The volcanics of Strawberry Crater have been divided into four geologic units (Plate 1) based on physical characteristics and physical location. These four units consist of a cinder cone (Tscc), rafted mounds of cinder cone material (Tscm), a block lava flow (Tscf), and a silicic vitrophyre plug (Tscp). The volcanic complex rests on volcanic substrate. The cone is located at the edge of the Deadman Mesa flow (Moore and Wolfe, 1987); only a portion of the cinders are deposited on this unit. The lava flow and remainder of the cone are underlain by the Deadman Wash flow (Moore and Wolfe, 1987). Most of the volcanics of Strawberry Crater have been mantled by ash and lapilli from the eruption of Sunset Crater, which began eruption in 1064 A.D. (Smiley, 1958). The eruptive sequence at Strawberry Crater is (1) building of the cinder one, (2) eruption of the lava flow and breaching of the cone, (3) extrusion of the plug, and (4) post-eruption erosion and deposition.
The general appearance of Strawberry Crater is a red colored, "C" shaped cone in plan view, capped by a mantle of welded agglutinate (Figure 3). At Strawberry Crater the breach is located on the east side of the cone. The summit of the cone is at an altitude of 1989 meters (6526 feet) and rises a maximum of 170 meters (550 feet) above its base.
The exterior of the cone is circular in plan view, except for the eastern portion, and has an average slope angle of 30°. The lower portion of the slope is composed of loose, unconsolidated lapilli and bombs (Figure 4). Irregular bombs and lapilli dominate, with ribbons and fusiforms being commonly found. The base of the cone consists primarily of bombs, and blocks of agglutinate. The break in slope with the surrounding topography is generally sharp, with only a small to virtually non-existent colluvial apron or mantling of Sunset Crater ash and lapilli covering the base of the cone.
The upper exterior slope and rim of the cone is mantled by agglutinate beds and rootless flows. Thickness of the beds varies from less than 1 m to greater than 3 m. The degree of welding in these beds ranges from poorly welded through highly welded and rootless flows. Much of the mantling has the appearance of flow lobes, reflecting the expected lobate nature of small rootless flows (Figure 5).
The exterior does not show evidence of erosion other than numerous, small scree piles of unconsolidated bombs and lapilli on the slope. Only a small colluvial apron has developed, which is the result of scree piles having reached the base of the cone, instead of remaining on the slope, as most of the scree has done. On both the north and south exterior slopes, Sunset Crater ash and lapilli deposits have begun to be covered by scree lobes (Figure 6).
The rim of the cone, where it is still present, is completely mantled by agglutinate. The agglutinate beds of the north rim dip at an average angle of 53° in a radially outward direction (Figure 7). Dissecting the north rim are a number of radially oriented extension cracks. These cracks range in size from less than 1 meter to 1.5 meters wide, and 1 meter to 3 meters deep (Figure 8). The south rim is broken by a large slump gap (Figure 9). Outcrops of agglutinate beds in the gap shows preservation of both inward and outward dipping beds (Figure 10). The average dip for these beds is 30° (Figure 7).
The interior of the cone is amphitheater shaped, and because of the breach, cannot properly be termed the crater. On the southern and northern interior slope beds dipping inward suggest a crater, but do not form a complete crater. The north and west interior slopes display a spectacular series of welded agglutinate and rootless flows (Figure 11). As with the exterior beds, the agglutinate is composed of loosely to highly welded lapilli and bombs. Individual beds show varying degrees of sorting, ranging from moderate to well sorted. Bombs and lapilli are generally basalt to basaltic andesite in composition (Moore and Wolfe, 1987; Bloomfield, 1988 written comm.), with plagioclase and augite dominating the phenocryst assemblage. Olivine phenocrysts range from common to rare. Textures of the clasts range from scoriaceous to vesicular, with the texture of the matrix ranging from hypohaline to hypocrystalline. Bombs with compositions similar to the lava flow and central plug are also found (Moore et al., 1974) and are generally blocky in form.
Rootless flows are common and are generally massive and structureless with occasional scoriaceous pockets. Composition is the same as the clasts. Upper and lower contacts of the flows tend to be gradational with loosely welded to highly welded scoria. Individual flow units are discontinuous laterally, suggesting that each is a distinct lobe.
The strikes and dips of both the rootless flows and agglutinate beds in the north slope range considerably (Figure 7). Extension cracks similar to those found in the rim break the beds. In the western and northwestern interior slope bedding is not continuous, appearing as individual, mildly deformed units surrounded by unconsolidated cinders and debris (Figure 12). Many of these show bedding characteristics similar to the bedding at the rim.
