Academic journal article
By Bailey, Geoff; King, Geoff; Sturdy, Derek
Antiquity , Vol. 67, No. 255
Tectonics and human behaviour
Humans live at the interface between the solid earth and the unstable atmosphere. The earth appears constant and safe, while the atmosphere is changeable and seems to have the greatest effect on environment. Droughts or floods, erosion or deposition of sediment and the consequent fertility or barrenness of soils are most readily explained by climate, or in recent millennia by human interference. Large-scale tectonic processes of mountain building or subsidence, driven by the motions of the continental plates, seem to operate too slowly, and to belong to a too far-distant geological past, to have any impact within the time span of human occupation. Even the most rapid changes -- earthquakes and volcanic eruptions -- appear disruptive and temporary in effect unless they seem to trigger some far-reaching social change.
Here we argue that this perspective is too simple. The inexorable changes in landscape geometry caused by deformation of the solid earth can determine the way in which climate affects the landscape, and the way in which the landscape in its turn affects, and is affected by, human activity. In many parts of the world, an appreciation of tectonics is needed to understand the distribution of agricultural soils or of animal herds. On the longer time-scales of prehistory, tectonically driven landscape change may become an active agent of selection, creating pressures or opportunities for changes in behaviour. It is easy to see that tectonic uplift can destroy an environment by triggering erosion, but active deformation is by no means always negative in effect. Tectonic subsidence can create well-watered sediment traps and the accumulation of fertile soils. Uplift and subsidence also create natural barriers to animal movement that can be exploited by humans. It is interesting to note that many early Palaeolithic sites are in regions of tectonic activity. The East African Rift Valley, North Africa, the Levant and Sub-Himalayan India are obvious examples. Undoubtedly the ease of site discovery can be assisted by tectonic changes. Tectonically active areas are those most likely to develop thick, rapidly formed sequences of terrestrial sediments favouring burial and fossilization of archaeological evidence, followed by rapid erosion and exposure to discovery. But the reverse is true and tectonically active regions can destroy or obscure human evidence within decades or centuries. In our discussion of Epirus, we discuss why ease of discovery is not the main reason why a correlation between early human settlement and tectonic activity is to be expected.
Since the advent of Plate Tectonics, it has been evident that deformation of the Earth is a continuing process. Relative motions at major plate boundaries have rates that can approach 20 cm per year (Demets et al. 1990). In oceanic regions deformation can be localized to ridges, trenches or transform faults with widths of less than 100 km. Where plate boundaries cross continental crust, contraction, extension or sideways motion (strike-slip) can be spread over regions with widths exceeding 1000 km. The overall strike-slip motion of 5 cm per year between the Pacific and North American plates, commonly regarded as being accommodated by the San Andreas fault, actually extends from a short distance off the Californian coast to central Utah (Slemmons et al. 1991). At most, 50 % of the deformation (2.5 cm per year) occurs as strike-slip on the San Andreas, while the remainder is distributed on contractional, extensional and strike-slip structures which have lower rates. These lower rates can nonetheless result in substantial changes. The Coastal Ranges south of San Francisco, rising at 1 mm per year (Valansise 1992), are 1000 m high and less than 1 million years old, while the floor of Death Valley has dropped relative to its uplifting flanks by a similar amount in the same period of time (King & Ellis 1990). …