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Ten terrestrial impact structures were imaged during two flights of the 1994 Space Radar Laboratory (SRL) experiment. These craters include Wolf Creek, Australia; Roter Kamm, Namibia; Zhamanshin, Kazakhstan; B.P. and Oasis, Libya; Aorounga, Chad; Amguid, Algeria; and Spider, Connolly Basin and Henbury, Australia. SRL contained two co-registered instruments; the United States SIR-C polarimetric radar system operating in L-band (λ=24 cm) and C-band (λ=5.6 cm), and the joint German/Italian X-SAR vertically-polarized radar operating in X-band (λ=3 cm). Comparisons show SRL images to be complementary to, or in some cases superior to, corresponding optical images for evaluating size, location, and relative age of impact features. Regardless of wavelength or polarization, craters with significant relief appear prominently on radar as a result of slope and roughness effects. In desert regions, longer wavelengths penetrate dry sand mantles to reveal hidden crater dimensions or associated buried landforms. Radar polarities and wavelengths are particularly sensitive to vegetation, surface roughness, and soil moisture or electrical properties. In the more temperate environments of Kazakhstan and Australia, SRL images show detailed stream patterns that reveal the location and structure of otherwise obscured impact features.

We have recovered 18 kg of layered tektites from 10 tektite-bearing localities in Laos and central Vietnam, including 5 localities around the town of Muong Nong (Laos). Several of these deposits originally contained several hundred kilograms of layered tektite fragments, and one fragmented mass may have been as large as 1000 kg. This is the largest single deposit of tektites yet reported. In this region, layered tektite fragments are found in isolated clusters usually associated with a pebbly laterite horizon that is 0-1 m below the surface. Near Khe Sanh, Vietnam, we estimate the abundance of layered tektite fragments to be -100 g/m2. This is greater than five times the abundance estimated for northeast Thailand (Fiske et al., 1996). In a region that extends from northeast Thailand, through central Laos, and into central Vietnam, we found only layered tektites, which confirmed the existence of a large (>50 000 km2) subfield of the Australasian strewn field with only layered tektites. The east-west extent of the "layered-only" subfield is well constrained. but little field data exist to constrain its north-south extent.

Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coefficient σ0 are dependent on topographic roughness at the submeter scale, and correlation between, these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between σ0 and z0 for L band (λ = 24 cm) HV (H, vertically and V, vertically polarized modes) L band HH, and C band (λ = 5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation-free areas from orbit using radar systems.

— The site of an impact event that spread ejecta in the form of tektites and microtektites over ∼5 × 107 km2 of the southern Pacific and Indian Ocean area has not yet been discovered. A number of lines of evidence point toward a source in eastern Indochina. From an examination of a digital topographic data set and Landsat imagery, we identified four candidate structures in southern Laos, and we visited these sites in 1995 February. No evidence of impact origin of these structures could be found; flat-lying, undisturbed Mesozoic sedimentary rocks similar to those on Thailand's Khorat Plateau were found over the region. Small layered tektite fragments are relatively common in a lateritic horizon that is characterized by the presence of quartz pebbles. This scene is identical to the situation found several hundred kilometers to the southeast in Thailand. New tektite sites identified on this trip support a previous suggestion that there is a large region in southern NE Thailand and Laos that is rich in Muong Nong-type (layered) tektites but seemingly devoid of the splash-form type tektites.

