VOLCANIC NECKS

COLORADO

The first time I drove to Pueblo, Colorado I noticed a black-rock hill sticking up out of the prairie that looked very out-of-place. It was about 35 miles south of Pueblo on the east side of the I-25 freeway. Some time later on one of my exploratory excursions I found a dirt road that went by it. Once I got closer to it I recognized it for what I suspected it was, a volcanic neck.

Huerfano Butte

A volcanic neck, or plug, is the subterranean pipe that once fed an ancient volcano. It is the cold, hardened core exposed after the original volcano and surrounding sedimentary rock has eroded away over many millions of years. Being more resistant to erosion the igneous rock remains as the last one standing.

Huerfano Butte is 150 feet tall.

Over the eons Huerfano Butte has crumbled away into a blocky scree slope but a solid core still remains at the center of the pile.

Huerfano Butte Thinking Rock

As I sat on one of the larger basalt blocks that had tumbled down the slope I started thinking about all the volcanic necks I had seen in Arizona and other western states. There are quite a few ranging from Wyoming, to New Mexico. and Colorado. Most are hardly noticed and others are famous and one even became a national monument.

WYOMING

In the summer 1978 I traveled up to Wyoming with friends to visit the three national parks and monuments there: The Grand Tetons, Yellowstone and Devils Tower. Devil's Tower National Monument is in the northeast corner of the state. It had recently been made famous by the 1977 movie, Close Encounters of the Third Kind by Steven Spielberg.

Devils Tower is about 40 million years old and 1,253 feet tall.
(internet image)

This was my first encounter with a volcanic neck although this one looks much better than most. It appears better preserved and is certainly the most impressive. There are several hypothesis as to how it formed and I lean towards the one that it was a molten lava lake inside a larger volcano. I consider two characteristics that support this idea; the flat top and the columnar jointing. A flat top would be indicative of a fluid lake and the columnar joints are long crystals that can only form, or grow, if the lave cools very slowly and remains undisturbed. A surrounding volcanic mass and earthen pipe would insulate the lava from cooling quickly and a recently dormant magma plume below would continue to cook the pool for decades or centuries to come allowing time for the crystals to form.

Devils Tower columnar jointing.
(internet image)

I also believe the long, vertical crystals add strength to the structure like the stanchions in a skyscraper. The crystals certainly contribute to the necks pristine condition and longevity. If it didn't look so good would they have made it a National Monument?

ARIZONA

In the southern part of Arizona southeast of Phoenix and east of the town of Apache Junction is an extinct volcano called the Superstition Mountains. It is famous for the legend of the Lost Dutchman gold mine which was "lost" in the 1870's somewhere in the rough dessert terrain of those mountains. There were many books, movies and a TV series made about it as any story of buried treasure always does.

Superstition Mountains
(internet image)

Supposedly the mine is in the vicinity of a landmark known as Weaver's Needle behind the mountain. As the name suggests, Weaver's Needle is a volcanic neck. At least that is what it appears to be. However, there is not sufficient geologic evidence to support the theory it is. It could be just an erosional anomaly according to one geologic study I read. If it coincides with the original eruption of the Superstition Mountain it could be 29 million years old. I could not fine a date assigned to the needle itself.

Unfortunately, you can't drive to the needle or get to a location where you can get a close up view unless you hike 1,300 feet up a challenging foot path to a ridge called Fremont Saddle. This should not be attempted in the summer but is a pleasant hike in the winter. I did it in 1976 and remember it as an "ankle breaker" because of the loose, round rocks that peppered the trail. Maybe it is better now. It is only 1,000 feet tall but looks to be much more because of the base it sits on, just like the Statue of Liberty on the pedestal in New York Harbor. Apparently it is a class 5.0 climb to the top of the needle but a long arduous walk to get there and suggested for advanced climbers only.

Weaver's Needle as seen from Fremont Saddle
(internet image)

The small city of Prescott in central Arizona was established at the foot of a volcanic neck. It served well as a landmark for early settlers to help find the place. There seems to be some debate whether Thumb Butte is a volcanic neck or a dome, a thick lava flow that extruded from the volcano below it. It erupted about 12 million years ago so it isn't that old which may contribute to the idea it isn't a neck. In the geologic time scale that isn't a lot of time for the necessary erosion to take place. The majority of what I have read consider it to be a neck so I'll run with that.

Thumb Buttes proximity to Prescott make it a popular hike.
It is 1,500 foot tall including the base.
(Internet image)

THE NAVAJO VOLCANIC FIELD

ARIZONA, NEW MEXICO, UTAH, COLORADO

In an area encompassing 20,000 square miles of the Four Corners states is the Navajo Volcanic Field. 25 to 30 million years ago a hot spot lay beneath this area and was the source of magma for over 80 volcanic vents.

Navajo Volcanic Field

In the map above a distinction is made between tuff pipes and diatremes. Tuff pipes form the typical cinder cones and lava flows that pile up around vents. Tuff being the new strata that may be formed if the ejected material is hot enough to weld itself back together when it hits the ground. .

How a tuff pipe neck would form.

