Mare Domes Research Articles

New maps of major oxides and Mg # of the lunar surface from additional geochemical data of Chang'E-5 samples and KAGUYA multiband imager data

In the past, global maps of major oxides and magnesium number (Mg #) on the lunar surface had been derived from spectral data of remote sensing images, combined with “ground truth” geochemical information from Apollo and Luna samples. These compositional maps provide insights into the chemical variations of different geologic units, revealing the regional and global geologic evolution. In this study, we produced new maps of five major oxides (i.e., Al2O3, CaO, FeO, MgO, and TiO2) and Mg # using imaging spectral data from the KAGUYA multiband imager (MI) and the one-dimensional convolutional neural network (1D-CNN) algorithm. We took advantage of recently acquired geochemical information from China's Chang'E-5 (CE-5) samples. We used the coefficients of determination (R2) and Root Mean Squared Error (RMSE) as model evaluation indicators. We compared the results with the previous machine learning algorithm models. Our study shows that the 1D-CNN algorithm model used in this study had a higher degree of fit and smaller dispersion between the “ground truth” value of geochemical information and the predicted value of spectral data. The 1D-CNN algorithm generally performs better in describing the complex nonlinear relationship between spectra and chemical components. In addition, we present regions of mare domes in Mairan Dome (43.76°N, 49.90°W) and irregular mare patches (IMPs) in Sosigenes (8.34°N, 19.07°E) to demonstrate the geologic implications of these new maps. With the highest spatial resolution (∼ 59 m/pixel), these new maps of five major oxides and Mg # will serve as an important guide in future studies of lunar geology.

Identification and Geomorphometric Characterization of Volcanic Cones in the Marius Hills, the Moon

AbstractInvestigation of the detailed geological characteristics of lunar volcanic cones can peer into their formation and emplacement processes, thus deepening our understanding of pyroclastic eruptions on the Moon. In this work, we focus on the Marius Hills region (centered at 13°N, 55°W), a volcanic plateau in south Oceanus Procellarum, which hosts the most abundant volcanic cones on the Moon, along with other volcanic features. The range of volcanic features indicates various eruption styles and complicated sequences of regional magmatic activities. Based on the SLDEM2015 topography data, with assistance from other multisource topography and image data sets, we identified 360 known and suspected volcanic cones and 22 volcanic domes in the Marius Hills. Following detailed morphological characterizations, the volcanic cones show various morphology types, including C‐shaped, elliptical‐shaped, elongated‐shaped, irregular‐shaped, and rounded‐shaped. The cones (median basal width ∼2.7 km, height ∼115 m, and volume 0.25 km3) contain summit pit craters (15%; median width ∼358 m), exhibiting smaller basal widths and summit pit crater widths compared with lunar mare domes. The mean slope (3.5°–15.6°) and rock abundance (0.3%–3.8%) of volcanic cones in Marius Hills are larger than those of the mare dome, suggesting different eruption mechanisms. Marius Hills cones also show larger basal widths and smaller height/width ratios than terrestrial volcanic cones, which are attributed to multiple factors, including subaerial environment, volatile content of the magma, eruption rate, and lava viscosity.

Mare Domes in Mare Tranquillitatis: Identification, Characterization, and Implications for Their Origin

AbstractMare domes, small shield volcanoes typically <∼30 km diameter, are part of the spectrum of lunar volcanic features that characterize extrusive basalt deposits. We used new spacecraft data to document these in Mare Tranquillitatis, among the oldest maria and the site commonly interpreted as an ancient degraded non‐mascon impact basin. We found 283 known and suspected mare domes, with the majority (n = 229) concentrated on a broad, ∼450 km circular topographic rise in eastern Mare Tranquillitatis. The domes (median diameter 5.6 km, height 68 m, volume 0.7 km3) contain summit pits (74%; median diameter 0.8 km), and exhibit minor compositional variability between domes and surrounding flows, suggesting that domes both supply and are embayed by these flows. Based on their characteristics and associations, we interpret the small shield volcanoes to have been built from individual low‐volume (<∼10–100 km3), low volatile content, short duration, cooling‐limited eruptions. The ∼450 km broad volcanic rise is ∼920 m high (volume ∼1.6 × 105 km3) and is interpreted to be built from multiple occurrences of small shield eruptions, a shield plains volcanism style. This implies a shallow mantle source region capable of supplying distributed dike‐emplacement and eruption events over an area of 1.75 × 105 km2 early in mare volcanism history (∼3.7 Ga). The difference between Mare Tranquillitatis and younger mare‐filled mascon basins is attributed to the more ancient thermal state and crustal structure of the viscously relaxed Tranquillitatis basin, and a shallower broad magma source region present in earlier lunar thermal history.

