Transcript History in Geography

Evidence for
Continental
Movement
Evidence of Continental Drift
- The shape of
coastlines
match

Philosopher Sir
Francis Bacon
remarked
about this in
1620
Evidence of Continental Drift
- The plants and animals
match

Fossil records
a. Mesosaurus
(aquatic reptile)
b. Glossopteris flora
(plant fossils)
Late Paleozoic seed
ferns
Gondwanaland
(India, Africa,
Australia, S.
America, Antarctica)
Evidence of Continental Drift
Map showing distribution of fossils on the southern
continents. These distributions caused Wegener to join the
continents as shown in the map.
Evidence of Continental Drift
- The rocks match


Geologic similarities between S. America and
Africa. Same stratigraphic sequence (i.e. same
sequence of layered sedimentary rocks
Mountain belts and folded rocks line up
{
Stratigraphic
sequence
S. America
Africa
Evidence of Continental
Drift
- The rocks match
Evidence of Continental
Drift
- Match in glacial landform


Continental ice sheet covered parts of South America,
southern Africa, India, and southern Australia about 300
million years ago
Resultant glacial landforms exist in parts of the world today
that are not covered by ice – like U-shaped valleys
History in Geography
(...the interesting bits!)
Developing the Continental Drift Theory…
Poor Wegener…
• Wegener proposed Continental
Drift Theory (CDT) and published
his book The Origins of the
Continents and the Oceans in 1915
• Wegener's book translated into
English, French, Spanish, and
Russian in 1924  ridiculed!
• Data based on continents, but the
oceans cover 70% of the earth's
surface
However…
• Major technical and scientific developments
allowed scientists to:
▫ Map the ocean floor
▫ Detect paleomagnetic reversals in the rocks on the
ocean floor
Before 1920s…
• Ships did not have sonar
• Crust below seas thought to be flat and featureless
After 1920s…
• Sonar maps showed very exciting seafloor
• Tremendously deep valleys, mountain chains and vast plains
• A long ridge was found to run down the middle of the Atlantic
Ocean
▫ Rising 1 to 2 km above the surrounding ocean floor and paralleling the
continental coasts on both sides
• Similar mid-ocean ridges were mapped in the eastern Pacific Ocean
and the western Indian Ocean
1962
• Harry Hess, geologist
(History of Ocean Basins)
▫ Proposed that the mid-ocean
ridges marked regions where
hot magma rose close to the
surface
▫ This extrusion pushed the
ocean floor away from the
ridges like a conveyor belt
▫ In deep trenches (e.g. off the
coast of South America and
Japan) spreading ocean floor
forced down below the thick
continents into
subduction zones.
▫ Hess's theory of “seafloor
spreading” offered a driving
mechanism for Wegener's
CDT but needed more proof
A not very hairy Hess
1962
• 1962: U.S. Navy published a report that
summarized findings concerning seafloor
magnetism
• During WWII, ships dragging devices
(magnetometers) to measure magnetism to
locate submarines
▫ Found bands of alternating strong and weak
magnetism in the rocks of the seafloor
 Irregular in width
 Paralleled the mid-ocean ridges
 Patterned symmetrically about the crest of these
ridges.
(Just to note)
• Magnetism caused by presence of magnetic minerals in
the rocks
• Scientists were not surprised to learn that seafloor rocks
contain the magnetic mineral magnetite
▫ When magma from the ridges rises and cools, it crystallizes,
locking the magnetite crystals into alignment with the
earth's magnetic field like the needle of a compass
• Existence of the earth's magnetic field had been known
since ancient times, but only after WWII did scientists
realize that the magnetic field is not constant –
▫ Fluctuates in intensity
▫ Reverses itself
• Today we consider the field to have “normal” polarity,
and our compass needles point to the north
1963
• Fred Vine and Drummond Matthews (British
geologists) joined the map of the Mid-Atlantic Ridge
with the symmetric bands of magnetism on the
seafloor
• Strongly patterned paleomagnetic reversals
recorded on the seafloor provided the necessary
proof of Hess's seafloor spreading
▫ Proved that new crust was continuously being
generated at the mid-ocean ridges, where magma
cooled and magnetite crystals “locked in” according to
the orientation of the earth's magnetic field at that
time
▫ Continents no longer had to “drift” to their present
locations-they were driven by the slow and steady
magma “conveyor belts” at the mid-ocean ridges
The work of Hess, Vine, and Matthews resulted in a new map of the
earth, one that included plate boundaries in addition to coastlines.
Boundaries were drawn at mid-oceanic ridges and subduction zones.
Blue = mid-ocean ridges, Red = subduction zones.
Today…
• Satellite technology
• Global positioning system (GPS) and other
satellite-based data collection techniques
▫ Scientists can directly measure the movement and
speed of plates on the surface of the earth
▫ Speeds range from 10 to 100 mm per year,
confirming the long-held belief that plates move at
a slow but constant rate
Before Wegener, few had conceived of such a world. His
continental drift theory was the first step in the
development of plate tectonic theory, the foundation
upon which modern geology is built.
The work goes on, the cause endures, the hope
still lives and the dreams shall never die.
- Edward Kennedy