JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. E9, PAGES 20,527-20,546, SEPTEMBER 25,2001
Icelandic pseudocraters as analogs to some volcanic cones on Mars
Ronald Greeley and Sarah A. Fagents Department of Geology, Arizona State University, Tempe, Arizona, USA
Abstract. Pseudocraters are rootless vents formed by the interaction of lava flows with surface or near-surface water. This interaction can produce mild explosions and the accumulation of scoria and spatter into small constructs. Pseudocraters in several localities in Iceland were examined in the field and compared to similar appearing features observed on Mars. The Icelandic pseudocrater cones in this study range in size from 6 to 70 m in diameter, have summit craters which range from 2 to 28 m in diameter (many cones lack craters entirely), and have flanks that are either con cave-up or convex-up. The size and spacing of Icelandic pseudocraters might be a function of the availability of water, in which larger, closely spaced features result from efficient lava-water inter action, as suggested by the environments in which the features formed. Possible Martian pseudo crater cones in Amazonis Planitia range in diameter from 30 to 180 m and have craters 12 to 80 m in diameter. A numerical model for volcanic explosions was adapted to study the formation of pseudocraters under terrestrial and Martian conditions. The results suggest that explosions forming Martian cones require significantly less water (calculated masses are less by a factor of 4 to 16) than those forming Icelandic pseudocraters, despite their larger sizes. This is attributed to the low gravity and atmospheric pressure in the Mars environment and is consistent with the likely lower abundance of water, which might be present as interstitial ice at shallow depths in the regolith. Locations of potential pseudocraters on Mars at latitudes as low as _8°N, imply the presence of crustal ice stores at the time of their formation.
1. Introduction Surveyor (MGS) spacecraft show cone-shaped features located on inferred lava flows (Figure 3). Many of the cone-shaped fea Pseudocraters are small volcanic features that form as a re tures have small summit craters and could be pseudocraters. sult of steam explosions from the interaction of lava flows Some of these features occur in the same areas speculated by with surface or near-surface water. They were first described in Frey and larosewich [1982] to contain pseudocraters. northeastern Iceland where lavas flowed over marshy ground In order to assess the mechanism of pseudocrater formation associated with Lake Myvatn [Thorarinsson, 1953], as shown as an analog for these features on Mars, in this report we in Figure 1. In some respects, pseudocraters are similar to Iit describe Icelandic pseudocraters based on field work and pho toral cones, which form where lava flows enter the sea and togeological analyses, use numerical models to assess the me generate local phreatic explosions, building small constructs chanics and energetics of their formation, and then apply the (Figure 2), as discussed by lurado-Chichay et al. [1996] and results to observations of the features on Mars. Mattox and Mangan [1997]. Mars exhibits a wide variety of volcanic features, first re vealed by results from Mariner 9 [McCauley et al., 1972; Carr, 2. Icelandic Pseudocraters 1973] and seen in greater detail in Viking Orbiter images [Carr et al., 1977]. Martian volcanism has been reviewed by Greeley Pseudocraters occur in several areas of Iceland and Spudis [1981], Mouginis-Mark et al. [1992], Wilson and [Thorarinsson, 1953], including the Myvatn, Landbrot, and Head [1994], and Greeley et al. [2000]. The potential interac Alftaver Districts, and at Raudh6lar, southeast of Reykjavik tion of water and magma on Mars has been considered previ (Figure 4). The first three sites were examined by RG during ously. For example, the Mariner 9 results showed evidence for field work in 1975, 1977, and 1981 to determine the mor abundant volcanic features in regions that had been eroded by phology and morphometry of the pseudocraters and their rela inferred fluvial processes and maar craters were suggested to be tionships to the associated lava flows and pre-flow con present [Greeley, 1973]. Subsequently, Frey et al. [1979, ditions. Subsequently, pseudocraters in the Alftaver District 1981] and Frey and Jarosewich [1982] suggested that Icelandic were analyzed on aerial photographs to obtain statistics on pseudocraters could be analogous to many of the small knobs their planform dimensions and general size frequency distribu mapped in the northern plains and other areas of Mars. tions. Recently obtained high-resolution images from the Mars Orbiter Camera (MOC) [Malin et ai., 1992] on the Mars Global 2.1. Myvatn Pseudocraters The type locality for pseudocraters is the Myvatn District. Copyright 2001 by the American Geophysical Union. Myvatn, a lake in northeastern Iceland, is readily accessible Paper number 20001E001378. by roads from Akureyri and is shown on the 1:50,000 scale 0148-0227/01/20001E001378$09.00 sheet (Myvatn) by the Icelandic Geodetic Survey.
20,527 20,528 GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS
Figure 1. Oblique aerial photograph of a small cluster of pseudocraters on the south shoreline of Myvatn in Iceland; roadways and buildings indicate scale (Arizona State University (ASU) photograph 2240-D-20, August 1980).
Pseudocraters are found mostly on the northwestern margin Raudh6lar result from a style of phreatic eruptions governed of the lake and have been described by many workers, as re by the concentration of ground water in lake basins, and the viewed by Thorarinsson [1953]. These features have been con relatively confined boundaries of the active lava flows. The sidered as potential analogs for extraterrestrial features by large diameter of the Myvatn craters with respect to the cone Fielder and Wilson [1975] and others. Thorarinsson coined the might also result from efficient thermal interaction with rela term pseudocraters to distinguish them from features formed di ti vely abundant surface water. rectly over a volcanic conduit. He also recognized that the Myvatn features and similar structures in other parts of Iceland 2.2. Landbrot District Pseudocraters shared some characteristics with other "rootless" vents, such as hornitos. Pseudocraters consist primarily of pyroclastic The Landbrot District is on the south coast of Iceland where materials ejected from steam explosions but also include the Skafta River empties into the sea and is accessible by the spatter, which formed agglutinate in the summit area. Ring Road (Route 1). The interior of much of the pseudocrater The Myvatn pseudocraters occur in several groups around field is relatively accessible by Secondary Road 204 and asso the lake and as islands within the lake. Many of the groups ciated fann tracks. The area is covered by the 1: 100,000 scale appear to be associated with the 2500-year-old Laxardalshraun sheet 78 (Kirkjubcejarklaustur) of the Icelandic Geodetic lava which flowed into the Myvatn lake basin [Francis, 1993]. Survey. The pseudocraters are raised-rim depressions which range in As described by Thorarinsson [1953], the Landbrot pseudo diameter from a few meters to >100 m and are as deep as 15 m, craters occur in a lava flow 'Nhich predates the well-known measured from the rim crest. The craters and their associated flows which were erupted from Lakagfgar in 1783. In some ejecta deposits include circular and elliptical planforms, with areas the pseudocraters form ki pukas surrounded by the the craters being either centered within the cones or offset Lakagfgar flow. The present course of the Skafta river marks [Fielder and Wilson, 1975]. Many of the cones and craters the northern and eastern boundaries of the Lakagfgar flows and stand "shoulder-to-shoulder" with overlapping,r-ejecta deposits the flow containing the pseudocraters [Kjartansson, 1962]. It (Figure 5). This close-packed arrangement is similar to the seems likely that the river was displaced by the emplacelncnt pseudoctater distribution in the Raudh6lar area, where the of these flows and that HUlny of the pseudocraters formed over pseudocraters are also associated with a lake basin, Ellidvatn. w'ater-saturated sediments along the stream bed. Thorarinsson [1953] diagrammed a cross section through The pseudocratersform a cluster covering an area of -50 the Myvatn basin and showed a proposed sequence of forma km2 . As shown in Figure 6, these structures tend to form tion for the pseudocraters, including the inferred relationship Inounds with convex-upward slopes and rounded summits. The to the ground water regime. We suggest that the morphology Landbrot pseudocraters tend to be closely spaced, similar to and close packing of the pseudocraters at both Myvatn and in those at Myvatn, but often lack the SUllllnit craters seen in the GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS 20,529
Figure 2. Oblique aerial photograph of the littoral cone, Pu'u Hou, on the island of Hawaii, a few kilometers west of South Point. The cone (smooth, light gray areas) formed when lava flows (dark, rough areas) erupted from Mauna Loa in 1868 and entered the sea, forming local phreatic explosions. Waves have eroded the sea- ward part of the cone (ASU photograph 373-F-9, June 1974).
