Lecture 5
CONTINENTAL DRIFT
SEA-FLOOR SPREADING

Plate Tectonics -- brief introduction

Scientific "revolutions": Plate Tectonics as an example

1. "Normal Science": At the start, a collection of theories and observations that are accepted

Geology 100 years ago: Theories were developed to explain the earth's features

The idea that the continents move was not really considered- not really "needed"

 

2. "Crisis": These theories are unable to explain new observations

Unexpected geological discoveries in the 20th century

 

3. "Revolution": New paradigm adopted, old theories eventually given up

New paradigm- plate tectonics- explains many, many aspects of the earth

In the end, a scientific revolution greatly advances science by developing a new, more accurate model with which to understand nature.

Continental Drift
Basic concept (Alfred Wegener)
Single supercontinent, "Pangea" ~250 m.y. ago
Slow "drifting" to present position
Evidence
Fit of continents
Distribution of rocks and mountains
Paleo-climates
Fossils
Acceptance?

Sea-Floor Spreading (Harry Hess, l962)
New oceanic crust is created at mid-ocean ridges and "spreads" away
Old oceanic crust is destroyed by sinking into mantle at trenches
Convection currents in mantle drives the process

 

End of brief notes.

Scientific "revolutions": Plate Tectonics as an example

1. "Normal Science": At the start, a collection of theories and observations that are accepted; new data are fit into those theories by expanding the theories incrementally

Geology 100 years ago: Theories were developed to answer questions such as:

Why are ocean fossils found near the top of Mt. Everest and in many mountain ranges?

Why are mountain chains found where they are?

Why are volcanoes found only in certain zones of the world?

The idea that the continents move was not really considered- not really "needed"

 

2. "Crisis": These theories are unable to explain new observations

Unexpected geological discoveries in the 20th century

New data, much of it from the oceans

Many attempts made to fit these data into existing theories, i.e., to continue doing "normal science"

 

3. "Revolution": New paradigm adopted, old theories eventually given up

New paradigm- plate tectonics- explains many, many aspects of the earth

Completely new way of looking at the earth- hard for many to accept

Old theories given up- but not without careful weighing of the competing theories

Healthy skepticism is the rule in science

Eventually, the evidence FOR plate tectonics became overwhelming

 

Note: Technology had advanced enough to put a person on the moon by 1968, and yet the most basic principle describing how the earth works was just gaining acceptance! This is partly because much of the evidence was hidden in the ocean depths, which were not explored until the mid-1900's.

 

For more information on scientific revolutions, see:

The Structure of Scientific Revolutions, by Thomas S. Kuhn

The Road to Jaramillo, by William Glen

Continental drift, sea-floor spreading and plate tectonics

In 1968, a revolutionary concept was proposed by marine geologists at Columbia University -- the theory of plate tectonics. Prior to that theory, many geological phenomena were simply not explained -- Why do earthquakes and volcanoes occur where they do? How are mountain ranges formed. Why do the continents on either side of the Atlantic appear to fit together? Why are there mid-ocean ridges and marginal trenches on the sea-floor? Plate tectonics is able to explain these, and many other features of our planet.

Plate tectonics proposes the following:

• Earth's lithosphere (the rigid, outer layer, roughly 100 km thick) is made up of a series of slabs (or plates) that move about.
• Plates can separate from one another, grind together, slide by one another, or one plate can slide down under another.
• These relative motions result in earthquakes, volcanoes, and mountains at plate boundaries.

Plate tectonics was developed from two previous notions about global processes:

• "Continental drift" -- continents migrate (drift) over the surface of Earth
• "Sea-floor spreading" -- oceanic crust forms at mid-ocean ridges, spreads laterally, and then descends back into to the mantle at marginal trenches.

To understand and appreciate the significance of plate tectonics, we begin with those concepts which led to its development.

