Monday, May 25, 2015

Anatomy of a Cinder Cone Roadcut and Its Tale of Farallon Plate Geo-Gymnastics - Part I: The San Bernardino Volcanic Field

"Nothing in geology makes sense except in terms of plate tectonic theory."
Lynn S. Fichter, Ph.D., Department of Geology, James Madison University (here)

In spite of their global ubiquity and profusion, cinder cones don't typically reveal their internal plumbing, even when degraded by erosion or mined for the production of aggregate. Their simplicity of construction likely explains why they haven't attracted the attention of geoscientists in the literature. For these reasons, geologist Wayne Ranney and I couldn't resist the allure of an anatomy-exposing roadcut through a cinder cone catalogued as V2009, while on a geological journey through southeast Arizona.  

Facing northeast on Arizona State Road 80, the highway slices through the northwest apron of cinder cone V2009.

Our rather ordinary and diminutive cinder cone is situated on the San Bernardino volcanic field in the northern third of a valley with the same name. Launch Google Earth, paste the following coordinates into search, and it will take you there: 31°32'07.35" N, 109°17'21.40" W. Fortuitously, the cone is transected by Arizona State Route 80, which runs from northeast to southwest across the San Bernardino Valley. Not to be confused with the one in southern California, the valley is situated in the extreme southeastern corner of Arizona's Cochise County, the adjoining corner of southwestern New Mexico's Hidalgo County, and across the international border into northeast Sonora, Mexico.

The San Bernardino volcanic field lies in the northern third of the San Bernardino Valley in the extreme southeast corner of Arizona. Clockwise, it is bordered by the mountain ranges of Chiricahua, Peloncillo, San Luis, Perilla and Pedregosa. Nearby are other similarly-situated basins and ranges such as the Animas, Sulfur Springs, San Simon and San Pedro. Modified from Shawn Blissett thesis, 2010 (left) and Drewes and Thorman, 1978.

The San Bernardino Valley is also within the Basin and Range physiographic province that extends across southern and western Arizona, northwest through the entire state of Nevada into eastern California, up to southern Oregon and Idaho, southeast into New Mexico and across the border into central Mexico. The geologic province differs from neighboring provinces in terms of topography, elevation, climate, population demographics, water availability, agriculture, industry and mineral resources, yet they share a commonality of evolution. Suffice it to say for now, the region is typified by crustal extension, which is responsible for the repetitive basins and ranges on the landscape, grabens and horsts in proper geological parlance.

Notice the locale of the San Bernardino volcanic field (arrows) within the Basin and Range province tucked into the extreme corner of Arizona. Modified from Wikipedia

The shrubby desert and grass covered plains of San Bernardino Valley are bordered by forested mountain ranges, while beyond in every direction, there are similar juxtapositions. The landscape is typical of the Basin and Range province. The ranges, called Sky Islands, have distinctive Spanish and Apache names - Chiricahua, Peloncillo, San Luis, Perilla and Pedregosa - that hint at the rich history of the region in this, the Land of Cochise and homeland of the Chiricahua Apache. 

Their story, like that of so many other Native Americans, is one of intrusion, oppression and subjugation. First came the Spanish, followed by the governments of Mexico and the United States. It's a story of bloody confrontation and retaliation between Apache Chief Cochise and warrior Geronimo, and the U.S. military stationed at Fort Bowie within Apache Pass between the Dos Cabezas and Chiricahua ranges. This extreme southeast corner of Arizona also tells a tale of the "Old West", about the gunfight at the O.K. Corral between Wyatt Earp, Doc Holliday and the outlaw Cowboys, and about Tombstone's Boothill Graveyard and a rush for Late Cretaceous silver, gold and copper. Geology and history. They're forever inseparable, wherever you go.

From the Geronimo Surrender Monument (below) just off highway 80 between the San Simon and San Bernardino Valleys, we're facing the Peloncillo Mountain range to the southeast. Nestled in the hills is Skeleton Canyon, the surrender site of Geronimo to the U.S. Army in 1886 and the pass to Animas Valley of New Mexico, another basin. Drive just a few kilometers south, and you enter the San Bernardino volcanic field, where we're headed. The terrain looks the same, but there are cinder cones and lava flows everywhere.

Why is the San Bernardino volcanic field peppered with cinder cones? If tectonics explains the location of magmatic systems along plate boundaries, what is the explanation for intraplate magmatism on the field? What did the cinder cone road-exposure reveal about its construction and emplacement mechanism?

