Cochise College Student Papers in Geology
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Roger Weller, geology instructor
Chiricahua Mountains
Vivian Lewis
Physical Geology
Fall 2005
Hiking
in the Chiricahua National Monument, Arizona
Twenty-seven
million years ago during the Tertiary age, from a caldera of a resurgent volcano
in the southeastern corner of what is now known as Arizona; eruptions occurred.
Over a long period of time, ash from the Turkey Creek Caldera (Marjaniemi xvi)
was spewed out in the form of air flows and pyroclastic flows. The ash hardened
into an igneous rock known as rhyolite. At Chiricahua National Monument, the
Rhyolite Canyon layer is about 1,000 feet thick and is the youngest (Marjaniemi
xvi). As the layers increased in height; they slowly cooled, and compaction,
assisted by heat, rain, and freezing temperatures, attacked the hardened flows,
causing the rhyolite to separate and pillar-like columns known as tuff formed (Bezy
9).
In
the last few millennia, man has discovered this wonderland of rocks and used
this area for various reasons while admiring the numerous and sometimes bizarre
formations. In 1924, what is now known as the Chiricahua National Monument was
set aside for all mankind to enjoy. But what are the unusual shapes called and
how were they formed? On this virtual journey, we shall find out.
As
with any hike in Arizona we will be at a high altitude. Take plenty of water
and electrolyte replacements in the form of liquids and snacks. Always pace
yourself; leave plenty of time to hike and enjoy the scenery. We will start at
the top of the Monument and return to our vehicle at the Visitor Center Parking
lot.
Our park ranger is dropping us off at the parking lot for Sugar Loaf Mountain.
The top of this mountain is formed of the youngest rock, a lava flow that came
from the Turkey Creek caldera millions of years ago (Bezy 20). Except for the
mountain peak, the dacite has eroded away. Hiking upwards, we come to our first
wonder, an arch carved by the Civilian Conservation Corps (CCC) when they
built the trails in the Chiricahua National Monument in the 1930’s during the
Great Depression. The arch, the buildings, the roads, the trails; all the rock
fortifications that we are seeing before us, were done during that time period.
Just past the arch, to the left we see an ancient steam vent called a fossil
fumarole. Look to your right and you will see rock spires geologists call
welded tuff.
Dr. Darwin Marjaniemi, who in 1969, researched and located the source for all
the rhyolite in the Chiricahuas, defined in his thesis, welded tuff as, “a rock
or rock body in which vitric particles have some degree of cohesion by reason of
having been hot and viscous at the time of their emplacement.”(Marjaniemi 3)
The best examples of welded tuff are located in the Monument.
Further up, we have come to an overhanging area that is
to our left. The closest layer to us is very chalky, grey almost white. It
flakes easily and when placed in water, breaks down. When it was dry, it
crumbled to the touch. This is known as a surge bed. It was formed by hot
fiery rock and gases that flowed down the slopes of the caldera (Bezy 18). In
the winter of 2001, I tried to hike to the top but found it was closed due to a
landslide caused by a lot of rain and snow. The slope was overloaded with
moisture and weight, undercut for the trail, so the slope collapsed.
The Park
Service has since repaired it.
When you walk past it, look back and you will see a fossil fumarole.
Surrounding us are the shrubs of the Mexican Locust Bean. They have very sharp
thorns and lovely gold- colored leaves in autumn.
At last we have hiked up to the top and we see the fire
lookout. The view all around is spectacular. To the West we can see the
Sulphur Springs Valley and the Dragoons with Cochise Stronghold. To the
Northwest we observe a large white area that is the Wilcox Playa, the remains of
an ancient lake called Lake Cochise( Bezy 22). The shallow lake had water
during the last ice age that supported pines, as well as fauna such as the
mammoth, horse, and camel, until about 10,000 years ago when the climate became
more arid. To the south you will notice the tallest peaks in the Chiricahuas.
That is the Turkey Creek caldera. A thousand times more ash and rock as well as
lava, was ejected from this volcano than was released from Mt. St. Helens in
1980 (Bezy 21).