Extraction of lunar domes from Chang'E-2 data with new method

Lunar domes, which are distributed mainly in the lunar mare, were generated by lunar volcanism several billion years ago. In this context, identifying lunar mare domes and studying their morphological properties enhances the study of lunar volcanism. Traditionally, most lunar domes have been identified by manually analyzing images or topographic maps of the lunar surface; this approach has identified several lunar domes in specific local areas. To learn more about global lunar domes, it is necessary to identify them from the global lunar mare. In this paper, we present a method of identifying lunar domes using Chang'E-2 data. We demonstrate the identification of domes and analyze their morphological and geological properties.

Eruptive history of a low-frequency and low-output rate Pleistocene volcano, Ciomadul, South Harghita Mts., Romania

Based on a new set of K–Ar age data and detailed field observations, the eruptive history of the youngest volcano in the whole Carpathian-Pannonian region was reconstructed. Ciomadul volcano is a dacitic dome complex located at the southeastern end of the Călimani-Gurghiu-Harghita Neogene volcanic range in the East Carpathians. It consists of a central group of extrusive domes (the Ciomadul Mare and Haramul Mare dome clusters and the Koves Ponk dome) surrounded by a number of isolated peripheral domes, some of them strongly eroded (Balvanyos, Puturosul), and others topographically well preserved (Haramul Mic, Dealul Mare). One of the domes (Dealul Cetăţii) still preserves part of its original breccia envelope. A large number of bread-crust bombs found mostly along the southern slopes of the volcano suggest that the dome-building activity at Ciomadul was punctuated by short Vulcanian-type explosive events. Two late-stage explosive events that ended the volcanic activity of Ciomadul left behind two topographically well-preserved craters disrupting the central group of domes: the larger-diameter, shallower, and older Mohos phreatomagmatic crater and the smaller, deeper and younger Sf. Ana (sub)Plinian crater. Phreatomagmatic products of the Mohos center, including accretionary lapilli-bearing base-surge deposits and poorly sorted airfall deposits with impact sags, are known close to the eastern crater rim. A key section studied in detail south of Băile Tusnad shows the temporal succession of eruptive episodes related to the Sf. Ana (sub)Plinian event, as well as relationships with the older dome-building stages. The age of this last eruptive event is loosely constrained by radiocarbon dating of charcoal pieces and paleosoil organic matter at ca. 27–35 ka. The age of the Mohos eruption is not constrained, but we suggest that it is closely related to the Sf. Ana eruption. The whole volcanic history of Ciomadul spans over ca. 1 Myr, starting with the building up of peripheral domes and then concentrating in its central part. Ciomadul appears as a small-volume (ca. 8.74 km3) and very low-frequency and low-output rate volcano (ca. 9 km3/Myr) at the terminus of a gradually diminishing and extinguishing volcanic range. A number of geodynamically active features strongly suggest that the magma plumbing system beneath Ciomadul is not completely frozen, so future activity cannot be ruled out.

Lunar intrusive domes: Morphometric analysis and laccolith modelling

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 km 3. 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.