On a grander scale, does a geologic relationship exist between the cinder cone, the volcanic field, and the sedimentary basin within which it emplaced, the physiographic province where it's situated, and even neighboring provinces? Is there a commonality of evolution that exists among these seemingly disparate entities? If so, how did it affect the geological development of the American Southwest? Lastly, is volcanism and deformation on the field active, dormant or extinct?

In this post, I address the details of V2009's construction and emplacement, and its relationship to the San Bernardino volcanic field. In Part II, which will follow, I'll discuss the tectonic big picture of the American Southwest and its relationship to V2009 and the volcanic field. 

Let's investigate.

Wayne Ranney rejoices in his position atop the west slice of cinder cone V2009. Notice the contour of the cone's slope across the road and three or four eroded cinder cones off to the northeast on the volcanic field.

It's a steep, straight-sided, low-profile, symmetrical volcano built of pyroclastic fragments. It's the most common type of volcano, the least destructive and the most simply constructed. It's found in a variety of tectonic regimes worldwide, both singly and in groups. Cinder cones range in diameter from .3 to 2.5 km, and vary from tens to 300 meters (1,000 feet) in height with a few exceeding 700 meters.

Their glassy, furnace-like "cinders", referred to as scoria (and hence the alternative name of scoria cone), contain voids from gas bubbles (1-2%) entrapped as molten magma forcefully explodes into the air. In contrast, pumice, along with ash, is the vesicular, light-colored, light-weight ejecta generated when magmas of intermediate (andesite) and felsic (rhyolite) composition erupt explosively. 

Pu'u 'O'o is an example of a large, active cinder cone with voluminous outpourings of magma that erupted from vents on the eastern flank of shield volcano Kilauea on the Island of Hawaii, the Big Island, in 1983. During its prolonged history, it has fed lava flows from crater-rim spillovers, vents along its flanks and from subterranean lava tubes that extend far downslope. Many flows have reached the sea to the south and recently advanced into the village of Pahoa to the southeast.

Cinder cone eruption occurs when pressure builds in magma reservoirs and ascends as a melt within a cylindrical conduit that feeds the developing cinder cone at a vent or eruptive center on the surface. Ascending magma may forcefully eject from cinder cones as a spectacular fiery fountain in a series of pulses or a continuous jet. The gas-charged magma violently blasts ejecta airborne, which quickly fragments, cools and falls back to earth solidified. 

The combination of short-lived, non-sustained spurts and prolonged eruptive events - mostly within a month but often up to a year and on occasion, several years - conspire to built a well-defined, and exquisitely symmetrical cone of tephra around and downwind of the central vent. Anointing the summit, a bowl-shaped crater represents the area above the vent from which material was explosively ejected.  

Cross-section of an idealized cinder cone. Modified from a Wikipedia image

As fallen pyroclasts avalanche downward around the vent, they form a conical apron of deposits on the flanks of the growing cone, some welded and others not. "Loose" cinders can't tolerate a slope greater than 30 to 40 degrees without slumping, called the angle of repose. The angle also varies with clast size and angularity. It's an example of how cinder cones, as with all volcanoes, are characterized by their compositional material, which also dictates their behavior. The following cross-section of an idealized cinder cone illustrates its external and internal architecture.

Schematic cross-section through a typical cinder cone showing the volcano-sedimentary processes and geomorphologic structures. Modified from Kereszturi and Nemeth

Classified by size, tephra (Greek for "ash") ranges from meter-sized, aerodynamic bombs, that form blocks when hardened, down to pea- and walnut-sized lapilli, and even fine-grained, millimeter-sized ash. The unconsolidated, pyroclastic fragments may weld together into an agglutinate and/or become compacted and cemented into a coherent volcaniclastic mass of agglomerate, which is mostly bombs (75%). 

The "cinders", as they are commonly called, are typically vesicular (pitted with cavities of "frozen" gas bubbles) and dark gray to black in color, due to a high iron content, which may oxidize to a deep reddish-brown. As the tephra rains down, the cinder cone becomes centrifugally layered into strata that reveal the history of their emplacement. Typically, fine-grained ejecta in buoyant plumes is transported by the wind, while coarser fractions are mostly ejected along ballistic paths.

Generally basaltic in composition, a potpourri of tephra conspire to build the cinder cone.

