1. Spring course selection? Consider GE254: Geomorphology! Meets MWF @ 9:00 a.m., lab on Weds. or Thurs., 1:00-4:00 p.m. Rivers! Deserts! Landslides! Glaciers! Volcanoes!Coasts! Can be used as a 2 nd science course for area requirements, & counts towards majors in Environmental Studies or Geology

2. Today's Geology Seminar: 1:00 p.m. Keyes 105

4. Faulting Review: JOINTS are simple fractures in the rocks, along which essentially no movement has taken place.  Normal faults are those in which the hanging wall has moved down relative to the footwall. They are produced by tension. Reverse faults are those in which the hanging wall has moved UP relative to the footwall. They are  produced by compression. Both are dip-slip faults.

5. Thrust faults are low-angle reverse faults, often with displacements measured in km. This is the Keystone Thrust of western Nevada, with the dark rocks pushed over the yellow, orange and pink rocks, which are some 300 million years younger. Thrust faults are created by collisional tectonics, and are one of the major ways the continental crust has been thickened over geologic time.

6. A close look at a canyon wall in Ecuador reveals some of the complexity forced by compression. overturned fold Figure is 5' (1.5 m) tall (1.5 m) tall

7. B. Strike-slip faults are those whose movement is mostly or entirely in a horizontal direction. This a product of horizontal shearing. Defining a fault as left-lateral or right-lateral is based on what relative direction the opposite side has moved relative to the other block - to the right, or the left - as shown in the above diagrams. ( right-lateral )

8. Imperial Valley, southern California Guatemala, 1976 Hollister, California

9. C. The most difficult of all to understand, initially, are the transform faults, named only in the 1960s by Canadian geophysicist J. Tuzo Wilson. These are features that exist SOLELY because of plate tectonics. Transformfault Locations of Earthquakes X X X X XX X X X X X

10. Earthquakes on the MORRS help to outline the plate margins and transform faults. Transform fault Spreading center (normal faulting from tension) Subduction zone (  thrust fault!)

11. The San Andreas Fault of California is a transform fault, extending from the Gulf of California to Cape Mendocino.

12. At its northern end is Cape Mendocino, beyond which the remnant of the East Pacific Rise known as the Gorda Ridge continues.

13. Mt. Lassen Mt. Shasta Juan de Fuca Ridge  Gorda Ridge  Mendocino Fracture Zone  Crater Lake Mt. Hood Mt. Rainier Cascadia Subduction Zone San Andreas Fault

14. DUE east of Cape Mendocino is Mount Lassen.

15. Farther to the south, the San Andreas looks like a rip across the land.

16. The fault itself is NOT that wide a zone in many places. At Cajon Pass, in southern California (right), it seems relatively minor, as sandstones are juxtaposed against a granitic rock. But in 1906, near the town of Olema, north of San Francisco, the Pacific Plate moved 22 feet (7 meters) relative to the North American plate. This is a replica of a fence that was offset.

17. Earthquakes on the San Andreas, like those on other transform and strike-slip faults, tend to be VERY shallow. Hollister, on the Calaveras Fault Olema

18. Next: How old is the Earth? How do we know? Geologic Time and Dating Techniques

19. Tectonic processes – e.g., sea-floor spreading, subduction, etc. – take place very slowly... about as fast as your fingernails grow. Given enough time, lots can happen.... just think of 27 years in your life.....

20. Dating Techniques in Geology NAH! We just wanna know how OLD the rocks are!

21. STRATIGRAPHY will give us relative ages - older vs. younger (and you've done THIS in lab already!) : Principle 1: Sedimentary rocks are, in general, derived from sediments originally deposited horizontally. Principle 2: In the sedimentary pile, those on top are younger than those beneath.

22. Principle 3: Any rock unit or structure that cuts across another MUST be younger than that which it interrupts.

23. As in all the sciences, we also apply what is commonly known as "Occam's Razor" – i.e., "Pluralitas non est ponenda sine necessitate." or "Plurality is not proposed without necessity." This is usually translated for modern times to "All things being equal, the simplest explanation is usually the right one." William of Ockham, (c. 1287–1347)

24. But HOW do we know how OLD they are, in years? We have what can be considered extremely reliable "geological clocks." How do they work? Isotopes... are different "varieties" of the same element. e.g., there are three isotopes of Hydrogen. Very often, the heaviest and lightest isotopes of elements are unstable - radioactive. The heaviest elements (e.g., uranium, plutonium, etc.) have NO stable forms; ALL isotopes of these elements are radioactive.

26. Where do they originate? a.Those with LONG half-lives (millions or billions of years) have been around since the formation of the Earth. b.Those with shorter half-lives (100s or 1000s of years) are forming in the environment today. One of each of these types is widely used in many settings, though there are literally dozens of different other specialized techniques: 1. Potassium-Argon (& Ar-Ar) Dating 2. Radiocarbon Dating

27. 1.Potassium-Argon dating 40 K - 40 Ar The calcium is "common calcium" - indistinguishable from any other "normal" calcium. BUT the argon can be trapped in crystal structures. SOME minerals hold it very well, others not so well.

28. KEY to the success of this method is that argon is a noble gas - one that will not combine with anything else. Minerals that hold the Argon well:microcline KAlSi 3 O 8 sanidine KAlSi 3 O 8 muscovite, biotite hornblende Orthoclase does NOT hold the 40 Ar well - because the crystal lattice is too open! The ratio between 40 K and 40 Ar in a mineral is therefore a function of its age. The half-life (t 1/2 ) of 40 K is 1.27 billion years.

