Detailed Course outline:

• What is science and how does it operate?
  • The essence of "doing science" is really simple - broken dorm room window example.
  • Diagram showing the process (how does the thought process in our "broken dorm window" example fit into this schematic diagram?)
  • "Types" of science:
    • frontier science
    • primary scientific literature science
    • secondary literature science
    • textbook science
    • For more info, check out the "what is science?" link here.
  • Some key characteristics of science:
    • Based on observation
    • makes predictions - a measure of the "power" of a scientific hypothesis
    • the predictions must be testable
      • As emphasized by Kuhn, a key characteristic of a scientific statement is that it can (at least in principle) be falsified (whereas a statement of faith cannot). Science is therefore always evolving and self-correcting. Carl Sagan noted that "... the human condition would be greatly improved if such confrontations and willingness to reject hypotheses were a regular part of our social, political, economic, and religious & cultural lives."
  • Science and religion
    • Albert Einstein said "Science (is) ... a methodical thinking directed toward finding regulative connections between our sensual experiences ... Religion is concerned with Man's attitude towards nature at large, with the establishment of ideals for the individual and commercial life, and with mutual human relationship." (click here for other favorite quotes about science and religion)
  • Dealing with quantitative data - many aspects of the sciences deal with quantitative data like rock ages, densities, lengths, speeds, etc. Here we look a bit closer at some aspects of measurements in science.
    • In homework 1 several students made measurements on the Billiken statue. The results are shown here. Observations:
      • Several of the measurements were not the same - there is inherent variability in the measurements
      • Some measurements were obviously way off - these are called outliers. Although outliers may be rejected, it is a good idea to ask yourself why these measurements were way off. Was it a gross error in measurement? (In this case it was; a couple of students measured the wrong thing.) Was your measuring instrument faulty? Or more importantly, do the outliers indicate some new phenomenon? 
      • What do we do with all these measurements?
        • The mean, or what most people call the average, is the best estimate for the height of the statue. It is a measure of central tendency. Although some measurements may be higher than the "true" value, others will be lower, and the mean ends up being a better estimate for that "true" value than any of the individual measurements.
        • Now, imagine if we do this whole exercise once more, it is likely that each student will measure the statue slightly differently on the second run and we will end up with a slightly different mean. Now we have two estimates (two averages) for the statue. Which one is "correct"? 
        • What we need is a measure of "confidence" in our mean, or a measure of "how close" it is to the "true height" of the statue. We want to be able to say something like "I am 95% confident that the 'true' height lies between 75 inches 77 inches". This is provided by the standard error (of the mean). 
        • It turns out, that if we do our exercise of measuring the statue (eight students doing the measuring) an infinite number of times, our calculated mean will fall within 2 standard errors of the 'true value' 95% of time. Hence, plus or minus 2 standard errors is called a 95% confidence interval (for example "76 ± 1", is a mean of 76 with a confidence interval of 2 inches, where the standard error is 0.5 inch).  (Strictly speaking, it is not exactly correct to say that "we are 95% confident that the true value lies between 75 and 77inches"; the proper things to say is "using our method, we will 'capture' the true value of the statue 95% of time using a confidence interval of 2 standard errors." That is a subtle but important difference.)
        • Given this whole discussion, can you tell me why I have always placed true value of the statue in quotes? If one student argues that the statue is 75.5 inches and another argues that it is 76.0 inches, who is right? Does it make sense to talk about the "true" height of the statue? Given that every measurement has an uncertainty, does that make them useless? What is the measurement needed for? How precise does one need it to be?
• A brief look at Earth's gross structure and composition
  • First, a brief look at Earth's gross internal structure and gross surface features
  • Layers based on gross compositional differences:
    • core - mostly iron
    • mantle - dense silicon- and magnesium-rich minerals
    • crust - less dense silicon- and aluminum-rich minerals
      • oceanic crust - composed primarily of basaltic rocks; denser and thinner than continental crust
      • continental crust - composed primarily of granitic rocks; less dense and thicker than oceanic crust.
  • Layers based on physical properties
    • inner solid core
    • outer liquid core
    • asthenosphere - the part of the mantle that can flow or deform like a plastic
    • lithosphere - the outer layer of the Earth which includes the crust and the uppermost mantle. It is stronger than the asthenosphere and behaves as a fairly rigid layer. The "plates" of plate tectonics are different pieces of lithosphere.
  • Gross features of Earth's surface:
    • shield area
    • (relatively) young mountain belts
    • continental shelves
    • continental slopes and rises
    • abyssal plains
    • trenches and ridges (discovered later)
  • Isostasy - the state of flotational equilibrium between blocks of the earth's crust and the underlying more plastic interior
    • The concept of isostasy arose from surveys adjacent to major mountains such as the Great Trigonometric Survey of India which approached the Himalayan mountain range. In a nutshell, the surveyors were smart enough to know that the plumb bob they were using to determine a vertical line would be deflected towards the massive mountains due to the gravitational pull of the mountains. They were so smart that they calculated how much it would be pulled. However, when they approached the mountains, they found the pull on the plumb bob was less than they expected. To explain the less than expected deflection of the plumb bob, they suggested that the high mountains were underlain by deep roots of the same less dense crustal material extending into the denser mantle. That way, there would be less total mass in the area of the mountains and therefore the plumb bob would not be deflected as much as initially calculated. Further work showed that in general, the higher the mountain, the deeper the roots that were required to explain the (less-than-expected) deflection of the plumb bob. This simple relationship would be expected if the crust was actually "floating" on the mantle like an iceberg in water. That's isostasy. (You can read a neat book about the survey of India here; you can read more about the concept of isostasy and how it has evolved here.)
