Within north central New Mexico is a strange geologic oddity.
There, layers of primarily white rock seemingly jut out of the ground in abundance, forming
hundreds if not thousands of hoodoo like features.
These fascinating structures are relatively thin, having a near vertical cliff on their
edge.
Even more interesting is the top of these structures, which has a cone like shape to
it.
This cone is how these features get their name, which are referred to as the Tent Rocks.
Although they might look like hoodoos seen in Bryce Canyon National Park, they are not
sedimentary in origin.
Rather, they formed through a sequence of highly explosive eruptions which deposited
thick layers of volcanic rock.
The Tent Rocks can be found in central New Mexico where they are 16 miles or 26 kilometers
south-southwest of the city Los Alamos.
These numerous volcanic features are largely located within Kasha-Katuwe Tent Rocks National
Monument, which spans an area of just under 22 square kilometers.
The formation of the tent rocks began 14 million years ago, when the region was undergoing
frequent explosive volcanic eruptions.
This volcanism occurred because the area was at the intersection of two important geologic
features in the region which were both associated with the formation of new volcanoes.
For several million years in New Mexico, the crust has been spreading apart to the east
and west due to a rift zone known as the Rio Grande rift.
At the site of what would become tent rocks, the Rio Grande rift intersects with a feature
known as the Jemez Lineament.
The lineament represents a regional weak point in the crust due to the collision of two tectonic
plates at this very location 1.7 billion years ago.
Since the crust is thin in the area, magma at depth can easily travel upwards and erupt
onto the surface, thus forming volcanic fields such as Zuni Bandera, Red Hill, and Mount
Taylor.
Due to the intersection of these volcanically active provinces, explosive volcanism became
inevitable.
While volcanism began in the Jemez Mountains 14 million years ago, the eruptions which
would form the Tent Rocks did not begin until 7 million years ago.
As a volume of high silica rich rhyolite composition magma intruded into the crust, it interacted
with overlying groundwater.
This groundwater subsequently flashed to steam, built up in pressure, and generated a powerful
volcanic explosion.
From the newly formed crater emerged an ever-growing dome of rhyolite lava, which slowly flowed
downslope at a rate of several meters a day.
This dome experienced frequent partial collapses, resulting in additional pyroclastic flows.
Pyroclastic flows from this highly explosive eruption traveled several miles, depositing
layers of superheated rock which combined with ash to form a hardened rock type known
as tuff.
Over the next 500,000 years, the area experienced at least 35 similar volcanic eruptions, which
continued to deposit a very thick layer of rock.
By 6.5 million years ago, the sequence of explosive volcanism ceased, which by then
had left behind more than 1,000 feet thick of primarily volcanic ash.
The top layer contained a mixture of much harder rhyolite lava flow fragments and welded
pyroclastic flow deposits known as ignimbrite.
However, these much harder fragments had an uneven distribution.
Over the following several million years, areas without the overlying harder layer quickly
eroded away as volcanic ash is highly prone to erosion.
Meanwhile, the ash layers beneath the harder material were partially protected from erosion.
This led to the creation of hundreds if not thousands of towering ash columns, where the
top cone like feature represents the harder caprock.
In some of these ash layers you can find shiny black volcanic glass surround by a gray material
known as perlite.
These represent features known as "apache tears", which are common finds within large
volume explosive eruption deposits in the southwestern United States.
As a final note, similar pillars of volcanic ash can be found within the La Garita caldera
in Colorado, and at the remote Hoodoo Mountain volcano in British Columbia.
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He works at the Utah military academy, where he is a teacher.