Molten lava may eject from a cinder cone's crater in a tall, fiery fountain or spillover from a breach in the crater, but typically exudes from a vent located at the base of the cone or an independent vent on the field. Following a path of least resistance, basaltic lava follows the topography of the landscape downslope in broad, thin sheets or stream-like ribbons in a manner that reflects its high fluidity (low viscosity), which in turn is related to eruption temperature (in excess of 950° C) and mafic chemistry (high ferromagnesium content). Individual flows associated with volcanic fields tend to be ~1-10 km long and several tens of meters thick. 

Against a backdrop of the San Luis range in Mexico, the eroded cinder cone (below) on the San Bernardino field possesses an amphitheater-like morphology largely attributable to agglomerate within the rim of the crater (Arizona Geological Survey map-verified here). Slumping, a type of post-eruptive cone degradation, can also contribute to the horseshoe shape, especially if an associated flow rafts pyroclastic material from the base of the cone. What appears to be an elongate, abruptly-terminating tongue of lava emanating from a breach in the crater or from its lower flank is in reality a large lava platform that surrounds the cone and its neighbors. The San Bernardino volcanic field was gradually built over a few million years from intersecting flows that have interbedded with Quaternary alluvium and colluvium. Notice the height of the flow front above the valley floor, an indication of the mass that has added to the field. The subtle concavity on the steep southeast (left) flank represents human excavation into the slope for aggregate. 

A few kilometers west of V2009 is this eroded cinder cone and its thick tongue of lava. 

Once the eruption of a cinder cone has ceased, surficial processes gradually begin to degrade the cone. Unconsolidated and highly permeable pyroclastic deposits are susceptible to erosion, which is highly contingent on rainfall, temperature and climate. Morphological variations of cinder cones are not caused so much by erosion but by eruption characteristics such as the nature of the pyroclasts that blanket the cone. Retardation is contingent on the degree of welding, agglutination, and cover of compacted ash and lava. A resistant rim of agglutinate around the crater may delay erosion and a lessening of the slope angle. 

On a larger scale, tectonics is a factor if post-orogenic chemical weathering decreases atmospheric carbon dioxide concentrations. Wind direction during construction affects cone symmetry by distributing ejected ash downwind, but once eruption has ceased wind deflation may degrade the edifice by stripping the windward side. During the lifespan of a volcanic field, which can last millions of years, erosive processes on cones may vary as the climate changes. Cinder cones on the San Bernardino field are in a relatively good state of preservation, which attests to the youthful age of the field. They emplaced during the wetter, erosion-inducing climate of the Pleistocene, but today are experiencing degradation at a seasonally-intermittent rate due to the Southwest's semi-aridity.

On the San Bernardino field, an eroded cinder cone languishes against a backdrop of the Peloncillo Mountains and San Luis range across the International Border. Animas Valley is interposed between the two ranges and has extensive lava flow with a chemical signature and age similar to the San Bernardino field, hence, is related tectonically. The grass-covered field is constructed of many inter-layered flows, which have been partially identified by drilling for water into the aquifer. Flank flows along the range are interbedded with eroded deposits from the mountains and interbed with valley flows on the field.

Young cinder cones are generally steep with little scoria oxidation. Older cones lack large-scale erosive features and exhibit sparse vegetation but begin to demonstrate clay formation as silicate-bearing (feldspar and pyroxene), basaltic sediments are reworked. More aged cones are vegetated and support rills and gullies that begin to reveal internal dikes and ridges. Although vegetation retards erosion by anchoring the soil, it hastens rill and gully development. Degradation diminishes the slope angle and cone height, but the ratio of crater diameter to basal diameter doesn't change appreciably. Large cones, which are associated with more volatile-rich magmas, more intensive eruptions and finer particles, appear to erode more quickly than small ones; whereas, smaller cones tend to be welded by hotter, erosion-resistant particulates. Thus, erosivity is also related to cone size and explosivity. The final stage of erosion may expose the cone's innermost plug, but only a roadcut can provide an architecture-preserving transect through the body of a cinder cone for direct observation.

In all of Arizona, I can't think of a more pristine cinder cone and associated lava flow than 71,000-year old SP Crater on the San Francisco volcanic field. Located 25 miles north of Flagstaff, this spectacular photo was captured by well-known geologist and author Wayne Ranney with glider pilot and photographer Ted Grussing at the controls. Its unweathered appearance is due to its youth, the semi-arid climate of the Colorado Plateau and the erosion-resistant agglutinate in the rim. Symmetricality implies minimal prevailing winds resided during emplacement. Notice older, eroded cones on the field.