Continental Drift: Alfred Wegener (German meteorologist) developed the concept of continental drift in the early 20th Century (although the idea had been around for centuries before-hand)

Tasa, Plate Tectonics CD-ROM, Chapt. 2, Advance through all frames

Continents were assembled in a single landmass ("Pangea") about 250 m.y. ago.
With time they have slowly moved to their present positions.
The idea was well thought out and based on the following data:

Evidence for Continental Drift

1. Fit of the continents across the Atlantic Ocean
Tasa PT Chapt. 3, Frames 1, 2, 3, 4, 5.

Atlantic coastlines fit together like a jig-saw puzzle. But if the pieces fit, the "pattern" must match also. Wegener explored whether geologic features also matched-up in his recontruction.
Tasa -- PT Chapter 5 (Rocks...) Frames 1 & 2

2. Distribution of mountains and rock sequences
Tasa -- PT Chapter 5 (Rocks...) Frames 3 to end

Appalachian mts. and similar range in Europe connect up when continents are reconstructed. Distinctive types of rocks and patterns of folds on Africa and S. America match up well for a reconstructed Pangea.

3. Distribution of sedimentary rocks reflecting paleoclimate
Tasa- PT Chpt. 6 (Paleoclimates) Frames 1 to end

Rocks deposited at the Earth's surface (sedimentary rocks) reflect the climate (and hence latitude) where they form. For example, ancient coral reefs and coal swamps form in warm, humid, low latitudes. Glacial deposits should form at cold, high latitudes. Wegener found evidence for glaciated regions widely scattered in the Southern Hemisphere, and for coal deposits. His reassembled continents explains the paleoclimatic record in those sedimentary rocks.

4. Distribution of fossils:
Tasa -- PT Chapter 4

A number of identical fossil organisms are found on widely separated continents.
o Cynognathus -- land reptile (couldn't swim at all) in S. Amer. & Africa
o Mesosaurus -- fresh water reptile (couldn't swim far and not in salt water) in
S. Amer. & southern Africa
o Lystrosaurus - fat land reptile (would sink faster than a lead ship)
in Africa, India, Antarctica
o Glossopteris - fern with heavy seeds (couldn't be blown across the ocean)

The distribution of those organisms only makes sense for a reconstructed Pangea

Wegener believed that the evidence for continental drift was overwhelming. But the idea was not accepted at the time. It was even ridiculed. The reasons were:

1) Wegener didn't have a reasonable mechanism for movement of the continents.

He suggested that attractive forces of the moon caused continents to crash
through the oceans (like a ship breaking through ice). Therefore, ocean crust
should be deformed, but it isn't.

2) Wegener's geological evidence was derived mostly from observations located in
the Southern Hemisphere.

North American geologists were unfamiliar with the observations so they discounted them. Prejudice based on ignorance (scientists are human).

Therefore most scientists rejected the idea of continental drift at the time. But that idea was ressurected when detailed information about the sea floor was gathered and analyzed in the 1950's and l960's. This work lead to the concept of sea-floor spreading.

Sea-Floor Spreading was proposed by Harry Hess (Princeton geologist) in 1962 based on his observations of sea-floor topography. The basic components of the concept are:

New ocean crust is created at mid-oceanic ridges, moves laterally ("spreads") away from ridges with the motion like that of a conveyer belt, and is destroyed at trenches, where it sinks into the mantle and melts. Hess suggested that convection currents in the mantle was the driving force of sea-floor spreading.

During the l960's, a number of research projects were done to test this new hypothesis. Many of these dealt with properties of the sea floor. As we shall see, these studies confirmed the concept of sea-floor spreading.

Study and Review Questions

• Why was the theory of plate tectonics such a "revolutionary" concept in the earth sciences?
• Briefly describe the basic idea of plate tectonics.
• In a sentence, describe Alfred Wegener's concept of continental drift. What did he name the super-continent?
• Identify and describe briefly the four lines of geologic evidence that Wegener used in support of his ideas.
• Although the evidence for continental drift was compelling, Wegener's theory was not generally accepted by other scientists. Why not?
• Based largely on the topography of the ocean floor, Harry Hess proposed the idea of sea-floor spreading in 1962. In a sentence or two, describe Hess's ideas of how and where new oceanic is created and how and where old oceanic crust is destroyed.
• What are the three phases of scientific revolutions?
• How do scientists normally respond to new evidence that seems to threaten existing theories and how does this response change in a scientific revolution?