Field Guide – Oak Creek-Mormon Lake Graben Northern Arizona

The Oak Creek-Mormon Lake Graben lies between Flagstaff and Sedona, Arizona. The Arizona Geological Society and geologist Paul A. Lindberg have produced a 13-page field guide to the geology of the area (shown on the map below).

This geologic field trip guide circumnavigates a loop of ~120 miles from Flagstaff to Sedona along Highway 89A and returns to Flagstaff along the Lake Mary Road. The guide contains many illustrations and photographs and may be downloaded from:

Lindberg introduces us to the local geological setting:

“The Oak Creek-Mormon Lake graben (a rift valley formed by extension of the earth’s crust) has been faulted into the southwestern margin of the Colorado Plateau as basin and range crustal extension has migrated eastward across Western U.S. over time. The graben may be as young as 2-3 million years old, based upon the youthful appearance of numerous V-shaped canyons (Oak Creek, West Fork, Munds, Woods and Rattlesnake Canyons) that cut the minimally eroded original surface of the largely basalt covered core of the graben. That morphology is in sharp contrast to more maturely eroded landforms along the northeast margin of 10 Ma Verde graben near Sedona. Timing of the genesis of the Oak Creek-Mormon Lake graben may be contemporaneous with the main eruptive cycle of San Francisco Peaks north of Flagstaff, Arizona.”

The 12 geologic stops focus on recent faulting and the encroachment of Basin and Range extensional structures on the Colorado Plateau. Each stop is detailed in the text, which is amply illustrated with photographs and colored geologic sketches.

Oak Creek Graben map

Earthquake videos from Arizona Geological Survey

Most earthquakes in Arizona are low magnitude and go unnoticed.  However, there is  potential for a big one.  The Arizona Geological Survey has several videos featuring earthquakes in and near Arizona.  The newest is a time-lapse animation of the Brawley earthquake swarm that occurred on 26-29, August, 2012. Brawley is in southern California, just south of the Salton Sea.

To see this video go to:  http://www.youtube.com/user/azgsweb

That link also contains several other earthquake-related videos (4 to 6 minutes long) so scroll down the page and you will see:

Earthquakes in Arizona

Time-lapse video animation of earthquakes in and around Arizona from 1852 to 2011. The apparent increase in seismic events in the latter part of the 20th century and early 21st century stems from improved seismic monitoring.  It is interesting to see where the most earthquakes occur.

Lake Mary Fault

The Lake Mary Fault, located immediately south of Flagstaff, Arizona, represents the greatest earthquake hazard to the more than 70,000 people of Flagstaff and environs. Dr. David Brumbaugh, Arizona Earthquake Information Center (AEIC) at Northern Arizona University, narrates the, “Lake Mary Fault — Potential Earthquake Threat to Flagstaff, Arizona.”

Little Chino Fault and Big Chino Fault

Filmed on location in Chino Valley, two separate  six minute videos describe the geometry and timing of seismic activity on these faults. This is an active fault area with a moderate recurrence rate on the order of tens of thousands of years. It is capable of yielding earthquakes in the range of magnitude 6 to 6.5 and presents a hazard to residents of Chino Valley and nearby Prescott, Arizona.

Earthquake Monitoring in Arizona, the role of the Arizona Integrated Seismic Network

Arizona has earthquakes. Geologist Dave Brumbaugh and seismic technician Lisa Linville, both of the Arizona Earthquake Information Center (Northern Arizona University), describe the role of the Arizona Integrated Seismic Network in monitoring earthquake activity in the Grand Canyon State. And Lisa deconstructs one of the broadband seismic stations that form the backbone of the system.

The 1887 Sonoran Earthquake

On 3 May 1887, a magnitude 7.2 earthquake rippled across Sonora, Mexico, and southern Arizona. Phil Pearthree, Chief of the Environmental Geology division of the Arizona Geological Survey, revisits that event in this video. The earthquake killed dozens of people and damaged or destroyed several hundred structures. A similar event today would disturb and disrupt population centers in northern Sonora and southeastern Arizona and New Mexico.

