Volcanic Construction Research Articles

Surface of Venus: Evidence of Diverse Landforms from Radar Observations

Recent radar images of the surface of Venus reveal a complex and varied terrain. By applying a set of simplifying assumptions about the nature of the surfaces returning the radar signal, it is possible to make a number of plausible interpretations. In one region on Venus, several circular features have the gross morphology of degraded impact craters. If they are indeed of impact origin, these features suggest that there exist on Venus areas which are ancient and where erosion or resurfacing has not been as intense or as pervasive as on the earth. In other regions there are intriguing features that may evidence active internal processes. One is a large trough-like depression (0 degrees , 76 degrees W; measuring 1400 by 150 by 2 kilometers) planimetrically suggestive of both the Valles Marineris on Mars and the East African Rift on the earth. Another feature, about 250 kilometers in diameter and of positive relief, includes an 80-kilometer-diameter circular depression at its summit, suggestive of a large volcanic construct. A third region, near 0 degrees , 10 degrees E, contains roughly parallel ranges of mountains separated by valley-like features, with relief varying from small isolated hills several hundred meters high to low ranges on the order of 1000 meters to large mountains approaching 2 kilometers in height. If Venus has a mobile crust similar to the earth's, these mountains may have been produced by compressional tectonics. These interpretations of the radar data indicate that Venus has been a geologically active planet which has developed diverse landforms and therefore is an exciting candidate for future exploration.

Near-bottom geophysical study of the Mid-Atlantic Ridge median valley near lat 37° N: Preliminary observations

Near-bottom geophysical observations made over the median valley of the Mid-Atlantic Ridge near lat 37°N show that the median valley can be divided into four provinces: the outer walls, terraces, inner walls, and floor. Block faulting along normal faults with as much as 600 m throw is the dominant mechanism for creating median-valley relief, particularly at the inner and outer walls. Most of these faults dip toward the median-valley axis an average of 45°. Many of the block tops slope 3° to 10° away from the valley axis. Some of the major fault scarps are linear for more than 20 km. Large-scale block faulting occurs less than 1 km from the axis of spreading. Inversion of near-bottom magnetic anomalies suggests that the principal zone of intrusion may be no wider than 1 to 2 km and is along the valley axis. This spreading center is marked by either a central ridge or central depression, which suggests either discrete magma sources or discrete taps of a linear source beneath the median-valley axis. The central ridge is a volcanic construction, although faulting and (or) volcano-tectonic uplift may contribute to its relief.

A proposed origin of the olympus Mons escarpment

Olympus Mons (Nix Olympica) is delimited by a step nearly circular scarp that is unique to this largest of Martian volcanic constructs. The origin of the scarp is most probably by erosion; such an origin is difficult to explain if the volcanic pile is assumed to be made up exclusively of basalt flows. Although interpretive evidence is accepted for basalt flows underlying the central slope areas, additional criteria are presented here to support the hypothesis that the outer reaches of the slopes comprise dominantly ash-flow tuffs which presumambly were deposited by nuées ardentes. Because of cooling during emplacement, the average degree of compaction of each ash sheet would normally decrease with radial distance from its source vent. It is suggested that the tuffs originally extended greater distances from the central caldera complex and merged the slopes with the surrounding substrate. At the distal edges eolian erosion worked on the relatively noncompacted tuffs and was a major factor in the development of the scarp. The height of the scarp increased as it retreated toward the central caldera complex and eolian undercutting caused slump which in turn furthered the migration of the scarp. The rate of scarp migration would have decreased as the scarp increased in height and as more densely compacted ash nearer to the center of the construct was encountered. By this model, then, the circularity of the scarp in plan reflects an approximately concentric distribution of the zones of average degree of compaction about the center of Olympus Mons.

Variable features on Mars. IV. Pavonis Mons

A remarkable set of albedo changes has been uncovered by Mariner 9 photography of the upper slopes of the shield volcano Pavonis Mons, near its summit caldera. The most likely explanation of the event is aeolian transport of fine-grained particles. Since the atmospheric pressure in this locality is ∼ 1.5 mb, minimum wind velocities above the surface boundary layer of about 110 m/s are necessary, corresponding to 0.51 of the speed of sound. Slope winds in this velocity range are expected near the upper flanks of major Martian volcanic constructs.

A Discussion on volcanism and the structure of the Earth - Aspects of magmatic evolution of Réunion Island

As with the Hawaiian islands, the volcanic construction of Reunion can be related to two main phases of activity—a shield-form ing stage of predominantly olivine-basalt composition, and a declining stage comprising more varied products (basalt-trachyte). The older of the two Reunion volcanoes (Piton des Neiges) appears to have completed both these stages, but the younger volcano (Piton de la Fournaise) is still in the shield-forming stage, and is lagging approximately 1.5 Ma* behind Piton des Neiges in its evolution. The chemical data indicate a considerable degree of coherence between the various rock types produced during the different stages of development, and it is concluded that they all stem from essentially the same hypersthene-normative, picritic, primitive magma, generated by partial melting of peridotite in the low-velocity layer of the upper mantle. The shield-forming lavas are believed to represent relatively rapid ascent of this magma, only modified by olivine fractionation, but the declining stage seems to require intermediate-depth fractionation (olivine + pyroxene) to account for its initially nepheline-normative character, followed by high level fractionation (olivine+plagioclase + pyroxene + magnetite, etc.) to produce the hawaiite—mugearite—benmoreite—trachyte sequence. Volume considerations appear to favour an open system of basalt extraction, involving a relatively modest (7 to 15 %) degree of partial melting and continuous replenishment of the mantle source beneath Reunion, rather than a closed system with its restricted basalt potential even if as much as 50 % partial melting is postulated.