Arizona has a wide variety of geological features relevant to planetary geology. The “Holey Tour” is a 2 d field trip (Phoenix-Flagstaff-Phoenix) that introduces participants to crater forms (hence the “holes” of the tour), including a maar, karst sinkhole, pit crater, cinder-cone craters, a volcano-tectonic depression, and the classic impact structure Meteor Crater. The Apollo astronaut field training site near Flagstaff is examined, which includes a terrain that was artificially generated to simulate a cratered lunar surface. In addition, planetary volcanism is discussed with stops that include a shield volcano, composite cone, silicic dome, and cinder cones; considerations include key variables in volcanic morphology, such as lava composition and rates of effusion. The general geology of Arizona is discussed throughout the trip and includes parts of the Colorado Plateau, the Basin and Range Province, and the Central Highlands (also called the “transition” zone). The trip can be adapted to meet the needs of any group, from secondary school students to established planetary scientists.

Chicxulub is the only known impact structure on Earth with a fully preserved peak ring, and it forms an important natural laboratory for the study of large impact structures and understanding of large-scale cratering on Earth and other planets. Seismic data collected in 1996 and 2005 reveal detailed images of the uppermost crater in the central basin at Chicxulub. Seismic reflection profiles show a reflective layer ~1 km beneath the apparent crater floor, topped by upwardly concave reflectors interpreted as saucer-shaped sills. The upper part of this reflective layer is coincident with a thin high-velocity layer identified by analyzing refractions on the 6 km seismic streamer data. The high-velocity layer is almost horizontal and appears to be contained within the peak ring structure. We argue that this reflective layer is the predicted coherent melt sheet formed during impact, and it may be comparable with the unit known as the Sudbury Igneous Complex at the Sudbury impact structure. The Sudbury Igneous Complex, interpreted as a differentiated impact melt sheet, appears to have a similar scale and geometry, and an uppermost lithological sequence consisting of a high velocity layer at the top and a velocity inversion beneath. This comparison suggests that the Chicxulub impact structure also contains a coherent differentiated melt sheet.

This study is a first attempt to compile sedimentological features of synimpact to postimpact marine sedimentary successions from marine-target impact craters utilizing six well-studied examples (Chesapeake Bay, Gardnos, Kärdla, Lockne, Mjølnir, and Wetumpka). The sedimentary formations succeed autochthonous breccias and, in some cases, allochthonous suevites. These late synimpact and early postimpact depositional successions (on top of the suevites) appear to be in comparable stratigraphic developments and facies in marine-impact craters. Their composition reflects common mechanisms of sedimentation; they were developed from avalanches/scree, slides, and slumps through sequences of mass-flow–dominated deposition before ending with density currents and fine-grained sedimentation from fluidal flow and suspension. With detailed study, it may be possible to separate the late synimpact and early postimpact successions based on their clast composition relative to target stratigraphy. The process-related comparisons presented here are highly simplified, including characteristics of moat, central peak, and marginal basin sedimentation of both simple and complex craters.

Pseudotachylitic breccias are the most prominent impact-induced deformation phenomenon in the Vredefort Dome, the eroded central uplift of the 2.02 Ga, originally 250-km-wide Vredefort impact structure in South Africa. Controversy remains about the origin of these melt breccias, and the most popular hypotheses are genesis by (1) shearing (friction melting), (2) shock compression melting, (3) decompression melting immediately after shock propagation through the target or slightly later during the modification phase of cratering, (4) combinations of these processes, or (5) intrusion of allochthonous impact melt. A resolution to this problem requires detailed multidisciplinary analysis in order to characterize the nature of different occurrences of such breccias with the aim of identifying the melt-forming process. Past work has focused mainly on orientation and geometry of Vredefort pseudotachylitic breccia veins, besides a few whole-rock geochemical investigations of mostly decimeter- to tens of meter-sized occurrences, whereas detailed geometric and micro-chemical analysis has not yet been adequately related to microdeformation studies of such melt breccias. Here, we report the results of detailed microchemical analyses of small- to meso-scale pseudotachylitic breccias in a polished 3 × 1.5 m granite slab from a dimension stone quarry in the western core of the Vredefort Dome, supplemented by data for samples from the Rand Granite Quarry in the northern sector of the core. The veinlets selected for analysis do not provide textural evidence for shearing/faulting. Electron microprobe analysis of pseudotachylitic breccia groundmass and X-ray fluorescence bulk chemical analysis of both pseudotachylitic breccias and their host rocks reveal that pseudotachylitic breccia commonly displays a close chemical relationship to its direct wall rock. If groundmass compositions are corrected for the inherent microclast content, correspondence of breccia groundmass and immediate host rock composition is further enhanced. For small veinlets (<1 mm width), melting appears to have occurred locally, with compositions of melt and immediately adjacent host rock minerals commonly being identical. It is, thus, suggested that larger breccia zones could be sites of pooling of melt generated in places throughout the wider environs of dilational sites. For millimeter-scale veinlets, local melt formation and also a lack of lateral mixing are indicated. In contrast, pseudotachylitic breccia veinlets <1 mm, and quite possibly also some larger veins, could conceivably have formed by shock or decompression melting. As previous shock experimental work has demonstrated, this local melting could have been accomplished with or without a friction component.

Catastrophic and unusual events on Earth such as bolide impacts, megafloods, supereruptions, flood volcanism, and subice volcanism may have devastating effects when they occur. Although these processes have unique characteristics and form distinctive features and deposits, we have difficulties identifying them and measuring the magnitude of their effects. Our difficulties with interpreting these processes and identifying their consequences are understandable considering their infrequency on Earth, combined with the low preservation potential of their deposits in the terrestrial rock record. Although we know these events do happen, they are infrequent enough that the deposits are poorly preserved on the geologically active face of the Earth, where erosion, volcanism, and tectonism constantly change the surface. Unlike the Earth, on Mars catastrophic and unusual features are well preserved because of the slow modification of the surface. Significant precipitation has not occurred on Mars for billions of years and there appears to be no discrete crustal plates to have undergone subduction and destruction. Therefore the ancient surface of Mars preserves geologic features and deposits that result from these extraordinary events. Also, unlike the other planets, Mars is the most similar to our own, having an atmosphere, surface ice, volcanism, and evidence of onceflowing water. So although our understanding of precursors, processes, and possible biological effects of catastrophic and unusual processes is limited on Earth, some of these mysteries may be better understood through investigating the surface of Mars.

Impact cratering has affected the surfaces of all bodies in our Solar System. These short-duration but energetic events can drastically affect the regional and occasionally the global environment of a planet. The cratering record is better preserved on Mars than on Earth due to longer-term stability of the Martian crust and lower degradation rates. Impact cratering had its greatest effect early in Solar System history when bombardment rates were higher than today and the sizes of the impacting objects were larger. The record from this period of time is largely lost on Earth. High bombardment rates early in Solar System history may have eroded the Martian atmosphere to its present thin state, causing dramatic climate change. The regolith covering much of the Martian surface and the large quantities of dust seen in the atmosphere and covering much of the ground have been attributed to fragmentation of target material by impacts. Heating associated with crater formation may have contributed volatiles to the Martian atmosphere and initiated some of the outflow channels. The effects of an impact event extend far beyond the crater rim, and the planet’s volatile-rich environment likely contributes to the greater ejecta extents seen on Mars than on the Moon. The cratering record of Mars thus holds important implications for how impacts may have affected the geologic evolution of Earth.