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Photos taken in Chester in 2012, 2013 and 2014. Features Blue Planet Aquarium, Cheshire Oaks Designer Outlet and Chester City Centre.
FIELD GUIDE TO "A" MOUNTAIN AND DESCRIPTION OF SURROUNDING REGION
by
Thomas G. McGarvin
ARIZONA GEOLOGICAL SURVEY
I have heard that this hill was called Dynamite Hill by a longtime Tucson resident.
The Geology of “A” Mountain
If you drive up Sentinel Peak Road, you will travel past a series of rock layers that formed between twenty and thirty million years ago. These multicolored strata produce the bedrock that constitutes “A” Mountain as well as the nearby Tumamoc and Powder House Hills. These hills and “A” Mountain are the visible remains of a former landlocked peninsula that was anchored in the west by the Tucson Mountains and extended beyond the present day Santa Cruz River to the east. “A” Mountain is an erosional remnant of this land prominence sculpted by the forces of ice, wind and water. Four distinct and interesting rock layers are easily visible on the face of this 550-foot mountain.
Flowing lava created the dark red rock strata at both the top and the bottom of “A” Mountain. Nearby volcanic pipes and fractures supplied the basaltic magma that created these beds, each one separated by a span of nine million years from the formation of the other. None of this lava came from the large “volcano like” crater on the northeast side of the mountain. Quarrymen from the Griffith Construction Company dug this basin at the turn of last century in the pursuit of its stone, used for building Tucson homes, walls and other structures.
The dark red color of these two basalt layers is due to a high concentration of iron and magnesium in the original magma. These elements reduce the lava’s viscosity and the explosive tendency of the sourcing eruptions while allowing the molten rock to flow more uniformly across the surface. Much of the basalt layer at the base of “A” Mountain contains small cavities known as vesicles. The magma, in this case, erupted out of the ground just fast enough for the dissolved gasses to vaporize in the decompressing molten rock and then cooled fast enough to retain the holes formed by these gas pockets. The same bubble forming principle occurs when opening a bottle of beer. An example of this vesiculated basalt is visible on the west side of Sentinel Peak Road between the parking lot at the bottom and the beginning of the one-way road around the summit. The basalt cavities are at the top of this dark layer, presumably because the bubbles floated upward before the cooling lava locked them in place.
The two rock layers sandwiched between the basalts were the result of more violent volcanic activity about 27 million years ago. The older of these two light colored rock strata is composed of rough dark pebble size cinders (basalt) embedded in silt, sand and ash. This material fell from the sky in the form of a volcanic cinder fall. The light brown agglomerate layer with its dark embedded pebbles is visible on the left side of the one-way lane just beyond the split in the road.
Finally, the most visually striking layer of “A” Mountain is composed of tan and pink rock, known as tuff. This layer resulted from one or more volcanic ash falls. The magma for this ash also went through decompression near the earth’s surface. In this case, however, the eruption occurred so suddenly that the expanding gas in the magma shattered the molten minerals and rock into very fine pieces and threw them forcefully into the air. After settling back to the ground, the combination of heat, pressure and time welded this bed of ash into the light colored rock layer that we can see today. Look for a sharp color transition between the tan rock and pink rock as you drive up the southern slope of “A” Mountain. This will identify the tuff layer that is also visible as a large light horizontal bed on Tumamoc Hill to the west.
This introduction to the geology of “A” Mountain is an invitation to explore and enjoy our hilly community with an understanding of its primordial past. The vestige of this beginning is locked within every pebble and stone of our iconic mountain.
Photos taken in Chester in 2012, 2013 and 2014. Features Blue Planet Aquarium, Cheshire Oaks Designer Outlet and Chester City Centre.