Publisbed by tbe Arizona Geological Survey

ARIZONA

THE STATE AGENCY FORGEOLOGIC INFORMATION

MISSIONTo collect and archive infor­mation about the geologiccharacter, processes, haz­ards, and mineral and en­ergy resources of Arizonaand to inform, advise, andassist the public in order tofoster understanding andprudent development of theState's land, water, mineral,and energy resources.

GOALS

• Increase understandingof the geology ofareas with potentialpopulation growth andeconomic development.

Volcanism in ArizonaLarry D. Fellows

Director and State Geologist

Figure I. Sunset Crater (arrow) uortheast ofFlagstaff iu Cocouino County, Arizona. Note cindercones in leji foreground aud background. Basalt that flowed ji·OIll the base ofSunset Crater can beseen in the IOlver right portion of the photo.

• Improve effectiveness ofadministering Arizona'soil and gas statutes.

• Expand the customerbase of the ArizonaGeological Survey.

• Improve access todigital geologicinformation to all users.

Blobs of red-hot, moltenrock sprayed into the air

like a huge Roman candle,cooled as they fell to theground, and fonned a cindercone. Ash and cinders werescattered over the adjacent areaand covered the pit houses ofthe Native Ameticans wholived there. Lava oozed fromthe vent and cooled to formlobes of a dark-gray to blackbasalt. Residents discovered

that when they pressed an earof com into the soft lava it leftan impression before it ignited.

This eruption began atSunset Crater northeast ofFlagstaff in 1064 A.D. andcontinued intetmittently forabout 150 years (Figures Iand 2). Nearly 600 cindercones were already formed inthe area during the precedingfour million years. Noeruptions have occurred in

Arizona since then, and thereis no evidence that any areimminent.

Volcanic eruptions werecommon. Volcanic eruptionstook place sporadicallythroughout Arizona, butespecially during the last 40million years (Figure 3).During the first 15 millionyears of this time periodvolcanism generally

Arizona GeologyWinter 2000

Figure 2. Basalt (darkest areas). alld cillders cover the grolllld west oISlIlIset Crater. The base ofSlIlIsetCrater is ill the IIpper leli o.fthe photo. Note all old cillder calle (arrow) ill the IIpper right.

progressed across the Statefrom southeast to northwest.Most of the volcanic rocksyounger than IS millionyears are located near thesouthwestern boundary ofthe Colorado Plateauphysiographic province.The youngest volcanic rocksin Arizona, those that arefour million years old or less(shown in red on Figure 3),are clustered in six areasthroughout the State. Thosethat crop out along theInternational Bordersoutheast of Yuma are onthe northernmost edge of thePinacate volcanic field,which is a large,spectacular, young field thatis almost entirely innorthern Sonora, Mexico.

Non-explosive eruptions.Most eruptions during thepast 15 million years hadcomparatively minorexplosive activity. The lava

flowed like hot tar and cooledto form basalt. During thepast two million years lavawith that consistency flowedinto the westem part of theGrand Canyon near Vulcan'sThrone at least a dozentimes, temporarily danill1ingthe Colorado River. Lava alsoflowed into and along theLittle Colorado River anddammed it in two places.Grand Falls, on the NavajoReservation about 40 milesnortheast of Flagstaff, is themost easily accessible place toview the results (Figure 4).Several times from about 8 to 2million years ago lava spi liedinto what was then a large lakebut is now the Verde RiverValley between Clarkdale andCamp Verde. Basalt there isinterbedded with lake-bedsediment. Between CampVerde and Strawbeny lavacascaded southward over theMogollon Rim and completelyobscured it.

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Explosive eruptions.Many eruptions wereexplosive. The lavas thataccompanied them werericher in silica and, therefore,more viscous (less fluid) thanthe basaltic flows. Viscosityof the lava was determined byits composition, which, inturn, determined the easewith which trapped gasescould escape. Much of thislava solidified as rhyolite orassociated rock types. Ash,pumice, and rock fragmentswere ejected into the air. SanFrancisco Mountain, 10 milesnorth of Flagstaff, is theremnant of one of Arizona'sbest-known explosivevolcanoes (Figure 5).Humphreys Peak, which hasan elevation of 12,633 feet, isthe highest point on SanFrancisco Mountain and thehighest point in Arizona.Sometime between 200,000and 400,000 years ago,however, San Francisco

Mountain was 15,000-16,000feet high before the top andnortheast side blew off duringa violent eruption. About2,500-3,000 feet of thesummit was destroyed.(That's essentially whathappened to Mount St. Helens

in Washington in May 1980,except that elevation of St.Helens plior to the eruptionwas 9,677 feet and 1,300 feetof its sUlmnit was blown off.)Pumice and volcanic ash fromSan Francisco Mountain werescattered toward the northeastand accumulated in low places.