Myvatn examples. Although some of the Landbrot pseudo phreatic explosions occurred. In some respects, this would be craters are more than 100 m across and as high as 20 m, more comparable to littoral cones which were "fed" by tubes and commonly they are 50 to 60 m across and less than 10m high. channels in Hawaii (Figure 2), described by Jurado-Chichay et Figure 7 shows the interior of one of the Landbrot pseudo al. [1996] and Mattox and Mangan [1997]. craters exposed in a borrow pit. This example is typical and is 2.3. Alftaver District Pseudocraters composed primarily of scoria, with fragments as large as 30 cm. The scoria is overlain by a deposit of agglutinate which The Alftaver District is also on the south coast of Iceland, forms a carapace .....40 cm thick. In turn, the carapace is covered in the area where the Skalm River flows into the sea. The area with a deposit of fine grained ash and soil which thins over the is covered on the 1: 100,000 scale sheet 69 (Hjorleifshofdi) summit area. Exposures here and elsewhere in this area suggest and is accessible from the Ring Road (Route 1), Secondary that lava tubes fed molten lava into some of the sites where Roads 211 and 212 and local tracks. 20,530 GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS
cave-up profile (Figure 8). The steeper, upper component is composed of agglutinated spatter. Some reworking of cone material may have resulted from postemplacement floods (T. Thordarsson, personal communication, 2000). Figure 9 shows a pseudocrater in cross section where a small stream has dissected it. Exposed are the scoria deposits which make up most of the structure, the agglutinated spatter which forms the upper part of the cone, and the "throat" feeding the rootless vent. The Alftaver pseudocraters were analyzed stereoscopically on aerial photographs. Nine groups of pseudocraters occur in a zone I to 3 km wide by 11 km long (Figure 10). This zone is inferred to mark the approximate course of the Skalm River prior to the emplacement of the lava flow in which the pseudo craters formed. The current course of the Skalm River adjacent to the pseudocrater field is highly braided with anastomosing channelways. It is likely that this morphology is representa tive of the conditions when the lavas were emplaced and the pseudocraters formed. Pseudocrater Group 10 forms an irregular patch -3 by 7 km in area in the western part of the field. It might have developed in marshy ground outside the main course of the ri ver. Alftaver pseudocraters in each group were measured to de termine the diameters of the outer flank, the inner steep zone, and (when present) the summit crater. Figure 11 shows histo grams of these parameters and the ratio of crater and cone di ameters for all of the cones identified. Plotting each cone group separately yielded no statistically significant differ ences between the groups. Taking the stereo measurements as a whole, the diameters of the main cone structures are gener ally less than -50 m. The peak of the size-frequency distribu tion of these diameters lies near 10m, and -80% of cones have diameters less than 20 m. The smallest measured cone is 6 m wide; the largest is 70 m, although one asymmetric cone has minimum and maximum diameters of 46 and 113 m, respec tively. The diameters of the summit craters, where present, lie in the range 2 to 28 m, with a peak in the distribution near 7 m. The outer flanks or aprons are typically 1.2 to 10 times the inner cone width. The crater/cone diameter ratio has a peak in its distribution at 0.35-0.44 (Figure lId), which is somewhat smaller than the values found for the Myvatn craters (0.45- Figure 3. Possible Martian pseudocraters associated with platy sheet lava flows in Amazonis Planitia near 24.8 0 171.1 oW (Mars Orbiter Camera image M03-03958).
Pseudocraters in the Alftaver District were described briefly by Thorarinsson [1953] and mapped by Kjartansson [1962]. The features occur in several local fields, all associated with lava flows which spread over the outwash plains of the Skalm River. The plains and the drainage are derived from K6tluj6kull, a glacial arm of the larger glacier, Myrdalsj6kull. The lava flows in which the pseudocraters are found are thought to have erupted from the Eldgja fissure, north north east of Myrdalsj6kull (Thoroddsen [1894] as cited by Thorarinsson [1953]). ....", Unlike the Myvatn and Landbrot structures, the pseudoc '--"~"""-' raters in the Alftaver District are much more widely spaced and o 100 km consist of small cones, most of which either have small sum mit craters, or lack craters. In profile they typically have a Figure 4. Map showing the location of pseudocrater fields broad outer flank zone of low slopes (less than a few degrees), in Iceland described in this report. Dotted outlines show the with a break in slope and a steeper summit area, giving a con- locations of glacial fields. GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS 20,531
Figure 5. Ground view of pseudocraters in the Myvatn area showing close-packed arrangement of the con structs. Figures in foreground and on horizon to the right indicate scale (ASU photograph 2282-F, August 1980).
0.54 [Frey and Jarosewich, 1982]). Because flooding might [Morrissey and Thordarson, 1991; Thordarson et aI., 1992]. have modified the cones, potentially decreasing their di The primary differences observed among the fields of pseudo ameters and thus increasing in the crater/cone diameter ratio, craters studied here are the presence or absence of craters, the the original pristine cone ratios would have been significantly slopes of the flanks, and the spacing of the individual centers. less than the present day Myvatn ratios. The classic features at Myvatn (and the features in Raudh6lar, as deduced from the literature) have prominent summit craters, 2.4. Summary of Field Observations convex-upward flanks, and tend to be closely spaced. The In general, Icelandic pseudocraters are small rootless vol crater/cone diameter ratios are intermediate between those of canic constructs composed of scoria with minor but varying cinder cones and tuff rings and maars [Frey and Jarosewich, amounts of spatter. Lava tubes and small lava channels appear 1982]. Pseudocraters in the Landbrot and Alftaver Districts to have been involved in the formation of some pseudocraters have either small craters or lack craters entirely and are more
Figure 6. View of pseudocrater mounds in the Landbrot District, showing generally convex-up slopes and rounded summits (also visible are some mounds with more peaklike summits). These mounds are somewhat closely spaced and commonly lack summit craters. The power line poles seen on the right and extending to ward the horizon are 8 m high and indicate scale (ASU photograph 2258-D 17-18, September 1981). 20,532 GREELEY AND FAGENTS: ICELANDIC PSEUDOCRA1ERS
Figure 7. View of the interior of one of the Landbrot District pseudocrater mounds, showing the outer cara pace of agglutinate overlying interior deposits of scoria. This cone was excavated to use the cinder for local road fills CASU photograph 2259-D 3-5, September 1981). widely spaced. The crater/cone diameter ratios are similar to 3. Martian Cones those of cinder and spatter cones. We suggest that differences in cone morphology and 3.1. Description crater/cone diameter ratio might be attributed at least partly to the environment at the time of formation. Both the Myvatn Fields of conical mounds were identified in Viking images and Raudh6lar pseudocraters formed in lake basins where pre of Eastern Acidalia, Utopia, Isidis, and Elysium Planitia and sumably there was an abundant supply of water. In contrast, were tentatively interpreted as pseudocraters [Allen, 1979; the Landbrot and Alftaver structures formed along valleys and Frey et al., 1979; Frey and Jarosewich, 1982]. The largest braided stream systems where the supply of water might have cone diameters exceed a kilometer, with a typical modal size of been more limited. As we discuss in section 4, water availabil 500 to 700 ill [Frey and Jarosewich, 1982]. However, features ity plays an important role in governing the phreatic eruption smaller than a few hundred meters probably could not be iden dynamics and the resulting morphology of the deposits. tified because of limited image resolution.
