Geology

Plate 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

• Catastrophism• Ortelius• Hutton• Pellegrini• Suess• Wegener

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

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

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

Antonio Snider-Pellegrini

• Geographer, 1858

• Made two maps• Before (avant la separation)• After (apats la separation)

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 • Frequent earthquakes

Alfred Wegener

• Young German meteorologist

• First proposed continental drift in1915

• Published 2 articles:• The Origin of Continents and Oceans • Theory of Continental Drift

Continental drift hypothesis Scientific Process

• Wegener

•1st scientist to go out an look for evidence of continental separation •Use of the Scientific method

Continental Drift

Figure 15.2

•Wegener stated that:•Continents move apart and together over time•All continents moved together about 300 mya

•They formed a supercontinent - Pangaea•They began breaking apart about 200 mya•Continents "drifted" to present positions •Continents "broke" through the ocean crust

Continental Drift Hypothesis

Wegener used four lines of evidence to support his hypothesis:

1. Continental Puzzle

2. Matching fossils on continents

3. Matching mountain ranges across the Atlantic Ocean

4. Ancient climate • N. Continents located over the equator• S. Continents covered in giant ice sheet

1) The Continental Puzzle

• Wegener• Saw a similarity between the coastlines on opposite sides of the South Atlantic Ocean.• Thought that the continents likely had been joined. • Used present-day shorelines to show how the continents fit together. • Where did he get this idea from?

•Ortelius and Pellegrini

2) Fossil Evidence

• Several fossil organisms found on different landmasses• Organisms could not have crossed the vast oceans presently separating the continents • Organisms evolving on separate continents does not fit with Natural Selection

3) Matching Mountain Ranges

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

America into Newfoundland • Similar in age and structure in the British Isles

and Scandinavia.• Landmasses form a nearly continuous belt.

4) Ancient Climates•Northern Continents

•Likely located over the equator•Evidence:

•Found coal deposits formed from tropical plants.

•Southern Continents•Ice sheets likely covered large areas of S.Africa, S.America, India and Australia. •Evidence:

•Layers of glacial till•Scratched and grooved bedrock carved by the ice

4) Ancient Climate Continued.

Assumption: •Tropical forests would not exist in the Northern hemisphere at the same time as a glacial ice sheet covering most of the southern hemisphere•Instead: present day northern continents were nearer the equator and all continents were connected

Summary