Questions for Discussion. Plate Tectonics II: The Ocean Floor and the Seafloor Spreading Hypothesis

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1 Plate Tectonics II: The Ocean Floor and the Seafloor Spreading Hypothesis Arthur Holmes British Geologist ( ) Used radiometric dating to determine an accurate age for the Earth and to create a geologic timescale. Proponent of the continental drift hypothesis. Proposed that convection currents in the mantle pushed continents apart, creating new ocean basins in between. Acknowledged that no hard evidence existed to support his hypothesis. Prior to 1950, humans were almost completely ignorant of the structure of 2/3 of the surface of the Earth - the Ocean Floor We knew more about the surface of the moon than about the deep sea floor. Questions for Discussion Historically, what human activity has generated the most amount of funding for scientific research? L What major global event of the mid-20th Century led to the invention of radar, sonar, and advanced magnetometry? R

2 Technology developed during World War II Sonar - image the deep ocean using reflected sound. Magnetometer - detect faint magnetic fields. Developed to search for German U- boats in the north Atlantic. World War II Submarine warfare Cold War uclear Submarines SS autilus 1st nuclear powered submarine Military funding of earth science during the Cold War (1950 s) Detailed mapping of the topographic features of the deep ocean floor. Detailed mapping of the magnetic signature of the deep ocean floor. World Wide Seismic Monitoring network (detection of atomic tests, monitoring of above ground testing ban).

3 Study of the ocean floor overturned many accepted ideas: The deep ocean is a flat, featureless expanse. False. The ocean floor is covered with seamounts and guyotes, many of which form linear chains called aseismic ridges.

4 Emperor Seamounts Hawai ian Ridge Hawai ian Islands Study of the ocean floor overturned many accepted ideas: The deepest regions of the ocean are found in the center of ocean basins. False. The deepest regions of the ocean floor are elongate trenches found along the margins of some, but not all, ocean basins.

5 Study of the ocean floor overturned many accepted ideas: Mountain ranges are only found on continents. Mid-ocean ridges are the largest mountain ranges on Earth.

6 Study of the ocean floor overturned many accepted ideas: The ocean floor is covered in a deep pile of marine sediments, accumulated over billions of years. False: The deep ocean sediment cover is very thin at the mid-ocean ridge and, although it thickens toward the continents, is much thinner than would be expected after billions of years of accumulation. Thin cover of sediments Map of the ocean floor - circa Marie Tharp and Bruce Heezen - Columbia University Making sense of the deep ocean Harry Hess Harry Hess - Geologist at Princeton U publishes geopoetry. Suggested that the ocean floor itself might be moving. Continents are carried along, embedded within the ocean floor. Hypothesis is named Sea Floor Spreading by Robert Dietz at Scripps Institute.

7 Sea floor spreading mid-ocean ridge trench mantle convection How would seafloor spreading expain... LR LF The absence of thick sediments on the deep sea floor? The existence and location of deep ocean trenches? The existence and location of mid-ocean ridges? RR The existence of extinct, eroded volcanoes on the deep ocean floor? RF The seafloor spreading hypothesis was not immediately embraced by geologists Robert Dietz Evidence was largely circumstantial Definitive proof of seafloor movement was still lacking However, research into the history of the Earth s magnetic field was convincing many geologists that the continents had moved through time.

8 Remnant Magnetism 1940 s - geologists discover that the Earth s magnetic field is recorded by iron-bearing minerals during the formation of rocks. Remnant magnetism is trapped in the rock. Magnetic minerals act like tiny compasses, pointing to the former position of the magnetic pole (declination). Magnetic minerals also record the polarity of the Earth s magnetic field in the past. Remnant Magnetism Apparent Polar Wandering Observation that progressively older rock records different positions for the north magnetic pole at different times in Earth history. Two possible explanations: Apparent Polar continents, stable poles

9 Apparent Polar continents, stable poles Apparent Polar continents, stable poles Apparent Polar continents, stable poles Apparent Polar poles, stable continents

10 Apparent Polar poles, stable continents Apparent Polar poles, stable continents Apparent Polar poles, stable continents Question for Discussion What evidence could geologists use to distinguish between the two hypotheses proposed to explain polar wandering?

11 Apparent polar wander paths for Europe and orth America - They DO OT match! Polar wander assuming no continental drift.am Europe Polar wander with continental drift The continents had to have moved! Magnetic field reversals 1940 s - geologists discover that the Earth s magnetic field randomly flips so that the and S magnetic poles become reversed. Reversals occur randomly and last for 100 s of thousands to millions of years. Pattern of reversals recorded in the rock through time is unique - like a bar code.

12 Supercomputer simulation of the Earth s magnetic field S S Magnetic field becomes weakly polarized and unstable Magnetic polarity reversal S

13 R Reversals of the Earth s magnetic field are recorded in rocks as intervals of ormal() and Reversed (R) polarity. Pleistocene Pliocene Miocene Oligocene Cretaceous Paleocene Tertiary Eocene Pattern through time of polarity reversals is random and forms a unique bar code that can be used to identify different intervals in Earth s past. Seafloor Magnetic Polarity Stripes Geophysicists mapping the magnetism of the ocean floor notice a peculiar pattern of magnetic stripes in sea floor crust around mid-ocean ridges. Raff and Mason, 1961 Results from a magnetic survey off the west coast of orth America. What could explain the peculiar pattern of magnetic polarity stripes in the crust of the ocean floor? Stripes are centered on the mid-ocean ridge. normal reversed

14 normal reversed Question for Discussion: How could sea floor spreading explain the pattern of magnetic stripes seen in the ocean floor? normal reversed Magnetic field reversals Lawrence Morley (Canadian geophysicist) proposes that seafloor spreading creates magnetic stripes. His paper is rejected by two scientific journals and is never published! Fred Vine and Drummand Matthews (British geophysicists) propose essentially the same hypothesis in a paper that is published in ature. The Vine and Matthews paper does not generate much interest at first.

15 Question for Discussion: How could you prove that magnetic stripes were caused by sea floor spreading over geologic time? Think about the bar code! normal reversed Continent Seafloor 1966: Fred Vine publishes in Science on the correlation between magnetic stripes on the seafloor and the magnetic timescale developed from continental rocks. Seafloor spreading becomes an accepted fact, leading the way to plate tectonic theory.