Figure 8. Photograph showing field of pseudocrater cones in the Alftaver District. Many of these cones have concave-up slopes and lack summit craters CASU photograph 776-H, June 1975). GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS 20,533
Figure 9. Cross section of a pseudocrater cone exposed by a small stream in the Alftaver District. The crater is lined with spatter and is deeper than typical for most pseudocraters (ASU photograph 2261-D 3-5, Septem ber 1981).
A high-resolution MOC image was acquired in July 1999, lava issuing from the cones [McEwen, 1999; Lanagan et aI., which shows at least 20 distinct conical constructs located 0 n 2001]. The exception to this last point might be cone F volcanic plains near 24.8°N, 171.3°W in Amazonis Planitia (Figure 3), from which several possible short lava flows (Figure 3). With a resolution of ....4 m/pixel the cone morphol emanate to the south and west. Jurado-Chichay et aI. [1996] ogy is revealed clearly. The features were interpreted as pseu noted similar spatter-fed lavas issuing from rootless littoral docraters based on their association with, and apparent super cones on Mauna Loa flows but generally such lavas are volu position on, a lava flow, their well-defined conical form, the metrically insignificant when compared to lavas erupted from presence of summit craters, the apparently random distribution cones formed over primary vents. Thus a rootless origin for (i.e., lack of alignment along primary vents), and the lack of the Martian features is possible.
Figure 10. Map showing the 10 groups of pseudocraters analyzed in the Alftaver District. Groups 1-9 repre sent clusters of pseudocraters which are thought to define the primary course of the Skalm River at the time of the lava flow emplacement. Group 10 occurs on the western margin of the lava flow in which they formed. Basemap is from sheet 6 (Midsudurland) of the Geodetisk Institute, Copenhagen. North is upward. 20,534 GREELEY AND FAGENTS; ICELANDIC PSEUDOCRAlERS
(a) 200 -+-.i...J.-'-...... I-.i...J.-'-...... L...... L...+-
40 150 ~ u> c c 30 Q) Q) ::::l 5- 100 cr ....Q) ~ 20 LL LL 50 10
0 0 0 10 20 30 40 50 60 70 80 0 4 8 12 16 20 24 28 32
Cone Diameter (m) Crater Diameter (m)
(c) 40 (d) 40
30 30 >- >- u u c c Q) Q) ::::l 20 ::::l 20 cr cr ....Q) ....Q) LL LL 10 10
0 0 l() 0 20 40 60 80 100 120 0 ~ """C\l (')""" l() """<0 I'- co """m 0 0""" 0 0 """0 """0 0 """0 """0 0 v Outer Apron Diameter (m) l() l() l() l() l() l() l() l() l() 0 ~ C\l (') l() <0 I'- co 0 0 0 0 0""" 0 0 0 0 Crater Diameter/Cone Diameter Figure 11. Size frequency distributions for Alftaver pseudocraters; (a) inner cone diameter, (b) summit crater diameter, (c) outer flank (apron) diameter, and (d) ratio of crater diameter to cone diameter.
There is no apparent alignment along, nor other morpho While none of the lines of evidence discussed above is con logical expression of, potential lava tubes within the flow. clusive proof of a rootless origin, a synthesis of all the evi The platy lava surface texture is interpreted to be evidence of dence is suggestive of pseudocrater formation. However, the broad sheet flow emplacement characterized by fluctuations in possibility that the features are members of a monogenetic effusion rate, leading to rafted plates of solidified crust which cone field lacking tectonic control cannot be definitively ruled formed on relatively stagnant flow and was later disrupted by out. surges in flow rate [Keszthelyi et aI., 2000]. This mechanism Measurements made on main cone diameter (steep-sided por would ensure a continued supply of lava to interact with the tion), crater diameter (where present and resolved), and outer ground ice. flank (apron) diameter (as shown in Figure 12) are summarized Surrounding each cone is a smooth halo or apron which in Figure 13. The peak of the main cone diameter frequency subdues the underlying lava surface texture, suggestive of a plot lies near 100 m, -10 times greater than that of the mantle of fine-grained ash or scoria (such as is typical for Alftaver cones (Figure 13a, cf. Figure lla). The smallest terrestrial scoria cones). As noted in section 2, some Icelandic cones are -30 m wide, and the largest cone has a diameter of pseudocraters commonly show a similar break in flank slope -180 m, comparable to the larger Icelandic examples within which most of the cone volume is confined (Figure 8). [Thorarinsson, 1951, 1953]. Diameters of the outer aprons These observations support the interpretation of these features are difficult to define due to the gradually decreasing deposit as volcanic cones, as opposed to other possibilities, such as thickness, but are generally in the range 100 to 300 m. The pingoes, or exhumed impact craters. summit craters are typically 40 to 60% of the width of the GREELEY AND FAGENTS: ICELANDIC PSEUDOCRAlERS 20,535
main cone, and the peak of cone/crater ratio distribution lies at 0.45-0.54. This is similar to the distributions for other Mars cones as well as for the Myvatn pseudocraters [Frey and Jarosewich, 1982] but larger than that of the Alftaver pseudo craters described in section 2.3. MOC images of other candidate pseudocraters exist for Elysium Planitia and the Isidis basin [Lanagan et aI., 2001], although the cones are rather less well defined and are therefore probably less pristine. In Isidis the cones range between 160 and 800 m in diameter and have summit craters 60 to 400 m across. Figure 12. Pseudocrater cone morphology shows a well de fined main cone (e.g., Figure 3) capped by spatter, which 4. Model of Pseudocrater Formation probably represents the waning energy of the explosions (due either to decreased availability of water and/or molten lava). 4.1. Model Description The outer apron probably contains a fine-grained material de posited from a small convecting eruption cloud, as well as a It is generally accepted that pseudocraters form as a result of proportion of larger scoria ejected at anomalously low angles hydrovolcanic explosions when lava flows over water-satu or high velocities. rated ground [Thorarinsson, 1953; Allen, 1979; Frey et al.,
(a) 8 (b) 8
6 6 >. >-C,,) C,,) c: c: 0) 0) ::l ::l 4 4 0- 0- ....0) ....0) u.. u..