After hiking back down the trail, we walk along the
drive to get to the Echo Canyon Parking Lot. To the northeast, we notice an
unusually shaped peak. This is the Head of Cochise. It is the remains of
a rhyodacite flow and shaped by erosion.
We are traveling on the Echo Canyon trail with
views of Sugar Loaf Mountain on our right. Tall, silent sentinels of stone
surround us on almost all sides. Many of them are rounded off at the corners,
others have horizontal lines or ribs incised into the rock. Soon, to our right
is Echo Grotto. The welded tuff seems to be leaning toward a center point,
leaving an open space.
Our trail becomes a series of switchbacks and descends
down among the tuff. We come out into an open area surrounded by skyscrapers
made of tuff, or if we use our imagination, we could see giant Easter Island
statues.
As we follow the trail, it narrows until
we are walking between two towering walls of stone, the beginning of Wall
Street. If the hiker will look up, you will see huge vesicular-looking cavities
geologists call case hardening. The interior of the holes have a protective
coating of silica that has extruded from the interior of the tuff. According to
John V. Bezy , who works for the National Park Service, ”The silica then is
reprecipitated on the surface.” (Bezy 29) In Figure 5, above the case
hardening, we notice some small holes known as tafoni. Found along the cracks
or joints, this is another weathering process similar to case hardening. They
are more likely to be seen in the desert, but are indicators of a past cooler,
wetter climate.
After exiting Echo Park, an area with more trees than
we have usually seen, we see an open space that is clearly a canyon. We can see
trails running parallel to the canyon and it is time to make a decision. Which
trail to take? We decide to hike to the Heart of Rocks. Now the trail is
almost flat. After a mile, we pass a mass of grey balls about the size of an
ordinary marble. This segment of the Rhyolite Canyon Trail is known as the
Hailstone Trail. They are not fossilized hailstones; but formed as the ash
sheet cooled, crystals of feldspar and quartz fanned out in almost equal
directions (Bezy 32). As we walk past the columns of stone, we notice that many
are wide at the base, narrow at the center and begin to spread out again at the
top. Others are wide at the base and narrow at the top.
Others are narrow at the base and then widen a lot at
the top. Many seem to do a balancing act.
To the left is the huge balancing rock. Looking down we
notice a trail of painted brown footprints, so we follow in the footsteps of
past hikers as we are guided into the Heart of Rocks. The welded tuff, composed
of various ash flow layers of rhyolite, seem to be carved into familiar
fantastic shapes here on this wind-swept plateau. The generation of the Great
Depression gave the tuff such names as Punch and Judy, Duck on a Rock, Thor’s
Hammer, etc. But each generation may see something different.
In addition to the painted footprints, there are the stone staircases. All
too soon, you have come out of the Heart of Rocks, and we begin our descent down
the Sarah Deming trail to the Visitor Center. Although the hiker can not see
much in the way of welded tuff during this part of the hike, toward the end of
the Sarah Deming trail, there is an unusually tall spire of welded tuff that
seems to soar out of sight. At the trailhead, we look back from whence we came,
and then across the canyon to see the tributaries of Rhyolite Canyon. Across
the Canyon, the hiker can see three distinct layers of what is known as Rhyolite
Canyon Welded Tuff Formations (Marjaniemi 18-20). Walking through the conifers,
we can see our vehicle in the distance.
After the violent volcanic eruptions quieted down; Basin and Range block faulting caused the hardened ash flows to uplift, tilt, shift, crack; forming joints. Rainwater, wind, freezing temperatures, trees and other plants also joined in the weathering and erosion of the Chiricahuas until almost twenty-seven million years later, the land continues to shape itself into the wonderland of rocks we all have come to love.
Works Cited
Marjaniemi, Darwin, PhD. “Geologic History of an Ash-Flow Sequence and Its Source Area
In the Basin and Range Province of Southeastern Arizona”. 1969,xvi-171.
Bezy, John V. National Park Service. “Rocks in the Chiricahua National Monument and the
Ft. Bowie National Historic Site”.2001. 7-33