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1.
The Marius Hills region, a volcanic plateau in Oceanus Procellarum, contains numerous rilles, rille-like structures, and chains of elongate craters. Most of these structures characteristically: (1) originate on or near irregular shaped craters associated with features previously interpreted as volcanic domes, (2) trend downslope onto Plateau Plains, (3) generally taper in width and become shallower, (4) are often discountinuous, (5) occupy the center, or apparent crest of a broad ridge, (6) may contain cut-off branches and distributary structures, and (7) may have local reversals in longitudinal slope. Structures having these characteristics are interpreted to be lava channels or partly collapsed lava tubes. Terrestrial lava tubes form exclusively, and commonly, in fluid basalt flows. Recent evidence indicates that viscosities of lunar mare basalt lava flows were conducive for lava tube formation.Terrestrial analogs are offered for structures described in the Marius Hills. The analogs are comparable in qualitative and quantitative geomorphic aspects, excluding that of width. Scaling consideration of lunar reduced gravity accounts for increased width of the lunar structures. Linear and curvilinear rilles trending along equal elevations are interpreted to result from fracturing or faulting.  相似文献   

2.
Karl R. Blasius 《Icarus》1976,29(3):343-361
Mariner 9 images of the four great volcanic shields of the Tharsis region of Mars show many circular craters ranging in diameter from 100mm to 20 km. Previous attempts to date the volcanoes from their apparent impact crater densities yielded a range of results. The principal difficulty is sorting volcanic from impact craters for diameters ?1 km. Many of the observed craters are aligned in prominent linear and concentric patterns suggestive of volcanic origin. In this paper an attempt is made to date areas of shield surface, covered with high resolution images using only scattered small (?1 km) craters of probable impact origin. Craters of apparent volcanic origin are systematically excluded from the dating counts.The common measure of age, deduced for all surfaces studied, is a calculated “crater age” F′ defined as the number of craters equal to or larger than 1 km in diameter per 106km2. The conclusions reached from comparing surface ages and their geological settings are: (1) Lava flow terrain surfaces with ages, F′, from 180 to 490 are seen on the four great volcanoes. Summit surfaces of similar ages, F′ = 360 to 420, occur on the rims of calderas of Arsia Mons, Pavonis Mons, and Olympus Mons. The summit of Ascraeus Mons is possibly younger; F′ is calculated to be 180 for the single area which could be dated. (2) One considerably younger surface, F′ < 110, is seen on the floor of Arsia Mon's summit caldera. (3) Nearly crater free lava flow terrain surfaces seen on Olympus Mons are estimated to be less than half the age of a summit surface. The summit caldera floor is similarly young. (4) The pattern of surface ages on the volcanoes suggests that their eruption patterns are similar to those of Hawaiian basaltic shields. The youngest surfaces seem concentrated on the mid-to-lower flanks and within the summit calderas. (5) The presently imaged sample of shield surface, though incomplete, clearly shows a broad range of ages on three volcanoes—Olympus, Arsia, and Pavonis Mons.Estimated absolute ages of impact dated surfaces are obtained from two previously published estimates of the history of flux of impacting bodies on Mars. The estimated ranges of age for the observed crater populations are 0.5 to 1.2b.y. and 0.07 to 0.2b.y. Areas which are almost certainly younger, less than 0.5 or 0.07b.y., are also seen. The spans of surface age derived for the great shields are minimum estimates of their active lifetimes, apparently very long compared to those of terrestrial volcanoes.  相似文献   

3.
The Rümker Hills, a volcanic dome-flow complex in the northern Oceanus Procellarum, is characterized by overlapping plains-forming units with lobate scarps, volcanic domes, a 60 km ring, and a scarp which separates the plateau from surrounding mare materials. Plains-forming units are interpreted as fluid volcanic flows, and domes as viscous extrusions. One dome may be a stratovolcano. The ring system is discordant with regional structural trends and probably has a local origin. The Rümker Hills is the closest lunar analog to the large martian shield structures revealed on the Mariner 9 photographs of Mars.  相似文献   