For more information on the 1887 earthquake, see my post: The Great Arizona-Sonora Earthquake of 1887

For more information on earthquakes and other geologic hazards, visit the AZGS Geologic Hazards Center: http://www.azgs.az.gov/hazards_earthquakes.shtml

Earthquake hazard near Flagstaff assessed, Video

Flagstaff, Arizona sits in the middle of the San Francisco volcanic field and at the northern end of the Lake Mary fault which poses a potential earthquake hazard for the city. The Lake Mary fault (yellow lines in map below) extends about 25 miles from Mormon Lake in the south into the city of Flagstaff on the north.


Dr. David Brumbaugh, Arizona Earthquake Information Center (AEIC) at Northern Arizona University, estimates that there is 50% chance for a magnitude 6.9 to 7.0 earthquake sometime in the next 30 years.  Such an earthquake could do great damage especially to older structures in town.  Three such earthquakes have occurred in the region between 1906 and 1912.   A swarm of very small quakes occurred along the fault this past June.

Flagstaff lies astride the Northern Arizona Seismic Belt and is considered second only to Yuma,  Arizona for potential seismic hazard.

The Arizona Geological Survey is currently featuring a 5-minute video by Dr. Brumbaugh which explains the geology and the hazard at Flagstaff. (Link to video).

The Arizona Geological Survey has two featured sections near the bottom of its webpage dealing with earth fissures and earthquakes.  Take a look at the earthquake hazards page here for some information on past Arizona earthquakes.


Young Volcanic Fields of Arizona

The volcanic history of Arizona spans more than one billion years. There are seven volcanic fields which have erupted within the last four million years. One of them entombs pottery of local inhabitants of the time. One dammed the Colorado River in the Grand Canyon several times. Another produces world-class gemstones, and another is associated with the 1887 earthquake which shook Southern Arizona and Northern Sonora.

The map below, from the Arizona Geological Survey shows the location of these recent and older volcanic fields. This article discusses those shown in red. For more information on rock names, see the Igneous rock naming page to the left.


Uinkaret Volcanic Field (UI on the map)

The Uinkaret volcanic field lies on the north rim of the Grand Canyon in northwestern Arizona. Four major eruptions of basalt from this field flowed into the Grand Canyon and dammed the Colorado River between 725,000 and 475,000 years ago, between 400,000 and 275,000 years ago, between 225,000 and 150,000 years ago, and between150,000 and 75,000 years ago. (Source).

Dating was done by radiometric methods using the ratio Argon-40 to Argon-39.

Some of the eruptions flowed down the canyon as much as 75 miles. Some of the dams reached more than 700 feet high. When the river eventually over-topped and broke the dams there were great floods. There is no consensus on how long the dams lasted. Some think they may have lasted up to 20,000 years and formed large reservoirs.

According to the Smithsonian Institution, “One lava flow, from Little Springs, south of Pliocene Mount Trumbull, has a cosmogenic helium age of 1300 +/- 500 years BP. Pottery sherds dated at between 1050 and 1200 AD were found within the Little Springs lava flow, which occurred about the same time as the Sunset Crater eruption in the San Francisco volcanic field to the SE.”

San Francisco Volcanic Field (FL near Flagstaff)

SFpeakThe San Francisco volcanic field near Flagstaff has been active for about 6 million years. The oldest eruptions occurred near the town of Williams. Sunset Crater, a cinder cone east of Flagstaff, is less than 1,000 years old.

spThe US Geological Survey says, “It is likely that eruptions will occur again in the San Francisco Volcanic Field. With an average interval of several thousand years between past periods of volcanic activity, it is impossible to forecast when the next eruption will occur. U.S. Geological Survey (USGS) scientists believe that the most probable sites of future eruptions are in the eastern part of the field and that the eruptions are likely to be small. These future eruptions may provide spectacular volcanic displays but should pose little hazard because of their small size and the relative remoteness of the area.”