2 2
0 0 o 20 40 60 80100120140160180200 0 16 32 48 64 80 96 112
Cone Diameter (m) Crater Diameter (m)
(c) 3 (d) 10
8
>. 2 C,,) >. c: C,,) 6 0) c: ::l 0) 0- ::l 0) 0- .... ~ 4 u.. u..
2
O-t--r-,-,- 0 o 40 80 120 160 200 240 280 320 ,.... C\J "'"C'l "'" "'"lO CD I' "'" Outer Apron Diameter (m) lO lO lO lO lO lO lO lO 0 ,.... C\J C'l lO CD I' 0 0 0 0 "'"0 0 0 0 Crater Diameter/Cone Diameter Figure 13. Size frequency distributions for Amazonis cones: (a) inner cone diameter, (b) summit crater di ameter, (c) outer flank (apron) diameter, and (d) ratio of crater diameter to cone diameter. 20,536 GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS (a) (b) 1. LAVA HEATS SU8STRATE, VAPORIZES WATER DETAILS OF EXPLOSION MODEL 2. THRESHOLD PRESSURE EXCEEDED, LAVA EXCAVATED atmosphere p" P" (ii) gas expansion (i) gas accumulation and pressurization Figure 14. (a) Proposed stages of pseudocrater formation. Repeated cycles of vaporization, pressurization, excavation, and infilling eventually lead to the cone form shown in Figure 12. (b) Details of the explosion process. 1979; Thordarson et ai., 1992, 1998; Thordarson and Self, duced by multiple, intermittent expulsions of material, rather 1993]. The morphologic attributes of Icelandic pseudocraters than as a consequence of one large explosion. This is also suggest that their associated section of lava flow was emplaced supported by the cone volumes; more material is apparently and had cooled somewhat prior to the formation of the cone present in the cone than would be represented by simple exca structures. The lava must have been sufficiently strong to bear vation of the overlying lava in a maar-type event. If, how the weight of the forming cones, because they are evidently ever, the core of the lava flow were still partly molten (per not rafted or deformed by subsequent lava motion. Most of the haps occupying a lava tube), additional flow from beneath the lava flow therefore was probably stationary and had developed surface crust into the explosion site would effect a repeated a competent surface crust, although the some fraction of the filling of the excavated void and provide a mechanism for flow interior must have remained molten to supply the spatter. cyclic pressurization and ejection of the lava, with gradual This would be facilitated by flows with lava tubes [Morrissey accumulation of the resulting debris to form a cone structure. and Thordarson, 1991; Thordarson et ai., 1992]. The super This process is probably somewhat analogous to the forma ficial resemblance of many of these features to scoria cones tion of littoral cones [Mattox and Mangan, 1997]. associated with primary vents suggests that they were pro- We therefore envision an initial energetic explosion which GREELEY AND FAGENTS: ICELANDIC PSEUDOCRA1ERS 20,537 would require a significant accumulation of pressurized gas to Table 1. Notation overcome the confining pressure due to the weight of the whole depth of flow plus the strength of the solid crust. At Symbol Meaning subsequent times, depending on the balance between the rate of inflow of lava from the core (which may not entirely replace acceleration due to gravity, m s·2 the initial depth of the flow) and the timescale for vaporiza mass of atmospheric gas, kg tion of substrate water, less energy would be required to exca mass of water vapor, kg mass of lava, kg vate smaller volumes of confining lava, so that these repeated mass ratio of gas to lava smaller explosions would be responsible for the main cone atmospheric pressure, Pa building stage. The locations within the flow field of individ initial (threshold) gas pressure, Pa ual explosion sites are probably determined partly by irregu distance from explosion source, m larities in the strength/thickness of overlying lava and/or gas region radius, m lava thickness, m topography of the substrate. distance at which maximum gas velocity is attained, m The formation of pseudocraters thus progresses through a time, s series of stages (Figure 14a): (1) initial emplacement of lava initial gas temperature, K heats the substrate, vaporizing water on the surface and/or in gas velocity, m s·l maximum gas velocity (=clast launch velocity), m s·l pores of substrate material (as either liquid or ice); pressure atmospheric density, kg m·3 builds due to accumulation of gas in a volume confined by gas density, kg m·3 country rock and overlying lava; (2) once a threshold pressure lava density, kg m·3 defined by a combination of the weight and strength of the clast ejection angle (above horizontal), 0 overlying lava is exceeded, the gas expands rapidly into the lava tensile strength, Pa lava yield strength, Pa atmosphere, excavating and entraining the overlying lava and velocity decay constant, s some (minor?) proportion of substrate material; (3) inflow of lava from the fluid flow core and accumulation of other debris seals the void; (4) repeated vaporization, pressurization, ex cavation, inflow, and resealing, leading to the final cone mor phology shown in Figure 12. Particularly efficient heat trans (3) fer between the lava and substrate might promote bursts of Once the explosion begins, the gas expands adiabatically more sustained fountaining activity [Mattox and Mangan, from the vent, ejecting the caprock, and displacing the atmos 1997]. phere ahead of it. The mass of atmospheric gas displaced is A model for transient volcanic explosions was developed by Fagents and Wilson [1993] and applied to Mars for erup =~nPa[(r+r/)3 tions resulting from accumulations of gas above near-surface ma -erg +r/)3] (4) magma bodies [Fagents and Wilson, 1996]. In that scenario, in which Pais the density of atmospheric gas. volatiles derived from the magma body, or from vaporization The equation of motion of the caprock and displaced atmos of ice/water in the country rock, would be confined beneath the pheric gas is surface either as a discrete pocket or in a porous medium. Once the gas pressure exceeds the tensile strength of the confining caprock, the explosion is initiated, the gas expands out of the [p,o(; -p+"+{~;J vent, accelerating the caprock into the atmosphere. The tra r jectories of the fragmented debris are susceptible to the motions of the expanding gas and those induced in the atmos X{P/[(rg +'i)3 -r/]+Pa[(r+r/)3 -erg +r/)3]) (5) phere. by the explosion. The starting point for the model assumes a pressurized gas pocket (so the source of heat is ir in which r is the ratio of the specific heats of the driving Hp relevant) and here we adapt the model to the formation of gas. This equation is integrated numerically to obtain the pseudocraters (Figure 14b). velocity of lava caprock as the gas expands from the explo Gas accumulates in a region of thickness r beneath a depth sion site. It is assumed that clasts of excavated lava are r/ of lava, the masses of which are given by g launched at the maximum velocity uo' which is reached at dis tance R . Beyond R the gas decelerates according to 3 (1) o o m=.!.nprg 3 g g R -t/r u=uo - O)2 e (6) m/ = inp/[(rg+r/)3 - r/] (2) ( r in which Pg and p/ are the gas and lava density, respectively, in which r is a time constant defined as the difference between and n is the solid angle subtended by the explosion region. the time taken for the maximum velocity, uo' to be attained All notation is listed in Table 1. The mass ratio of gas to lava and the time required for the gas velocity to decline to zero is given by n=m/m/, which has been determined to be a key [Fagents and Wilson, 1995]. factor in influencing the energetics of the hydrovolcanic ex The trajectories of ejected material, which is subject to the plosions [Sheridan and Wohletz, 1981; Wohletz, 1986]. drag forces imposed by the explosion-induced atmospheric The onset of the explosion takes place when a threshold motions, are computed using a fourth-order Runge-Kutta pressure Pgo is exceeded. PgO is the pressure required to over scheme [Wilson, 1972; Fagents and Wilson, 1993]. The re come the weight of the overlying lava, the yield strength a: of sulting ejection distances of various clast sizes launched at a the lava, the tensile strength aT of solid crust, and the atnios range of angles can then be synthesized to determine the di pheric pressure Pa such that ameter of the resulting construct. 