A Mountain Fact Sheet: Is A Mountain a volcano?
Side bar panel with a historical picture?? Modern picture? The crater-like feature is not
the result of a meteor impact, it’s the remains of an old quarry. Quarry history, picture,
uses…. Local construction uses, basalt around town, on U of A campus, where else?
A Mountain is made up of a series of volcanic rocks that represent different types of
volcanic activity. The mountain itself is not a volcano. The layers of rock you see once
extended out west towards the Tucson Mountains and into the Tucson Basin (where the
city is now). Erosion and faulting are responsible for A Mountain’s place in Tucson and
its conical shape. The mountain continues to erode.
As volcanoes erupted near A Mountain, around 25 million years ago, they left evidence
of their activity in the form of different rocks. As you go up the mountain, the rocks get
younger. Rocks at the base of A Mountain represent a lava flow, which is the earliest
volcanic event evident at A Mountain. Rivers carried volcanic debris to the A Mountain
area and covered the cooled lava flow. A large rain of volcanic ash then covered the river
deposits. And another lava flow that you can see at the top of A Mountain (near the “A”)
poured over the ash. The different rocks at A Mountain rocks demonstrate that different
parts of a volcanic system (and perhaps different volcanoes) affected A Mountain through
time. We can examine these rocks in detail and reconstruct the history of events that
formed A Mountain.
Basalt
Rocks at the base of A Mountain are basalt – cooled lava. You can see these rocks
exposed in the quarry off Mission Road and near the entrance gate on the road that leads
up to the top of the mountain. Lava travels up to the surface from underground magma
chambers through fractures and channels, the volcano’s plumbing system. The lava
reaches the surface through vents or craters on the top or sides of the volcano. Once the
lava leaves the volcano it flows outward or explodes upward depending on its viscosity
(thickness) and water content (explosivity). The bubbles, rough texture, and outlines of
distinct flows show us that the lava that formed the basalt at the base of A Mountain was
non-explosive as it flowed from the source volcano and covered A Mountain.
Conglomerate
The red rocks you see above the basalt are part of a conglomerate, which is a collection
of pebbles and sand. This red conglomerate is made of volcanic debris that came from a
nearby volcano and was carried to this spot by water, likely by an underwater mudslide.
The angular shape and sharp edges of the pebbles suggest that they could not have
traveled very far. If water had carried the volcanic material a great distance we would
expect the pebbles to be well-rounded and smooth, like marbles. The black and gray
pebbles in this conglomerate are small pieces of basalt that have been removed from
wherever the lava had initially cooled. The red coloring of the sands in between the
black basalt pieces is due to oxidation of the iron rich minerals in the sands. Oxidation
also happens when metal rusts and turns orange – something that’s not too common in
Tucson.
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Tuff
The white, pink, gray and tan blocks of rock above the red band of conglomerate formed
when volcanic ash from nearby (or perhaps far away) eruptions accumulated in the
Tucson area. A tuff is a rock that forms from an ash fall deposit, like we see here. The
tuff at A Mountain varies in hardness. In some places it crumbles and in other places it is
solid and blocky. The hardness of the tuff depends on how much it has been welded
together. [Sidebar??: You can test the hardness by banging a hammer against the rock.
Can you easily break off a piece of tuff? What sound does the rock make when you hit it
with the hammer? Usually harder rocks will make a ping sound with a higher pitch than
softer rocks.] Tuff is made up of ash which itself is composed of many small pieces of
glass. This glass formed when hot magma shoots up from a volcano. If the glass falls
when it has not completely cooled it will stick and weld together becoming harder and
more durable. If you look closely at the tuff you can see small crystals, pumice, or glass
fragments. These pieces are usually the first parts of the ash fall to hit the ground
because they are more dense than the rest of the ash material. Following this reasoning
you should expect to find a greater proportion of crystals and pumice fragments at the
bottom of an ash deposit.
Basalt
On the top of A Mountain, beneath the “A” and near the parking lot, there is another
basalt. This basalt has many vesicles (holes) in it that tell us that the lava was full of gas
bubbles as it cooled. Most of the vesicles are not round but are instead elongated or
irregular ovals. Vesicle shape tells us that the lava was still slowly moving as it cooled,
squishing the bubbles. In some places at the top of A Mountain you can see folds within
the rock that gives you a sense of the lava’s movement which slowed as the lava cooled
and got thicker. If you ever try using a compass near this rock, you might realize that you
get incorrect readings. Why? The magnetic properties of the iron in this rock will deflect
a compass needle, up to 4 feet away from the rock. Try it out.
Is A Mountain a volcano?
The insides of volcanoes have unique features – and if A Mountain itself were a volcano
we should see these features. All volcanoes, regardless of type -- strato, shield, cinder
cone, must have some sort of pipes or plumbing system that provide a path for the
magma to escape and come to the surface as the volcano erupts. When the volcano is no
longer active the magma cools and leaves plutonic rocks (rocks from magma
underground) in the place of this “plumbing”. [Diagram of the inside of a volcano.]
There are no plutonic at A Mountain – instead we see evidence of lava cooled aboveground, debris transported by water from nearby volcanoes and ash falls. A Mountain is
not a volcano!
The rock layers show us that A Mountain is made of volcanic rocks that formed from
many different parts of a volcanic system. There was a lot of volcanic in the Tucson area
19 to 27 million years ago. Several eruptions from one or more volcanoes in the Tucson
area formed the rocks from A Mountain.
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So where were these volcanoes that covered the A Mountain area with lava flows,
mudslides, piles of ash and more lava? Geologists do not know exactly where the
volcano responsible for each episode was located. However, you can get a sense for how
far away the volcanoes were by knowing how far each volcanic process can take material
from the source material. How far can flowing lava travel before it stops and hardens to
form basalt? A less viscous lava flow can travel 10 miles and spread out several miles
wide. A pyroclastic eruption can cover areas in ash hundreds of miles away from the
source volcano, depending on the size of the eruption. And water can carry volcanic
debris for hundreds of miles although the angular rocks in the conglomerate likely did not
travel more than 50 miles.
[Sidebar]
Caliche
The white cement-like material draped on top of the A Mountain rocks in places is
common in the desert. Caliche is not a volcanic rock. It forms in the soils of A
Mountain, well after the volcanic rocks were deposited. Caliche is common in soils of
arid environments and is made of calcium carbonate -- the same chemical composition as
limestone, marble and stalagmites. Calcium carbonate that is dissolved in the soil water
precipitates and forms a white chalky material when the water evaporates. You can find
caliche underneath most backyards in Tucson. Try digging a hole. If you hit caliche
you’ll know it – it’s pretty difficult to dig through.
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