Transcript 12.1 Evidence for Continental Drift

12.1 Evidence for Continental Drift
 Wegener
came up with the continental
drift theory
•
the idea that the continents used to be connected as
one big supercontinent but slowly moved apart over
time
 He
named the original supercontinent Pangaea.
Pan - gaea
“all” “Earth”
Putting the Pieces Together...
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)
The
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
Provincial Exam Question
Provincial Exam Question
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”
Spreading Ridge
An animation....
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!)
Provincial Exam Question
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!
“magnetic striping”
Provincial Exam Question
A HOT SPOT....
A
geological area where molten rock rises to
Earth’s surface!
Provincial Exam Question
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!
Take the Section 12.1 Quiz