Mark Settle

Volcanic eruption clouds and the thermal power output of explosive eruptions

Abstract The maximum height attained by a volcanic eruption cloud is principally determined by the convective buoyancy of the mixture of volcanic gas + entrained air + fine-sized pyroclasts within the cloud. The thermal energy supplied to convection processes within an eruption cloud is derived from the cooling of pyroclastic material and volcanic gases discharged by an explosive eruption. Observational data from six recent eruptions indicates that the maximum height attained by volcanic eruption clouds is positively correlated with the rate at which pyroclastic material is produced by an explosive eruption (correlation coefficient r = + 0.97). The ascent of industrial hot gas plumes is also governed by the thermal convection process. Empirical scaling relationships between plume height and thermal flux have been developed for industrial plumes. Applying these scaling relationships to volcanic eruption clouds suggests that the rate at which thermal energy is released into the atmosphere by an explosive eruption increases in an approximately linear manner as an eruptions pyroclastic production rate increases.

Radial variation of lunar crater rim topography

Abstract The variation of rim topography as a function of range from the crater rim has been determined for a group of morphologically fresh lunar craters ( D = 10–140 km) using the recent series of Lunar Topographic Orthophotomaps. The rate at which exterior crater topography converges with the surrounding surface is highly variable along different radial directions at individual craters as well as between different craters. At several craters, oblique impact appears to have contributed to azimuthal elevation/range variations. The topographic expression of a crater above the surrounding surface typically decreases to one-tenth of the estimated rim height at a range of 1.3 R –1.7 R , well within the rough-textured ejecta deposit surrounding the crater. Comparisons with terrestrial craters suggest that the topographic crater rim is predominantly a structural feature. In most craters large portions of the hummocky facies and virtually all of the radial facies, in spite of their rough appearance and local topographic variations, provide no significant net topographic addition to the preexisting surface. The extreme variability of crater rim topography strongly suggests that ejecta thicknesses are highly variable and that a unique power-law expression cannot truly represent the radial variation of ejecta deposit thickness.

Statistical analysis of persistent explosive activity at stromboli, 1971: Implications for eruption prediction

Abstract In September 1971 three vents within the summit crater of the volcano Stromboli intermittently ejected molten lava and hot gas to heights of several hundred meters. Field observations were conducted over four consecutive days (5–8 September 1971) and a chronological record of 399 discrete eruptions was compiled. Frequency distributions of intereruption (repose) periods at each vent can be statistically represented by a normal (Gaussian) distribution at the 99.9% confidence level. This implies that the eruption triggering mechanism operating at each vent is a completely random process, if the vents are individually considered to be independent volcanic systems. Regression analysis of the complete chronological record reveals, however, that the length of a vents repose period is generally correlated with the number of eruptions occurring at the other two summit vents. Such intercorrelations can be used to increase the accuracy of eruption predictions by 18–87% compared with eruption forecasts that simply anticipate the average repose period between successive eruptions at each vent. The results of this study imply that in the case of persistent explosive activity at several vents, it may be possible to predict the outbreak of individual eruptions quite accurately on the basis of parameters reflecting the nature and intensity of eruptions occurring at other vents. In addition, intercorrelations between the explosive activity occurring at several active vents can be used to develop physical models of subsurface volcanic systems.

The role of fallback ejecta in the modification of impact craters

Abstract Fragments of target material ejected from an impact craters transient cavity may be deposited within the cavity as fallback due to in-flight particle collisions and atmospheric deceleration. The characteristics of aerodynamically decelerated fallback ejecta are inferred by determining the size of the smallest fragment that is aerodynamically capable of traveling beyond the transient cavity rim at successive stages of cavity excavation. This technique has been applied to Meteror Crater, Arizona (MCA), a 1.2-km diameter crater that formed within a group of flat--lying, sedimentary rock units. Model results predict that: (i) ∼5% of the primary ejecta excavated from the crater was deposited as fallback (volume percent), (ii) average fallback fragment size is 1.4 cm, (iii) fallback ejecta is derived from a region encompassing all sedimentary formations intersected by the crater, and (iv) portions of the fallback source region were subjected to large shock stresses, on the order of several hundred kilobars. These results are in excellent agreement with empirical studies of the MCA fallback deposit. This agreement is interpreted to be an indication that atmospheric deceleration is the principal mechanism responsible for fallback deposition on the Earth. Model results for larger terrestrial craters indicate that as excavation cavity diameter ( D e ) increases from 1 to 100 km, the proportion of primary ejecta deposited as fallback increases from 5 to 11% (average fragment size increases from 1.5 cm to 1.3 m over the same cavity diameter range). Fallback deposition is inferred to be a major cavity modification process during Venusian cratering events. Model estimates suggest that more than half of the primary ejecta excavated from large scale Venusian cavities ( D e > 10 km) should be deposited as fallback. In contrast, relatively small quantities of fallback are anticipated within lunar and Martian craters formed in coherent target media.

Origin of Olympus Mons Escarpment by erosion of pre-volcano substrate

OF all the large shield volcanoes of Mars, Olympus Mons is unique in being fringed by a nearly continuous scarp rising 1–4 km above the surrounding terrain1,2 (Fig. 1). We present evidence that the scarp was caused by preferential erosion of fragmental cratered terrain material previously adjacent to, and at present underlying the shield volcano. Removal of this material caused undercutting and collapse of the shield flanks, producing the observed terraces and scarps.

Emplacement of Fahrenheit crater ejecta at the Luna-24 site

The Luna-24 site is situated in Mare Crisium at a range of 18.4 km from Fahrenheit, an Eratosthenian-aged crater 6.4 km in diameter. Fahrenheits ejecta deposits have been degraded to such an extent that secondary craters and rays cannot be unambiguously identified in the vicinity of the Luna-24 site. On the basis of an analogy between Fahrenheit and Lichtenberg B (a much younger crater of comparable size located in northern Oceanus Procellarum) Fahrenheit ejecta deposits near the sample site are inferred to have consisted of secondary crater clusters, subradially aligned secondary crater chains, and lineated terrain furrowed by fine-scale radial grooves. At the range of the Luna-24 site more than 80% of the mare surface should have been morphologically disturbed by the ballistic deposition of Fahrenheit ejecta. Blocks and fragment clusters of primary Fahrenheit ejecta ranging up to 5–20 m in diameter are inferred to have impacted the local surface at velocities of 165–230 m s−1 forming secondary craters ranging up to 100 m in diameter. The maximum depth of excavation of primary Fahrenheit ejecta deposited near the sample site is estimated to be at least 100 m. Primary Fahrenheit ejecta is expected to constitute a substantial fraction of the exterior deposits emplaced at the range of the Luna-24 site. Microgabbro and monomineralic fragments discovered in the Luna-24 drill core may have been derived from gabbroic rocks transported to the sample site by the Fahrenheit cratering event. This hypothesis is consistent with the widespread occurrence and characteristics of Fahrenheit ejecta anticipated in the vicinity of the Luan-24 site. Current interpretations of the drill core sample suggest that the Luna-24 regolith was deposited in its present configuration sometime during the last 0.3 AE implying that at least one local cratering event has occurred since the emplacement of Fahrenheit ejecta ∼2.0±0.5 AE ago.