4.
A number of Martian volcanoes, especially Ceraunius Tholus, Uranius Tholus, Uranius Patera, and Hecates Tholus, show morphological features strikingly different from those of shield volcanoes but analogous to those of terrestrial cones and composite volcanoes such as Barcena Volcano, Mexico. The most distinguishing overall features are steep slope angles, and Krakatoa-type caldera morphologies. Erosional features comprise numerous radial channels which extend from below the rim toward the base of the dome, and in some cases, patterns of anastamosing gullies which contribute to the main radial channels. Constructional features include blanketed flanks interpreted as dune or fan-like deposits of ash, and perhaps lava deltas. A possible explanation for the morphological features associated with these volcanoes is that they were formed by explosive volcanic density currents. Such eruptions would be expected on Mars where a rising magma came in contact with a thick layer of permafrost generating a base surge or after a Vulcanian explosion of a separate gas phase producing a nuée ardente. Crater age data from the surface of Martian domes and shields indicate that such explosive activity occurred more frequently early in Martian geologic history. This is more consistent with the view that the volcanic density flows were base surges rather than nuées ardentes, the melting of permafrost supplying the water required in base surge generation. If atmospheric conditions were more clement at the time, allowing the recycling of water back to the ground water, then the length of duration of phreatic activity would have been longer, not being limited by depletion time of the local permafrost reservoir.  相似文献   

5.
New topographic maps of six large central volcanoes on Mars are presented and discussed. These features are Olympus Mons, Elysium Mons, Albor Tholus, Ceraunius Tholus, Uranius Tholus, and Uranius Patera. Olympus Mons has the general form of a terrestrial basaltic shield constructed almost entirely from lava flows; but with 20 to 23 km of relief it is far larger. Flank slopes average about 4°. A nominal density calculated from the shield volume and the local free-air gravity anomaly is so high that anomalously dense lithosphere probably underlies the shield. Uranius Patera is a similar feature of much lower present relief, about 2 km, but its lower flanks have been buried by later lava flood deposits. Elysium Mons has about 13 km of local relief and average slopes of 4.4°, not significantly steeper than those of Olympus Mons. Its upper flank slopes are significantly steeper than those of Olympus Mons. We suggest Elysium Mons is a shield volcano modified and steepened by a terminal phase of mixed volcanic activity. Alternatively, the volcano may be a composite cone. Albor Tholus is a partially buried 3-km-tall shield-like construct. Ceranius and Uranius Tholus are steeper cone-like features with relief of about 6 and 2 km, respectively. Slopes are within the normal range for terrestrial basaltic shields, however, and topographic and morphologic data indicate burial of lower flanks by plains forming lavas. These cones may be lava shield constructs modified by a terminal stage of explosive activity which created striking radial patterns of flank channels. Differences among these six volcanoes in flank slopes and surface morphology may be primarily consequences of different terminal phases of volcanic activity, which added little to the volume of any construct, and burial of shallow lower flanks by later geologic events. Additional topographic data for Olympus Mons, Arsia Mons, and Hadriaca Patera are described. The digital techniques used to extract topographiv data from Viking Orbiter stereo images are also described.  相似文献   

6.
Over 300 previously unrecognized volcanic domes were identified on Lunar Orbiter photographs using the following criteria: (1) the recognition of land forms on the Moon similar in morphology to terrestrial volcanic domes, (2) structural control, (3) geomorphic discordance, and (4) the recognition of land forms modified by dome-like swellings. Many terrestrial volcanic domes are similar in morphology to lunar domes. This analogy suggests that some lunar hills are in fact extrusive volcanic domes. Many of the domes identified in this paper seem to be related to basins and craters, and with the exception of local tectonic grid control few domes are related to any observable Moon-wide pattern. Domes are not uniquely found on maria. Dome formation probably spans a wide range of lunar time and activity in areas where domes are located may be continuing to the present as revealed by the close correlation of dome distribution with the distribution of lunar transient events. The overall morphology of a lunar dome is a poor indicator of the composition of the rock that forms the dome. Contribution No. 33 Planetary Geology Group, University of New Mexico.  相似文献   

