Introductions:

Readings for the week:

• USGS - The Dynamic Earth - This site provides an overview of plate tectonics. Some of this you will have likely encountered before, but it is a good place to start as a refresher.

Definition of tectonics: science of trying to understand the large scale movements and deformation of the earth's crust.

Definition of plate tectonics:

The earth's outer shell, the lithosphere, is broken up into distinct plates that have moved large distances with respect to each other over geologic time spans. Average rates of motion are in the range of cms/year. Plate interiors are relatively stable. Geologic activity is concentrated at plate boundaries, and the type of relative motion and the type of lithosphere involved are primary influences on the type of activity.

Caution: We will get into an interesting discussion on what defines plate boundary vs. plate interior material later on.

Plate tectonics as a major paradigm in earth science - what does paradigm mean?

• How do we know this?
• How well do we know this?
• What are the geologic results of this motion at different scales?
• Why do the plates take the paths they do?
• How has tectonic behavior changed over time?
• Chris Scotese's plate movement animations - http://www.scotese.com/newpage13.htm.

• Retreat of Niagara knickpoint. Niagara River started cutting into the Niagara escarpment some 10,000 years ago, and has since cut a notch back some 10 km. Compute the rate of retreat in cm/yr and comment on the rate. Info source Bates, R. L., 1986, Laboratory Manual in Physical Geology, AGI, NAGT; Merrill Publishing Company, 177 p.
• Uplift of marine terraces in a tectonic environment, Fiordland, New Zealand. Terraces at 370 m elevation above sea level are about 130,000 years old. Compute a rate in cms/yr and comment on this rate. Info from: Keller, E. A. & Pinter, N., 1996, Active Tectonics, Prentice Hall, p. 162.
• Sedimentation rates in Svalbard: Some 1900 m of strata were deposited in a Tertiary basin from roughly 65-32 million years ago, during a time of active tectonism. Some 380 m of strata were deposited from 265-252 million years ago, during a time of relative tectonic stability. Compute rate of sedimentation in millimeters per year and comment on them. Info from: Dallman, W. K. (ed.), 1999, Lithostratigraphic Lexicon of Svalbard, Norsk Polarinstitutt.
• Rate of movement on the San Andreas fault. Volcanic units of identical age, 23.5 million years, and chemistry are separated by 315 km along the San Andreas fault. Assuming they were once part of the same volcanic construct that come up along the fault, what is there rate of separation in cms/yr. Along another stretch of the San Andreas a stream channel was offset 250 m from 5,900 to 3,900 years ago. What is the offset rate? Info from: Mathews, V., 1976, Correlation of Pinnacles and Neenach Volcanic formations and their bearing on San Andreas Fault problem: AAPG Bulletin, v. 60. Keller, E. A. & Pinter, N., 1996, Active Tectonics, Prentice Hall, p. 162.
• Rate of sedimentation in Ridge Basin, next to the San Andreas. In Late Miocene to Pleistocene, some 10 million years, has a structural sediment thickness of 4000 meters. What was the rate of subsidence in cm/yr. Info from: Crowell, J. C., 1984, Origin of Late Cenozoic Basins in Southern California; in Sylvester, A. G., Wrench Fault Tectonics; AAPG Reprint Series # 28.
• Speleothem growth from Northern Oman. Using U-Th techniques layers were date in speleothems. The purpose was to also look at the oxygen-18 isotope values and pull out a paleoclimate record, but it also allows computation of how fast speleothems grow (longer term average). In 9,700 years some 320 mm of material was added to one stalagmite, and in 101,000 years some 2,200 mm were added. Calculate the rate of speleothem growth in mm/yr. Info from Burns, S. J., Matter, A., Frank, N., Mangini, A., 1998, Speleothem-based paleoclimate record from northern Oman; Geology, 26, 499-502.
• Beach ridges on Spitsbergen, glacioisostatic rebound: On Kong Karls Land raised beaches about 10,000 years old are now at 130 m elevation above sea level. Another beach some 6500 years old in central Spitsbergen is at an elevation of 11 m above sea level. Calculate the rate of relative uplift in cms/yr. Info from: Hjelle, A., 1993, Geology of Svalbard, Norsk Polarinstitutt, Oslo, Norway, 163 p..
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  This is a photograph of the Festningen section area along the west coast Spitsbergen. Note the modern beach deposits and bench in lower right. Several geologists are standing on an old marine platform with beach deposits. Above them is yet another. Altogether there may be 5 distinct paleoshorelines preserved here. These are from glacio-isostatic rebound, or the uplift of the crust after ice has been removed by melting processes. Organic matter (e.g. driftwood, whale bones) that is found in the beach deposits can be dated and from this information a history of uplift obtained.