In the SuperstitionMountains, about 23 mileseast of downtown Phoenix, avolcano exploded about 18mi)lion years ago. Volcanicash beds associated withthese eruptions are exposedspectacularly along theApache Trail, which followsthe Salt River northeast ofApache Junction. Geologistsnamed the thickest and mostwidely distributed ash depositthe Apache Leap Tuff.

About 27 million yearsago a series of explosiveeruptions took place in theChiricahua Mountains insoutheastern Arizona andresulted in deposition of theRhyolite Canyon Tuff andother ash beds to a thicknessof about 2,000 feet. Theseunits were subsequentlyeroded to form spectacularpinnacles and columns(Figure 6). The Organ PipeCactus National Monumentis also an excellentlaboratory in which toobserve rhyolite, volcanicash, and other rocksassociated with explosivevolcanism.

(COli till lied all page 4)

.~.~ .

N

I

30 0 30~~~~-.·~__liii! Miles

VOLCANIC FIELDS

Uinkaret (UI)

Flagstaff (FL)

Springerville (SP)

San Carlos (SC)

San Bernardino (SB)Sentinel (SE)Pinacate (PI)

(

... -;'")

AGE OF VOLCANISM(million years)

• 0-4

II 5-16

D 11-38

50-82

II 160-200

Figure 3. Volcanic rocks in Arizona, compiled from the Geologic MapofArizona (Map 35, published by the Arizona Geological Survey).

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Arizona GeologyWinter 2000

Figure 5. San Francisco Mountain as viewed/rolll the northeasttolVard the inner basin (IB). This side a/the IllOuntailiwas blown outduring a violent eruptioll. Note volcanic debris and cinder cones in the center/oreground. Sunset Crater (arrO\I') is at the left a/thephotograph. Photo by J. Dale Nations

Figure 6. (Right) Colulllns o/rhyolite/orllledby erosioll 0/ cOlllpacted volcallic ash, pUlllice,alld rock ji·aglllellts. This explosive volcanicactivity occurred in what is 11011' theChiricalllla Natiollal MOllulllent ill CochiseCOUllty, southeastern Arizona. Colulllns andother geological /eatures present within theMonulllent are described in Field Guide to theGeology of the Chiricahus National Monument.See Selected Reji"ences.

Figure 4. (Left) Gralld Falls all the Little ColoradoRiver lIortheast a/Flagstaffall the NavajoReservatioll. Tile river origillallyflolVedfi"Olll theupper right toward the lowerleji. A lobe a/lavaflowedji"olll right to left, plugged the chaliliel(arrow), alld extellded a short distallce to the left.The red lille lIIarks the call tact betweell basalt alldthe ullderlyillg bedrock. The river evelltuolly flowedaroulld the basalt lobe alld retumed to its origillalchmlllel,/orlllillg the Gralld Falls. Basalt in the/oregrollild is all eroded relllilallt a/part a/the flow thatextellded dowlistrealll.

Does Arizona have avolcano hazard?Geologically, the volcanicactivity that took place inArizona during the lastfour million years is veryyoung. Those volcanicfields that are less thanfour million years oldshould probably beconsidered dormant, butnot extinct.

Volcanoes do not eruptwithout giving some type

of warning. Warningsignals include frequentsmall earthquakes causedby upward movement ofmolten rock (magma).The land surface mighteven bulge slightly.Shallow magma heatsground water, commonlyenough to cause it to boil.Hot springs are common.In addition, gases such ascarbon dioxide that areassociated with magma

might bubble from theground and, if sufficientlyconcentrated, even causeplants to die. Although"swarms" of smallearthquakes have occurrednear the Grand Canyon,seismologists do notattribute them to themovement of magma. Noevidence suggests thatmagma is currentlymoving toward the surfacein Arizona.

SELECTEDREFERENCES

Volcanoes of northernArizona, sleepinggiants of the GrandCarlyon region: W. A.Duffield, 1997, GrandCanyon, Arizona,Grand CanyonAssociation, 68 p.

Field guide to thegeology of theChiricahua NationalMonument J.v. Bezy,1998, ArizonaGeological SUNeyDown-to-Earth 8, 32 p.

A guide to thegeology of OrganPipe Cactus NationalMonument and thePinacate BiosphereReserve: J.V. Bezy,J.T. Gutmann, andG.B. Haxel, 2000,Arizona GeologicalSurvey Down-to­Earth 9, 63 p.

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