26 January 2011
patterns of historic human-caused erosion
I am pretty enthusiastic at the geologic direction being taken by certain segments of the architecture/built environment/landscape blogs, leading up to the "Landscape Futures Super Workshop" Bldgblog put on recently in Los Angeles (see also Friends of the Pleistocene's work on the Geologic City for more of this long-view exploration of the geologic origins of the built environment). The geologic space of the contemporary city is a fascinating and highly relevant direction for geographically-minded artists and scholars to turn. While not directly infrastructural nor relevant to digital communication or the spaces of mobile connectivity, I am posting below some research I have been doing on anthropic geomorphology, human-caused erosion.
As earth moving and manipulation technologies have evolved, humans have gone from hunting and gathering, to using iron ploughs, to industrial agriculture, and from walking footpaths to commuting on continent-spanning freeway systems. Humankind’s impact on global-scale geomorphology has grown with the exponential population gains. The rates of anthropic erosion that are a by-product of modern, global living now greatly exceed rates of natural soil formation. Anthropogeomorphology has over the last century attempted to quantify the extent of human-induced earth change. More recent scholarship has attempted to predict future change. Since humans are now the primary agent of geomorphology, the global scale and short time-frame of change will be as damaging to humanity’s long-term survival as global climate change.
Below is a review section of recent and not-so recent scholarship into anthropogeomorphology.
HISTORIC HUMAN-CAUSED EROSION
Although the extent of anthropogenic geomorphology has grown enormously since the Industrial Revolution, it is a process that has occurred since the Paleolithic period 1,000,000 years ago. The first anthropogeomorphic activity was the making of seasonal shelters from rock through the moving of boulders for the walls, and “foundations and floors from small rubble” (Hooke 2000, 843). Constructing rock shelters was most likely the only major anthropogeomorphic activity until “ten thousand years ago, in the late Paleolithic, [when] humans quarried flint” (Hooke 2002). This need for flint for stone tools initiated the first instances of mining (Hooke 2000, 843).
The initial permanent human settlements have been traced back to around 14,000 BC. Agricultural cultivation started between 6,000 and 8,000 BC and led to the first centralized, urban spaces (both Morris 1994, 3). The development of villages then cities, meshed with the rise of agrarian societies, together initiated the use of more building materials for the permanent dwellings and the need for irrigation projects to water the crops. The construction of canals and dikes followed, and became the first large-scale earth-moving activities (Hooke 2000, 843). In roughly the same time period the wheel was invented, which “facilitated transport of geologic materials, both ore and stone, as well as other trade goods. Because loads in carts cannot be moved efficiently over rough terrain, roads were invented to make maximal use of the increased hauling capacity provided by the wheel (Hooke 2000, 843).
Copper mining and then bronze smelting, combining copper with tin, initiated new mining techniques. It was not until the Iron Age, 2,500 years ago, that iron became cheaper and thus available to a larger population. This initiated a positive feedback loop where, as one activity required the other, iron plough blades as well as iron hand tools allowed agriculture, mining, and stone masonry to advance upon the earth, creating the need for more iron ore to be mined, and so on. The more iron cast, a greater manipulation of the natural landscape was made possible (Hooke 2000; 2002). With more processes, more materials, more efficient technologies, and most importantly, a growing global population, anthropogeomorphology has continued unabated, growing exponentially with the passing years (Haff 2003).
The primary cause of global-scale erosion became anthropogenic “sometime during the latter part of the first millennium A. D.” Human-caused erosion is not just a symptom of industrial society—it predates that by a millennium (Wilkinson 2005, 161). Through an examination of “prehistoric denudation rates imposed on land surfaces solely by natural processes,” Bruce Wilkinson determined that:
“Mean denudation over the past half-billion years of Earth history has lowered continental surfaces by a few tens of meters per million years. In comparison, construction and agricultural activities currently result in the transport of enough sediment and rock to lower all ice-free continental surfaces by a few hundred meters per million years. Humans are now an order of magnitude more important at moving sediment than the sum of all other natural processes operating on the surface of the planet” (Wilkinson 2005, 161).