A lunar cryptomare dome near Mee H and Drebbel F

In this study, we examine the lunar mare dome Mee 1 situated near the craters Mee H and Drebbel F in a region showing evidence of ancient (pre-Orientale) mare volcanism and cryptomare deposits. Regional stratigraphic relations indicate that Mee 1 was formed prior to the Orientale impact at the beginning of the Imbrian period. Based on a combined photoclinometry and shape from shading technique applied to telescopic CCD images of the dome acquired under oblique illumination, we determined a diameter of Mee 1 of 25 km, a height of 250 m, a flank slope of 1 . 15 ∘ , and a volume of 44 km 3 . Based on rheologic modelling of the dome and a viscoelastic model of the feeder dike, we obtained a magma viscosity of 5.1 × 10 5 Pa s , an effusion rate of 869 m 3 s - 1 , a duration of the effusion process of 1.6 years, a magma rise speed of 1.9 × 10 - 4 ms - 1 , a width of the feeder dike of 32 m, and a horizontal dike length of 144 km. A comparison of Mee 1 with domes with similar morphometric properties, which are located near Milichius and inside the crater Petavius, reveals strong similarities with respect to the viscosity of the dome-forming magma and the feeder dike geometry, while the effusion rate and magma rise speed of Mee 1 are somewhat higher. The pronounced morphometric differences between Mee 1 and a smaller dome situated close to the crater Doppelmayer and characterised by a similar magma viscosity suggest that the growth of that dome was limited by exhaustion of the magma reservoir, while Mee 1 and the other larger domes display morphometric properties presumably coming closer to the cooling limit. The comparison of the ancient dome Mee 1 with the younger (Eratosthenian) edifices near Milichius and Doppelmayer suggests that the conditions in the upper mantle and the crust favoured high eruption volumes, effusion rates, and magma rise speeds, implying the occurrence of large magma reservoirs preventing the limitation of dome growth by magma exhaustion. On the other hand, we observe similar general morphometric, rheologic, and feeder dike characteristics and, thus, conclude that the formation conditions of lunar mare domes did not change fundamentally during the Imbrian period.

Lunar domes in Mare Undarum: Spectral and morphometric properties, eruption conditions, and mode of emplacement

In this study we examine five lunar domes in Mare Undarum. Four domes termed Condorcet 1–4 are located between the craters Condorcet P and Dubiago, immediately east of Dubiago V and W. The fifth dome, termed Dubiago 3, is located about 35 km further south. The region under study is situated in a major trough concentric to the Crisium basin. The domes Condorcet 1–3 are aligned radially with respect to the Crisium basin. Similar dome configurations aligned radial to major impact basins are known from other lunar mare dome fields. The spectral signature of the domes derived from Clementine UV/VIS imagery reveals that they consist of basaltic lava with a low TiO 2 content below 2 wt% and with a FeO content around 10 wt%. Three examined domes exhibit highland components in their soils, which we attribute to lateral mixing between the material in the mare ponds and the surrounding highland material due to random impacts. All five domes have moderate diameters between 10 and 12 km. Condorcet 1–3 are similar to effusive domes of intermediate flank slope between 1 ∘ and 2 ∘ like those situated in the Hortensius/Milichius/T. Mayer region, while Condorcet 4 has an exceptionally steep flank slope of 2 . 8 ∘ and a large volume. With its low flank slope of 0 . 9 ∘ , the dome Dubiago 3 is morphometrically very similar to a known intrusive dome in the west of Mare Serenitatis. Hence, this structure is possibly of intrusive origin, but with the available data an effusive origin cannot be ruled out. Based on a rheologic model, we infer the physical conditions under which the domes were formed (lava viscosity, effusion rate, magma rise speed) as well as the geometries of the feeder dikes.

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

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 TiO 2 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 10 4 Pa s < η < 10 6 Pa s 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 10 2 Pa s < η < 10 4 Pa s comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with 10 6 Pa s < η < 10 8 Pa s 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 TiO 2 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 10 8 – 10 9 Pa s , 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.

