Friday, October 2, 2009
Back in April I started exploring questions from a series called "Where You At?" (see my April 10th post, "Agua Dulce"). Returning to that series, we are asked this question:
#13 What primary geologic event/process influenced the landform where you live?
This question took me a long time to think about. Perhaps this is because the idea of landform is a bit difficult to suss out in a place where the land has been bulldozed and paved in a way that makes it tough to recognize the natural form of the land. But therein lies the answer! Humans are the primary geologic force at work here in Phoenix. The valley has been transformed from a vast undulating desert of bajadas and arroyos into canals, dams, bridges, reservoirs, mines, and sprawling development that recent reports declare we will add 400 more miles of highway to by the year 2050! Four hundred!!!
Volcanism has also had a big influence on the landscape here. Where the land has not been reworked, paved over, and re-landscaped, our neighborhood is circled by a ring of six hundred foot high hills that rise above the valley floor. Like many of the surrounding desert peaks, Deem Hills is made of rubble leftover from volcanic eruptions that occurred between two and five million years ago, which is relatively recent by geologic standards. The dark rocks, called basalt, glow deep purple and orange in the evening sun. Basalt is basically cooled off lava. Here, many of the rocks have pits and holes in them, evidence that the lava was almost frothy when it flowed and then quickly cooled, leaving small air bubbles in the rock.
Ironically, water has had an equally large influence on the landscape here in one of the most arid regions of North America. Whenever rains fall, flash floods and sheet flow move thousands of tons of rock, sand and silt, gradually eroding the hills. Bajadas are the alluvial fans, or outwash of debris that flows out of the mountains creating a gently sloping landscape at the base of the hills. Arroyos are the beginnings of canyons, like the Grand. Over time, water and wind probably move more earth than any other geologic force other than plate tectonics.
But right now, humans may outweigh even plate tectonics in the effects on landform when you count the accumulated impacts of our civilization on the movement of air and water combined with actual earth moving here in Phoenix.