The breached portion of the cone is now located on the surface of the flow as rafted mounds of agglutinate, rootless flow and unconsolidated cinders. The mounds range in size from individual blocks less than 1 meter and small patches of cinders, to mounds greater than 30 meters (100 feet) high. Material found in the mounds is indistinguishable from that found in the cone. Beds of agglutinate and rootless flow are rarely continuous, however, and are generally broken into large blocks.
Clustering of mounds on the flow is common. The largest mounds are clustered proximally to the cone. Bedding in these mounds shows the greatest degree of continuity, but the mounds are dominated by unconsolidated cinders, with agglutinate and rootless flow being a minor constituent (Figure 13). Distally located mound clusters are dominated by agglutinate and rootless flow, with lesser amounts of loose cinders (Figure 14).
The lava flow at Strawberry Crater is classified as a block flow of basaltic andesite composition (Moore et al., 1974). Hand sample examination shows a porphyritic texture, with an aphanitic matrix. Phenocrysts of plagioclase and pyroxene are common, with olivine also being found. Quartz is rare. The plagioclase phenocrysts show both sieve texture and solid crystal texture. Thin section examination shows hypocrystalline and intersertal textures with plagioclase and augite as the common phenocrysts and groundmass minerals. Olivine and hypersthene are also present as phenocrysts and groundmass crystals but are less common. Quartz in samples containing olivine shows reaction rims of pyroxene, suggesting that the crystals are not in equilibrium with the melt. Vesicles are common, but not abundant, irregular in shape, and often lined with calcite.
The flow consists of polyhedral blocks, which are equidimensional and angular. Hodges' (1960) description of the block flow at S P Crater indicated areas on the flow of flat jointed surfaces with the appearance of columnar basalt, which she termed polygonal pavement. Numerous examples of polygonal pavement are present throughout the flow at Strawberry, and can be found on the surface at orientations ranging from horizontal to vertical (Figure 15).Most of the flow, however, consists of piles of loose blocks (Figure 16). Many of these piles are elongate and are oriented perpendicular to the direction of flow (Figure 17). Most of the flow is covered with Sunset Crater ash and lapilli deposits (Figure 16), which has filled in low areas of the flow and spaces between blocks.
The flow covers an area of 6.6 km2 (2.55 mi2) and has a maximum flow length of 3.8 km (2.38 mi). Thickness of the flow varies considerably, but generally increases distally. On the south side of the cone the flow is no more than 8 meters (26 feet) thick (Figure 18). At the distal portion of the flow, however, thicknesses range from 30 to 40 meters (100-140 feet) (Figure 18). Volume of the flow is estimated at .142 km3 (.035 mi3).
Rare outcrops of the flow interior are present where the mantling blocks have fallen away. The outcrops show the same characteristics as the blocks. Ramping structures, and platy joints are found in the interior of the otherwise, massive flow unit (Figure 19). Minor contortions of these joints are occasionally seen, suggesting that the flow was still mobile after shearing had occurred.
The plug at Strawberry Crater is located in the breach of the cinder cone (Plate 1). Few outcrops of the plug are found, and they are scattered over a small area (Figure 20). The largest outcrop is approximately 2 m high, with most outcrops less than 1 m. Outcrops are generally blocky, but commonly show a rounded, weathered morphology. Friable samples tend to crumble easily. Inclusions of basaltic andesite, lava flow-like material and gabbroic rocks occur.
Moore and others(1976) classify the plug as a rhyodacite vitrophyre, while Bloomfield (1988, written comm.) classifies it as a trachyte, based on the alkali-silica diagram of LeBas and others (1986). Hand sample examination shows a porphyritic texture, with a black, aphanitic to glassy matrix. Phenocrysts of plagioclase dominate the assemblage, reaching 1 cm and showing sieve and solid crystal textures. Pyroxene phenocrysts are common, while quartz is uncommon and olivine is rare. Vesicles are present and are less than 2 mm. Thin section examination reveals a hyalopilitic to hypocrystalline texture, with plagioclase as the dominant phenocryst. The plagioclase is generally anhedral and shows embayed, sieve and solid crystal textures. Hypersthene is the abundant pyroxene, while augite is less common. No olivine was found in thin section.
A number of eruptive models for cinder cone growth indicate that the initial eruption is generally a fissure eruption, which subsequently closes to a single vent (McGetchin et al., 1974; Gutmann, 1979; and Crowe et al., 1981). At Strawberry Crater, no evidence is found to indicate that the initial eruption occurred along a fissure. If the initial eruption was a fissure eruption, the resulting deposits have been buried by the lava flow, which seems unlikely. Study of cinder cone morphology and vent shape by Breed (1964) can be applied to the symmetrical, unbreached portion of the cinder cone to indicate that the eruption occurred through a circular vent.
