18-1: Craters on the Moon, Mercury, Venus, Mars, and most of the satellites of the outer planets are nearly all circular in outline (unless distorted by superimposed craters) and have raised rims, concentric inner terraces, and other features that are seldom found in terrestrial craters of a volcanic nature. The closest analogs on Earth that are truly volcanic are the maars, which are almost invariably small, have only gently raised rims (such as Crater Elegante), and occur in a clearly volcanic environment. Lunar craters in the highlands are imposed on rocks that are fragmented (brecciated). Those in the lunar lowlands (maria) are associated with the basaltic lavas that fill these areas but again their morphology is more like terrestrial impact craters. Some lunar craters are much larger than any known on Earth. There are literally millions of such craters on the Moon, quite unlike the limited numbers of terrestrial volcanic craters or even volcanos that form stratocones; constant bombardment by countless numbers of asteroids and meteorites known to frequent space (especially in early solar times) is a far better explanation of such large numbers than any volcanic model. Also, innumerable similar craters occur on the ice crusts of most outer planet satellites; no known mechanism for ice "volcanism" has yet been put forth. Finally, shock effects (see statement elsewhere on page 18-1) in lunar rock samples returned to Earth afford direct evidence that impacts (which cause such effects) have been commonplace on the Moon. BACK

18-2: The distribution of craters in the above maps seem non-uniform, a fact explained by variations in ages of surficial rocks, distribution of mountain systems, and differences in the extent of exploration. Obviously, any impacts on the ocean floor would not show up. Impacts are absent in Greenland and the Antarctic since these regions have been ice-covered over the last few million years. Impacts are also sparse in major tectonic belts as folding, faulting, etc. may have separated or obliterated them. Craters as such are not likely to survive the erosion that affects the crystalline shields of the world, although scars or remnants of large craters have been found there and a few young craters have been cut into the shields over the last 100 million years or so. Craters are hard to find in tropical forests as exploration conditions there are difficult and aerial photography usually can't yield evidence under the trees. The observed distribution is largely controlled by accessibility to favorable terrain on which the impacts are preserved and, to a lesser extent, on the desire by geoscientists in certain countries to search diligently for them. BACK

18-3: All nuclear warheads exploded simultaneously at one place would just about make another Zhamanshin crater (there are a number of larger craters in the geologic record, so bigger, more destructive ones will occur infrequently [seldom enough so one is not likely in a single human lifetime]). A somewhat smaller impact crater would produce an equivalent of a nuclear winter, but probably on a regional rather than global scale. But, in each million year span, the probability of at least one world-affecting impact is real (though statistical, i.e., may not actually occur in any given million year span but will follow that time frame on average; more than one such impact can happen in that span, followed by a dearth in the next million years). The movie questions are yours to answer - if you missed them, rent a video. BACK

18-4: a) 1-2 seconds; b) 7 sec; c) 11 sec; d) 25 sec; e) 30-50 sec; f) 26 sec; g) 35-60 sec; h) 1-30 minutes. BACK

18-5: After passing through glacial materials, a) the drillers would encounter moderately shocked Precambrian igneous and metamorphic rocks making up the fractured and uplifted central peak; some of this rock may be brecciated; b) they would bring up core that consists mostly of breccia fragments and blebs of melt; the fragments would contain some Paleozoic clasts (term refers to "small to large pieces of rock"), metasedimentary rock clasts from the red clastic units, and occasional igneous/metamorphic clasts; c) at the rim position, the drillers would extract core containing fractured and perhaps further tilted (from the downsliding as the rim rocks slipped along faults) bedrock in the sequence: Paleozoic; Proterozoic red clastics; Precambrian crystalline rocks (the normal stratigraphic sequence); the rocks probably show little or no obvious shock effects except maybe shatter cones (see page 18-3). BACK

18-6: Quartz begins to convert to coesite (another polymorph or atomic-structural form of silica) at about 20 kilobars (that pressure is reached in the solid Earth at about a depth of 70 km (43 miles). Solids begin to convert to glass at 400 kb, to melt at ~500 kb, and to vapor (gas) at a megabar. Thus, the range of shock metamorphism is from 0.02 to 1 megabar - these pressures are known to occur in the Earth only in its mantle and core but the rock types affected by shock metamorphism are dominantly those of the crust where the natural pressure gradient achieves values less than those affecting shocked rocks. BACK