7.
Mariner 9 pictures indicate that the surface of Mars has been shaped by impact, volcanic, tectonic, erosional and depositional activity. The moonlike cratered terrain, identified as the dominant surface unit from the Mariner 6 and 7 flyby data, has proven to be less typical of Mars than previously believed, although extensive in the mid- and high-latitude regions of the southern hemisphere. Martian craters are highly modified but their size-frequency distribution and morphology suggest that most were formed by impact. Circular basins encompassed by rugged terrain and filled with smooth plains material are recognized. These structures, like the craters, are more modified than corresponding features on the Moon and they exercise a less dominant influence on the regional geology. Smooth plains with few visible craters fill the large basins and the floors of larger craters; they also occupy large parts of the northern hemisphere where the plains lap against higher landforms. The middle northern latitudes of Mars from 90 to 150† longitude contain at least four large shield volcanoes each of which is about twice as massive as the largest on Earth. Steep-sided domes with summit craters and large, fresh-appearing volcanic craters with smooth rims are also present in this region. Multiple flow structures, ridges with lobate flanks, chain craters, and sinuous rilles occur in all regions, suggesting widespread volcanism. Evidence for tectonic activity postdating formation of the cratered terrain and some of the plains units is abundant in the equatorial area from 0 to 120° longitude.Some regions exhibit a complex semiradial array of graben that suggest doming and stretching of the surface. Others contain intensity faulted terrain with broader, deeper graben separated by a complex mosaic of flat-topped blocks. An east-west-trending canyon system about 100–200 km wide and about 2500 km long extends through the Coprates-Eos region. The canyons have gullied walls indicative of extensive headward erosion since their initial formation. Regionally depressed areas called chaotic terrain consist of intricately broken and jumbled blocks and appear to result from breaking up and slumping of older geologic units. Compressional features have not been identified in any of the pictures analyzed to data. Plumose light and dark surface markings can be explained by eolian transport. Mariner 9 has thus revealed that Mars is a complex planet with its own distinctive geologic history and that it is less primitive than the Moon.  相似文献   

8.
In this study we examine a set of lunar mare domes located in the Hortensius/Milichius/T. Mayer region and in northern Mare Tranquillitatis with respect to their formation along crustal fractures, their rheologic properties, the dimensions of their feeder dikes, and the importance of magma evolution processes during dome formation. Many of these domes display elongated summit vents oriented radially with respect to major impact basins, and several dome locations are also aligned in these preferential directions. Analysis of Clementine UV/VIS and Lunar Prospector gamma ray spectrometer data reveals that the examined mare domes formed from low-Si basaltic lavas of high FeO and low to moderate TiO2 content. Based on their morphometric properties (diameter, height, volume) obtained by photoclinometric and shape from shading analysis of telescopic CCD images, we derive rheologic quantities (lava viscosity during eruption, effusion rate, duration of the effusion process, magma rise speed) and the dimensions of the feeder dikes. We establish three rheologic groups characterised by specific combinations of rheologic properties and dike dimensions, where the most relevant discriminative parameter is the lava viscosity η. The first group is characterised by and contains the domes with elongated vents in the Milichius/T. Mayer region and two similar domes in northern Mare Tranquillitatis. The second group with comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with the relatively steep domes near Hortensius and in the T. Mayer region. The inferred dike dimensions in comparison to lunar crustal thickness data indicate that the source regions of the feeder dikes are situated within the upper crust for six of the domes in northern Mare Tranquillitatis, while they are likely to be located in the lower crust and in the upper mantle for the other examined domes. By comparing the time scale of magma ascent with the time scale on which heat is conducted from the magma into the host rock, we find evidence that the importance of magma evolution processes during ascent such as cooling and crystallisation increases with lava viscosity. We conclude that different degrees of evolution of initially fluid basaltic magma are able to explain the broad range of lava viscosities inferred for the examined mare domes. The spectral data reveal that differences in TiO2 content may additionally account for the systematic difference in lava viscosity between the two examined lunar regions. We show that the described mechanisms are likely to be valid also for other lunar mare domes situated near Cauchy and Arago, regarded for comparison. On the other hand, we find for the Gruithuisen and Mairan highland domes that despite their inferred high lava viscosities of , no significant magma cooling in the dike occurred during ascent, supporting previous findings that the highland domes were formed during a specific phase of non-mare volcanism by highly silicic viscous lavas.  相似文献   

9.
Extra-mare basalts occupy 8.5% of the lunar basalt area and comprise 1% of the total mare basalt volume. They are preferentially located where the crust is thin and topographically low. In terms of age, eruption style, and composition they are as variable as the mare basalts. In some instances extrusion in extra-mare craters was preceded by floor-fracturing whereas in other cases it apparently was not. The volume of lava erupted may have been controlled more by the volume of magma produced than by hydrostatic effects. A minimum of nearly 1300 separate basalt eruptions, is indicated; the true value could be nearer 30 000 separate eruptions.  相似文献   