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• Glacial surge movement, Usherbreen, Svalbard: In 1978-1980 (3 years) the glacier front moved forward 300 m. In 1985 it had surged a total of 1.5 km. Calculate glacier movement rates in km/yr and . Info source: Hagen, J. O., 1987, Glacier surge at Usherbreen, Svalbard; Polar Research, 5, p. 239-252.
• compare the various rates computed, and draw some general conclusions

Fundamental divisions of the earth to be familiar with:

• atmosphere, hydrosphere, biosphere, cryosphere, lithosphere.
• continental crust vs. oceanic crust.
  • continental crust:
    • up to 3.9 billion years old
    • granitoid and heterogeneous in composition. Quartz and feldspar dominant minerals.
    • 30-70 km thick.
    • relatively weak, and exhibits considerable internal deformation.
  • oceanic crust:
    • 180 million years maximum in-place age.
    • basaltic and relatively homogeneous in composition.
    • 6-7 km thick with some 4 km average water thickness.
    • relative strong, and distinctly layered.
• crust vs. mantle vs. core:
  • image to right from USGS Dynamic Earth. Much has been learned since this was drawn which we will discuss as the course proceeds.
  • planetary differentiation: crust is partial melting froth that has come out of the convective turmoil and partial melting of the mantle.
  • mantle:
    • primary differentiate from very early accretionary history of earth.
    • ultramafic in composition; olivine and pyroxene are dominant minerals at shallow levels.
    • under tremendous range of P-T conditions and can go through a myriad of phase changes.
    • upper and lower mantle
  • core:
    • outer core liquid iron; therefore convects readily.
    • a lot going on at core-mantle boundary that we are just learning about.

Some historical points:

• Da Vinci, 1400s - "From the two lines of shells we are forced to say that the earth indignantly submerged under the sea and so the first layer was made, and that the deluge made the second."
• Nicolas Steno, 1660s - Collapse of large subterranean hollows was cause of tectonism and the source of waters for the deluge.
• Hutton, 1770's - Uplift and subsidence very important part of his rock cycle "machine".
• Charles Darwin, in Voyage of the Beagle: earthquakes and atolls.
  • writing of an earthquake he experience in 1822 in western South America
  • "There can be no doubt that the land round the Bay of Concpecion was upraised two or three feet .... The elevation of this province is particularly interesting, from its having been the theatre of several other violent earthquakes, and from the vast numbers of sea shells scattered over the land, up to a height of 600, and I believe 1000 feet ..... it is hardly possible to doubt that this great elevation has been effected by successive small uprisings, such as that which accompanied or caused the earthquake this year, and likewise by an insensibly slow rise, which is certainly in progress on some parts of the coast."
  • Below is show a picture of an atoll (source: http://en.wikipedia.org/wiki/File:Atafutrim.jpg). One of Darwin's earlier contributions was to realize that these were the end result of a volcanic island, that once volcanism ceased, started to sink, while the coral reef continued to build up on its own foundation.
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  • Below is a diagram from Darwin capturing this basic idea. Source: http://commons.wikimedia.org/wiki/File:Britannica_Coral-reefs_Atoll_Formation.png
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What are various indications of vertical movement, uplift and subsidence?

• a major proponent was James Dana in 1890s.
• three stage general history:
  • development of linear trough of subsidence with peripheral bulges, and infilling with sediment.
  • sediments at depth get heated, producing magma, and weakening roots.
  • trough collapses and inverts to form a mountain belt.
• the focus was on vertical tectonics, with subsidiary horizontal or contraction motions. Collapse and contraction was due to earth's thermal cooling over time and associated shrinkage. Often two parallel belts (miogeosyncline and eugeosyncline) were envisioned, one characterized by igneous activity.
• many unsatisfactory aspects recognized at the time. What might some of these have been?
• used as a basic paradigm in Russia right up to the 1980s (interesting history of science project right there)!

