INTRODUCTION

During our previous discussions of the early Earth, we noted the heavy bombardment of the planets by bolides. These bolide impacts were greatest between 4.6 and 3.9 b.y. and are still visible on other planets and moons (e.g. our Moon, Mercury, and Mars; see photos in impact folder). The dense cratering history of Earth is no longer evident because of dynamic forces which constantly change the surface of the Earth.

In this unit, we discover the nature of these dynamic forces which are embraced in Plate Tectonic Theory, a single unifying theory which accounts for the means of crustal formation, destruction, and modification. Of our subjects this semester, this is the big one! Understanding plate tectonics will give you a new view of your immediate and distant surroundings. Plate tectonics accounts for or influences many things, including:

the distribution of earthquakes

the distribution and composition of volcanoes

long term climate and sea level changes

evolutionary processes

the distribution and abundance of resources

and so much more.

Plate tectonic theory originated in the 1960's with modern geophysical examination of the ocean basins. Our discussion of plate tectonics will take the following course:

Part 1: Examination of historical writings suggesting that the continents had moved through time and the introduction of the concept of "continental drift".

Part 2: Examination of some evidence from the continents that suggests continents have moved through time.

Part 3: During this phase of our discussion, we examine observations about the features and characteristics of ocean basins and margins which must be adequately explained in any theory accounting for the dynamic nature of the Earth's crust. Many of these observations have been made only during the last 25 years and were keys to the theory of plate tectonics.

Part 4: Next, we will present the principles of plate tectonic theory.

Part 5: Finally, we will examine how plate tectonic theory accounts for the observations we made (in Parts 2 and 3) about characteristics of the continents and ocean basins.

It has been long suggested that continental positions have changed through time. Early suggestions were based on matching continental outlines of Atlantic bordering continents (A and B); however, later naturalists based such arguments on continental fossil and geologic evidence (C-E).

A. Frances Bacon - suggested W. Hemisphere once joined 1620 with Eurasia

B. P. Placet - before the deluge, America was 1668 separated from other parts of the world

C. Antonio Snider - noted similarities between America and

1858 European fossil plants of Carboniferous

(300 m.y. B. P.), therefore, all continents

once together

D. E. Suess - noted correspondence of geologic

1885 formations in S. Hemisphere landmasses, suggested once formed single continent called Gondwanaland

E. F.B. Taylor (1908)

A.L. Wegener (1915)

common historical record for both sides of

Atlantic therefore joined approximately 200

m.y. ago; introduced the concept of Pangaea

The movement of continents through time (continental drift) was a concept not widely excepted until long after Wegener. The major problem was that no mechanism could be explained to move continents. Biologists and geologists could not ignore similarities of fossils no separated by oceans so they invoked the "stepping stone" mechanism of biologic dispersal.

Acceptance of "continental drift" by the development of plate tectonic theory awaited new discoveries that would provide a mechanism for continental movement.

Examination of some evidence from the continents that suggests movement through time. Much of this type of information was known by Wegener but was not adequate to be the basis of a dynamic theory of the Earth.

A. Fit of continents

B. Compatible ages of geologic provinces on both sides of

Atlantic and between other continents. For example:

1. Paleozoic Geologic Activity - N. Atlantic

a. 550 m.y. - Mass., Conn., Is. off France, Normandy, Scotland, Norway

b. 450 m.y. (Taconic) - New England, British

c. 360 m.y. (Acadian) - Appalachians, Nova Scotia, England, Norway

d. 250 m.y. (Appalachian) - S. Europe, Africa, New England, etc.

2. South Atlantic

Matching structural trends (SW) between Africa and South America

-2 B.Y. in Ghana and Ivory Coast separated near Acra, Ghana from 600 m.y. rocks in Nigeria

-same age provinces divide Brazil margin near San Luis in NE Brazil

3. Matching basins and tectonic age provinces between SW Africa, India, Madagascar, Australia, and Antarctica

C. "Juxtaposition" of mineral resources, e.g. manganese, iron ore, gold, and tin deposits

D. Distribution of climate sensitive deposits

1. glacial

2. evaporite

3. coal and oil

4. reefs

E. Distribution of fossil plants and animals

Any major new advance in science, such as the theory of evolution or plate tectonic theory, requires a set of observations that the theory is intended to explain. The theory must be consistent with these observations and new additional observations. Below are some observations about the features and characteristics of ocean basins and margins which must be adequately explained in any theory accounting for the dynamic nature of the Earth's crust. Many of these observations were made after World War II when geophysical exploration of the ocean exploded. The theory of plate tectonics was made possible by many of these observations, which provided new insights into Earth processes.

A. The ocean ridge systems

Many thousands of miles long

High standing above surrounding ocean basin

Central valley

Hydrothermal fluids

Mineralization

High heat flow

B. Trenches

Many border continental margins

Associated with island arcs (volcanic islands)

Zones of earthquake and volcanic activity

Shallow to deep earthquakes

More in Pacific than Atlantic or Indian Ocean

C. Seismic (earthquake) activity

Non-random!

Found concentrated in long-linear belts

Shallow to deep near trenches

Ocean ridges (shallow)

Faults (transforms)

D. Volcanoes

Concentrated greatly around the Pacific "Ring of Fire"

On oceanic ridges

Associated with island arcs near trenches

On continental margins near trenches

Some linear chains

E. Island chains and seamount chains

Linear

Increasing age with distance

F. Age of oldest marine sediment very young (150-160 m.y.) compared to age of continents (4 B.Y.)

G. Age of ocean crust and sediment immediately above crust

increases with distance from ridge crest

H. Dead coral reefs and atolls now far below sea-level

I. Origin of magnetic anomalies

Magnetic anomalies parallel to ocean ridges

Magnetic polar-wandering paths of continents