SP is composed of basaltic andesite, while its flow bears a somewhat different chemical signature. That's not unusual, although not well understood, considering that lava generation is generally a late-stage development during cinder cone construction. Flow direction is dictated by the slope of the landscape, which dips slightly to the northeast. There are over 600 volcanoes on the Miocene to Holocene-age volcanic field. Please read my post about the San Francisco volcanic field here.

Does a genetic relationship exist between the San Francisco volcanic field in northern Arizona and the San Bernardino field in southeastern Arizona?

Photograph of SP Crater and associated 4-mile lava flow on the San Francisco volcanic field north of Flagstaff, Arizona. Courtesy of geologist and author Wayne Ranney and photographer and glider pilot Ted Grussing.

In addition to cinder cones and lava flows, the San Bernardino field contains at least eight maar craters and associated tuff rings. They are the hydrovolcanic or phreatomagmatic (Greek for "well of magma") equivalent of cinder cones that erupt when ascending magma interacts with aquifers within basin-fill sediments or fracture-controlled groundwater. Molten contents are explosively evacuated via steam-blast eruptions leaving the funnel-shaped maar crater and tephra ring cut into the landscape, and a diatreme (Greek for "through perforation") as the substructure. When the rapidly expanding, superheated water contacts the confined space of the country rock, it breaks into fragments forming a microbreccia.

Schematic cross-section through a maar-diatreme (top) and a tuff ring (bottom) showing the typical volcano-sedimentary processes and geomorphic features. The left side of the diagram represents the characteristics of the volcaniform formed in a hard substrate, while the right side is a soft rock environment. Modified from Keresturzi and Nemeth

Seven miles due east of V2009 is horseshoe-shaped Paramore, the largest maar crater on the San Bernardino field at 1.5 km in diameter. Surrounded by a fine-grained, light-colored ring of laminar tuff beds, its depressed crater is covered with Holocene playa deposits. The steam-blast explosion has lifted fragments of older basalt flows along with unconsolidated detrital material out of the crater. As the conduit and fissure system gradually cooled down, post-eruptive subsidence of the crater occurs due to diagenetic compaction and lithification. That places the crater below the level of the surrounding bedrock, which accommodates subsequent playa formation and deposition. Interestingly, the initial phase of cinder cone emplacement may involve an unsorted, xenolith-rich basal phreatomagmatic layer associated with initial magma-water interaction.  

Surrounded by younger eruptions that somewhat obscure its surface morphology, the phreatomagmatic crater of Paramore is situated below the level of the bedrock. The Pedregosa (left) and Chiricahua (right) Mountain ranges frame the horizon.

Cinder cones typically reside: 1.) as satellites or parasitic cones on the flanks of shield volcanoes located over hotspots and rift zones; 2.) on the flanks of composite (strato-) volcanoes, their back-arc spreading regions and calderas in subduction zones, and 3.) isolated or in clusters of 10 to 100 within flat-lying volcanic fields in intraplate, continental settings, such as the San Bernardino field. Their global abundance makes cinder cones the most frequent volcaniform, while phreatomagmatic volcanoes reign second. Conical structures that resemble terrestrial, basaltic cinder cones have been tentatively identified on the Moon, Mars and Venus. They may provide valuable information regarding volcanic processes and planetary evolution, which has ironically spurned an interest in cinder cones back on Earth.

An example of a satellite cinder cone is Pu'u ka Pele, on the flanks of Mauna Kea, one of five shield volcanoes that comprise the Island of Hawaii, the "Big Island." Cinder cones and lava flows typically erupt from vents on the flanks of the parent volcano in Hawaii. Beyond the cone is a young flow distinguishable by its dark color that emanated from a vent on nearby Mauna Loa, the world's largest volcano measuring from the ocean floor. Vegetated, older flows are in abundance on the shield's almost imperceptible slope, which is, flow-upon-flow, how the Hawaiian Islands were built from beneath the sea. 

Pu'u  ka Pele, on the southeast flank of Mauna Kea, is 95 m in height and 400 m in diameter. Over 300 cinder cones pepper the upper slopes of Maina Kea largely along three principal rift zones. Hawaiian cinder cones generally don't emit lava flows, which emanate from vents on the flanks of the parent shield volcano.