Significance of geologic mapping in 1800s. This mapping documented significant contraction and extension of crust. Glarus thrust in Switzerland - 50 km of movement on one subhorizontal fault! Seuss, at turn of century recognized many more nappes and contractional structures in the Alps. How could this be explained in the world of vertical tectonics?

Image to right is a 1812 painting of Glarus Thrust by Hans C. Escher von der Linth showing the Glarus Thrust as the somewhat inconspicuous, subhorizontal contact between the darker and lighter colored units. Since this contact could be traced for quite some distance and was a fault, it had important implications for the amount of thrusting that has occurred in mountain belts. Image source: http://en.wikipedia.org/wiki/File:Escher_Martinsloch.jpg.

1881 Reverend Osmond Fisher, Physics of the earth's crust:

• developed idea of isostatic compensation.
• postulated fluid interior with convection currents
• oceans expanding and continents contracting
• moon born from earth.

The importance of isostasy:

• what is isostasy ?
• will explore the significance of isostasy shortly in another exercise.

Alfred Wegener's contributions to continental drift theory.

• 1929, Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans).
• received a lot of attention, and hence worth scrutiny.
• preceded by Taylor, 1910.
• continental drift was generally rejected by geologic community with a few minority community exceptions. What has to wonder what the reasons for rejection, given the present acceptance of plate tectonics?

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  Figure originally from du Toit (1921), but used by Wegener to show the geologic similarity between South America and Africa.

Each group will read a select portion of Wegener's book, and report to the rest of the class on the following. Spend about 15 minutes reading the selection, and 5-10 minutes discussing the answer to the following questions. Select a spokesperson to report on your findings.

• What was the basic argument Wegener was presenting in the section you read?
• To your knowledge is it correct?
• If not how is it incorrect?
• Considering only this one argument is there anyway to try and counter it; e.g. is there an alternate explanation to continental drift?

The science of paleomagnetism:

The magnetic field at any point is a combination of an internal core component, a rock component (NRM), and an "astronomical" component. The former usually dominates, which is lucky for us as it permits navigation. In our case we are interested in where the rock's magnetic field component comes from?

Two major ideas that come out of looking at NRM histories:

• polar wandering and drift.
• polarity reversals.

Image of recent polar wandering which contributes to the small yearly changes in magnetic declination and inclination. This recent polar wandering is different from the longer term polar wandering associated with plate motions (as we will see). Image source: http://www.nationalatlas.gov/articles/geology/a_geomag.html

Background: Usually a good magnetometer is needed to sense a rock's NRM, but for some rocks particularly rich in magnetic minerals the field the rock generates is strong enough to effect a standard compass. Banded iron formations with magnetite are a good example of such a rock.

Activity:

• Note the geologic character of the specimen.
• Take the compass and find north direction at a distance of at least several feet from the specimen. Note the orientation with respect to the table or building walls.
• Note how the direction changes as you bring the compass closer to the specimen.
• Note and draw a diagram of how the compass changes as you circle it around the specimen.

What can you conclude from this exercise?

Introduction to next week's topic - A geotechnical and geophysical revolution - seafloor spreading.

Bibliography for this week:

• Berkland, J. O., 1979 , Eilisee Reclus -- Neglected geologic pioneer and first (?) continental drift advocate: Geology, 7, 189-192.
• Biram, J. - 1966 - Translation of Alfred Wegener, 1929, The Origin of Continents and Oceans; Dover Press, 246 p.
• Hallam, A., 1983, Great Geologic Controversies - Chapt. 5, Continental Drift; Cambridge Press, p. 110-156.
• Holmes, A., 1944, The Machinery of Continental Drift: the Search for a Mechanism; in Principles of Physical geology, p. 505-509, Thomas Nelson and Sons Ltd.
• Lowman, P. D., Jr., 1983, faulting Continental Drift, The Sciences, p. 34-39.
• Milnes, A. G., 1979, Albert Heim's general theory of natural rock deformation (1878); Geology, 7, 99-103.