10.
Jeffrey B. Plescia 《Icarus》1981,45(3):586-601
The Tempe volcanic province of Mars is a broad region adjacent to and superimposed upon the western Tempe Mareotis plateau. The volcanic region is a relatively low plain of probable basaltic flood lava affinity. Superimposed upon this plain are a variety of small constructional and nonconstructional volcanic features, all of which appear structurally controlled, along an orientation of approximately N60°E. Constructional features included low shields, composite (?) cones, and low irregular hills which may be silicic domes. The nonconstructional features include linear depressions which may be rift zones, fissure flows, and pit craters. The Tempe volcanic province appears comparable in many aspects to features of the Snake River Plains of Idaho. While the Tempe region may cover a significantly larger area than the Snake River Plains the same type and scale of features are observed on both areas. The tempe region includes three styles of volcanism: flood basalts, a capping of fissure- and vent-fed flows with scattered low shields, and areas where low shields have coalesced into plains-type volcanism. The Snake River Plains are represented by one style, plains volcanism. This represents one of the few examples where direct comparison can be made between Martian and terrestial geologic-geomorphic features and where a scale problem is not encountered.  相似文献   

11.
This study examines a set of lunar domes with very low flank slopes which differ in several respects from the frequently occurring lunar effusive domes. Some of these domes are exceptionally large, and most of them are associated with faults or linear rilles of presumably tensional origin. Accordingly, they might be interpreted as surface manifestations of laccolithic intrusions formed by flexure-induced vertical uplift of the lunar crust (or, alternatively, as low effusive edifices due to lava mantling of highland terrain, or kipukas, or structural features). All of them are situated near the borders of mare regions or in regions characterised by extensive effusive volcanic activity. Clementine multispectral UVVIS imagery indicates that they do not preferentially occur in specific types of mare basalt. Our determination of their morphometric properties, involving a combined photoclinometry and shape from shading technique applied to telescopic CCD images acquired at oblique illumination, reveals large dome diameters between 10 and more than 30 km, flank slopes below 0.9°, and volumes ranging from 0.5 to 50 km3. We establish three morphometric classes. The first class, In1, comprises large domes with diameters above 25 km and flank slopes of 0.2°-0.6°, class In2 is made up by smaller and slightly steeper domes with diameters of 10-15 km and flank slopes between 0.4° and 0.9°, and domes of class In3 have diameters of 13-20 km and flank slopes below 0.3°. While the morphometric properties of several candidate intrusive domes overlap with those of some classes of effusive domes, we show that a possible distinction criterion are the characteristic elongated outlines of the candidate intrusive domes. We examine how they differ from typical effusive domes of classes 5 and 6 defined by Head and Gifford [Head, J.W., Gifford, A., 1980. Lunar mare domes: classification and modes of origin. Moon Planets 22, 235-257], and show that they are likely no highland kipukas due to the absence of spectral contrast to their surrounding. These considerations serve as a motivation for an analysis of the candidate intrusive domes in terms of the laccolith model by Kerr and Pollard [Kerr, A.D., Pollard, D.D., 1998. Toward more realistic formulations for the analysis of laccoliths. J. Struct. Geol. 20(12), 1783-1793], to estimate the geophysical parameters, especially the intrusion depth and the magma pressure, which would result from the observed morphometric properties. Accordingly, domes of class In1 are characterised by intrusion depths of 2.3-3.5 km and magma pressures between 18 and 29 MPa. For the smaller and steeper domes of class In2 the magma intruded to shallow depths between 0.4 and 1.0 km while the inferred magma pressures range from 3 to 8 MPa. Class In3 domes are similar to those of class In1 with intrusion depths of 1.8-2.7 km and magma pressures of 15-23 MPa. As an extraordinary feature, we describe in some detail the concentric crater Archytas G associated with the intrusive dome Ar1 and discuss possible modes of origin. In comparison to the candidate intrusive domes, terrestrial laccoliths tend to be smaller, but it remains unclear if this observation is merely a selection effect due to the limited resolution of our telescopic CCD images. An elongated outline is common to many terrestrial laccoliths and the putative lunar laccoliths, while the thickness values measured for terrestrial laccoliths are typically higher than those inferred for lunar laccoliths, but the typical intrusion depths are comparable.  相似文献   

