Evidence for Evidence for Continental Continental MovementMovement

Evidence of Continental DriftEvidence of Continental Drift

- The shape of - The shape of coastlines coastlines matchmatch

Philosopher Sir Philosopher Sir Francis Bacon Francis Bacon remarked about remarked about this in 1620this in 1620

Evidence of Continental DriftEvidence of Continental Drift

- The plants and animals - The plants and animals matchmatch Fossil recordsFossil recordsa.a. MesosaurusMesosaurus (aquatic (aquatic

reptile) reptile) b.b. GlossopterisGlossopteris flora flora

(plant fossils)(plant fossils)Late Paleozoic seed Late Paleozoic seed fernsfernsGondwanaland Gondwanaland (India, Africa, (India, Africa, Australia, S. Australia, S. America, Antarctica)America, Antarctica)

Evidence of Continental DriftEvidence 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 DriftEvidence of Continental Drift

- The rocks match - The rocks match Geologic similarities between S. America and Geologic similarities between S. America and

Africa. Same stratigraphic sequence (i.e. Africa. Same stratigraphic sequence (i.e. same sequence of layered sedimentary rockssame sequence of layered sedimentary rocks

Mountain belts and folded rocks line up Mountain belts and folded rocks line up

S. America

Africa

{Stratigraphicsequence

Evidence of Continental Evidence of Continental DriftDrift

- The rocks match - The rocks match

Evidence of Continental Evidence of Continental DriftDrift

- Match in glacial landform- Match in glacial landform Continental ice sheet covered parts of South Continental ice sheet covered parts of South

America, southern Africa, India, and southern America, southern Africa, India, and southern Australia about 300 million years agoAustralia about 300 million years ago

Resultant glacial landforms exist in parts of the Resultant glacial landforms exist in parts of the world today that are not covered by ice – like U-world today that are not covered by ice – like U-shaped valleysshaped 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: 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.

1962

(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