20,538 GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS 4.2. Model Constraints range for rr, from the minimum required to produce an explo sion for a given flow thickness, up to a maximum of r =30 m. Many workers have considered the influence of the Martian g The initial gas pressure P fl,0 includes the ambient atmos environment on effusive and explosive eruptions [Wilson et pheric pressure P (_105 Pa for Earth, -600 Pa for Mars), the al., 1982; Mouginis-Mark et ai., 1992; Wilson and Head, a weight of the overlying lava (PIgrl)' the lava yield strength q, 1994; Fagents and Wilson, 1996; Glaze and Baloga, 1998]. and the solid crust tensile strength aT (equation (3)). The pres~ The low atmospheric pressure and acceleration due to gravity sure due to the overlying lava would range from -2.5x104 to exert strong influences on magma ascent, eruption, and em 7.5x105 Pa for confining lava thicknesses of 1 to 30 m (these placement. For a given set of initial eruption conditions, values are 60% lower for Martian gravity conditions). Lava Martian explosions are likely to be larger than on Earth and yield strengths range from zero (for a low crystallinity result in more widely dispersed eruptive products. Therefore Newtonian rheology) to -8x103 Pa for cooler, high-crystal the amount of gas and the initial pressure required to produce a content lavas [Shaw et ai., 1968; Shaw, 1969; Gauthier, 1973; given size of pseudocrater cone should be less on Mars than on Pinkerton and Sparks, 1978; McBirney and Noyes, 1979; Earth. Murase, 1981; Pinkerton and Stevenson, 1992], although val For Martian pseudocraters we are interested in the amount of ues up to -5x 104 Pa are inferred from solidified flow mor gas required to excavate the lava and produce the cones, be phologies [Hulme, 1974; Moore et ai., 1978; Fink and cause this is an indication of the amount and availability of Zimbelman, 1986]. The tensile strength might range from crustal volatile stores on Mars. Therefore the model pa near zero (i.e., no significant crust) to a maximum of -107 Pa r , rameters we must investigate are the lava thickness l the gas (representative of pristine, dense basalt [Touloukian et al., region size rfl,' and the initial gas pressure, PfI,O' 1981 D. However, the tensile strength is not likely to be this The thickness of the confining lava layer could potentially high due to the thermal and mechanical stresses in the scenario be quite different for the initial explosion than for subsequent under consideration. The range of PgO (=Pa+p/gr/+ar-a) to be ejections. In Iceland the lava thicknesses range up to -15 m considered is therefore 1.25xl05 to 1.86x106 Pa for Earth and [Thorarinsson, 1953]. Thicknesses of Martian lava flows are 2.56xl04 to 1.28xl06 Pa for Mars. not known, but are typically thought to be 10 to 200 m Other model parameters are the lava density (taken as 2500 [Moore et al., 1978; Zimbelman, 1985; Cattermole, 1987; kg m- 3), eruption site altitude (which defines atmospheric pa Head et al., 1997; Mouginis-Mark and Tatsumura Yoshioka, rameters such as pressure, density, and temperature), and gas 1998]. After the initial excavation of the lava flow, inflow temperature immediately prior to the explosion (Tr,O)' This from the mobile core might seal the void with as little as a last quantity might lie anywhere between the vapor point to meter of lava. Thus we will consider values for rl in the range 1 near the lava temperature, depending on the temperature of the to 15 m for Earth and 1 to 30 m for Mars. lava at the interface with the H20-rich country rock, and the The size of the pressurized gas region is difficult to con length of time taken to exceed the threshold pressure. How strain. Heat transfer calculations suggest that a thermal wave ever, because the model output is sensitive to the product of might penetrate the substrate to produce a column of water and the temperature and the gas-solid mass ratio (nTfI,O)' we arbi vapor equal to roughly one third to one half of the lava flow trarily adopt a temperature of 800 K (52TC) and discuss the in thickness [Allen, 1979; Squyres et al., 1987]. However, this fluence of different initial temperatures in section 4.5. assumes an instantaneously-emplaced, stationary, cooling In computing the trajectories of ejected material we must flow. If the core of the flow were mobile, or contained a tube, specify the atmospheric properties appropriate to Icelandic or advective heat transfer could prolong the heating time and ex Martian conditions, clast size (-1-10 cm is typical for cone tend the thermal influence to greater depths. Thus we explore a forming scoria, consistent with field observations in Iceland), 800 .....----.....,.--.....,.--.....,.--.....,.--~~-'r'"-r-..,...r-:--....,-_;..., 1000 .,Jf<> 400 'I . / / 300 / 2 3 4 5 6 7 8 10 Specific Entropy (kJ K-1 kg'1) Figure 15. Temperature-entropy diagram for H20, illustrating possible thermodynamic paths for terrestrial and Martian pseudocrater explosions. GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS 20,539 clast density (2500 kg m-3), and ejection angle, 8. For the last pressure explosions under consideration here, this effect item we assume that the clasts are ejected with a minimum ejec should be at a minimum; however, we discuss the implications tion angle of 70° from horizontal, consistent with observa of potential early onset of condensation in section 4.5. tions of strombolian explosions in which >90% of particles are ejected within 20° from vertical [Chouet et ai., 1974]. 4.3. Application to Icelandic Pseudocraters The conditions outlined above suggest that the substrate Our assumption is that clasts are ejected with a continuous heating and explosion are not as violent as deep-seated hydro range of dispersal angles within 20° of vertical. Clasts ejected volcanic magma-water interactions, in which runaway frag at the corresponding minimum elevation of 70° above hori mentation and mixing in fuel-coolant interactions leads to the zontal will attain the greatest distance, equivalent to the outer very high pressures required to excavate tens to hundreds of margin of the main cone structure, whereas all clasts ejected at meters of overburden [Wohletz, 1986]. We also argue that greater elevations will land closer to the vent, building the pseudocraters form somewhat differently from littoral cones in upper flanks of the cone. We assume that material comprising which bench collapse and wave action are key factors in pro the outer apron has a fine grain-size component, which is sub moting magma-water mixing and magma fragmentation ject to more complex motions in the eruption cloud and [Mattox and Mangan, 1997]. Rather, pseudocrater formation