Cinder cones constitute one of the most common volcanic features on earth (Basaltic Volcanism Study Project, 1981), as well as one of the least studied. The breaching of cinder cones, due to the eruption of an associated lava flow, has been noted by a number of geologists, but has received little attention. Breaching is defined as the removal of material from the cone due to erosional, gravitational, or volcanic mechanisms, such that the interior of the cone is exposed. The Dictionary of Geological Terms (1976) defines a breached cone as a cinder cone in which lava has broken through the sides and carried away the broken material. The "broken material" is either pushed away by the flow or rafted on the surface of the flow.
Detailed examination of breaching events and rafted material of cinder cones is, in general, lacking. This investigation will examine the rafted material and cone structure and morphology at Strawberry Crater and O'Neill Crater to determine the sequence of events in the breaching of these cinder cones.
Strawberry and O'Neill Craters are located in the eastern section of the San Francisco volcanic field (SFVF), at the southern edge of the Colorado Plateau physiographic province, north-central Arizona. (Figure 1). The SFVF is dominantly basaltic in composition with local centers of silicic rocks. The basaltic rocks show a strong alkaline affinity (Eastwood, 1974; Moore et al., 1974) and are distributed throughout the volcanic field as flows and pyroclastic deposits. A total of 568 basalt, basaltic andesite and benmoreite scoria vents have been identified (Table 1, Ulrich, unpub. data).Thirty-six of these vents have been identified as being breached due to lava extrusion during the eruption.
Volcanism in the SFVF has been active since the late Miocene. Extensive K-Ar dating of the volcanics has shown a regular pattern for the migration of vent location from the southwest to the east at a rate of 1.2 cm/yr (Tanaka et al., 1986). The location of the volcanic centers and individual cone morphology within the SFVF are strongly controlled by the NW-SE, NE-SW, and N-S fault systems of the southern Colorado Plateau (Figure 2; Shoemaker et al., 1974; Breed, 1964). The current stress regime for the SFVF is extensional and is attributed to current Basin and Range deformation (Luchitta, 1974; Zoback and Zoback, 1980) with a least principal horizontal stress direction at approximately N50E (Brumbaugh, per. comm.).
The SFVF rests upon the Permian Kaibab Formation and the Triassic Moenkopi Formation. The Kaibab Formation is a dolomitic limestone, deposited in a near shore to open marine environment (Cheevers and Rawson, 1979), generally showing a blocky, jointed, cliff/ledge forming morphology. The Moenkopi Formation is a distinctly colored red mudstone/sandstone, deposited in a fluvial system (Stewart et al., 1972), showing a ledge/slope forming morphology. Neither formation directly contacts the volcanics at Strawberry Crater or O'Neill Crater. However, the close proximity of the Kaibab Formation outcrops to O'Neill Crater suggests that the strata may directly underlie much of this complex.
Colton (1937, 1967) appears to be the first to have described the geologic features of Strawberry and O'Neill Craters. Strawberry Crater is described as a breached cinder cone which was initially breached on the south side, followed by a major breach on the east side. Colton described the associated flow as being "fractured into blocks" and comparable to S P Crater's associated flow. He classified it as the earliest known Stage IV flow. The description for O'Neill Crater is even shorter, stating that it is a breached cone with an associated "broken fragments" flow that is classified as Stage III. This flow is also described as being similar to the S P flow.
Colton's (1937) Stage I-Stage V classification of cinder cones and Cooley's (1962) addition to this system of the Black Point-Sunset Age classification, later modified by Moore (1974) (Table 2), is based on geomorphic examinations of the volcanic field, and thus provide only relative ages. Damon and other's (1965, 1974) K-Ar dating of volcanic flows gave absolute dates for both Strawberry and O'Neill Craters. Strawberry Crater is dated at 46,000±46,000 yrs. and O'Neill Crater is dated at 50,000±14,000 yrs. (these dates are uncorrected for recently revised decay constants, Ulrich et al., 1984). Moore and Wolfe (1987) give dates of 51,000±46,000 yrs. and 55,000±14,000 yrs., respectively.
