Plate Tectonics The Unifying Theory of Geology
The 4 “Big Ideas” in Geology • The Rock Cycle (Friday’s lecture) • By the mid-1700s • Antiquity of Earth - “Deep Time” (Wednesday’s Lecture) • By the early 1800s, millions of years • Faunal Succession (Fossil Record – Core 6) • By mid-1800s • Plate Tectonics • By late 1960s
The Big Ideas Earth is Old and Dynamic Rather than Young and Static
Recall: The Scientific Method • Observations • Hypothesis (a testable explanation) • Includes testing by prediction • More observations (testing) • If ALL observations fit ---> Theory
The Ancient Greeks • Knew that Earth was round (not discovered by Columbus) by 300 B.C. • In fact Eratosthenes (ca. 250 B.C.) measured its circumference PhotoNot Available
Early Observations • First European explorers started to gather knowledge on world geography • Early 1500s: E.g., Magellan & da Gama
Early Observations • Francis Bacon (1561-1626) first noted how coasts of Africa and South America fit. Yikes!!!… bacon!
Today it’s known: Fit best along their continental shelves
The 1800s • Better maps available • Some people proposed that all continents could fit together • In 1872, British Challenger began mapping ocean floor by soundings • Similarities in rocks of NW Europe and NE America were discovered
Paleontology The study of fossils
Evidence from Paleontology Similar fossils in South America and Africa
Glossopteris, a seedfern whose seeds are too large to be carried far by wind
Mesosaurus, a fresh water reptile that couldn’t swim across the open sea
Paleoclimatology The study of ancient climates
in South America, Africa, India, and Australia...At same time the Northern Hemisphere had lush swamps...
and the pieces fit together like a puzzle... “Gondwanaland”
Coal • Deposits found in Antarctica brrrrr…. • Coal requires a warm, lush climate ===> • What’s Antarctica like today? ===>
Paleomagnetism • Magnetic minerals in molten rock align with Earth’s magnetic field
Paleomagnetism • When igneous rock cools, magnetism is “frozen in”, like little compasses...
Polar Wandering • Different continents indicate different position of North Pole - very strange... • Only logical explanation is that the CONTINENTS have moved since the rocks were formed
WEGENER and CONTINENTAL DRIFT • In the 1910s Alfred Wegener put all these pieces of evidence together and made the hypothesis of CONTINENTAL DRIFT... • The Origin of Continents and Oceans (1915)
CONTINENTAL DRIFT • When Wegener proposed to fit the continental shelves together, rather than the coastlines, we got a better fit for all modern continents...
All continents have moved to their present positions from one “supercontinent” he called PANGAEA - 200 Ma
Scientific Consensus is that Earth is about 4.5 billion years old (4.5 Ga)
1912 - 1945 • A few geologists looked for more evidence during this period... • Seismologists began studying the deeper layers of the earth and discovered a dense mantleand liquid outer core.
WWII and SONAR • Sonar, developed to find enemy subs, was used in the decade after WWII to map the deep sea floor...
SONAR • Previously, most geologists thought the sea floor was rather flat and featureless • They were wrong... • A diverse topography was discovered
Topography of the Sea Floor • Researchers found an undersea mountain range 40,000 miles long. • And a trenches seven times deeper than the Grand Canyon. • Even more striking were the geophysical findings...
Some Geophysics Thousands of drilling samples were taken...
Geophysics • Paleomagnetism in the deep sea floor rocks indicated that many episodes of magnetic reversals had taken place...
These reversals occur in parallel paired bands on opposite sides of a mid-ocean ridge. Radiometric dating showed the rocks get older the farther you get from the ridge.
Sea-Floor Spreading • In 1962, these data were collected into a theory called SEA-FLOOR SPREADING • New crust forms at mid-ocean ridges... • Oceanic crust pushes outwards from the ridge and (perhaps) takes the continents along
Earthquakes and Volcanoes • Seismology is the study of earthquakes
Earthquakes and Volcanoes • Data indicate that earthquakes and volcanoes do not occur at random locations, rather...
Earthquakes and volcanoes occur mostly along or near trenches and mid-ocean ridges...
Seismology studies • located a zone of weakness where seismic waves travel more slowly • It was called the asthenosphere. • The rigid lithosphere sits on top of this
Finally, a unifying theory... • In 1968, seismologists at Columbia put all the evidence together and came up with the theory of PLATE TECTONICS... • This combined the sub-theories of CONTINENTAL DRIFT and SEA-FLOOR SPREADING
PLATE TECTONICS The rigid upper 35 miles or so of the earth (lithosphere) is broken up into a dozen or so plates, which can slide around on the zone of weakness.