Still an unclear process, magma is generated at great depth within the Earth's convecting mantle. In order to reach the surface at the continental crust, it must pass through the lithospheric mantle. Partial melting (in that only a fraction of the available mass forms a melt while the remainder stays solid) in the upper mantle occurs and forms molten material with a mafic composition (described below) that buoyantly rises toward the surface through the lithosphere and ponds forming a magma chamber. 

On the flanks of shield and stratovolcanoes...
Cinder cones form when the supply of magma within the upper mantle begins to diminish or cease, and the magma chamber begins to cool and crystallize. First-formed minerals are high-temperature, olivine-rich mafics, which are mantle-abundant, rich in magnesium and iron, and silica-poor. Depleted magma minerals remain in the chamber and endow it with silica, which makes it viscous. Eventually, back-pressure forces a mafic eruption that emplaces cinder cones as satellites on the flanks of its parent volcano, which in turn may fuel mafic lava flows downslope.

On volcanic fields...
Investigations of mineral composition and thermodynamic calculations indicate the source region of volcanic fields at a depth of 67 km at a temperature of 1400°C, and any local magma chambers are at a depth of 33 km beneath the presumed crust-mantle boundary. Volcanic fields are characterized by a thin crust and lithosphere created by extension above an anomalously shallow asthenosphere with high heat flow. Volcanic fields can be formed by products of every composition, although they are most commonly basaltic. On the San Bernardino field, following Basin and Range uplift and extension, widespread basaltic volcanism formed pockets of melts. 

As pressure increased, magma began its buoyant ascent through the crust, dissecting to the surface along faults, structural weaknesses and sub-surface dike complexes. Episodic extension, variations in the geochemical, temperature and pressure states of the mantle, shifts in the locus of volcanism and magma supplies can add to the complexity of vent distribution. Thus, the emplacement of cinder cones on the field can form in a variety of distributions and geometries.

Cinder cones can form in a variety of geometries along faults or their intersections (A), offset laterally from the fault as a function of dip (B), and in an en echelon array along the fault trace. Modified from 

In northern Arizona, at 11,820 feet on the saddle between Mounts Humphreys and Agassiz of the San Francisco Peaks north of Flagstaff, we're facing the eastern portion of the San Francisco volcanic field through the Peak's caldera. It contains many linearly-distributed and clustered cones including the O'Leary Peak lava dome (left) and historically recent Sunset Crater (right) cinder cone.

Temperature, gas content and chemical composition of magma directly influences the size, shape and activity of all volcaniforms. To varying degrees, these factors affect the magma's mobility or viscosity. In regards to cinder cones, its magma is "thin and runny" with a low resistance to flow, since it's very hot, gas volatile-rich (1-6% by weight of water vapor, carbon dioxide and others such as sulfur dioxide) and silica-poor (largely of basalt but even some andesite). Thus, cinder cones tend to erupt effusively and are constructed with a symmetrical, low-profile, layered architecture about a central vent. When present, lavas flow readily in thin, broad sheets for considerable distances. 

The behavior of lava depends primarily on its viscosity (resistance to flow), the slope of the ground cover and the rate of lava eruption. Because basalt contains the least amount of silica and erupts at the highest temperature compared to the other types of lava, it has the lowest viscosity (the least resistance to flow). Thus, basaltic lava moves over the ground easily, even down gentle slopes. Dacite and rhyolite lava, however, tend to pile up around a vent to form short, stubby flows or mound-shaped domes. Modified from USGS and J. Johnson illustration

Geoscientists, true masters at categorization, distinguish types of volcanoes by their eruption behavior, which are named after volcanoes where the behavior has been observed. Icelandic eruptions are typified by effusive eruptions of basaltic lava from long, parallel fissures. Hawaiian eruptions are similar, but lava exudes from the summits of shield volcanoes and from radial fissures along the flanks. Strombolian eruptions consist of initial moderate bursts of expanding gases with later continuous small eruptions. Vulcanian involves moderate eruptions of gas laden with ash in dark clouds that rapidly ascend and expand. Pelean are explosive outbursts with pyroclastic flows and dense mixtures of hot fragments and gas that pour down slopes with great velocity. Plinian are intensely violent eruptions of gas-rich magma that rocket gases and fragments into the stratosphere often generating lightning.

