Wegener found fossils of
a fresh-water reptile
‘Mesosaurus’ in two
places:
- SE South America and SW
Africa
This small freshwater reptile
could not have swam this
distance.
Fossil Evidence
• Glossopteris (a fern)
Found
In:
South
America
Africa
Australia
India
Antarctica
Fossil Evidence
• Problem: Ferns do not grow in cold climates
(Antarctica)
Glacial Evidence
• Evidence of glaciers
formed in ice ages
provides support to
CD theory
• During ice ages,
glaciers covered
large areas of land
• When the glaciers
retreat/ advance, they
leave behind proof
– Deeply scratched rocks
(striations), U-shaped
valleys, patterns of
rock formations
“Paleoglaciation”
More Evidence
• Coal Deposits in Antarctica
- Coal forms from dead
matter (usually tropical
swamp material)
- Since the South Pole has
never had a tropical
climate....it must have
been in a warmer location
than it is now
How the Continents Moved
• The current theory is
that the continents
moved due to the
movement of
tectonic plates.
Tectonic plates =
movable slabs of rock
that make up the
Earth
Tectonic Plates
• These large rigid plates slide over the surface
of Earth (over a layer of partially molten rock)
magma
partially molten surface
• Tectonic plates explain the location of
volcanoes and earthquakes....
Volcanoes/ Earthquakes
Volcanoes are openings
in the Earth’s surface
that spew gases,
chunks of molten rock
Earthquakes are sudden,
ground-shaking
releases of built up
energy under Earth’s
surface
Earthquake zones and volcanoes follow a
pattern.... They occur along the boundaries
(edges) of tectonic plates (form an outline)
Mid-Atlantic Ridge
• Mapping of the ocean floor
revealed a long mountain
range running down the
middle of the Atlantic Ocean.
• It is called the “Mid-
Atlantic Ridge”
How it was formed...
• The tectonic plates of the Earth
slide apart (sliding is called
“conduction”)
• Another way to think of this is
“seafloor spreading”
• Something must
replenish this lost
crust!
Occurs at
rate of ~ 1
inch /yr
Magma to the Rescue!!!!
• Areas of oceanic crust are replenished by
volcanic activity....
– Magma (molten rock) rises up
– Cools and hardens when it reaches surface
– Forms the new sea floor
Magma and More Magma
• This process keeps going.....
– Convection currents cause more magma to rise
– Magma forces apart hardened material
– Older rock is pushed aside as new rock forms!
“Sea Floor Spreading”
An animation....
Spreading Ridge
Proof of Sea Floor
Spreading!
• In the
1940’s...oceanographers
took samples of rock from
the Mid-Atlantic Ridge
- Younger rocks were found closer to ridge
- Older rocks were found farther from ridge (and in thicker layers...built up!)
More evidence in the
rocks!!
• Scientists have also found
that the rocks themselves
have a “striped pattern”
• To understand this, we must
understand the Earth’s
magnetism....
Earth’s Magnetic Field
N geographic Pole
S geographic Pole
Every few thousand years....the direction of magnetic field REVERSES!
“MAGNETIC REVERSAL”
Consider a rock.....
Rock contains metals like iron
(Fe)
Little pieces of iron “line up”
with magnetic field and then
they STAY that WAY!
When MAGNETIC REVERSAL
happens...they line up in opposite
direction!
Rocks “preserve” the direction
of the Earth’s magnetic field
that existed when the rock
formed! Amazing!
Studying the magnetic properties of ancient
rocks....
“Paleomagnetism”
Back to the Sea Floor....
• So the sea floor is
spreading and new
rocks (from magma)
are formed...
• Looking closely at the
sea floor, scientists
found a striped
pattern!
Normal magnetic polarity
Reverse magnetic polarity
“magnetic striping”
What is a HOT SPOT?
What is a HOT SPOT?
A HOT SPOT....
• A geological area where molten rock rises to
Earth’s surface!
HOT SPOTS....
Hawaiian Islands were
formed by a tectonic
plate passing over a
hot spot.
The plate slides in the
direction of the
arrow....
Forms a CHAIN of
islands!