atmosphere, and therefore is not amenable for accurate treat might be more analogous to the milder "bubble burst" mecha ment with this model. However, by focusing our attention on nism of littoral cone construction observed by Mattox and the larger clasts comprising the main cone, for which the Mangan [1997], in which intermittent contact of lava with model is most accurate, we can gain a good sense of the erup relatively small amounts of water results in repeated low-en tion conditions. ergy explosions. For pseudocrater explosions, emplacement The plots in Figure 16 show the pyroclast ejection range of lava over saturated sediments or shallow standing water plotted as a function of gas mass for a maximum ejection angle would involve limited mixing, leading to relatively low pres of 70°, and for several lava thicknesses and strengths. The sure explosions. This is even more likely to be the case for dashed curves represent the radius of a hemispheric region of Martian explosions, in which longer heating periods might be gas surrounding the explosion source. The lowest gas mass required for an additional phase change (solid to liquid), and to value for each curve represents the minimum required to pro 0 access more limited crustal H2 stores which may only be pre duce an explosion for that specific set of initial conditions. sent at some minimum depth beneath the ground surface Table 2 presents the gas mass, gas region size, and the lava [Paige, 1992; Mellon and Jakosky, 1995; Mellon et al., strength (which strongly controls the gas pressure needed to 1997]. produce the explosion) that are required to produce typical 10, The temperature-entropy diagram for H20 (Figure 15) illus 20, and 50 m Icelandic cones with different confining lava trates some of the possible thermodynamic paths for the ex thicknesses. As noted above, we favor lower lava thicknesses plosion scenario described above. Curve ABCDE represents which might represent the repeated infilling of the explosion the heating conditions at the base of a 5 m thick Icelandic site by lava from the flow core. For example, a 20 m cone lava. From A to B, a volume of water is heated until vaporiza produced by repeated ejections of 1 m of lava requires 14 to tion commences. Segment BC indicates conversion of liquid 300 kg gas accumulating in a hemispheric region 1 to 6 min to vapor taking place at constant pressure. The consequent radius, assuming expansion starts from 800 K. The require volume change might be accommodated either by displace ments rise to 700 to 2400 kg and 5 to 12 m for each explosion ment of liquid or vapor through the porous substrate or, as ob in a 5 m thick lava. served in some instances in Iceland, by hummocks produced With increasing lava thickness, the greater pressures and by upwarping (without excavation) of the overlying lava gas masses required to initiate the explosion imply much [Allen, 1979], which might represent gas-filled pockets. On larger gas regions, which in turn imply a larger minimum clast Mars the possible absence of interstitial ice in the very near ejection range. Explosions through the greater lava thick surface regolith could accommodate vapor from deeper ice nesses (Figures 16c and 16d) are less capable of producing stores. Segment CD indicates continued superheating of the large cones because elevation of greater overburden requires vapor. Continued addition of vapor with no volume accom more of the energy of gas expansion, at the expense of kinetic modation could cause the pressure to rise until the lava energy; thus lower maximum velocities and short trajectories strength was exceeded (dashed curve indicates uncertainty in are the result. The consequences are that thicker lavas promote the path), thus triggering the explosive isentropic gas expan larger minimum cone sizes (>20 m), and smaller maximum sion down to atmospheric pressure (portion DE of the curve). cone sizes, for a given gas mass. Conversely, a greater range Curve AB'C'D'E' illustrates a similar path for Martian condi of cone sizes is possible for smaller confining lava thick tions. The total pressure at the base of 5 m lava is less than 0 n nesses (Figures 16a and 16b). The typical Icelandic cones we Earth due to the lower atmospheric pressure and acceleration measured (:S;20 m) must be produced by smaller confining lava due to gravity. thicknesses, which is consistent with our conception of re One possible complication arises when expansion begins peated infilling of the initial explosion cavity from the flow from significantly higher pressures or temperatures (curve core by relatively small volumes of lava. AB "C"D"E"). The shape of the condensation curve is such that Greater lava strengths (which result from significant cool condensation takes place before atmospheric pressure is ing and development of high yield strength or tensile achieved, thus causing the expansion to cease earlier, produc strength, and are independent of lava thickness) also act to ing lower velocities and shorter clast trajectories. This is produce the greater range of potential cone sizes. This would likely to be a more significant problem for Mars, where at require repose periods between explosions to provide suffi mospheric pressures lie well below the condensation curve for cient time for thermal interaction with the water to achieve fur much of the pressure-temperature field. For the relatively low- ther vaporization and pressurization. Alternatively, efficient 20,540 GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS (a) Lava Thickness: 1 m (b) Lava Thickness: 5 m 10000 50 -+-----I------L...---:f"__-+--...- 50 -f------1------L."Tf"--r-;- 40 40 g g ~ 30 Q) 30 c OJ ell c a:: ell a:: -~ 20 ti 20 (3 ell (3 10 10 Lava Strength (Pa) O-+---r-..--r...... ,."T"TT~---r---r-r-.....,...,rTTT--I- 10 100 1000 A 1x103 100 1000 10000 Gas Mass (kg) ~ 3x104 Gas Mass (kg) T 1x105 (c) Lava Thickness: 10 m o 3x105 (d) Lava Thickness: 15 m • 1x106 10000 50 -1- 10..L-00 50000..L--;- 50 -t----'------'------'--+_ 40 40 g g Q) 30 Q) 30 OJ OJ c C ell ell a:: a:: ti 20 ti 20 ell ell (3 (3 10 10 O-+----r--r---r-..,...... ,r'"'"'T'""T""T'".------.,..--_+_ O-f-___r----,r---r---r-...... ,.------,.----r----I- 1000 10000 30000 3000 10000 40000 Gas Mass (kg) Gas Mass (kg) Figure 16. Clast ejection range as a function of gas mass for Icelandic pseudocrater explosion simulations. The lava thicknesses are (a) I m, (b) 5 m, (c) 10 m, and (d) 15 m; solid curves show lava strengths (yield strength or tensile strength) ranging from 103 to 106 Pa; dashed curves indicate size of gas region. Lava den sity is 2500 kg m- 3, ejection angle is 70', lower axis for gas initial temperature of 800 K, upper axis for gas temperature of 500 K. mIxing of the lava and substrate water could produce high 4.4. Application to Martian Cones pressures on short timescales and more sustained activity [Mattox and Mangan, 1997]. However, even if the larger A similar analysis for Martian pseudocrater formation is pressures were unable to develop, longer clast ranges could be presented in Figure 17 and Table 3. It is clear that much produced with lower gas pressures (i.e., lava strengths) pro greater clast ejection ranges are produced under Martian condi vided that sufficient driving gas was drawn from a larger vapor tions, with significantly less gas required. The average 100 m rich region. Developing a quantitative understanding of ther cone measured in Figure 3 requires only 2 to 18 kg of gas in a mal interactions in the substrate