29. The long half-life of 40 K means it's only applicable to relatively OLD specimens, generally >100,000 years (more commonly >1,000,000 years). consider : a rock weighing 500 grams (~1 lb.), 2% K by weight, and 100,000 years old, will only have generated 0.00001 mm 3 of 40 Ar! In igneous rocks, potassium-argon dating can tell you the time of crystallization - when the minerals crystallized. In metamorphic rocks, it will tell you the when metamorphism occurred, but NOT the age of the original parent material. WHY?? Why wouldn't this work to give you the age of a sedimentary rock, even if it was an arkose with feldspars, biotite and hornblende in it?

30. One of the oldest rocks known from Earth, the Acasta Gneiss of northwestern Canada, is 3.96 billion years old - a little more than three half-lives of 40 K. Less than 1 / 8 of the original 40 K is left.

31. But if the Earth's sedimentary record is the basis for our geologic time column, how do we go about putting ages on those rocks? The oldest specimens from the Apollo missions to the Moon (ultramafic plutonic rocks from the lunar highlands) have been dated at ~4.5 billion years.

32. To use Olduvai Gorge as an example....

33. For sections with fossils and NO datable materials, we use biostratigraphic correlation, drawing on known other sites.

34. Radiocarbon Dating: The great tool of the Quaternary ( Plus a few special wrinkles in dating techniques thrown in for fun! )

35. Radiocarbon ( 14 C) Dating * 14 C forms in the upper atmosphere from the interaction of cosmic rays and atmospheric nitrogen ( 14 N 2 ) * 14 C converts to 14 CO 2, and becomes incorporated in plant matter. Decay is relatively simple: As the neutron decays, the 14 C is converted back to 14 N.

36. The half-life (t 1/2 ) of 14 C is only 5730 years, however, so its practical limit is generally only up to ~ 40,000 years b.p. (7 half-lives or so) (though some specialty research labs have gone back to 70,000 b.p.). Useful ONLY on organic materials preserved in sediments - not sedimentary rocks. ( WHY NOT ?? )

37. Aniakchak Volcano erupted ~3400 years ago to devastate the SW Alaskan coast. Lower sediments in the section revealed that numerous volcanic ashes had buried the regional landscape previously. 11,250 +/- 50 6800 +/- 60 7900 +/- 50 8800 +/- 70 3400 +/- 80

38. Radiocarbon dating is also of paramount importance in archeology - the study of human cultural remains from the prehistoric past. People were living semi- permanently at this Alaskan riverbluff site some 8500 years ago.

39. Radiocarbon dating has also been used to determine the ages of human bodies found in peats in northern Europe. (Museum reconstruction of bog body burial) Note the rope tied around the neck. This one was an execution!

40. Conventional radiocarbon dating requires 30 grams (1 ounce) of organic matter, or more. A new technique, developed in the past 30 years, allows us to determine the 14 C content of a sample directly. This is called Accelerator Mass Spectrometer (or AMS) radiocarbon dating. It is a specialized 14 C dating technique specifically developed for very small samples. Costs: Conventional: $345; AMS: $595 per sample

41. A small mass spectrometer used in such analysis.

42. Arguably, no single human artifact has received more study than the Shroud of Turin. AMS dating of the shroud by three separate scientific laboratories showed that it is 690 +/- 30 14 C years in age. Thus, it dates only from ~1260-1385 of the Common Era. Only one thread fragment from the shroud was sacrificed for this age determination.

43. Cosmogenic nuclide dating - relies on interaction of cosmic rays with surface minerals - creation of specific isotopes reflects age of exposure of surface to the atmosphere The technique was developed by Paul Bierman at the University of Vermont ( http://geology.uvm.edu/morphwww/cosmo/lab/cosmolab.html)

44. Cosmogenic dating requires gathering samples of minerals (usually quartz) from exposed rocks - such as boulders on a glacial moraine.

45. We know from the presence of glacial erratics that the late Pleistocene ice (about 20,000 years ago) went over the tops of both Mt. Washington (at right) and Katahdin (below). HOWEVER, the cosmogenic dating of rocks from the summit areas indicate that the ice was probably quite thin over Mt. Washington.

46. Fission-track dating relies on spontaneous fission - splitting - of uranium atoms in minerals. The large particles created rip through the crystal structure. The density of the "tracks" through the grain is a function of its age and the uranium content. This technique is being used not only to determine ages of rocks, but also rates of landscape erosion and uplift.

47. Other dating techniques are particularly useful for special situations. Three that are fairly widely used include: Uranium-lead dating ( 238 U  206 Pb, 235 U  207 Pb) is useful for old shield rocks, either igneous or metamorphic. Rubidium-strontium dating ( 87 Rb  87 Sr) is also useful for the oldest metamorphic shield rocks, and can give ages of both the parent rock and the time of metamorphism. (T 1/2 of 87 Rb is ~ 50 BILLION yrs.) Uranium-series dating (relying on intermediate decay products) can be used sometimes to determine ages of corals, sedimentary units or soils up to 500,000 years old.

48. Rubidium-strontium dating ( 87 Rb  87 Sr) is also useful for the oldest metamorphic shield rocks, and can give ages of both the parent rock and the time of metamorphism. (T 1/2 of 87 Rb is ~ 50 BILLION yrs.) Uranium-series dating (relying on intermediate decay products) can be used sometimes to determine ages of corals, sedimentary units or soils up to 500,000 years old.

49. next: Mass Wasting: "Landslides" and related phenomena