    • Continents (composed of continental crust) are largely above sea level because they are less dense and thicker than the oceanic crust (they float "higher" on the mantle).
    • Within continental crust, higher mountain ranges tend to have deeper roots, much like tall icebergs have deeper roots underneath. Here is an analogy with blocks of wood in water.
    • Isostasy implies that vertical adjustments in the crust can occur if erosion for example thins crust in one area or if thickening of the crust occurs by some process. (Much as a boat will sink deeper in the water as people get in and rise as people get off the boat.)
    • So, vertical motions of the continental crust were quite well-accepted by scientists prior to continental drift. Large horizontal motions were not. Continental drift (large horizontal motions) was a radical idea.
• How was continental drift and plate tectonics "discovered"?
  • Alfred Wegener - Recognized as the first person to rally a variety of lines of evidence from paleontology, geophysics, geography, geology, etc. in support of continental drift
    • a meteorologist/astronomer
    • did meteorological research in Greenland where he eventually died
    • Initially wondered about the matching of coastlines of Africa and America, but dropped the idea, because it seemed absurd to him.
    • Revived the idea after reading a paper on "land bridges" between Africa and South America to explain the distribution of some fossils
      • Some land bridges are likely if sea level drops a few dozen meters. For example, in the last ice age, when sea level was lower, land was exposed in several areas allowing plants and animals to disperse across them. Then they were flooded when the glaciers melted and sea level rose. However, the land bridges that some proposed to have existed across the Atlantic ocean seemed unlikely to Wegener.
    • Proposed continental drift in a book published in 1912
    • Wegener's evidence for drift:
      • Matching of the coastlines of Africa and the Americas. The "Bullard fit" (from http://www.uwgb.edu/dutchs/202ovhds/platetec.htm)
      • The distribution of ancient climates (as recorded in rocks such as sand dunes, salt deposits, glacial deposits, tropical forests and coal swamps) made sense in terms of latitudinal control on climate if the continents had drifted apart from a single continent he called Pangea. (Here is a website that shows the distribution of climatic zones at various times in the past.)
      • The distribution of fossils, especially land dwelling critters, was difficult to explain unless the continents were once together.
      • When assembled as Pangea, geologic structures such as fold belts on the different continents matched up as single continuous belts.
      • NASA website on Wegener and evidence for drift 
    • Continental drift was only accepted after the mid 1960’s. Why?
    • Wegener was a meteorologist—not an "expert" in the field
    • Critical evidence from the ocean floor, yet unexplored, was not yet available.
    • Wegener’s proposal for how continents drifted—by "plowing" through the oceanic crust was unacceptable to most people—it was physically impossible. Believers in drift said "what did happen can happen" and doubters said "what can't happen didn't happen."!
  • Paleomagnetism provided further support for drift that began to turn the tide.
    • Earth has a magnetic field; a dipole field: there is a north magnetic pole and a south magnetic pole
    • At present, the magnetic poles do not coincide with the geographic poles (defined by the Earth's spin axis)
    • Over time though, the magnetic pole wanders around the vicinity of the geographic pole (called "true polar wander") and if averaged over time, apparently coincides with it.
    • The Earth's magnetic field, like that of a bar magnet (as shown in our in-class demonstration) has lines of force that a compass needle will align with.
    • We can describe the lines of force with two angles: the declination (the horizontal angle the compass needle makes with true north) and the inclination (the vertical angle the needle makes with a horizontal line).
    • As seen in our class demonstration, the inclination varies with latitude: It varies predictably from zero at the equator increasing progressively to vertical at the poles.
    • Many rocks, when they form, preserve a record of the Earth's magnetic field at the place they form. We can therefore measure the ancient magnetic field preserved in a rock to tell how far it was from the geographic pole (from the inclination) when it formed, and whether it rotated (from the declination) since it formed.
    • By measuring the ancient magnetic field in rocks of various ages from the same continents, it was found that the poles had wandered quite drastically through time (apparent polar wander) for a single continent, and poles of the same age from two separate continents did not coincide. If the continents did not move, the Earth's magnetic field must not a have been a dipole and it must have varied quite oddly through time. But if one allowed the continents to drift, then one could assume that the Earth's field is a dipole and has remained aligned with the geographic poles. Many preferred the second interpretation and therefore had to accept continental drift.
    • Still, a mechanism for drift was lacking.