The San Francisco mountains, which include Humphreys Peak, Arizona’s highest mountain at 12,633 feet, is a stratovolcano which erupted between 1 million and 400,000 years ago. This volcanic mountain consists of interspersed layers of andesitic lava, cinders, ash, and volcanic mudflows.

The younger volcanic cones and their flows are basaltic, such as SP crater (71,000 years old) and Sunset Crater. According to the USGS,

“Most of the more than 600 volcanoes in the San Francisco Volcanic Field are basalt cinder cones. Basalt has the lowest viscosity of all common magmas. Cinder cones are relatively small, usually less than 1,000 feet tall, and form within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity. SP Crater, 25 miles north of Flagstaff, is an excellent example of a cinder cone and its associated lava flow.”

SunsetCraterThe volcanic field also contains lava domes. These are dome-shaped, steep-sided piles of viscous dacite and rhyolite lava. These can expand like balloons when lava wells up inside the dome, or the dome can break and develop by buildup of successive layers.

A side story, the cinder cone gold scam: Among my duties as an exploration geologist was examining properties submitted to our company by third parties. One submitted property was a gold prospect in one of the basaltic cinder cones. It is extremely unlikely that gold occurs in cinder cones, but a friend of one of my company’s directors was interested, so I looked.

Upon arriving at the property and meeting the owner, I asked where he thought the gold was. I collected samples at those places, and others. The property owner just happened to have an assay lab on the property and offered to analyze the samples I collected. Just to see what would happen, I gave him a few handfuls of some of the samples. Miraculously, his lab found gold in the samples. When I got back to Tucson, I had the remaining material assayed by a reputable laboratory which failed to find any gold. Now, that result is a truly amazing “nugget effect.” Conclusion: the assayer was either incompetent or crooked.

Springerville Volcanic Field (SP)

Most of the basaltic flows in the Springerville volcanic field are between 2.1 million and 300,000 years old. Some older flows, 6- to 8 million year old flows occur to the south. There are no stratovolcanoes here, only about 400 cinder cones and their associated flows.

San Carlos Volcanic Field (SC)

The San Carlos volcanic field is on the San Carlos Indian Reservation south of Springerville. The basalt and peridotite lavas erupt between 7 million and 500,000 years ago. Peridotite, from whence the term peridot comes, is a coarse igneous rock consisting mainly of the minerals pyroxene and olivine. Peridot is gem-quality olivine, a magnesium-iron silicate. It is claimed that 90% of the world’s peridot comes from Peridot Mesa in this volcanic field. Peridot Mesa is a diatreme (a breccia-filled vent formed by gaseous explosion) and was followed by lava flows.

San Bernardino Volcanic Field (SB)

The San Bernardino volcanic field in the southeast corner of Arizona contains about 130 vents and cinder cones, maars, and flows of olivine basalt. These were erupted between 1 million and 27,000 years ago. An earthquake in 1887 was centered just south of the border in the San Bernardino Valley.

Maars are craters created by a steam blast. Paramore Crater is the largest of these, about 1 km by 1.5km and about 60 m deep.

In the northern extension of the San Bernardino Valley, the San Simon Valley, there are still many hot springs.

Sentinel Volcanic Field (SE)

The Sentinel volcanic field, west of Gila Bend, AZ, contains basalt flows 2- to 6 million years old. This field may be related to the larger Pinacate field to the south.

Pinacate Volcanic Field (PI)

Most of the Pinacate volcanic field is in Sonora, just south of the Arizona border. Pinacate contains eleven giant-maar craters and hundreds of cinder cones. (See photos here and here.) The field has been active for 2 or 3 million years and eruptions have occurred as recently as 13,000 years ago. (Source.)


A map showing young volcanic fields in Arizona and New Mexico may be found here. This is a PDF file. It shows a remarkable straight line of volcanic fields from San Carlos in Arizona through Taos, NM. It also gives ages of the volcanic units. (You have to enlarge the view.) The map was prepared as part of an assessment for geothermal energy.