would place additional con region of <3 m radius with I m lava thickness. These values straints on volatile amounts, and remains a subject requiring rise to 700 to 1000 kg gas in a 5 to 13 m region for a 10 m further study. lava thickness, still rather modest amounts. The largest cone GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS 20,541 Table 2. Summary of Conditions Required to Form Icelandic Cones of 10, 20, 50, and 100 m Diameter Lava Thickness rl' m 5 10 15 10 m Cone Gas mass,a kg 8-90 240-400 Lava strength,b Pa 106-105 106-3x105 Gas region radius,c m 1-4 3-5 20 m Cone Gas mass,a kg 14-300 700-2400 2200-3000 4400 Lava strength,b Pa 106-105 106-103 106-3x105 106 Gas region radius,c m 1-6 5-12 7-9 8 50 m Cone Gas mass,a kg 50-300 280-7000 9700-15700 20000 Lava strength,b Pa 106-3x105 106-3x105 106-3x105 106 Gas region radius,c m 2-5 3-13 11-16 13 100 m Cone Gas mass,a kg 500 3000 Lava strength,b Pa 106 106 Gas region radius,c m 4 16 aAdopting an initial gas temperature of 800 K; multiply given values by 1.6 if Td ) is 500 K. bEquivalent to tensile strength and/or yield strength. cCalculated assuming explosion draws on a hemispheric pocket of gas below a point at the base of the lava. Maximum possible values are -5 m for aIm thick flow, and -15 m for 5 to 15 m flows. Multiply values by 1.5 if gas is contained in country rock with 30% void space. (200 m) requires 4 times this amount of gas for each of the Other parameters that influence the model results are the series of cone-forming explosions. assumed lava density, clast size, and ejection angle. Figure 18 The influence of the planetary environment on the explo shows the influence of these parameters for lava with a yield sion process can be appreciated by comparing the conditions strength of 106 Pa. In each case, the deviation from the results required to produce a 50 m diameter cone on Earth and Mars given in Figures 15 and 16 is greatest for the highest initial (Tables 2 and 3). In order to project material to a distance of gas pressures (dictated by the lava strength) and gas region 25 m, 5 to 70 times more gas is required to drive the explosion sizes. For the clast ejection ranges appropriate for the on Earth than on Mars. The low Martian gravity and atmos Icelandic and Martian pseudocraters, relatively modest pres pheric pressure promote both greater velocities and longer sures and gas masses are required, so the deviation is less particle trajectories for a given gas mass. severe. The most important factor is the ejection angle (Figure 18a). Ejection angles of 60° to 50° produce ejection 4.5. Discussion of Results and Sensitivity to ranges 40 to 65% greater than an angle of 70°. In any given Input Parameters explosion, some proportion of material might be ejected at Tables 2 and 3 show how the model results vary with initial angles lower than 70° which would contribute to the outer apron, but most material comprising the inner cone is ejected gas temperature TgO and substrate porosity. The amount of gas required to produce a given clast ejection range varies with the within a narrow range of angles [Chouet et ai., 1974]. assumed initial gas temperature, a parameter which is difficult The lava density is also important in the model. In Figure 3 to constrain. However, adjustments to the gas mass values 18b, a density of 1500 kg m- (corresponding to a vesicular 3 shown in the Tables 2 and 3 and Figures 15 and 16 (calculated lava) was used .to plot the solid curves and 2500 kg m- for the dashed curves. Larger_. ejection ranges (i.e., wider cones) are assuming Tgo=800 K), can be made simply by multiplying by the quotient of the given and desired temperatures (i.e., xI. 6 expected for lower density lavas because there is less lava mass to accelerate upward during gas expansion, leading to for a desired TgO of 500 K). Thus a 60% greater gas temperature translates to 37% less gas. greater initial velocities. The differences are less pronounced The values for gas region size assume a discrete pocket for greater lava thicknesses, but for a lava thickness of 1 m filled with gas. If the gas is contained within country rock and large gas region sizes, ejection ranges up to 60% greater having a volume fraction of void space (cracks, pores, cavi for p/=1500 kg m-3 than for p/=2500 kg m- 3 are expected. ties) denoted by ljJ, the radius to provide enough gas for the ex Higher-density lavas are favored by volatile loss during em plosion would increase by a factor of ljJ-l/3 times the values placement. Furthermore, material ejected in magma-water in shown in Tables 2 and 3. For example, this implies 50 to 88% teractions tends to be poorly vesiculated and therefore of greater radii for substrates having 0.3 to 0.15 void space, i.e., greater density [Wohletz, 1983; Heiken and Wohletz, 1991]. a 50% decrease in porosity produces only a 25% increase in Ejection range is shown for clasts of 1, 10, and 25 cm radii the required radius of the gas-rich region. in Figure 18c. Smaller clasts are more readily decelerated by 20,542 GREELEY AND FAGENTS: ICELANDIC PSEUDOCRA1ERS (a) Lava Thickness: 1 m (b) Lava Thickness: 5 m 20 60 100 140 o 200 600 1000 1400 150 -1--"""r--'-....L_.J.-~L---L_-L.._ _\_ 150 -1-_l..-----'-_....L..-_I...----l.__...... _...l-_ _\_ " ::[100 ::[ 100 Q) Q) 01 OJ C C ell ell a: a: (j) (j) ell ell <3 50 <3 50 Lava Strength O-q-...... ,.-.--_--r-.--,-...... -.--...... -_/_ (Pa) o 20 40 60 80 100 Ll 1x103 o 200 400 600 800 1000 Gas Mass (kg) .3x104 Gas Mass (kg) T 1x105 03x105 (c) Lava Thickness: 10 m (d) Lava Thickness: 15 m • 1x106 o 2000 4000 6000 8000 10000 14000 150 -1------J-__..I...-__--L +_ ::[ 100 ::[ 100 Q) Q) OJ OJ C c ell ell a: a: (j) ~ ell <3 50 <3 50 O-l---,..-,--....,.....--,--.---,.-----r----,.--...... -_/_ o 1000 2000 3000 4000 5000 o 2000 4000 6000 8000 10000 Gas Mass (kg) Gas Mass (kg) Figure 17. Clast ejection range as a function of gas mass for Martian pseudocrater explosion simulations. The lava thicknesses are (a) I m, (b) 5 m, (c) IO m, (d) 15 m; solid curves show lava strengths (yield strength or tensile strength) ranging from 103 to 106 Pa; dashed curves indicate size of gas region. Lava density is 2500 kg m-3, ejection angle is 70°, lower axis represents a gas initial temperature of 800 K, upper axis represents a gas temperature of 500 K. aerodynamic drag, resulting in shorter ballistic ejection the cones modeled in this study, the gas mass is rather insensi ranges and smaller cones (although submiIlimeter particles tive to variations corresponding to the typical range of clast would become incorporated in the convection cJtmd and form a sizes comprising proximal scoria deposits (1-10 cm). more widespread fall deposit). Conversely, 25 cm clasts As noted above, the onset of condensation during explo travel -5% farther than IO cm clasts. The effect is more ex sion could potentially halt the gas expansion prior to atmos treme for thinner lavas because the ejection velocities are pheric pressure being reached. This would result in lower ejec greater. On Mars, atmospheric deceleration will be less, but tion velocities than are c2lculated with the current model in the same trend is apparent: 1 cm clasts travel -80% of the 10 which the condensation pressure Pc would take the place of Pa cm clast ejection range, whereas, 25 cm clasts travel up to in (5). Because Pc is itself a function of the initial pressure and 16% farther. However, for the conditions required to produce temperature (Figure 15), this introduces an additional