The volcanoes and associated lava flows in this area are among the youngest eruptions in New Mexico. The lava flows are unusually long lava flows and have covered most of the broad valley between the e Zuni Mountains on the west and Cebolleta Mesa on the east.
The youngest flow, the McCartys lava flow field, is the youngest eruption in New Mexico. An inflated pahoehoe type lava flow, it is 40 km long and retains characteristic surface glass (tachylite), and other millimeter-scale features typical of fresh lava flows. The extreme northern distal end of the lava flow lies within the Rio San Jose Valley at modern day I-40. Another young lava flow of mostly aa type lava characteristics, erupted from a small basaltic cone within the Zuni Mountains flowed north through a canyon and out onto the valley of the Rio San Jose near modern day Grants and I-40. And other flows erupted from the larger cinder cones within the Zuni-Bandera volcanic field at the southern margin of the Zuni Mountains.
General geologic map of the Zuni-Bandera Field.
Images of the McCartys Lava FLow (Age ~3900 years)- Eastern Side of the Field
The McCartys Lava flow is the youngest eruption in the Zuni-Bandera field. The lava flow parallels the modern route of NM 117 and retains most of the primary lava characteristics like these lava crusts or shells several centimeters thick that were broken, jumbled, and remain where they formed several thousand years ago.
Aerial, low altitude view of inflation pits near the south end of the McCartys lava flow field. Note that the low areas were sunsequently flooded with lava in this scene. Some of the deep cracks visible in the surface of the lava would have required cooling of the lava for months in order to create a crust thick enough to crack to depths visible. The view is directed northward. Photo L.Crumpler
McCarty lava flow field southern edge, NM 117 (from about 20,000'). The white spot on the right edge of the lava flow is the parking lot and trail head for the Lava Falls Trail. Photo L. Crumpler
Guide to Lava Falls Trail at the south end of the McCartys Lava flow field , A Brief Text Description Guide to the Lava Falls Trail area, McCartys Lava flow: Numbers refer to locations shown on overhead image above. -L. S. Crumpler
1 0 to 50 m The eastern margin of the lava flow from here to the southern tip consists largely of coalesced multiple divergent sheet lobes. As we will see on the traverse, the last emplaced part of the lava flows here were trapped between an existing steep margined sheet lobe and the eastward rising ground surface. This means that the flow margins on the east side represent about as far as the flow could advance with the hydraulic gradient resulting from the thickness of the initial sheet lobes here. The surface of the lava flow slopes eastward and consists of a sub-centimeter scale granular or knobby texture common on many pahoehoe lava flows. In addition, a prominent lineation occurs here distinguished by a parallel ridge-and-furrow pattern consisting of relief between ridges and adjoining grooves or furrows on the order of a centimeter to a few centimeters wavelength. The ridge-and-furrow texture is common across many of the broader elevated surfaces and it commonly strikes in a uniform direction over the space of a hundred meters or so. In this area near the beginning of the trail the lineations are oriented generally east-west or perpendicular to the local flow margin. Also, here at the trailhead, note that the surface of the lava flow is an oxidized brownish color and consists of exposed vesicles. This is somewhat atypical for the McCartys lava flow field. For most of the McCartys Lava flow field, the upper few millimeters consist of black glassy chilled surface (tachylite). In detail, this black glassy rind commonly consists of extremely stretched vesicles, but less commonly can include dripstone patterns and micro ropes. Here at the lava falls trail the black glassy outer layer is largely removed and a dark oxide brown vesicular surface prevails. Patches of dark glassy material are present , but just not widespread as on much of the McCartys lava flows. Much of the lithic debris from removal of the glassy rind is collected in hollows and forms a fine crunchy soil, the fragments of which often preserve the glassy surfaces. The origin of the differences in glass retention is unclear. There are many examples where two flow lobes, side by side, have dramatically different glass retention. This observation suggest that the conditions of the environment or the particular batch of lava itself at the time of emplacement may exert a strong influence on the surface characteristics and whether glass is retained over the long term.
[1 to 50 m]
2 50 m - 100 m The surface changes to generally smooth plates 3 to 5 meters across. Locally small squeeze-ups along the margins of individual plates are evidence that the surface crust was relatively thin when the plates were deformed. This is a type of surface fairly common to many pahoehoe flows and is characterized by upwardly convex plates, or "swales." The process of swale formation is as yet to be understood, but it is common in areas where the flow surface cooled initially with minimal shearing, such as in ponded areas or margins of newly spread sheet lobes.