Types of Eruptions Based on Behavior (Explosiveness)
Modified from Encyclopedia Britannica 2006

Based on their explosivity, plume height, frequency of eruption, and volume, cinder cones on the San Bernardino volcanic field are thought to have been Hawaiian style eruptions, that is "calm" (relatively speaking) from vents and fissures, or low-level Strombolian eruptions, short-lived but more explosive with increased plume height and with magmas of intermediate viscosity. It is conceivable that during construction of a cone, as magma fractions, temperatures and chemistries evolve, a combination of eruption types may occur. In the V2009 roadcut (discussed below) upward increases in the abundance of coarse blocks and bombs, and sequences of welded agglutinate imply an evolution in the growth process of the cone. Strombolian eruptions, as opposed to Hawaiian, tend to produce more sustained fountains of lava and more extensive welded facies, also seen.

San Bernardino Valley is a 21-mile long and 18-mile wide, northeast by southwest-trending, gently-sloping, sediment-filled intermontane basin. The surface extent encompasses about 1,000 square kilometers in Arizona and about 90 in Mexico. The topographic gradient averages 49 feet per mile. Geologically, the valley, which is divided into northern and southern portions, is classified as an asymmetric, down-dropped block of crust called a graben. The northern portion is divided into smaller half-grabens by four transfer faults that strike NW-SE, aligned with the structural lineament (more on that later). Geomorphically, the valley has also been described as a semi-bolson, which is a wide desert basin with ephemeral playa drained by intermittent streams, and, in the case of San Bernardino, that cumulatively funnel south into Mexico.

Specifically, the valley is bounded by roughly-parallel mountain ranges of the Perilla, Pedregosa and Chiricahuas to the west, and Peloncillo and Sierra San Luis to the east. The ranges are riddled with Oligocene-age calderas, which are likely buried beneath the intervening valleys as well. All this overlies a basement of Paleozoic and Mesozoic sedimentary rocks. Excluding the deeply-buried Proterozoic foundation, the landscape is comprised of these three distinctive elements - sedimentary rocks, Mid-Tertiary eruptive centers, and Tertiary to Quaternary basalt lava flows and alluvial deposits. 

Sketch map of southeastern Arizona and southwestern New Mexico showing structural features most recently from Basin and Range movement, although the region has been deformed many times as early as the Precambrian. For orientation, Tucson is 100 miles to the northwest. Modified from Drewes and Thorman, 1978

Our geo-journey through the San Bernardino volcanic field began from camp in the Turkey Creek caldera of the Chiricahua range. Facing east, the valley is the San Simon that abuts the San Bernardino Valley basin to the south and is half its width and separated from it by an ill-defined hydrologic divide. Beyond is the Peloncillo range and further back in New Mexico, the Animas Valley and the Pyramid range. Repeating basins and ranges - grabens and horsts - can be identified on the landscape in this, the Basin and Range geologic province of southern Arizona.  

Facing east from the Turkey Creek caldera within the Chiricahua Mountains, the San Simon Valley basin is backed by the Peloncillo Mountain range, and it by the Animas Valley and the Pryamid Range.

The valleys are alluvial basins filled with volcanic and sedimentary deposits shed from the surrounding mountains, and volcanic rocks that have erupted within the valleys. They formed from extension on the landscape during the Miocene-Pliocene period of high-angle faulting during the Basin and Range Disturbance starting at least 15 million years ago and continues to the present. The details are discussed in my post Part II.

And yes! There are turkeys in Turkey Creek.

The major geomorphic feature in the San Bernardino Valley is the San Bernardino volcanic field (named by Lynch in 1972), which dominates its northern third. In older literature, it was referred to as the Geronimo volcanic field. Measuring some 850 square kilometers, the field possesses over 130 separate basaltic volcanic vents, associated lava flows and pyroclastic deposits. Four radiometric dates for basalts exist for the volcanic field. Older basalts are dated at ~3.3 to ~4.7 Ma, while younger flows fall between ~750,000 and ~274,000 years ago of the late Pliocene to Pleistocene.

Lavas within the valley are distinguished by their location, age and composition. Flank lavas, which originate along the fronts of bordering ranges, are older and composed primarily of alkali-olivine basalts and have low ratios of magnesium to iron; whereas, valley lavas are predominantly basanites and fall within the younger range with relatively high magnesium to iron ratios. Overall, their composition suggests an anomalous mantle, whose properties are consistent with the presence of areas of partial melting.

