Archaeological Geology
A recent trend in geoarchaeology is the use of archaeological artifacts and features to date geological events such as earthquakes, volcanic eruptions, and floods. In many cases, the artifacts and organic matter associated with them provide a much more detailed diary of the geological history in an area than any deposits, fossils, or soils present. A number of geologists and archaeologists who collaborated on such studies presented their research at the 1994 annual meeting of the Geological Society of America.
Earthquakes
Archaeological sites are being used to date movement along the San Andreas Fault and other faults in California. Sites cut by the faults provide a single "piercing" point that can be used to determine the amount of offset along the fault. In addition, the age of the site provides a maximum age for the earthquake event. In some instances the sites were reoccupied after the quake and the date of reoccupation provides a minimum date for the seismic event. Fortunately for modern earthquake studies, native Americans settled along faults thus providing abundant sites for study. This settlement pattern may result from an abundant water supply along the fractured rock and ponds that form where stream channels are disrupted.
Archaeological sites can also be used to date earthquakes in the New Madrid Seismic zone of the central United States. In this area, there are almost no surface faults because of the thick unconsolidated sediment in the region; however, saturated sand liquefies during large earthquakes and provides a record of the events. The liquefied sand may erupt from depth disrupting and sometimes burying sediment, soil, and archaeological sites at the ground surface. The ages of the buried and disrupted sites provide a maximum age for large earthquakes (M³6.8). Sites and archaeologic features that overlie or are cut into sand blows indicate minimum ages for these large earthquakes.
Along a coast, earthquakes may cause drowning of coastal occupation sites due to ground subsidence and tsunamis. In the Pacific northwest, a coastal sequence that records slowly rising sea level during the last 4,000 years is abruptly disrupted by rapid submergence that was again followed by slowly rising sea level. Hearths that had been built along a beach a millennia ago are now 2 meters below sea level and buried by tsunami deposits.
Volcanic eruptions
A volcanic eruption in the present country of El Salvador in Central America has been dated to within a range of 180 years (590±90 years) by using radiocarbon analysis of organic matter associated with the buried Mayan village of Ceren. Archaeology, however, allows even greater precision about the season and time of day of the eruption. Using preserved vegetation, such as maize plants in the buried agricultural fields and guayba fruit, the eruption is interpreted to have occurred in late August. Ethnographic analogy with contemporary agricultural communities of the area indicates that the eruption probably occurred between 6 and 9 p.m. because the agricultural field implements had been returned to storage, the evening meal dishes had not been cleaned, cooking fires had been allowed to go out, and bed mats had not yet been unrolled.
Stream valleys and deposits
Archaeology can also be used to date less catastrophic events along streams. Rivers in karst areas down cut into caves. Initial stream incision to the level of a cave can be dated using organic materials within the alluvium in the cave. By using caves at various elevations along a valley, the incision history of a river may be determined. For instance, the Tennessee River of northeastern Alabama has incised at least 12 meters in the past 15,000 years.
Not all streams incise; some streams aggrade. In the process of aggradation a stream deposits sediment and may also incorporate or bury human artifacts such as pot sherds, which change through time as styles and technology change. These ceramics were initially dated using tree rings from archaeological sites. The dates were later corrobated with radiocarbon analysis. Changes in the artifacts within aggrading stream deposits are used to date and determine the rate of aggradation. Streams in the southwestern Unites States can aggrade very rapidly and several began their most recent aggradation 1000 to 1100 years ago.
Human artifacts are the index fossils of the last 10 to 12,000 years in the New World and may be used for even longer periods in the Old World. The advantage of human artifacts is that the relatively rapid evolution of human artifacts allows more precise dating than do fossils of the more slowly evolving organisms. In addition, human artifacts proved dating tools where other materials suitable for dating are absent. These examples of using archaeology to date geologic events show a range of applications of geoarchaeology.