12.
In this study we examine the spectral and morphometric properties of the four important lunar mare dome fields near Cauchy, Arago, Hortensius, and Milichius. We utilize Clementine UV-vis multispectral data to examine the soil composition of the mare domes while employing telescopic CCD imagery to compute digital elevation maps in order to determine their morphometric properties, especially flank slope, height, and edifice volume. After reviewing previous attempts to determine topographic data for lunar domes, we propose an image-based 3D reconstruction approach which is based on a combination of photoclinometry and shape from shading. Accordingly, we devise a classification scheme for lunar mare domes which is based on a principal component analysis of the determined spectral and morphometric features. For the effusive mare domes of the examined fields we establish four classes, two of which are further divided into two subclasses, respectively, where each class represents distinct combinations of spectral and morphometric dome properties. As a general trend, shallow and steep domes formed out of low-TiO2 basalts are observed in the Hortensius and Milichius dome fields, while the domes near Cauchy and Arago that consist of high-TiO2 basalts are all very shallow. The intrusive domes of our data set cover a wide continuous range of spectral and morphometric quantities, generally characterized by larger diameters and shallower flank slopes than effusive domes. A comparison to effusive and intrusive mare domes in other lunar regions, highland domes, and lunar cones has shown that the examined four mare dome fields display such a richness in spectral properties and 3D dome shape that the established representation remains valid in a more global context. Furthermore, we estimate the physical parameters of dome formation for the examined domes based on a rheologic model. Each class of effusive domes defined in terms of spectral and morphometric properties is characterized by its specific range of values for lava viscosity, effusion rate, and duration of the effusion process. For our data set we report lava viscosities between about 102 and , effusion rates between 25 and , and durations of the effusion process between three weeks and 18 years. Lava viscosity decreases with increasing R415/R750 spectral ratio and thus TiO2 content; however, the correlation is not strong, implying an important influence of further parameters like effusion temperature on lava viscosity.  相似文献   

13.
In addition to modes of formation offered by previous investigators, terrestrial drainage craters formed over lava tubes in Oregon are presented as analogs to lunar drainage craters. Craters associated with lava tubes result from: (1) drainage of surface material through roof fractures; (2) plastic collapse of the partially cooled lava tube roof; and (3) drainage of surface material into roof collapses. The former two categories result in shallow, often elongate craters; the latter category forms classic dimple shaped craters. Elongate dimple craters formed over volcanic fissures in southern Idaho are also discussed and presented in support of one mode of formation proposed by previous investigators for lunar drainage craters.Movement of surface material toward the orifice is initiated on Earth largely by wind and water; in lunar conditions, initial movement is attributed to micro- and macro-meteoritic bombardment, seismic disturbances generated by internal and external processes, and by thermal creep.Nat. Res. Council Postdoctoral Resident Research Associate.  相似文献   

14.
Wudalianchi volcanic field, located in northeast China, consists of 14 Quaternary volcanoes with each volcano as a steep-sided scoria cone surrounded by gently sloping lava flows. Each cone is topped with a bowl-shaped or funnel-shaped crater. The volcanic cones are constructed by the accumulation of tephra and other ejecta. In this paper, their geologic features have been investigated and compared with some Martian volcanic features at Ascraeus Mons volcanoes observed on images obtained from High-Resolution Imaging Science Experiments (HiRISE), Mars Orbiter Camera (MOC), Context Imager (CTX) and Thermal Emission Imaging System (THEMIS). The results show that both Wudalianchi and Ascraeus Mons volcanoes are basaltic, share similar eruptive and geomorphologic features and eruptive styles, and have experienced multiple eruptive phases, in spite of the significant differences in their dimension and size. Both also show a variety of eruptive styles, such as fissure and central venting, tube-fed and channel-fed lava flows, and probably pyroclastic deposits. Three volcanic events are recognized at Ascraeus Mons, including an early phase of shield construction, a middle eruptive phase forming a low lava shield, and the last stage with aprons mantling both NE and SW flanks. We suggest that magma generation at both Wudalianchi and Ascraeus Mons might have been facilitated by an upwelling mantle plume or upwelling of asthenospheric mantle, and a deep-seated fault zone might have controlled magma emplacement and subsequent eruptions in Ascraeus Mons as observed in the Wudalianchi field, where the volcanoes are constructed along the northeast-striking faults. Fumarolic cones produced by water/magma interaction at the Wudalianchi volcanic field may also serve as an analogue for the pseudocraters identified at Isidis and Cerberus Planitia on Mars, suggesting existence of frozen water in the ground on Mars during Martian volcanic eruptions.  相似文献   