Most volcanic activity occurs between converging and diverging tectonic plates or over hotspots, often far from plate boundaries. The latter is an example of intraplate magmatism, which is found at regions of lithospheric extension where mantle-derived asthenospheric melts are permitted to passively rise. This scenario is typical of continental rifting and may develop into a divergent tectonic regime. In a region that defies generalities, this is found within the Basin and Range Province of western and southwestern North America and what we see within the San Bernardino volcanic field. 

 Being bounded by mountain ranges, the San Bernardino volcanic field is located in the extreme southeastern corner of Arizona and dominates the northern third of the San Bernardino Valley. Cinder cone V2009 on SR 80 and well-known Paramore maar crater are labelled. Captured from Google Earth

Scattered across the field and onto the flanks of the neighboring ranges, monogenetic (short-lived, single-eruption, small-volume) cinder cones are the most common volcaniform. Volcanic fields typically consist of volcanic clusters and/or alignments along fissures, which are dictated by structural influences and tectonic regimes. Vents on the field are aligned along a NNE trend, which cuts across the dominant N-S Basin and Range tectonic trend. Taking the cumulative volume of pyroclastics generated by the entire field into account over periods of hundreds of thousands to millions of years, monogenetic activity can exceed that of individual composite volcanoes.

Facing southeast from atop V2009, notice its layered apron across the road, and many cinder cones and lava flows on the San Bernardino volcanic field. The backdrop is formed by the closer Pedregosa Mountains (right) and further Mule Mountains (left). Our evening destination for this day's geo-journey was the copper-mining town of Bisbee up in the Mules.

Multiple (at least seven encountered in drilling for water), thin (5-20 meters) alkaline basaltic lava flows lie beneath volcanic field's grass covered plain. The flows interbed and are partially covered by a veneer of middle and late Pleistocene alluvium, aeolian material, and near the mountain fronts, colluvium. In the center of the field, such as around V2009, it consists of dark to reddish brown soils and clay-rich vertisols, and is littered with subangular to angular boulders and cobbles of vesicular basalt (below). Closer to the fronts, rocks of newer alluvial deposits are interbedded with flank lava flows and include basalts, rhyolites and sedimentary rocks.

Concordant remnants of oldest lava flows along the flanks of the bordering mountains on both sides of the volcanic field indicate that the basalt flowed onto pediments or alluvial embayments in mountain fronts of much lower relief relative to the valley floor than today. Subsidence in the valley, which lowered the floor relative to the mountains, continued (or was renewed) not long after volcanism initiated, which left older flows as remnants flanking the valley.

On the Google Earth image below, Highway 80 slices through a thin portion of V2009's northwest flank, thereby creating two exposures, herein designated "east" and "west". Wayne and I pondered as to why highway 80 didn't circumvent the cone altogether rather than dissect through it. Apparently, the now-empty railbed of the Arizona and Southwestern Railroad (built in 1888 to transport copper mined in nearby Bisbee to El Paso and beyond) "forced" the highway to transect the cone rather than make a circuitous detour around it. 

On Google Earth, the cone's diameter measures ~1,371 (west to east) by ~1,342 feet (north to south), while its summit is skewed slightly to the south of center. This fact is confirmed within the east cut and is suggestive of a southwest prevailing wind during eruption. Its eroded crater is revealed on the 3x vertically-exaggerated image. It is located at 4,641 feet and rises some 86 feet above the field. By comparison, the highest elevation in the valley is 5,135 feet on a cone to the east, and the lowest point is 3,700 feet in Black Draw that drains the valley across the International Border into Mexico. 

A few gullies (see image below) have dissected the flanks of the cone and formed a small debris apron of colluvium around it, while an ephemeral stream has carried alluvial debris to the southeast and converged with one from another cone forming a small playa (far right of center on the Google image and the map below). No lava flows appear to have emanated from vents associated with V2009, although the entire edifice resides on a multi-layered platform of interbedded flows that constitute the volcanic field.

View of cinder cone V2009 sliced by SR 80 as it cuts through a small portion of its northwest flank. Notice the empty railbed that parallels the highway and the other vertically-exaggerated cinder cones on the field.