Near the 100 m point, over a distance of less than 10 m along the traverse, some of these plates are up-ended and locally jumbled by local shearing strains. To the immediate south there is a circular depression approximately 18 m in diameter. This is part of a north-south striking zone of similar slab deformation for several hundred meters north and south. Individual plates are several tens of centimeters thick. From basic heat transfer and observations of active lava flows, we know the approximate time intervals required to develop crusts of different thickness on basaltic lava flows (Hon et al.1994; Keszthelyi and Denlinger, 1996). Based on this we can estimate that the lava flow surface was less than an hour old when the deformation occurred.
Over the next 100 meters note the presence of shallow circular depressions, particularly one on the left (southwest) along the traverse at about 150 m. A common morphology for these is a depression from 10 to 20 meters across, encircled by deep cracks or clefts. The depression itself is frequently characterized by a V-shaped profile. Small lava features such as “squeeze-outs” and “break-outs” may occur on the marginal slopes leading into the depression. Deep clefts along the margins of a few rare examples of this type may even preserve tooth-paste-like squeeze-outs. Others, though less common, are escarpment-bounded along their inner margins and flat floored.
For many years the circular depressions like these were interpreted to be the results of collapse following draining of “lava tubes” during the waning stages of the flow. Recently it has been shown that these are lava rise pits and that the profiles and evidence for crustal thickness during deformation are more consistent with the processes of inflation (Walker, 1991) in which the pits literally represent uninflated zones where the upper crust and floor were not separated by melt. Such areas might occur particularly in association with underlying relief such as where an overflowing sheet lobe is thinner and the section more quickly chilled and solidified. A few examples of cleft ridges in the underlying older Hoya de Cibola flow occupy some depressions here on the south end of the McCartys flow field.
3 200 m The immediate lava surface here is relatively low in relief features and consist of a large flat mosaic of fractured surfaces. The strike of ridge-and-furrow lineations described earlier are here oriented southwest-northeast which is nearly at a right angle to those near the margin at the starting point of the traverse. This is probably the core of the main sheet lobe that was moving southwest at this site.
4 270 m The surface of the flow is again disrupted in a series of contorted slabs and ropes. Most of the deformation is again within sheets with thickness of a few tens of centimeters. Here we move off of the area of broad plates or swales, and into a part of the flow surface that begins to gradually descend to the west. In this area large deep cracks or clefts also begin to be more common. Some of these clefts are 4 to 5 meters deeps. If we explore the interior of the wider clefts, we will see evidence for some semi-plastic-to-brittle transition in the mechanical properties of the lava as though crack tips were keeping pace with the solidification. Where the clefts are several meters deep, this means that the flow was still deforming several months after initial emplacement.
Over the next one hundred meters note the two 20 to 30 m wide shallow depressions on the left. Several large clefts striking NE-SW through this part of the flow are deflected and encircle these depressions. Meanwhile the ridge-and-furrow texture is unaffected. Our interpretation is that the ridge-and-furrow patterns developed in the thin initial sheet lobe, and that the broad depressions developed during large inflation of the lava flow.
5 380 m The surface of the lava flow starts to descend to the west. Ahead is a low trough between this elevated part of the flow and a somewhat more elevated pine-tree-covered plateau in the distance. As we move forward note that the local relief appears more chaotic. Where ridge-and-furrow patterns are present they may change orientation rapidly within a few meters. This surface abruptly contacts the steep margins of a 5 to 6 m high rise in the terrain beyond (Fig 10).
6 430 m Note that the local relief appears much rougher at scales of a few meters. Where ridge-and-furrow patterns are present they may change orientation rapidly within a few meters. This type of pattern is common to lower areas within the flow field. Indicators of flow direction such as ropes, ridge-and furrow-lineations, and short flow lobes are frequently different from the inferred overall direction of flow within the regional field.
7 480 m A distinct contact occurs between the lava flows that we have just traversed and the lower slopes of the plateau ahead. The plateau is from 5 to 10 m higher than the flows we just traversed and the flows appear to have lapped onto a somewhat brownish flow surface forming the lower slopes of the plateau margin.
8 510 m If one traces out the local shapes it is apparent that the digitate-shaped brown flow or flow-lobe also appears to onlap the plateau. And the adjacent sloping surface of the lavas forming the plateau margin onlapped by this brown lobe are characterized by a distinctive aggregation of 10 to 20 cm diameter “balls” where not covered by ridge-and-furrow or ropey textures. The contact between the “brown” lava and the plateau is also considerably elevated from the lower margins of the plateau. First it is clear that there are at least three separate flow units here which would account for the two on-lapping contacts of lava bearing distinctive textures.
An alternative to the above interpretation is that the “brown” lobe initiated near its highest point on the margin as a type of late squeeze-out that flowed down the margins of the plateau, and was subsequently overlain by later flows as the surrounding flow field emplacement progressed. There are many similar occurrences throughout the flow field, some of which are more convincing than others.
