The mountains in the Phoenix area are mostly jagged faulted
peaks,with one exception: South Mountain. This mountain is unique
to the mountains in our area and represent a class called
metamorphic core complexes. Only about 20 or so of these mountains
have been identified within the US. What causes this type of
mountain is still under debate. Whereas most of the mountains in
Phoenix are the result of the land being torn apart, metamorphic
core complexes (MCCs for short) are the result of rocks that are
pushed up through the ground, stretching out the surface above
them. These mountains have a rounded dome shape to them, unlike the
sharp peaks of the nearby Estrella Mountains, or the familiar
Camelback Mountain.
In the US, MCCs can be found in the western US from Canada to
Mexico. It is believed that about 25 million years ago, the crust
started to stretch in a northeast to southwest direction. As it
thinned, thermal upwelling caused lighter rocks to surface to form
dome shaped wrinkles. Millions of years later, the crust began to
crack and break apart. These “faults” run perpendicular to the
direction of the original stretching. This breaking apart gave us
the valley of the sun – Phoenix – as well as the other mountains
around
Phoenix. Metamorphic Core Complexes are unique structures found
in the North American Cordillera. The basic structure of an MCC
consists of a metamorphic basement terrane and an unmetamorphosed
cover. Between these structures is a discontinuity, or decollement
(defined as the detachement of the upper structure from the lower
geological structure).The decollement consists of mylonitic fabric.
(Mylonite is defined as rock in which there has been differential
movement, at temperature usually exceeding 250-350 degrees Celsius
and pressure such that the mineral grains within it are crushed
against one another. The resulting rock shows a series of
lamination planes, which give it a stretched look, and a platy
appearance. The mylonite usually consists of more than one mineral,
and the minerals behave differently to the shearing
pressures.)Overlying this is detachement faulting of the structure,
with crustal displacement.
The coordinates take you to Dobbins Lookout. From the viewpoint,
look east. At the canyon wall below, the metamorphic core complex
is readily visible. To obtain credit for this earthcache, please
post a gps photo of the view of the MCC from Dobbins Lookout
(daytime photos only). Also, you need to email me your estimation
of how high the mountain is from this point. Once you see how it
looks, you can find this in other mountains in Arizona such as the
Catalinas (Catalina-Rincon Complex) and White Tank Mountains.
Please
visit AZGeocaching.com link here