15.
Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater. Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented. Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed, and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered. It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and that this process must have been important in the evolution of the lunar surface.  相似文献   

16.
Magellan radar image data of Sapas Mons, a 600 km diameter volcano located on the flanks of the Arla Rise, permit the distinction of widespread volcanic units on the basis of radar properties, morphology, and spatial and inferred temporal relations, each representing a stage or phase in the evolution of the volcano. Six flow units were identified and are arranged asymmetrically about the volcano. Although there is some evidence for overlapping of units, the stratigraphy clearly indicates a younging upwards sequence. The estimated volume of this 2.4 km high volcano is 3.1 × 104 km3, which is comparable to the largest Hawaiian shield (Mauna Loa, 4.25 × 104 km3), but it is significantly less than an estimated volume for the entire Hawaiian-Emperor chain (1.08 × 106 km3) and less than the lower diameter (100 × 150 km) island of Hawaii (11.3 × 104 km3). Although it is difficult to clearly identify a single lava flow, estimates of apparent single flow volumes range from 4 km3 (for an average unit 5 flow of 3.4 km width, 10 m thickness, and 121 km length) to almost 59 km3 (for a 17.8 km wide, l0 m thick, 330 km long unit 1 flow). Estimates of total volumes for the units show that four of the six flow units have volumes that are within a factor of 1.2 of each other, one unit is approximately three times more voluminous, and the latest unit has a very small volume. Flows within a given unit are very distinct relative to flows in other units with respect to average lengths, aspect ratio, radar brightness, and planimetric outline. There is a weak distinction in rms slope between units and emissivity is correlated with altitude, not unit boundaries. A pair of 25 km diameter scalloped-margin domes occur at the summit and are the source of the last stage of eruptions on Sapas; steep fronts and high aspect ratios suggest that associated flows may have had a high viscosity. Graben form a circumferential structure 75–100 km in diameter surrounding the summit domes and are interpreted to be indicative of subsidence over a central magma reservoir. Radial fractures with associated small edifices cut the lower flanks of the edifice but are not observed within the summit ring of graben; these are interpreted to be the expression of near-surface dykes and may have been emplaced during a period of enhanced activity that correlates with the most voluminous flow unit. Unlike at Hawaii, however, these dykes and small edifices do not seem to be the source of significant flank eruptions. Although some effusive activity may have accompanied their emplacement, the majority of lava flows at Sapas appear to be radial to a single, near-summit point located between the two summit domes.Calculated effusion rates range from 1.5 × 103 m3/s to 3.1 × 105 m3/s; these values suggest that rates were high compared with the Earth and decreased with time. These rates, and the volumes calculated, give eruption durations for the various units that range from 18 days to over 20 years. If eruption is caused by the influx of magma from depth and rupture of an overpressurized chamber, this suggests a variable flux over the history of the volcano. The late-stage eruptions which formed the summit domes are interpreted to be the result of fractional crystallization and/or volatile build-up in the chamber, following a period of decreased supply from depth.Local topography and gravity, as well as regional geology support the presence of a mantle plume at Sapas. The similar properties of large volumes of magma over the total history of the volcano, as well as the prolonged period of magma supply and gradual waning, are consistent with a plume origin. These inferences and the observations allow us to characterise the history of the volcano as follows: arrival of the mantle plume caused uplift of topography and surrounding plains formation: continued supply of smaller volumes of material permitted construction of the edifice; development of a magma reservoir (predicted by theory to form at shallow depths) modified eruption characteristics by permitting storage and homogenization of magma; unbuffered conditions prevailed for the majority of eruptions, producing flows of similar volumes but decreasing flow lengths; a period early on of enhanced supply led to buffered chamber conditions, resulting in the eruption of the voluminous flow unit and the emplacement of many lateral dykes; evacuations from the chamber and cooling towards the last stages caused distributed summit collapse and formation of the ring graben; and finally the gradual waning of supply allowed evolution of the magma which produced the late-stage, possibly viscous flows and dome construction. Preliminary observation of Sapas and two other volcanoes at different elevations suggests that altitude-dependent chamber development and growth may influence the complexity of lava flows and determine the existence of collapse calderas. Many features at Sapas are representative of large volcanoes on Venus and thus Sapas Mons is a good example of a typical plume-associated edifice. Sapas differs in many ways from Kilauea, a terrestrial type shield volcano, but these differences can be understood in the context of the Venus environment.  相似文献   