A study of the northern and eastern parts of the SFVF by Moore and Wolfe (1974, 1976) yielded a 1:362,000 scale map, petrologic data, and SiO2 content of basaltic andesite flows associated with Merriam age cinder cones. These were described as containing phenocrysts of corroded plagioclase, olivine, clino- and orthopyroxene and some occasional gabbroic, granulitic and sedimentary xenoliths. Both Strawberry and O'Neill Craters are mentioned as having small central plugs of rhyodacite vitrophyre. Silica content is given for O'Neill Crater only: cinder and spatter - 54.5%, associated flow - 59%, rhyodacite plug - 67%. This information was updated by Moore and Wolfe (1987) with whole rock chemical analyses (Table 3) and CIPW normative data, as well as detailed descriptions of the lithologies of cones, flows and plugs.
Bloomfield's (1988, written communication) petrologic and chemical analyses of Strawberry Crater (Table 3) indicate that the volcanics range in composition from basalt and basaltic andesite to mugearites, benmoreites and trachytes, using the classification of LeBas and others (1986). The wide range in compositions, with SiO2 varying from 49 to 64 weight percent, are accounted for through a magma mixing model. Mantle derived basaltic magma is suggested to have caused crustal melting, which was followed by subsequent mixing of the rhyolitic magma with a mantle derived or a differentiated basaltic magma.
Moore and Wolfe (1976, 1987) and Ulrich and others (1984) show each crater on U.S. Geological Survey maps, scale 1:50,000 and 1:250,000 respectively. Wood's (1980) study of cinder cone degradation uses Strawberry Crater in a crater width vs. cone width graph. Lastly, the two craters have been mentioned in works by Green and Short (1974), Wolfe (1984), and Tanaka and others (1986).
Despite the sparseness of detailed work on Strawberry and O'Neill Craters there has been a fair amount of general work done on the cinder cones of the SFVF. General studies of cinder cones include, most notably, Colton's (1937) work on relative dating of craters and flows based on geomorphic features and superposition. The earliest works concerning the volcanic field are Robinson (1913) and some remarks in pre-1900 surveys (see Robinson, 1913). However, these were more concerned with the San Francisco Peaks and only mention cinder cones as relatively minor features, if they are mentioned at all. More recent studies have primarily dealt with morphometric measurements (Babbitt, 1964; Breed, 1964; Colton, 1964), while others have dealt with volcanic processes and emplacement of cinder cone fields (Stoeser, 1974; McDonald, 1975; Settle, 1979; Lynch, 1982).The specific studies of individual cinder cones in the SFVF has been limited to S P and Sunset Craters (Colton, 1937; Smiley, 1958; Hodges, 1960; Amos, 1981, 1986; Holm, 1987; and Ulrich, 1987).
Discussion regarding the breaching of cinder cones due to volcanic events is primarily limited to statements indicating whether a cone is breached or that breaching occurred at a specific time during a witnessed eruption (Foshag and Gonzalaz, 1956; Porter, 1972; Williams and Moore, 1973; and Hammill, 1979). The Foshag and Gonzalaz (1956) account of the eruption of Paricutin, Mexico, gives descriptions of breach events. In all cases, however, the accounts lack details concerning the cause of the breach or detailed maps showing positions of the removed material. Three notable works are Macdonald (1972), Gutmann (1979) and Holm (1987). All three works indicate that the breached portion of a cinder cone is represented by rafted mounds of agglutinate and cinders on top of the associated lava flow. Holm's (1987) work specifically states that the presence of rafted agglutinate and cinder mounds can be used to indicate a past breaching event, even if the cone is subsequently rebuilt and appears unbreached. Scott and Trask (1971) observed that the breaching of the cinder cones in the Lunar Crater volcanic field, Nevada, show a preferred direction, parallel to the trends of the local fault system.
Several mechanisms for cinder cone breaching by the associated lava flow have been proposed. Macdonald (1972) states that breaching can occur due to burrowing of the flow through the cone or melting of the cone wall by the flow. Gutmann (1979) suggests that the outward directed pressure of the upwelling magma is sufficient to cause catastrophic failure of the cone structure, and results in a breach.