            EXAM 1

• Sea floor spreading
  • Before World War II, we knew more about the Moon's surface than we did the ocean floor!
  • The need for better maps of the seafloor for submarine navigation led to a variety of studies of the seafloor that led to several amazing discoveries about the ocean floor:
    • From careful bathymetric measurements the following were discovered:
      • The mid-ocean ridge system
      • deep sea trenches
      • fracture zones
    • The ocean floor was made up almost entirely of the volcanic rock basalt (whereas the continents are made up of lighter-colored "granitic" rocks), and the basalt were very young at the ridges.
    • Sediments resting on the basalt were nil on the mid-ocean ridges and became thicker away from the ridges.
    • The same sediments resting directly on the basalt were older away from the ridges.
    • measurements of heat flow from the ocean floor showed that the ridges were hotter and heat flow declined away from the ridges.
    • Magnetic anomalies formed symmetrical, striped patterns parallel to the ridges. This observation was one of the most perplexing of them all.
  • Harry Hess proposed a process called seafloor spreading and Vine & Matthews explained the striped magnetic anomalies by combining seafloor spreading and reversals of the Earth's magnetic field. (Note: these reversals are a phenomenon different from polar wander, but they are occurring during polar wander--north pole becomes south pole and vise versa. They lead to an ambiguity in whether a continent was in the northern or southern hemisphere.)
  • A website explaining sea floor spreading quite simply is here.
  • Movie on seafloor spreading.
• Intro to Earthquakes
  • Fault - a fracture in the Earth along which movement has occurred
  • Normal fault
  • Thrust fault
  • Strike-slip fault (left lateral and right lateral)
  • Elastic Rebound Theory
  • Hypocenter (or focus) and epiceneter
  • P and S waves
• The role of earthquake studies in developing plate tectonics -  seismology, the study of earthquakes, led to a couple of important observations:
  • Distribution of earthquakes - earthquake epicenters were not randomly distributed.
    •  They formed well-defined belts of earthquakes that coincided with the mid-ocean ridges, the trenches and associated volcanic zones, and large faults such as the San Andreas fault.
    • At the trenches it was found that the earthquake foci formed a well-defined zone dipping down from the trench (the Wadati-Bennioff zone).
    • The fracture zones displacing the ridges had earthquakes only along the portion between the ridges; the extension of the fracture zones beyond the ridges was aseismic (no earthquakes).
  • Earthquake first motion studies (in-class demonstration with for Slinkys) showed the relative motion along some of the major faults of the Earth:
    • The Wadati-Benioff zone was a big thrust fault; the upper block was being shoved over the lower block (Father William Stauder of SLU was a pioneer in demonstrating this).
    • The faults that apparently displace the mid-ocean ridges had a sense of motion opposite to that initially thought.
• Tying it all together:
  • Continental drift + sea floor spreading + earthquakes studies led to plate tectonics

• The basics of plate tectonics
  • What is plate tectonics?
    • Earth has rigid outer shell = the lithosphere
    • The shell is broken up into distinct "plates" ; the plate boundaries are defined by the distribution of earthquake epicenters; these correspond to major physiographic features
    • The plates are moving relative to each other; about a few centimeters a year (your fingernails grow at about the same rate). The motion of plates allows three basic boundaries types of boundaries to be defined:
      • divergent boundaries (also called constructive plate boundaries)
        • marked by mid-ocean ridges (like the mid-Atlantic ridge) or major continental rifts (like the East African rift).
        • marked by shallow earthquakes
        • has basaltic volcanism
      • convergent boundaries (also called subduction zones or destructive plate boundaries)
        • marked by deep ocean trenches and/or a major mountain range like the Himalaya
        • also has volcanism (of a different sort to be covered later)
        • major earthquakes, both shallow and deep (Wadati-Bennioff zone)
      • transform plate boundary (also called strike-slip plate boundary or conservative plate boundary)
  • Continental drift and seafloor spreading are consequences of plate tectonics
    • seafloor spreading occurs where two plates are moving apart at divergent plate boundaries; it creates new plate material
    • continental drift is a consequence of continent crust being part of a plate (although not all plates have continental crust on them, for example, the Pacific plate).
    • What of the problem of the mechanism for continental drift?
      • Continents are not "plowing through" the oceanic crust; they are simply "riding" on a plate.
  • Movie on convergent margins
• A closer look at divergent plate boundaries
  • A cross section of an oceanic plate from ophiolites
    • sediments
    • volcanics - basalt
    • dikes
    • plutonic rocks - gabbro and others
    • mantle rocks
    • black smokers; massive sulfide deposits, copper, and the Bronze age; life not reliant on photosynthesis
• A closer look at convergent boundaries
  • the accretionary prism, the trench, volcanic chain, and the Wadati-Benniof zone
  • Two broad types:
    • when an oceanic plate subducts
    • when a continent is present on the subducting plate –  "collision" and major mountain belts
  • possible plate reorganization; change in position of subduction zone. The example of the Indian plate and the Himalayan collision zone
  • An example of a convergent boundary through time: western Washington state