level of GREELEY AND FAGENTS: ICELANDIC PSEUDOCRATERS 20,543 Table 3. Summary of Conditions Required to Form Martian Cones of 50, 100, 200, and 500 m Diameter Lava Thickness rl' m 5 10 15 30 50 m Cone Gas mass,a kg 1-18 50-100 190-220 320-360 330-500 Lava strength,b Pa 106-104 106-103 106-103 106-103 106-103 Gas region radius,c m 0.6-5 2-7 3-7 4-7 4-7 100 m Cone Gas mass,a kg 2-18 170-350 700-1150 1400-1700 3200-3700 Lava strength,b Pa 106-105 106-105 106-103 106-103 106-103 Gas region radius,c m 0.7-3 3-7 5-13 6-13 8-13 200 m COYle Gas mass,a kg 8-77 570-970 2600-4700 5500-7000 17000 Lava strength,b Pa 106-105 106-3xl05 106-105 106-3xl05 106 Gas region radius,c m 1-5 5-8 7-16 9-14 13 500 m Cone Gas mass,a kg 40-165 2900 15000 Lava strength,b Pa 106-3xl05 106 106 Gas region radius,c m 2-5 3-13 11-16 aAdopting an initial gas temperature of 800 K; multiply given values by 1.6 if Tgo is taken as 500 K. bEquivalent to tensile strength and/or yield strength. cCalculated assuming explosion draws on a hemispheric pocket of gas below a point at the base of the lava. Maximum possible values are 5 m for aim thick flow, 8 m for a 5 m flow, and 15 m for 10 to 30 m flows. Multiply values by 1.5 if gas is contained in country rock with 30% void space. complexity in solving (5). We feel that the explosions re distances, and small cone sizes. Abundant water was probably sponsible for the relatively small features identified in this available in the Icelandic environment, but we infer that the study require relatively low pressures and gas temperatures, and lava t10w displaced any surface water, with relatively little that equation (5) is a good approximation. However, larger trapped and pressurized in pockets or sediments beneath the features such as the few anomalously large >300 m cones in t1ow. Within the range O (a) Influence of Ejection Angle low-gravity environment are sufficient to explain this differ 50 -t=:===::!:!!!:!!:;-'-4,...-t-'"...... ~...... u.uL.-'rj1l't ence; there is no need to infer that higher-pressure fuel-coolant Launch interactions were responsible. We therefore offer a cautionary Angle note for interpreting the origin and eruption conditions of • 50· 40 Martian cones (or indeed any Martian volcanic feature) based 060· on direct morphometric comparison with terrestrial structures; +70· the effects of the ambient environment can dominate the final I morphology. ~ 30 c There is considerable debate concerning the location of a:CI:l water on Mars. Various theoretical models have not reached a firm consensus. It was initially suggested that at low lati 1i5 20 CI:l tudes, the Martian regolith would have become dehydrated to (3 great depths over time, if not completely, due to the instabil ity of water under the low atmospheric pressure conditions and 10 warmer equatorial surface temperatures [Clifford and Hillel, 1983; Fanale et al., 1986; Kuzmin, 1988]. More sophisti cated thermal models incorporated heterogeneities in regolith properties [Paige, 1992], variations in orbital obliquity 10 100 1000 10000 [Mellon and Jakosky, 1995], and the effect of recondensation Gas Mass (kg) as water migrates upwards through the regolith [Mellon et al., 1997]. Each of these mechanisms allows for the existence of shallow ground ice, but there remains some uncertainty over (b) Influence of Lava Density some of the assumed regolith properties incorporated in the 50 -t;::::!:::::::!:::~~...... ~--r-'u..LLI.uL-"""""""""""'--'-"'T models. Nevertheless, there is ample evidence that water was a Density key factor in the formation of a variety of geologic features (kg m-S) seen at low latitudes, including rampart craters, outflow chan 1:J..1500 40 nels, collapse features, and potential subglacial volcanic fea +2500 tures. The volcanic cones highlighted in this and previous I studies [Allen, 1979; Frey et al., 1979; Frey and Jarosewich, ~ 30 1982; Lanagan et al., 2001] are also located within a few tens c of degrees north of the equator. If the pseudocrater CI:l a: interpretation is correct, these features are additional evidence -~ 20 of the existence of ground ice within a few meters to <20 m of (3 the surface at the time of cone formation. This is consistent with the prediction of Mellon et al. [1997] that subsurface ice 10 might lie at a depth of 4 to 300 m at the equator, and shallower still at higher latitudes. The pristine cone morphology and sparsely cratered lava surface (Figure 3) suggests a very young age, such that significant crustal volatile stores must have existed in the very recent past, and may well persist today. 10 100 1000 10000 Identification of additional pseudocraters in MOC images will Gas Mass (kg) help confirm our inference of low-latitude near-surface volatile stores. (c) Influence of Clast Size 50 -tr:~~!::;''''''''''''''''''''''''''''..L..L.Lw.u...... &&...-...... ,.. 6. Summary Clast Size (cm) I. Pseudocraters in the Alftaver District are smaller and +10 more widely spaced than those of the Myvatn District and have 40 'Y 1 smaller crater/cone diameter ratios. This might attest to less 025 efficient lava-water interaction (and hence less energetic ex I plosions) in a stream sediment setting versus a lake environ ~ 30 ment. Another factor limiting cone size might be a restricted c CI:l or waning lava supply. a: 2. All else being equal, pseudocrater-forming explosions U5 20 would produce a 5 to 10 times wider constructs on Mars than CI:l (3 on Earth. 3. Four to 16 times less gas for each individual explosion is 10 required to form the Amazonis cones than Alftaver pseudo- Figure 18. Sensitivity of model results to key input 10 100 1000 10000 parameters: (a) ejection angle, (b) clast size, and (c) lava den Gas Mass (kg) sity. GREELEY AND FAGENTS: ICELANDIC PSEUDOCRA1ERS 20,545 craters, despite their larger size. This is consistent with the teristics on Mars: First results from the Mars Orbiter Laser Altimeter (MOLA) data (abstract), Lunar Planet. Sci. COf1:f [CD-ROM], XXIX, notion that water (in the form of interstitial ice) was less ac abstract 1324, 1997. cessible in the Martian regolith than in Iceland. Heiken, G.H., and K. Wohletz, Fragmentation processes in explosive 4. If the cratered cones identified on Mars are pseudocraters, volcanic eruptions, in Sedimentation in Volcanic Settings, pp. 19-26, their distribution suggests that ice was available in the rego Soc. for Sediment. Geol., Tulsa, Okla., 1991. lith at depths of <10 to 20 m, and at latitudes from a few de Hulme, G., The interpretation of lava flow morphology, Geophys. J. R. Astron. Soc., 39,361-368,1974. grees to ±45°. In the case of the Amazonis cones (at 24.8°N), lurado-Chichay, Z., S.K. Rowland, and G.P.L. Walker, The formation the apparent young age of the host lava flow implies that ice of circular littoral cones from tube-fed pahoehoe: Mauna Loa, was present geologically recently and could persist today. Hawaii, Bull. Volcanol., 57, 471-482, 1996. Keszthelyi, L. P., A.S. McEwen, and T. Thordarson, Terrestrial analogs and thermal models for Martian flood lavas. J. Geophys. Res., 105, Acknowledgments. This work was supported by the National 15,027-15,049,2000. 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Head, Mars: Review and analysis of volcanic ([email protected]; [email protected]) eruption theory and relationships to observed landforms, Rev. Geophys., 32,221-264, 1994. (Received Septelnber 7, 2000; revised May 7, 200I: Wilson, L., J.\V. Head, and PJ. Mouginis-Mark, Theoretical analysis of accepted May 30, 2001.)