Half lives of different isotopes vary enormously from less than one second to billions of years. The effective interval for geologic dating using any particular isotope is from about 1/10th half life (minimum) to 10 half lives (maximum). This interval is related to the analytical limits of laboratory measurements of radioactive isotopes. After 10 half lives, less than 1/1000th of the parent isotope still remains--too little to detect (Hall 1995). Several geologically common isotopes have half lives of appropriate lengths for dating Quaternary materials. For example, the argon-argon method has proven quite effective for dating volcanic deposits of Pleistocene eruptions--see Fig. 10-2.

Laacher See volcano, Germany. Lake occupies the volcano caldera, and hills are formed by tephra rim around the caldera. This volcano was formed during the latest eruption from the East Eifel volcanic province, near the Rhine valley of western Germany. Photo date 7/95; © by J.S. Aber.

Laacher See tephra exposed in vertical wall of a pumice mine. The tephra consists of interbedded ash and pumice. Argon-argon and radiocarbon dates indicate an eruption age of 13,000 years BP (van den Bogaard 1995). This tephra is a key stratigraphic marker across western Europe in Italy, France, Switzerland, Germany, Benelux, Denmark, Sweden and Poland. It marks the Allerød, a warm interval near the end of the last glaciation. Photo date 7/95; © by J.S. Aber.

Indian Creek volcanic ash bed, southeastern Nebraska. Light-colored sediment in trench is ash and reworked ash mixed with local silt. The ash consists almost entirely of glass shards. Such ash beds are common across the Great Plains from Saskatchewan to Texas; most were derived from eruptions at Yellowstone. Volcanic ash can be dated using the fission-track method. Photo date 10/83; © by J.S. Aber.

Fission-track dating is complicated in two ways: (1) determining the uranium content of a sample and (2) fission-track annealing. Uranium content is determined by comparing the spontaneous (natural, ²³⁸U) fission tracks to induced (artificial, ²³⁵U) fission tracks--see Fig. 10-6. Inducing ²³⁵U fission tracks is done by irradiating the sample with high-energy neutrons in a nuclear reactor. The number of induced fission tracks depends on the sample's uranium content and the neutron dose. The two uranium isotopes always occur in the same ratio in nature; thus, the number of induced fission tracks is a measure of total uranium content in a sample.

Fission-track annealing takes place over time, as the minute damage tracks recrystallize and disappear. The rate of annealing depends on temperature and type of material. High temperature accelerates annealing; thus, fission-track dating gives the age since the sample has cooled below metamorphic temperatures. Glass is especially subject to annealing even at low temperature. This effect is demonstrated by the Huckleberry Ridge ash of the Yellowstone region (Naeser and Naeser 1988). Fission-track dates on glass usually fall in the 1.2 to 1.4 million years BP range; whereas, fission-track dates on zircon crystals are consistently 1.9 million years BP. Other types of dating support the 1.9 million year age for the Huckleberry Ridge ash.

The rate of production of ³⁶Cl depends on the concentrations of the target elements (K, Ca, Cl), sample elevation, surface orientation, and geomagnetic latitude (which governs the cosmic-ray flux). The rate of accumulation of ³⁶Cl depends on the production rate and on the rate of erosion of the rock surface, if the surface is not stable (Jackson et al. 1997).

The primary difficulties of this method are twofold--sample analysis and uncertainty of erosional history of rock surfaces. Sample analysis involves complete solution of the rock material in strong acids (HF and HNO₃) followed by accelerator mass spectrometric analysis of the ³⁶Cl/Cl ratio. These techniques require special laboratory procedures and equipment, which are expensive and not widely available. Erosional history of rock surfaces is often difficult to constrain, and so must be estimated based on geomorphic field evidence. In spite of these complications, cosmogenic dating has proven effective in situations where other types of dating were not feasible--see Figs. 10-7 & 8.

Giant erratic blocks of quartzite at Okatoks, Alberta, Canada. These erratics were transported from the Canadian Rocky Mountains by the Cordilleran ice sheet. They are part of the "foothills erratic train" that extends from Jasper southward to near the U.S.-Canada border. These erratics were successfully dated using cosmogenic isotopes--see Figs. 10-7 and 8. Photo date 8/84; © by J.S. Aber.