Products of an underwater eruption near Socorro Island in the NE Pacific were observed directly on January 29, 1993, ten days after precursors were first recorded by SOFAR (Sound Fixing and Ranging) hydrophones located in Hawaii and Tahiti. Eruptive activity was noticed from boats and ships as small steam plumes rising from the sea at an area centered at 18 °48′N, 111 °05′W, 2.4 km NW of Punta Tosca and 4.6 km SSW of Cape Henslow on Socorro Island. The observed steam was produced by 1–3-m-large blocks of hot, dark-grey, highly vesiculated basalt rising buoyantly to the surface from two submarine shallow vents at 210 and 30 m depth. Tens of blocks accompanied by bubbles could be observed rising to the surface in irregular pulses. These scoriaceous blocks remained floating at the surface until they would crack into smaller pieces by thermal contraction, emitting hissing noises from vapourizing seawater in contact with the hot interior of the blocks. Steam jets several metres in height were produced and occasionally blocks were propelled laterally by the steam jet. Depending on vesicularity and permeability, blocks remained floating and drifting with the surface current for 1–15 minutes before sinking back. Floating rocks covered an area of about 6000 m2. This intermittent activity has been observed ever since and has not stopped as of April 1994. Buoyant scoria and reticulite are indicative of volatile (mostly CO2) supersaturation and exsolution in the magma prior to rapid quenching, which inhibits loss of volatiles by bubble escape. A high-velocity ascent of low-viscosity magma in a relatively narrow conduit is also required to prevent substantial gas escape and allow formation of reticulite. The buoyant scoria is most probably ejected by intermittent lava fountaining at fixed vents as a result of changes in eruption velocities due to changes in the exsolved gas content of the lava. Between January and July 1993 floating blocks of scoria and reticulite were collected on several occasions from the surface of the sea for chemical and mineralogical analyses. Major- and minor-element analyses (including REE), as well as electron microprobe analyses of different phases revealed that the composition of the emitted lava has not changed through time. Blocks of basalt (SiO2 = 45–47%) are highly vitric and vesicular with tabular anorthite phenocrysts up to 3 mm in length and minor grains of forsteritic olivine. REE and trace-element composition of these rocks suggest an anomalous mantle source for the erupted alkali basalt lava. The ongoing eruption will either continue in a similar fashion as described and eventually cease or built up a mound that reaches the surface and forms an island with accompanying change in eruptive style.

We recovered Australasian tektites in place throughout a 40×130 km region in north-east Thailand extending from the Laotian border westward to a line connecting Na Pho Klang in the northeast through Det Udom to Nam Yun in the south. With two exceptions, in sites near the western edge of this region, all fragments are layered (muong-Nong-type) tektites. It appears that large layered tektites are mainly found by rice farmers in fields that were forested until the recent past. The presence of layered tektites in this 40×130 km area implies that impact melt that fell in these areas was hot enough to flow if it was deposited on a sloping surface. The absence of splash-form tektites from the region indicates that the layer was still molten when masses having shapes (teardrops, dumbbells, etc.) produced by spinning reached the ground. To account for this and to allow time for the melt to flow a few tens of centimeters requires that the atmosphere remained hot (>2300 K) for a few minutes. Tektites that are in place are almost always associated with a widespread 10-cm to meter-thick layer of laterite. In two cases (one involving layered, one involving splash-form tektites), where accurate stratigraphic control demonstrated that the tektites were still in section, they were sited on top of the laterite layer just below a layer of aeolian sand.

In November, 1981, NASA's first Shuttle Imaging Radar mission (SIR-A) began producing maplike photographic strips of Earth scenes from orbital altitude. A Saharan radar image acquired over Algeria clearly delineates two sedimentary basins, Erg Occidental and Erg Oriental, separated by an elongated zone of exposed bedrock, the M'Zab Chebka. At the NE margin of the Chebka, rimrocks, slopes, and ejecta deposits of Talemzane meteorite impact crater appear as a distinct two km wide radar-bright ring. This unique circle of strong radar backscatter distinguishes the solitary impact structure from numerous dayas (similarly appearing karstic depressions) which characterize the region. The crater is prominent on radar, but is obscure on optically obtained satellite and aircraft images, as are partly buried fluvial drainage systems and fault-block traces developed in bedrocks of the Chebka. Radar detection of an annular drainage system indicates possible presence of a ring graben at the crater. Brightest radar signals on the image are cultural features at recently developed gas fields near Hassi er R'Mel.