Based on the morphology of the cone and the cinders with which it is composed, the style of eruption is strombolian. Slight variations in the eruption through time are apparent in the internal cone structure. Lower stratigraphic sections of the cone are composed of unconsolidated lapilli and bombs, indicating a typical strombolian eruption for most of the cone building period. Higher stratigraphic sections, however, are dominated by deposits of agglutinate and rootless flow. This change in the deposits reflects a slight change in the magma and therefore a slight change in the character of the eruption. A number of factors control the physical parameters of the magma, including temperature, crystallization, volatile content, rate of eruption, and composition. Thin section examination indicates a decrease in vesicularity for samples found higher in the cone. This suggests that a slight decrease in the volatile content may be responsible for the change in the deposits.
Many of the eruption models for cinder cones indicate that late stage cone building is often characterized by a decrease in volatiles, thus producing a spatter style eruption, which deposits an agglutinate rim. At Strawberry Crater, it appears that a decrease in volatiles occurred earlier than the final cone building stage, resulting in a thick sequence of welded material. This decrease in volatiles was not great enough to cause the strombolian eruption to cease, but was enough to allow the ejecta to retain enough heat to weld and flow after impact. Observations at the eruption of Paricutin, Mexico, show that this late stage spatter is also a result of large bubbles bursting at the top of the magma column which has moved either to the level of the vent or within the crater, forming a lava lake (Foshag and Gonzalaz, 1956). This suggests the possibility that, not only is a decrease in volatiles partly responsible for the change in the deposits, but that the top of the magma column had probably risen to near, at or above the elevation of the vent.
In addition to variations in consolidation of the deposits, variations in the strength of the eruption are evident in the cone structure. Figure 21 shows an example where the lowest agglutinate bed, dipping toward the vent, is unconformably overlain by near horizontal agglutinate beds. This indicates that at times the strength of the eruption was insufficient to throw material clear of the crater. Agglutinate deposited during this sequence of eruptions began to build a cone within the crater.
After the strombolian-style eruption had ceased, the eruption style changed to lava extrusion. Structures in the cone suggest that the vent for the lava flow was the same as the cone vent. Complex internal cone structures can be partially explained by variations in the character of the eruption. Over-steepened agglutinate beds, radially-oriented extension cracks, and deformed bedding in the north rim, however, indicate modification of the cone structure by the erupting lava, and may be directly related to the breaching of the cone.
Before the various models that attempt to explain the cone structures and the initial breaching event can be examined, it is important to examine the over-all sequence of breach events, as inferred by the location of rafted material.
The locations of rafted mounds of cinder and agglutinate support the conclusion that the upper portion of the cone was removed first during the breaching event. Distally-located rafted mounds are dominated by agglutinate and rootless flow. This material is found only in the upper portions of the cone. Proximally-located rafted mounds are dominated by unconsolidated cinders, reflecting the lower stratigraphic sections of the cone. These proximally positioned mounds form a "barrier" across the proximal end of the flow (Plate 1). If the proximally-located mounds had been rafted first, there would be no way to move the remainder of the flow and rafted material to its present position. Therefore, the breach must have initially occurred at a stratigraphic position at or near the rim of the cone.
The models for initial lava extrusion and the initial breaching event must explain the following observations: (1) over-steepening of agglutinate beds in the north rim, (2) extension cracks in both the rim and agglutinate beds, and (3) initial breaching at or near the rim. Three models are presented and examined.
This model is based on the premise that the crater filled with a lava lake late during or after the strombolian eruption had ceased and prior to breaching. The final stage of the cone building period is marked by deposition of agglutinate beds and spatter. The top of the magma column, in this situation is believed to be at or near the height of the vent. If this is indeed the case, it would only require a continued upwelling of the magma to fill the crater with a lava lake, which would then be able to overtop the lowest point in the rim, and thus breach the cone. In order to explain the cone structures, the lateral pressure exerted by the lava on the cone must be examined. This is where the lava lake model, however, begins to break down.
Hydrostatic pressure within a column of liquid shows a linear increase with depth, with the greatest lateral pressure at the base of the column, while the least lateral pressure is at the top (Hwang and Hita, 1987). Over-steepened beds in the cone are those found only at the very top of the rim. Beds just below the rim do not appear to have been deformed. With the lateral pressure at the top of a liquid column as the least significant pressure component within a column of liquid, it is difficult to explain the deformation at the rim being a result of lateral pressure by a lava lake.
A model based on the intrusion of the rising magma as a dike or series of dikes works well for breaching the top of the cone first. It does not, however, explain the deformation of the rim deposits. If the rising magma was intruded as a dike into the cone structure and reached the surface of the cone just below the eastern rim, the rim deposits would be removed first. The main problem with this model is that there does not appear to be any way for the dike to cause the deformation of the north rim. If magma had been intruded at or near the north rim, causing deformation, there should be evidence of that intrusion, considering the exposure that is seen in the north slope. The presence of a radial dike would be expected, where none is found. It also seems unlikely that an intruding dike, which has deformed beds at the rim, would not also deform the beds just below the rim.