17.
The morphology of a series of highland volcanoes found on the floor of the crater Palisa is similar to that of mare volcanoes. The superposition of the cones and craters indicate that this volcano complex had a complex, multiphase history of development. A blanket of relatively dark ash, which is between 4 and 13 km wide, is observed surrounding these volcanoes.  相似文献   

18.
We studied a data set of 28 well‐preserved lunar craters in the transitional (simple‐to‐complex) regime with the aim of investigating the underlying cause(s) for morphological differences of these craters in mare versus highland terrains. These transitional craters range from 15 to 42 km in diameter, demonstrating that the transition from simple to complex craters is not abrupt and occurs over a broad diameter range. We examined and measured the following crater attributes: depth (d), diameter (D), floor diameter (Df), rim height (h), and wall width (w), as well as the number and onset of terraces and rock slides. The number of terraces increases with increasing crater size and, in general, mare craters possess more terraces than highland craters of the same diameter. There are also clear differences in the d/D ratio of mare versus highland craters, with transitional craters in mare targets being noticeably shallower than similarly sized highland craters. We propose that layering in mare targets is a major driver for these differences. Layering provides pre‐existing planes of weakness that facilitate crater collapse, thus explaining the overall shallower depths of mare craters and the onset of crater collapse (i.e., the transition from simple to complex crater morphology) at smaller diameters as compared to highland craters. This suggests that layering and its interplay with target strength and porosity may play a more significant role than previously considered.  相似文献   

19.
Material is ejected from impact craters in ballastic trajectories; it impacts first near the crater rim and then at progressively greater ranges. Ejecta from craters smaller than approximately 1 km is laid predominantly on top of the surrounding surface. With increasing crater size, however, more and more surrounding surface will be penetrated by secondary cratering action and these preexisting materials will be mixed with primary crater ejecta. Ejecta from large craters and especially basin forming events not only excavate preexisting, local materials, but also are capable of moving large amounts of material away from the crater. Thus mixing and lateral transport give rise to continuous deposits that contain materials from within and outside the primary crater. As a consequence ejecta of basins and large highland craters have eroded and mixed highland materials throughout geologic time and deposited them in depressions inside and between older crater structures.Because lunar mare surfaces contain few large craters, the mare regolith is built up by successive layers of predominantly primary ejecta. In contrast, the lunar highlands are dominated by the effects of large scale craters formed early in lunar history. These effects lead to thick fragmental deposits which are a mixture of primary crater material and local components. These deposits may also properly be named regolith though the term has been traditionally applied only to the relatively thin fine grained surficial deposit on mare and highland terranes generated during the past few billion year. We believe that the surficial highland regolith - generated over long periods of time - rests on massive fragmental units that have been produced during the early lunar history.  相似文献   

20.
We use multispectral reflectance data from the lunar Clementine mission to investigate the impact ejecta deposits of simple craters in two separate lunar mare basalt regions, one in Oceanus Procellarum and one in Mare Serenitatis. Over 100 impact craters are studied, and for a number of these we observe differences between the TiO2 (and FeO) contents of their ejecta deposits and the lava flow units in which they are located. We demonstrate that, in the majority of cases, these differences cannot plausibly be attributed to uncorrected maturity effects. These observations, coupled with morphometric crater relationships that provide maximum crater excavation depths, allow the investigation of sub-surface lava flow stratigraphy. We provide estimated average thicknesses for a number of lava flow units in the two study regions, ranging from ∼80 m to ∼600 m. In the case of the Serenitatis study area, our results are consistent with the presence of sub-surface horizons inferred from recent radar sounding measurements from the JAXA Kaguya spacecraft. The average lava flow thicknesses we obtain are used to make estimates of the average flux of volcanic material in these regions. These are in broad agreement with previous studies, suggesting that the variation in mare basalt types we observe with depth is similar to the lateral variations identified at the surface.  相似文献   

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