Shuttle radar images (SIR-A) of volcanic and impact craters were examined to assess their appearance on radar images. Radar characteristics were determined for: (a) 9 maarlike craters in the Pinacate volcanic field, Sonora, Mexico; (b) the caldera of Cerro Volcan Quemado, in the Bolivian Andes; (c) Talemzane impact crater, Algeria; and (d) Al Umchaimin, a possible impact structure in Iraq. SIR-A images were compared with conventional photographs and with results from field studies. Consideration was then given to radar images available for Venus, or anticipated from the Magellan mission. Of the criteria ordinarily used to identify impact craters, some can be assessed with radar images and others cannot be used; planimetric form — expressed as circularity — and ejecta-block distribution can be assessed on radar images, but rim and floor elevations relative to the surrounding plain and disposition of rim strata are difficult or impossible to determine. We conclude that it will be difficult to separate small impact craters from small volcanic craters on Venus using radar images, and suggest that it will be necessary to understand the geological setting of the areas containing in order to determine their origin.

While Part I was devoted to the study of bowl-shaped depressions in Algeria, the present article focuses on multi-ringed structures of possible impact origin on the Algerian Sahara platform. Four structures were selected from orbital, aviation and geologic documents but only three were visited. TIN BIDER (27°36′N; 005°07′E) is a concentric multiple ring structure at least 6 km in diameter. Upper-Cretaceous sedimentary beds outside the structure dip inward a few degrees at the periphery and become extremely folded nearer the center, yet a general circular symmetry is always retained. A clearly exposed contact between upper deformed beds and underlying on-deformed beds is remarkably flat, dipping less than 10° inward. The upper beds display strong centrifugal folding. In the center of the structure Lower-Cretaceous sandstones about 0.5 km above their normal stratigraphic position are exposed. Although no shatter cones, intensive brecciation, or fracturing were observed, there is definite petrographic evidence of shock metamorphism (planar elements) in the quartz grains of the central sandstones. Tin Bider is a probable impact structure. It is the only astrobleme known with such prominent ductile deformations, a characteristic which may be due to the nature of the target materials. Detailed studies are now required to understand the mechanism of deformation of this multi-ringed structure. Its formation may be early Tertiary in age. FOUM TEGUENTOUR (26°14.5′N; 002°25′E) is an 8 km diameter bull's eye ring pattern. Although the high circularity and the morphology are consistent with an impact origin, the prominence of ductile deformation, the nature of the formations (clay-gypsum with sandstone intercalcations), the type of folds, the relationships between the structure and a surrounding plateau, and the lack of any evidence of shock effects better support a diapiric origin. MAZOULA (28°24′N; 007°49′E) is an 800 m diameter multi-ring feature with a 300 m wide anticlinal dome which rises some 30–35 m above the surrounding horizontal strata. The dome is capped by a flat-lying massive carbonate layer dipping on the flanks in accordance to the surface topography. There are no special disturbances, no fractures or breccias. Mazoula is not impact in origin but may be a Cretaceous rudistid reef and/or a salt diapir.

Stishovite, a dense phase of silica, has become widely accepted as an indicator of terrestrial impact events. Stishovite occurs at several impact structures but has not been found at volcanic sites. Solid-state silicon-29 magic-angle spinning nuclear magnetic resonance (silicon-29 MAS NMR) and X-ray diffraction of samples from the Cretaceous-Tertiary boundary layer at Raton, New Mexico, indicate that stishovite occurs in crystalline mineral grains. Stishovite was indicated by a single, sharp resonance with a chemical shift value of -191.3 ppm, characteristic of silicon in octahedral coordination, that disappeared after heating the sample at 850 degrees Celsius for 30 minutes. An X-ray diffraction pattern of HF residuals from the unheated sample displayed more than 120 peaks, most of which correspond to quartz, zircon, rutile, and anatase. Eight unambiguous weak to moderate reflections could be ascribed to d-spacings characteristic of stishovite.