More important than the erosion alone, this erosion associated with agriculture and construction “exceed soil formation by an order of magnitude” (Wilkinson 2005, 161). Not only do human actions account for the majority of erosionary activities, it also greatly outpaces soil formation. To determine these rates Wilkinson set a baseline through estimating the “uplift and erosion [of sedimentary rock which] results in a progressive decrease in epoch-long interval rock volume with increasing age.” He goes on, stating that “data on surviving amounts of sedimentary rock therefore allow for estimation of epoch-long rates of sediment accumulation, which in turn relate to rates of physical and chemical denudation over Earth’s subaerially exposed surface for at least the past half-billion years” (Wilkinson 2005, 161). As the base level of all human actions, the amount of earth-change construction and agriculture accomplish in creating sustenance and shelter are so grand they are difficult to conceive of. We as humans both individual and as a species impact the Earth enormously: “soil and rock movement currently amounts to ~21t per person per year (6 from construction, 15 related to farming)” (Hooke 2000). Also, over-irrigation, which leads to soil salination, and fertilization of cropland are both instances of chemical weathering (Brown 1970, 79). Even if the erosion is balanced across continents, the overdeveloped north accounts for much more than the global south. Wood and other construction products are made elsewhere, and agricultural commodities and products travel to global markets. The links are complicated and only the earth-change is universal.
Construction sites cause significant erosion—in Japan alone urban land use development accounted for 1.3 x 10 (to the 9th) cubic meters a year in 1970 (Kadomura 1980, 138). The need for construction materials from offsite and out of the area, of a sort that modern modes of transportation make possible, is also impacting geomorphic processes greatly: “Demand in the UK [for aggregates for concrete] has gone from 20 million tones per annum in 1900, to 50 m tones in 1948, to 276 m tones in 1973. It is an increase per capita from 0.6 tones per year to 5 tones per year” (Goudie 2000, 269).
At the rate it is occurring today, “the amount of [eroded] material would fill the Grand Canyon of Arizona in ~50 years,” again a scale much larger than that of the landscape, which is the largest the human eye can take in. This matters geomorphologically because “over the interval of anthropogenic erosion, delivery of sediment to rivers quickly exceeds rates of transport that are possible in fluvial systems that have more or less attained geomorphic equilibrium over a significantly longer prehistory with an appreciably lower sediment flux” (Wilkinson 2005, 163-164). What humans do at every level is to disrupt geomorphic equilibrium; this is anthropogeomorphology’s end-result.
While much of this research is solidly academic, the statistical information about society's use of geologic materials such as the aggregates for concrete, is fairly stunning when considered as an agent in erosion and general geomorphic change. With every new building and road, let alone skyscraper or elevated freeway, a bit more of the earth's surface changes location, never to return to its point of origin. Seeing the city as the current end point of the flows of ores and aggregates, even down to the computer I am typing on and that, ostensibly, this blog will be read on, changes how one views the landscape. The asphalt paving on the roads again becomes dead dinosaurs, the brick facade of a row home no longer solely reddish rectangles stacked two stories high.
Brown, E. H. (1970). "Man shapes the Earth." The Geographical Journal 136(1): 74-85.
Goudie, A. (2000). anthropogeomorphology. Dictionary of physical geography, 3rd ed. D. S. G. T. Andrew Goudie. Malden, MA, Blackwell Publishing.
Goudie, A. (2000). The human impact on the natural environment. 5th ed. Cambridge, MA, The MIT Press.
Haff, P. K. (2000). "Prediction and the anthropic landscape." Eos Trans. AGU, 81 (48) Fall Meet. Suppl., Abstract xxxxx-xx.
Haff, P. K. (2003). "Modeling and predicting human impact on landscape." GSA 2003 Seattle Annual Meeting, abstracts.
Hooke, R. L. (2000). "On the history of humans as geomorphic agents." Geology 28(9): 843-846.
Hooke, R. L. (2002). Humans are geomorphic agents. Geological Society of America 2002 Denver Annual Meeting, Denver, CO.
Kadomura, H. (1980). "Erosion by human actvities in Japan." GeoJournal 4(2): 133-144.
Morris, A. E. J. (1994). A history of urban form: Before the industrial revolutions, third edition. New York, Longman Scientific & Technical/ John Wiley & Sons, Inc.
Wilkinson, B. (2005). "Humans as geologic agents: A deep-time perspective." Geology 33(3): 161-164.