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 2nd science course for area requirements, & counts towards majors in Environmental Studies or Geology

Today's Geology Seminar:

1:00 p.m. Keyes 105

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.

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.

A close look at a canyon wall in Ecuador reveals some of the complexity forced by compression.

overturned overturned foldfold Figure is 5'Figure is 5'

(1.5 m) tall(1.5 m) tall

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 )

Imperial Valley, southern CaliforniaImperial Valley, southern California Guatemala, 1976Guatemala, 1976

Guatemala, 1976Guatemala, 1976 Hollister, CaliforniaHollister, California

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.

TransformTransformfaultfault

Locations of Earthquakes

XXXX

XXXX XX XX XX

XXXX

XX

XX

Earthquakes on the Earthquakes on the MORRS help to outline MORRS help to outline the plate margins and the plate margins and transform faults.transform faults.

Transform fault

Spreading center (normal faulting from tension)

Subduction zone ( thrust fault!)

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

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

Mt. Lassen

Mt. Shasta

Juan de Fuca Ridge

Gorda Ridge

Mendocino Fracture Zone

Crater Lake

Mt. Hood

Mt. Rainier

Cas

cadi

a Su

bduc

tion

Zon

e

San Andreas Fault

DUE east of Cape Mendocino is Mount Lassen.

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

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.

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

Hollister, on the Calaveras FaultHollister, on the Calaveras FaultOlema

Next:Next:

How oldHow old is the Earth? is the Earth?How do we know?How do we know?

Geologic Time and Geologic Time and Dating TechniquesDating Techniques

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 .....

Dating Techniques Dating Techniques in Geologyin Geology

NAH! We just NAH! We just wanna know wanna know how how OLDOLD the the rocksrocks are! are!

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.

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

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)

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.

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:

1.1. Potassium-Argon (& Ar-Ar) DatingPotassium-Argon (& Ar-Ar) Dating2.2. Radiocarbon DatingRadiocarbon Dating

1. Potassium-Argon dating 40K - 40Ar

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.

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 KAlSi3O8

sanidine KAlSi3O8

muscovite, biotitehornblende

Orthoclase does NOT hold the Orthoclase does NOT hold the 4040Ar well - because Ar well - because the crystal lattice is too open!the crystal lattice is too open!

The ratio between 40K and 40Ar in a mineral is therefore a function of its age.

The half-life (t1/2) of 40K is 1.27 billion years.

The long half-life of 40K 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 mm3 of 40Ar!

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?

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 40K. Less than 1/8 of the original 40K is left.

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.

To use Olduvai Gorge as an example....

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

Radiocarbon Dating:Radiocarbon Dating:

The great tool ofThe great tool ofthe Quaternarythe Quaternary

( ( Plus a few special wrinkles in dating Plus a few special wrinkles in dating techniques thrown in for fun! techniques thrown in for fun! ))

Radiocarbon (14C) Dating

* 14C forms in the upper atmosphere from the interaction of cosmic rays and atmospheric nitrogen (14N2)

* 14C converts to 14CO2, and becomes incorporated in plant matter.

Decay is relatively simple:As the neutron decays, the 14C is converted back to 14N .

The half-life (t1/2) of 14C 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 ??WHY NOT ?? ))

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 +/- 5011,250 +/- 50

6800 +/- 606800 +/- 60

7900 +/- 507900 +/- 508800 +/- 708800 +/- 70

3400 +/- 803400 +/- 80

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.

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!

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 14C content of a sample directly.

This is called Accelerator Mass Spectrometer (or AMS) radiocarbon dating. It is a specialized 14C dating technique specifically developed for very small samples.

Costs: Conventional: $345; AMS: $595 per sample

A small mass spectrometer used in such analysis.

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 14C 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.

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)

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

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.

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 The density of the "tracks" through the grain is a function of its age and the uranium content. 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.

Other dating techniques are particularly useful for special situations. Three that are fairly widely used include:

Uranium-lead dating (238U 206Pb, 235U 207Pb) is useful for old shield rocks, either igneous or metamorphic.

Rubidium-strontium dating (87Rb 87Sr) is also useful for the oldest metamorphic shield rocks, and can give ages of both the parent rock and the time of metamorphism. (T1/2 of 87Rb 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.

Rubidium-strontium dating (87Rb 87Sr) is also useful for the oldest metamorphic shield rocks, and can give ages of both the parent rock and the time of metamorphism. (T1/2 of 87Rb 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.

next:

Mass Wasting:"Landslides" and

related phenomena