In this model, the magma is intruded into the cone structure as a large plug-like body, as opposed to a dike. Where the dike intrusion is not believed to be capable of the deformation seen, an intruding body of magma could possibly account for the structures that are found. Intrusion of the magma would cause swelling of the cone, which could oversteepen the flanks causing gravitational collapse. Because there is no noticeable displacement of beds throughout most of the cone, the swelling is believed to have occurred in the, now missing, eastern section. The intrusion would have had the effect of weakening the eastern portion of the cone, increasing its susceptibility to breaching. Swelling of the cone also accounts for the extension cracks in the beds of the north interior slope. Over-steepening of the north rim deposits without major deformation of the lower stratigraphic beds is believed to have been accomplished by transmitting outward-directed pressure through the ventward-dipping beds (Figure 22). These beds (Figure 23), which can be considered to be roughly parallel to the direction of pressure, could have been used to push the rim deposits outward without showing noticeable deformation. Unconsolidated cinders appear to compose the deposits below this bed and therefore do not show any preserved sense of movement. It is unlikely that gravitational collapse due to swelling is the main cause of the breach. If the eastern rim collapsed due to the swelling of the cone structure, then the rim deposits would most likely have slumped to the base of the cone where the erupting lava would flow over it. Because the slumped rim material deposits are found on top of the lava flow it is apparent that the extrusion began before slumping.
Breaching of the cone near or at the rim may be explained by a combination of the dike intrusion model and the magma body intrusion model. Intrusion of the main magma body into the cone and the accompanying swelling could have been accompanied by small dike intrusions off of the main body. One of these dikes reaching the surface of the cone near the rim would start the breach. If the initial breach occurred at an elevation below the rim, the cone deposits could slump or fall onto the surface of the flow and be rafted away.
Most of the rafted mounds on the flow are found in clusters. This is believed to support the conclusion that removal of cone deposits by the lava flow was due to the walls of the breach becoming over-steepened and collapsing or slumping, depositing material on the surface of the flow. Slumping and rotation of material is indicated by the mound at the eastern end of the north limb of the cone which has agglutinate beds dipping into the cone (Plate 1). Because of the high viscosity and flow mechanism of a block flow, cone deposits which collapse on the flow surface as a cluster are more likely to remain clustered on the flow. The clustering of rafted deposits suggests the slumping and widening of the breach occurred episodically.
Deformed beds in the northern and western interior slope are probably the result of the slumping of the breach walls (Figure 12). Large blocks of agglutinate can be seen on the slope, but do not appear to be in place, nor are they continuous with other beds. In the case of these units, the breaching lava ceased flowing before they could slump onto the flow surface.
The clustering of proximally located mounds indicates that the final event in the breaching of the cone was a massive removal of the lowest portion of the cone. Undermining of the cone by dike intrusions may be responsible for this large scale rafting event. The large mound along the southern limb of the cone, however, suggests that the lava flow may have simply pushed the mounds away from the cone. This final breaching was followed by a continued extrusion of lava which formed the small lobe to the south.
The final eruptive event at Strawberry Crater was the extrusion of the dacite vitrophyre plug. The vent for the plug is located under what would have been the eastern section of the cone, suggesting that the dacite used a different conduit than that which was used during cone building. The volume of magma erupted was small, involving an amount which was capable of barely reaching the surface.
Post-eruption erosion of the volcanics at Strawberry Crater appears to be minor. Outcrops of the lava flow are undissected and angular, with only minor amounts of surficial oxidation. Erosion of the cone has been limited to scree lobes caused mass wasting. The gap in the south rim of the cone appears to be the result of a slump block. Agglutinate and rootless flow mounds are scattered at the base of the cone to the south (Plate 1). Colton (1936) stated that the first breach of Strawberry Crater occurred on the south and was followed by the major breach on the east. Examination of the material to the south shows no evidence that the removal of material to the south was caused by a lava flow. It is believed that weakening of the cone structure by the erupting lava caused the south rim to become unstable and collapse during or after the lava flow extrusion. Whether the collapse occurred after all eruptions had ceased or was triggered by tremors caused by the erupting plug is unknown.
The eruption of Sunset Crater, approximately 8 km to the south, beginning in 1064 A.D. (Smiley, 1958), deposited a large amount of black, glassy ash and lapilli on Strawberry Crater. Most of this material has been washed off the cone, but has remained as thick deposits on the lava flow.