18-7: Two sets for sure, and maybe three (one at bottom may just be a rotated group of the dominant up-done set). There could be more - 5-6 sets is not uncommon and rarely up to 12 sets may be present. The way in which more sets are found involves the procedure by which their crystallographic plane orientations are determined. The glass slide containing the thin slice of rock is place in a Universal-Stage (or U-stage) which is designed to allow up to 5 directions of rotation of the slide from the usual horizontal position on the microscope platform. Thus, the slide can be systematically rotated into any possible position in three-dimensional space; as new positions are reached, additional planar sets can come into view. BACK

18-8: Belts of close-spaced folded mountains. In a sense, mica kinks can be likened to folding. BACK

18-9: Remember the concept of vertical exaggeration, done to emphasize smaller differences in relief. In making this perspective view, that process was done. What is unusual in the view is that a well-displayed central peak appears to be missing, although drilling indicates some basement uplift. BACK

18-10: The 10-meter granite interval most likely marks passage of the drill through a single very large clast, probably derived from the lower part of the rock target excavated by cratering. BACK

18-11: As they are generated, the shock waves diverge from the impact point (ground zero) along an expanding spherical wave front (actually, a hemisphere bisected by the ground surface). A line can be drawn from ground zero to a place where a shatter cone forms; this line is a radius of the spherical wave front. When the advancing shock wave begins to form the cone, say at a pebble, the wave is diverted by the impediment along conical stress surfaces causing the cone to develop under tension. The cone apex lies along the radial line connected to the site of shock wave origin and the cone is symmetrical around that extended line. Its orientation thus points like an arrow to the origin. BACK

18-12: From the upper left corner, go down the left margin about 1/4th the way, and then go in (using the upper margin as a guide) about 1/3rd. Look there for a faint ring of dark material. If you still can't find it, check its appearance in the enlargement link. BACK

18-13: The northern (upper) half: In the field this is marked by a line of low hills. The southern half, although a rolling surface, is relatively flat. Incidentally, the finest exposure of shatter cones at Sudbury is at the back of the parking lot behind the McDonald's fast food restaurant. To make that lot, it was necessary to blast into a small hillock, leaving a near vertical cliff face about 20 feet high. The Burger King in Sudbury doesn't have this marvelous exposure, which may be why they still are number 2. BACK

18-14: The center of Haughton is about at the center of the image. The rim, better defined on the right side but evident also on the left, is found about half way to the right and left margins of the image. But, on the left size there appears an outer row of hills that are curved parallel to the rim. While these may be structurally related to the impact event (thus making this a multi-ring crater), they may instead be the consequence of drainage control by streams that developed in a partial circular pattern outside the main (and probably only) rim. BACK

18-15: Sometimes the circularity of the inner crater, and often the rim as well, is affected by the regional joint (fractures) pattern, which commonly is dominated by two sets of joints crossing at right angles. As the crater grows, and shock pressures decay, the last excavation is influenced by these joints, so that the outer parts take on a square outline. This same control was very influential at Meteor Crater in Arizona, as we shall see on page 18-6. BACK

18-16: The east (right) edge of the crater is about 1/5th in from the right center image margin; the left edge is about 1/6th in from the left center. This Principal Components image is an excellent example of how special image processing by computer can enhance and thus reveal subtle image features not easily recognized in the standard false color composites. BACK

18-17: Based on the ejecta pattern, the meteorite came in from the west or southwest. Its outline tend to be squarish, owing to the controlling effect of joints in the sedimentary rocks. The tiny crater was produced by a chemical explosion as part of an experiment to duplicate on a small scale the Meteor Crater itself. The red line is a power line right-of-way. BACK

18-18: The usual response proposal is to send out a fleet of ballistic rockets with big nuclear warheads. The problem with that is that the number that can be sent to arrive simultaneously is not large and the total energy released if all struck at the same time is just a tiny fraction of the total energy of a moving asteroid. The classic analogue: bringing down an elephant with a peashooter. At best, the asteroid might be broken into several pieces but each should continue along its path and the fragments would still strike Earth like buckshot, releasing about the same amount of energy. More promising (perhaps) would be to have all the rocket warheads explode at once some distance from the oncoming (and very fast moving) asteroid, with the resulting shock wave deflecting it from its course. But calculations of such an effect aren't terribly encouraging. As an alternative, one could follow the plans so popular after the Second World War for survival of a nuclear holocaust: build a "bombshelter", provision it with food, water, and an air filter and expect to live in it for at least a few years, hoping the natural world can recover in that time. Might work? But, can't build enough for 5 billion people. And, one must be lucky and not have the asteroid hit nearby. Methinks in the long run, the best strategy: pray! BACK

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