Scientific Method at Work From Contracting Earth to Plate Tectonics

What components make up the Scientific Method Observations Questions/Purpose Hypothesis Data gathering/Experimentation Data analysis Conclusion Peer Review Verification More data gathering

An example of the Scientific Process Let’s review the Theory of Plate Tectonics from your 8th grade science class. What do you remember? How do you think scientists came up with this theory?

Early 1900’s Commonly held view of the formation Earth’s features at the time was the: Contracting Earth Theory

Question-How had the mountains and deep sea formed? Earth formed from a molten blob as it cooled- -dense materials like iron sank to the middle to form the core -less dense materials like aluminum stayed at the surface and formed the crust -in between was molten material

Evidence Materials contract when cooled, thus this would happen to the earth causing some areas to be pushed up (mountains) and others to sink (ocean) (ex-grape to a raisin) Isostasy-balance of forces (up and down) like an ice cube in water so it floats. Earth’s crust “floated” on the molten interior. The crust could move up and down as a result of mass changes.

Analysis Similar fossils found on different continents, how could this happen? Due to isostatic adjustments land bridges connecting continents would allow for migration of animals

Conclusion The Earth’s mountain’s and seas were formed as the Earth contracted as it cooled. End of Story or is it? BUT THEN some said how do we explain the shape of the continents or the similarities of mountain chains on different continents……

AAAHHHH! NEW QUESTIONS

Then along came Alfred Wegener He tried to answer some of those questions in his "Origin of the Continents and Oceans" which was published in 1915. His question went something like this-How do we explain these other connections between the continents because the Contracting Earth Theory can’t do this very well?

Evidence Looked at maps saw similarities of coastlines, not a good fit but similar.

Need to look off coastline(200 m depth) to continental shelves, better fit. More realistic because these areas were once above sea level and would have been the edge of the continent at one time.

Evidence Fossils-Mesosaurus-found in Eastern South America and Western Africa couldn’t have swam across the Atlantic Index fossils- have specific habitats, and lived during specific time frame.

Other Fossil Evidence

Geologic evidence Mountain chains -Appalachians follow to Greenland and Northern Europe.

Similar climate conditions- evidence of glaciers in areas that presently would not support glaciers in Africa and South America.

Coal deposits showing warmer climates that today would not support the plant life necessary to form these deposits.

Wegener’s Conclusion The continents have moved over time. They were once together forming a super continent he called Pangaea. Over time they have moved to their present locations.

Peer Review Nice evidence A lot of people would not buy this idea on the evidence provided. Here are some of their thoughts on Wegener’s hypothesis.

How could these huge continents move? The Problem How could these huge continents move? There is no explanation to answer this question in the early 1900’s, even though there is a lot of evidence to suggest the continents were together at one point in time.

Until the 1940’s What happened globally during the late 1930’s and 1945? World War II There were many crossings of the Atlantic Ocean by submarines and our understanding of the ocean floor changed from this United Europe States

TO THIS

Along with this new picture, new technology gave us more information about the ocean floor Magnetic properties of the rocks showed a unique pattern as we moved from the mid Atlantic to the coasts on either side.

Paleo-magnetism-change in magnetic orientation of rocks along either side of the mid-Atlantic rift zone.

An important piece of evidence Sea-Floor Spreading

Another supporting piece of evidence. As scientists looked at the ages of rocks along the ocean floor the Ocean floor rocks only 150 million years old at most close to continents and younger closer to the ridge. Continental rock up to 4 billion years old Minnesota Gneiss (nice)

There were still questions about how these huge plates moved, but now we knew they did move. Use of lasers placed on opposite sides of the ridge or rift zone can help us determine the rate of motion of the various plates that make up the Earth’s surface.

The Birth of a Theory A Theory in science is an explanation of observable facts with a large body of evidence to support the observed phenomenon. It has been peer reviewed by many individuals and is accepted as the explanation for the observations. New found observations fit the model, but the model may be adapted as new information is found.

The Theory of Plate Tectonics This theory grew out of observations that started in the 1500’s as maps were drawn of the earth. It continued as additional information was gathered in the 1800’s and early 1900’s. In the 1940’s and 50’s with new technology additional observations were able to be made that were not possible in the past. It still took more time until the late 1960’s before the hypothesis of continental drift became the Theory of Plate Tectonics.

Tectonic Plates

Tectonics comes from Greek, meaning “construction” so the Theory of Plate Tectonics- is the explanation for construction of features on the earth’s crust. (Mountains, earthquakes, volcanoes, deep sea trenches……) Even today we continue to gather information to test the validity of the theory.