A diatreme is a violently explosive eruption, often associated with water or high velocity gas (kimberlite) that blasted the earth away leaving a depression in the ground and a tephra ring around the pit. With enough erosion either type can form a volcanic neck but the diatreme would have been deeper underground. It may take an analysis of the rock chemistry to identify a kimberlite type magma which originates in the asthenosphere. It would contain a higher concentration of the mineral olivine.

The Four Corners region is home to the Navajo Indian Nation and the volcanic outcrops there are sacred to the tribe and part of creation stories. The locals do not appreciate non-natives on their land so stay on the paved or dirt roads and do not approach the volcanic features on foot. All of my photos are taken from the road. I travel between Colorado and Arizona once or twice a year and it is my habit to take a different route as much as possible. Doing this I have passed through some beautiful country and passed many of the renowned volcanic necks.

Starting in the northwest portion of the Navajo Volcanic Field we are going to visit some of the more notable volcanic features. First is Alhambra Rock in Utah. Heading south from the community of Mexican Hat on highway US-163 you can see it just one mile to the west of the road. Most necks are basically round like a pipe but this one is long and narrow. It is not very tall, just 250 feet and appears to be a part of a longer dike orientating north/south. Dikes are earth cracks radiating out from a volcano forced open by the pressure of the rising magma. Alhambra Rock appears to be the epicenter of a dike and probably produced a fissure eruption.

Alhambra Rock

Continuing south on US-163 through Monument Valley and thirty-three miles towards Kayenta is Agathla Peak. This is one of the larger volcanic necks jutting up from the desert floor and can be seen from a long ways off. Rising over 1,500 feet it is the remnants of diatreme eruption.

Agathla Peak diatreme 1500 feet tall

Most of the volcanic necks are thought to be monogenetic, meaning derived from a singe eruptive event. This is not true of Agathla Peak. There is a dirt road that passes closer to the back side of peak. In the morning light it is obvious a second eruption took place sometime after the first one had cooled.

Agathla Peaks secondary eruption vent.

The arrows point to an inner pipe with a darker color and a blocky texture that forced it way through the first plug. This suggests to me that the chemistry of the magma chamber had changed and it was at a lower temperature than the first eruption.

Chaistla Butte 400 feet tall.
(internet image)

Chaistla Butte can be seen just 3 miles south of Agathla Peak. It erupted as a diatreme some 30 million years ago. .

From Chaistla Butte go south to the community of Kayenta and head east on highway US-160. In 7 miles Church Rock is north of the road. Like Alhambra Rock, Church Rock it is long and narrow and is the feeder dike to the volcano that was once above it.

Church Rock diatreme.

Church Rock is named for the 400 foot tall steeple like tower at one end,

There must have been an aquafer in the area that caused Agathla, Chaistla and Church Rock to all be diatreme type volcanoes.

Part of the dike is buried in the wind blown sand.
(Google Earth image)

Leaving the western end of the Navajo Volcanic Field we travel 70 miles east into New Mexico. To get there I took a backroad Indian Reservation Route 13 through the Chuska Mountains. At one point the trees opened up I could see this view into New Mexico.

Mitten Rock & Shiprock from the Chuska Mountains.

Black Rock
(internet image)

As the road descends out of the mountains it turns north towards Black Rock still in Arizona. It's an impressive site where it protrudes out of the red sandstone cliffs. I couldn't find any information about it specifically but there it is.

Mitten Rock
(Internet image)

Route 13 crosses over the New Mexico state line and past The Mitten on the north side of the highway. It is another diatreme over 800 feet tall and the epicenter of a fracture zone. It is sometimes referred to as Little Shiprock.

Shiprock, eight miles to the east is the the largest, tallest and most impressive of the volcanic necks. Also a diatreme it went through multiple eruptions 27 million years ago.

Shiprock (Rock with Wings)
(internet image)

The Navajo Rock With Wings. If viewed by satellite three prominent dikes can be seen radiating out from Shiprock which look like wings. Route 13 passes through the largest of the three.

Google Earth satellite image

Shiprock dike 1
Shiprock in the distance

South of Shiprock about 20 miles, highway US-491 passes between a pair of volcanic necks just 1.5 miles apart. In this photo heading north Bennett Peak is on the left and Ford Butte on the right.

Bennett Peak (left) Ford Butte (right)

Bennett Peak 870 feet tall

Ford Butte 550 feet tall

The Chacoan civilization lived in this area between 800 - 1,100 AD, and this group of three volcanos: Shiprock, Bennett Peak and Ford Butte, were a part of their sacred beliefs. The Navajo arrived 200 years later and then they embraced similar beliefs about the volcanos.

There are many more than the volcanic vent I just mentioned but these are some of the more notable ones and the ones most likely you are to come across.

COLORADO

Leaving the Navajo Volcanic Field and back in Colorado is the Spanish Peaks Volcanic Field. It is in the south-central part of the state 60 miles south of Pueblo. One would think it to be part of the Rocky Mountains but it is actually a mush later later event. 24 million years ago an igneous pluton rose and became the source of magma for the Spanish Peaks. There are three volcanic necks associated with that magma plume.