Using Google Earth's Elevation and Profile tool, I ran a 1,574-foot, SW-NE linear transect through V2009 just south of its degraded summit crater. The average elevation gain/loss above the field was 73.7 feet, and the maximum slope was 21.1%. As with our observations in the field and on the Arizona Geological Survey map, the Google Earth profile of V2009 attests to its displacement to the northeast. 

In addition to the chemistry of its lavas, the life history of a volcano is preserved in its stratigraphy, structure and form provided by the "window" of the roadcut. The west exposure contains only pyroclastics, while the east exposure (below) exposes not one but two lava vents or possibly feeder dikes consisting of a dark-gray basalt. That fact is substantiated on Plate 3 of the Arizona Geological Survey map (here) and is designated as Qbvl, basaltic vent lava and agglutinate proximal to vent. It would be interesting to know if the cone's innermost plumbing are conjoined, the assumed emplacement architecture. The degraded summit is situated between the conduits, as anticipated geologically, since they both would have contributed to the rain of tephra that built the edifice. 

A few map observations, as previously mentioned, the pyroclastics are skewed to the northeast suggesting prevailing wind direction during emplacement. In addition, many of the valley lava flows progress in a northerly direction, while contemporary streams and alluvial deposits from the cones progress southerly, such as the serpentine drainage emanating from V2009, Qy on the map (arrow). This suggests, requiring substantiation in the field, that the basin has progressively tilted to the south following the cessation of volcanism, which directed drainage internally or externally to Black Draw across the border into Mexico. Recent tectonic activity could have renewed stream downcutting, while uplift would have entrenched older streams. Thus, drainage patterns are good indicators of recent and past tectonic activity. The implication is that Basin and Range deformation is on-going, whether or not volcanism is dormant or extinct on the field.

This AGS map shows V2009 transected by highway SR 80. Two vents are exposed within the roadcut on the east face. Also note the Quaternary alluvial deposits emanating from the apron of the cone to the southwest.  Relevant Map Units: Qbpc (Basaltic cinder deposits proximal to vent); Qbvl (Basaltic vent lava and agglutinate proximal to vent); and Qcb (Basalt-derived hillslope colluvium); Qy (Alluvial deposits in active drainages). Modified from the Geology and Geomorphology of the San Bernardino Valley, Southeastern Arizona, Arizona Geological Survey (map three of three) by Thomas Hi. Biggs et al 1999

Distal to the vents, and therefore circumferential to them, massive reddish-brown, oxidized agglomerate forms the internal bulk of the cinder cone. The agglomerate is highly brecciated and includes scoriaceous deposits of cinders, volcanic bombs of varying size and welded agglutinate. Progressing outward, the cinder cone possesses well-defined facies and a bedded, unsorted pyroclastic stratigraphy of brownish-red lapilli of varying size (2 to 64 mm by definition), which is layered and dips centrifugally (far left), reflective of the manner of emplacement. Lapilli are also somewhat interbedded with the agglomerate centrally. Large volcanic blocks of varying sizes are scattered on the surface along with various woody plants, forbs and grass typical of the field.

East face of the roadcut

Closeup of the northernmost vent in the east face of the roadcut

This view of the east exposure reveals dark gray to dark reddish brown basaltic vent lava and massive clumps of agglomerate proximal to it. The second vent (unseen and less well exposed) is off to the right in the photo. The flanks are composed of reddish-brown, indurated pyroclasts distal to and surround the vents. 

View of the east face of the roadcut

From atop the smaller west roadcut (below), angled beds of pyroclastics are visible across the highway on the cone's apron and a small portion of the second vent. Numerous volcaniforms and associated flows are scattered across the volcanic field. In the distance, the Pedregosa range is to the right, while the Mule Mountains, the location of copper mining town Bisbee (our destination), are to the left. Many of the lava flows on the field, if not all, contain sub-rounded to well-rounded, ultramafic xenoliths (rocks not from the parent magma but from the upper crust and mantle, and introduced during emplacement). 

Where might these foreign inclusions have been derived? Think "commonality of evolution" of the landscape!

Facing highway 80 south from atop the west slice of the roadcut

Having investigated V2009's construction, emplacement and relationship to the San Bernardino volcanic field, my post Part II will address the grander question. Does a geologic relationship exist between the cinder cone, the volcanic field and sedimentary basin on which it emplaced, the physiographic province where it's situated, and even neighboring provinces in the American Southwest? Is there a commonality of evolution that exists among these seemingly disparate features? 

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