9 580 m This station is located on the surface of the plateau and lies some 5 to 10 meters above the surrounding flow field. Unlike much of the flow field this surface is relatively flat-lying: surface relief across 100 m distances is less than 10 cm on average as measured by differential GPS methods [Zimbelman, personal communication]. Strong ridge-and-furrow textures are a dominant characteristic of this surface and imply that the initial sheet lobe flow directions were roughly northeast-southwest in this vicinity. Note that the strike of the lineation changes slightly over 100 m scales, but generally it is locally uncorrelated with the current existing large-scale relief. There are many localities where the local lineations strike right across significant relief features such as pits, depressions, and even plateau margins. The interpretation is that the lineations and the relief characteristics developed at different times.
10 780 m This is a good place to note the clear on-lapping contact between the small flow lobes constituting the surrounding lava field and the plateau, also sometimes known as a "lava rise." The surrounding flows clearly post-date the formation of the plateau margin.
11 840 m The “Lava Falls:” are actually a small (only meter-scale width) lava drip that poured over the rim of a local depression during the course of a local breakout from one of the many small toes or lobes that make up the chaotic valley floor. A walk up the slope of the plateau margin just west of this location is useful for examining the size and depth of clefts that occur in these plateau. Depths of cracks in excess of 8 m have been documented in this area. Some of the deepest cracks occur along the tip of the digitate-shaped ridges. Locally there are examples of lava breakouts that flood the floor of some clefts, or that spill out to flood surrounding surfaces. An example of the latter may be seen on the north wall of the large circular depression immediately to the west at this location. In still other locations the cracks opened in semi-brittle to semi-plastic solids resulting in distinctive wall textures.
The return route starts here at Lava Falls: From lava falls traverse 100m bearing N160E to a surface just east of a prominent cleft ridge. The cleft ridge in question is locally enshrouded by later lava flows.
12 940 m The lava flow surface slopes up and east. This is equivalent to station 5 and marks the western margin of the central elevated region on the east margin of the lava flow field. East of this point the traverse moves back onto the zone of large, generally horizontal surfaces consisting of plates and local deformation of centimeters-thick slabs.
13 1050 m Lava shatter zone. Here the surface consists of fragments of centimeter-thick scoriaceous crust broken and apparently tumbled along a zone that strikes roughly northeast-southwest, trending through a small local depression before striking more southerly down-flow ). This is an example of one of the many surface textures that are evidence for relatively thin (one to two meter thick) sheet lobes being dominated during the initial emplacement of the lava flow.
Similar features have been observed forming on active lava flows (Orr, 2010) and are a surface characteristic of lava tubes and channels associated with fast-moving lava. This particular example is aligned with some of the deformation zones we crossed on the flow earlier (station 4).
From here to station 14 the traverse crosses a surface consisting of a large mosaic of fractures enclosing 2- to 3-meter scale convex surfaces similar to that seen between station 2 and 4 on the out-going leg of the traverse.
14 1180 m The lava flow surface begins to gradually descend to the east. Here the surface morphology changes again and becomes more complex in relief, while gradually stepping down to the elevation of the pre-lava landscape. As at stations 1 and 2, the dominant ridge-and-furrow surface lineation indicators of flow direction here tend to be eastward, in contrast to the more southwesterly trend noted in the strike of ridge-and-furrow textures between station 13 and 14.
Traverse end. 1340m Margin of flow. Return to parking lot
Cleft in McCartys lava flow field tumulus. Photo L. Crumpler
McCartys flow field vent area (aerial). Photo L. Crumpler
McCartys vent as seen from the surface. Photo L. Crumpler
Margin of a lava rise plateau near the southern terminus of the McCartys lava flow field. Photo L. Crumpler
Ropy lava a few hundred meters east of the vent region. Photo L. Crumpler
Lava Tube near McCartys vent. Photo L. Crumpler
Images of the Bandera Crater ("Ice Caves") area - Western Side of the Field
Low altitude aerial view of Bandera crater, a young scoria cone (~ 10-20 Ka) on the western edge of the Zuni-Bandera volcanic field. Photo L. Crumpler
High altitude view of Bandera Crater and the lava flow. NM Highwya 53 cuts across the image north of Bandera Crater. Photo L. Crumpler
Lava channel in aa lava flows from Bandera crater (in the distance). Photo L. Crumpler
Lava tree mold. This example is located in the Bandera flow immediately behind the visitor's center. Bandera Crater, NM. Photo L. Crumpler
Lava tree mold in the Bandera lava flow next to western end of the Zuni-Acoma trail. Photo L. Crumpler
View McCartys Lava Flow in a larger map