Paleomagnetism

	Thick basaltic lava flows form the resistant ledge of the waterfall, Fagifoss, Iceland. Such iron-rich volcanic rock is excellent material for paleomagnetic measurements. Its magnetic character was acquired at high temperature as the lava cooled. Photo date 8/94; © by J.S. Aber.
	Iron-cemented sand and gravel exposed at Ristinge Klint, Langeland, Denmark. The orange-brown color indicates secondary iron cementation of the sand and gravel sediment. Such cement may preserve primary remnant magnetism that dates from the time the cement was chemically precipitated. Photo date 8/82; © by J.S. Aber.
	Collecting paleomagnetic samples from unweathered gray till at the Independence Formation stratotype, Atchison, Kansas. These samples possess weak, but stable, natural remnant magnetism with normal polarity. Ibrahim Abdelsaheb (left) and Brian Nutter. Photo date 10/87; © by J.S. Aber.
	Split core sample of subsurface till from Nemaha County, northeastern Kansas. This core sample preserves weak, but stable, natural remnant magnetism with normal polarity. Core is about 8 cm (3 inches) in diameter. Photo date 11/89; © by J.S. Aber.

The orientation of the Earth's magnetic field at any point is specified by two measurements: declination (trend) and inclination (plunge). The inclination varies from horizontal at the equator to vertical at the poles--see Fig. 10-9. The field is directed downward in the northern hemisphere and upward in the southern hemisphere at present. The magnetic field periodically reverses its polarity at intervals ranging from several 1000 to > 1 million years; during reversed polarity, a compass needle would point south.

Polarity reversals should be excellent stratigraphic markers. Reversals are global in extent, they are detectable in many kinds of rocks and sediments, and the ages of reversals can be dated by radiometric techniques. Remnant magnetism of samples is measured using either spinner or astatic magnetometers. Secondary remnant magnetism acquired long after the time of rock formation is a troublesome factor. The effects of secondary magnetism can usually be removed from samples by stepwise demagnetization. Various field tests may also be applied to determine the origin of natural remnant magnetism in samples.

A detailed record of paleomagnetic reversals is available for the Cenozoic Era--see Fig. 10-10. Positive (normal) intervals are numbered, while reversed episodes are unnumbered; the younger anomalies have been given individual names. The Neogene/Quaternary boundary is currently defined at the end of anomaly 2 (Olduvai event = 1.6 million years BP); and the Early/Middle Pleistocene boundary is set at the beginning of anomaly 1 (Brunhes epoch = 720,000 years BP). The Blake event was a short-lived reversal during the Brunhes epoch, about 123,000 to 117,000 years BP (Abrahamsen 1995). It corresponds to the Sangamon/Eemian high stand of sea level.

By itself, paleomagnetism cannot give an absolute date. However, in combination with other dating techniques it may further refine or verify the age of Quaternary sediments, rocks, and fossils.

1. Relative dating -- Average size of maximum lichen thalli can give an indication of relative age (young vs. old) when comparing different landforms. This information can be used in the absence of any other type of dating.

2. Absolute dating -- The sizes of lichen thalli are calibrated with some independent dating method, such as radiocarbon or dendrochronology, to determine lichen growth rates (mm per year). On this basis, lichen size is an indicator for absolute age of the rock surface; however, use of a lichen growth curve is restricted to the immediate vicinity in which it was developed.

The following examples demonstrate lichens on periglacial features in vicinity of Trinchera Peak, Sangre de Cristo Mountains, southern Colorado. Periglacial phenomena include inactive rock glaciers (Wallace and Lindsey 1996), as well as stone polygons, terraces, and talus streams. Lichen coverage shows considerable variation on these landforms, indicating different ages for most recent activity. The youngest periglacial forms are located on the steepest slopes. The Sangre de Cristo Mountains were subjected to a series of late Holocene cold climate intervals that resulted in cirque glacier advances and/or periglacial phenomena--see Table 10-1.

Table 10-1. Chronology of late Holocene cold climate episodes in the Sangre de Cristo Mountains of northern New Mexico.

Climatic episode	Age*
Periglacial event	ca. 4900 yr BP
Cirque glaciation	ca. 3700 yr BP
Periglacial event	ca. 2800 yr BP
Little Ice Age	ca. 120 yr BP

* Approximate date in ¹⁴C years before present.
Based on Armour et al. (2002).

	Jet Tilton kneels at the center of a stone polygon on the crest of Trinchera Peak, approx. 13,500 feet (4100 m) elevation. Photo date 8/01, © J.S. Aber.
	Closeup view of stones in polygon on crest of Trinchera Peak. Note dense coverage of lichens. Large, lime-green lichens are Rhizocarpon. These stones have been relatively stable for several centuries. Photo date 8/01, © J.S. Aber.
	Closeup view of stones in talus stream on steep slope. Note dense lichen coverage and growth of grass and shrubs. These features indicate a stable landform for some time. Photo date 8/02, © J.S. Aber.
	Stone terrace on steep slope below Trinchera Peak. Many of the upturned stones bear few lichens, and Rhizocarpon thalli are small in size. This terrace probably was active during the Little Ice Age in the 19th century. Photo date 8/01, © J.S. Aber.