Earth SciencePlate Tectonics: A Scientific Theory

Unfolds from Continental Drift

Objectives

Review the historical background leading to the Theory of Plate Tectonics

Discuss the Continental Drift hypothesis

Review evidence supporting the Continental Drift hypothesis

Historical Background on the theory of plate tectonics

Abraham Ortelius • Dutch map maker, 1596

• Suggested that the Americas were “torn away from Europe and Africa by great floods and earthquakes”

• 1st time suggested in written form

Historical Background on the theory of plate tectonics

Catastrophism • Until the 1700’s most Europeans

thought that a biblical flood played a role in shaping the earth’s surface.

• Geology was based on the belief that all changes were sudden and caused by a series of catastrophes.

Historical Background on the theory of plate tectonics

James Hutton• Father of modern Geology, 1785

• Doctrine of uniformitarianism• The present is the key to the past• Geologic forces and processes –

both gradual and catastrophic

Historical Background on the theory of plate tectonics

Antonio Pellegrini• Geographer, 1858

• Made two maps of the world • Before separation After separation)

Historical Background on the theory of plate tectonics

Eduard Suess• Viennese Geologist, 1880• Recognized an Atlantic type of margin

• Identified by abrupt truncation of former mountain belts

• Recognized a pacific type of margin • Identified by parallel mountain ranges, lines of

volcanic areas and frequent earthquakes

Historical Background on the theory of plate tectonics

Alfred Wegener • German meterologist

• 1912 Published 2 articles• The Origin of Continents and Oceans • Theory of continental drift

• 1915 proposed hypothesis of continental drift, Supercontinent - Pangea (next slide)

Continental drift hypothesis

Figure 15.2

•Supercontinent called Pangaea began breaking apart about 200 million years ago •Continents "drifted" to present positions •Continents "broke" through the ocean crust

Continental drift hypothesis Evidence used by Wegener

• Wegener •1st scientist look for evidence of continental separation using the Scientific method!

Continental drift hypothesis Evidence used by Wegener

Wegener -

Observation 1: He noticed the similarity between the coastlines on opposite sides of the South Atlantic Ocean• Thought that the continents might have been joined • He used present-day shorelines to show how the continents fit together.

- His opponents argued that erosion continually changes shorelines over time.

Continental drift hypothesis Evidence used by Wegener

Observation 2: Fossil evidence •Several fossil organisms are found on different landmasses in matching locations. •Organisms could not have crossed the vast oceans presently separating the continents•Example: Mesosaurus

•Aquatic reptile •Fossils limited to eastern South America and southern Africa• If the mesosaurs could swim the vast South Atlantic Ocean, the fossils should be more widely distributed

•

Continental drift hypothesis Evidence used by Wegener

Observation 3: Rock types and structures match• Appalachian Mountains eastern side of North

America continues into Newfoundland • Rocks are similar in age and structure in the

British Isles and Scandinavia.• Landmasses form a nearly continuous belt of

matching rock types.

Continental drift hypothesis Evidence used

by Wegener

Observation 4: Ancient climates

• The Northern Hemisphere was once tropical

• Evidence: •Coal deposits that were formed from tropical plants.

•Assumption: •Large change in climate could not have taken place without continental drift •Instead, N. Hempishere nearer equator

Continental drift hypothesis Evidence used

by Wegener

•More Paleoclimatic evidence•Glacial deposits 220-300 mya•Shows ice sheets covered large

areas of the Southern Hemisphere •Layers of glacial till

•Southern Africa, South America

India, Australia. •Below beds of glacial debris

•Scratched and grooved bedrock carved by the ice. •Looked like ice moved from sea onto land.

Satellite photo of parallel lakes and mountain ridges

Greenland Glacier