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The geology of the Superstition Wilderness Area can
be divided into three distinct categories. They
include (1) rock type, (2) structure and (3)
process. The rock type consists primarily of
extrusive igneous rocks associated with vulcanism.
Structure is consistent with the type of landforms
that result from volcanic action and tectonic
activity. The process is related to mass wasting,
erosion, faulting, mountain-building and volcanic
activity. All three of these categories contributed
to the rugged and spectacular beauty of the
Superstition Wilderness Area we enjoy today.
The western portion of the wilderness area is
dominated by igneous rock whose origin is primarily
volcanic action. There are examples of sedimentary
and metamorphic activity within the boundaries of
the wilderness; however this activity is very
limited. There are Pre-Cambrian granites in a few
isolated areas of the wilderness that were formed
from intrusive igneous rocks. Intrusive rocks are
formed from molten magma that cooled beneath the
surface of the earth. In other words, the melt
(magma) was not exposed to the atmosphere while it
cooled. Eventually intrusive rocks are exposed on
the earth’s surface by erosion.
 Erosion
is the wearing away process of the earth’s surface
by wind, water and ice. These intrusive rocks can
be found along the First Water Road and the Apache
Trail (SR 88) northeast of Apache Junction. Granite
diabase can be found east and south of the IV Ranch.
Common volcanic landforms found in the area include
calderas, blister cones, lava flows, mudflows and
ash deposits. Dacite, andesite, rhyolite, and
breccia are the common igneous rocks found in the
area.
During a more recent eruptive geologic period (3
m.y.a.) many erosional remnants were capped with
black basalt from the Black Mesa, Willow Springs and
Florence caldera systems. Most of these large lava
flows were the results of fissures; cracks in the
earth, not volcanoes.
North of the wilderness, sedimentary deposits of
volcanic debris have been deposited by the action of
running water during the eruptive stage. Many of
these deposits have graded bedding, which is
sometimes indicative of stream deposited gravel.
The water was produced by volcanic steam and vented
during the eruptions in this particular case. A hot
and very viscous mudflow formed the graded bedding
near Canyon Lake.
 Along
the Apache Trail, where the road crosses the
Superstition Mining District between Superstition
Mountain and the Goldfield Mountains, other rock
types are prevalent. These rock types include
coarse-grain granites, indurated conglomerates and
breciated granites. Arkosics can also be found in
this area. Faulting prevails throughout the area.
Small stringers of quartz that were caused by
epithermal and hydrothermal action intruded the
rocks in the Goldfield area.
These small veins carried small quantities of gold.
The Mammoth Mine, Bull Dog Mine and Black Queen Mine
were all exceptions to this. These three mines where
large pockets or veins and produced a considerable
amount of free-milling gold ore.
Near Roosevelt Lake, but still within the confines
of the wilderness area, true sedimentary rocks can
be found. These rocks are ancient Pre-Cambrian
sediments of the Apache Group found in the upper
Salt River Canyon some forty miles northeast of
Globe, Arizona. This group includes limestone,
sandstone, and conglomerates.
Outcrops of ancient metamorphics can be found in one
isolated area near the southern boundary of the
wilderness. These rocks i nclude
schist, marble and slate. The Hewitt Canyon
metamorphics are commonly intermingled with
volcanics. Most of this intermingling was the result
of intrusion by igneous rocks. This intrusion was
followed by severe tectonic activity or movement of
the Earth’s crust This area appears to be the
extreme eastern limits of the Superstition Mountain
eruptive field.
The geology of the Superstition Wilderness Area and
adjacent areas are very complex. For the most part
the region has a tremendous amount of geological
diversity. The actual wilderness area has never
proved to be highly mineralized near the surface.
Deep-seated deposits of minerals still remain
elusive to the modern prospector or miner. Major
mining companies have avoided the area for almost a
century. Extensive core drilling was done near the
Woodbury Trailhead in the 1970’s with no major
mineral discoveries being made or announced.
Some geologist believed Superstition Mountain is the
remains of a large collapsed caldera. This caldera
has experienced three basic geologic stages in its
evolution. The first stage involved the formation
of dacite cones on a granitic basement complex more
than twenty-nine million years ago. This resulted
from magma intruding
the granite and depositing basalt and ash in the
forms of cinder cones and lava flows. The
alternating layers of ash and basalt helped form the
present mountain we call
Superstition today.
After millions of years of eruptions the area was
leveled by
erosion. After a long period of inactivity the
magma below
the earth’s surface began to recede leaving a
massive cavity below the crust of the earth. The
second
 stage
in the formation
of Superstition Mountain then occurred. The entire
area collapsed into the cavity resulting in a huge
caldera some seven miles in diameter. As parasite
cone system developed within the caldera and began
to issue forth ash, the resulting deposition covered
more than one thousand square miles. Remnants of
these eruptions remain visible today. Picket Post
Mountain, west of Superior, is a classic example.
As the volcanic action of the second stage began to
subside, the third stage of the mountain’s evolution
began. Uplift and subsidence were both common
during this period. However, uplift dominated the
development scene for several million years
resulting in the familiar landmark we know today as
Superstition Mountain. Originally the mountain was
some three to four thousand feet higher, but water
erosion reduced it to its present configuration and
size.
The deeply incised canyons and large alluvial fans
are landforms resulting from millions of years of
erosion. This is the basic geologic evolution as
reported by Dr. Michael Sheridan, Vulcanologist at
Arizona State University from 1970-1989. Today
there are other theories as to how Superstition
Mountain formed and the other geologic formations in
the area. So far none of these theories are
conclusive.
Geologists have studied other theories as to how
Superstition Mountain formed. The degree of uplift
or resurgence as reported by Sheridan is in direct
contrast with the findings some
 U.S.G.S.
geologist. Some geologist report
Weaver’s Needle was
formed the same way as Superstition Mountain.
Weaver’s Needle was formed as an erosional remnant.
Some geologist believed this theory is
applicable to Superstition Mountain. However, there
is not sufficient geologic evidence to support the
theory Weaver’s Needle is volcanic plug. The lack of
mineralization is indicative of Weaver’s Needle and
its surrounding area.
The geology of the Superstition Wilderness Area is a
fascinating study of igneous rock and their various
formations. The rocks of the western portion of the
Superstition Wilderness are eruptive igneous in
origin. These rocks formed a wonderland of beautiful
volcanic formations deposited between 3 – 25 million
years-ago. We can only marvel the beautifully
textured landscape the Superstition Wilderness Area
provides for us. |