Lunar domes in the Doppelmayer region: Spectrophotometry, morphometry, rheology, and eruption conditions

In this study we examine a lunar volcanic region near Doppelmayer, composed of two domical structures previously not studied in detail and a well-known lunar pyroclastic deposit. Dome 1 is situated at selenographic coordinates 41 . 92 ∘ W and 30 . 08 ∘ S , dome 2 at 43 . 42 ∘ W and 30 . 66 ∘ S . We perform a spectrophotometric study of representative locations of the volcanic region based on Clementine UVVIS data. Relying on ground-based high-resolution CCD imagery, we furthermore examine the morphometric characteristics of the two domes, making use of a combined photoclinometry and shape from shading technique. Based on a rheologic model, we examine the physical conditions under which the domes were formed. Dome 1 is spectrally atypically red for mare domes and shows a very weak mafic absorption, implying a low TiO 2 and FeO content. The overall spectral signature corresponds to that of a mixture between mare and highland soils. Dome 1 was formed of lava with a relatively high viscosity value of ∼ 10 7 Pa s , situated between the ranges of values typically observed for mare domes and for the Gruithuisen and Mairan highland domes, respectively, erupting at moderate rates over a long period of time. Dome 1 is larger, steeper, and more voluminous than typical mare domes but has a much lower flank slope than the Gruithuisen and Mairan highland domes. It is an exemplar of a rare type of unusually steep and voluminous mare domes, similar to the well-known mare domes Hortensius 5 and 6 and Herodotus ω . We discuss the relevance of vertical (assimilation of crustal material) vs. lateral (distribution of material across mare-highland boundaries by random impacts) mixing mechanisms as being responsible for the observed spectral appearance of dome 1. The thermal conditions in the lunar interior did not favour the assimilation of crustal wallrock into the ascending magma. Due to the fact that dome 1 is located right on the boundary between hummocky terrain and a mare pond, lateral mixing of mare and highland soils is a much more natural explanation for the observed spectral signature. For dome 2, we find that it is a typical effusive mare dome, given its spectral and morphometric properties and inferred rheologic parameters. An estimation of the dimensions of the feeder dikes of the two domes reveals that the dike which formed dome 1 was five times as broad as the one which formed dome 2, while the dike lengths only differ by about one third. The dike dimensions suggest that their source regions were located below the lunar crust.

A combined spectrophotometric and morphometric study of the lunar mare dome fields near Cauchy, Arago, Hortensius, and Milichius

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-TiO 2 basalts are observed in the Hortensius and Milichius dome fields, while the domes near Cauchy and Arago that consist of high-TiO 2 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 10 2 and 10 8 Pa s , effusion rates between 25 and 600 m 3 s −1 , and durations of the effusion process between three weeks and 18 years. Lava viscosity decreases with increasing R 415 / R 750 spectral ratio and thus TiO 2 content; however, the correlation is not strong, implying an important influence of further parameters like effusion temperature on lava viscosity.

Spectral properties of the Marius Hills volcanic complex and implications for the formation of lunar domes and cones

We have used multispectral data from the Clementine UV‐visible camera to study the volcanic features of the Marius Hills complex and their comparison to other lunar domes and cones. There are several mare units identified in the complex, each with a unique Ti content, as indicated by their 415/750 nm value. The domes in Marius Hills are spectrally identical to the mare plains of the complex, supporting similar compositions. In contrast, most of the volcanic cones of the complex are lower in reflectance, bluer in color, and have weaker mafic absorptions than the mare and domes. The spectral characteristics of the cones can best be explained by fine‐grained crystallization in the spatter that compose the cones. Other lunar cones, such as Mons Esam in northern Tranquillitatis, have spectral properties similar to those at Marius Hills. The Rima Parry cones and their associated dark mantle deposit appear redder with a stronger mafic absorption than the Marius Hills cones. The cones Isis and Osiris in southern Mare Serenitatis, the domes of Rumker Hills, and several domes in northern Mare Tranquillitatis are spectrally similar to adjacent mare units. The Mairan and Gruithuisen domes in northern Oceanus Procellarum have a feldspathic signature characteristic of highland material although they are redder and brighter than adjacent highland soils. They appear to represent highland material that resembles domes rather than actual mare domes, like those at Marius and Rumker Hills. The diversity of lunar volcanic features can best be explained by differences in accumulation rates and cooling of ejected clasts from various eruption styles. Mare domes may have formed at lower effusion rates, thereby allowing lava to construct small shields with slopes &lt;3°. The steeper domes at Marius Hills require higher viscosities resulting from even lower effusion rates and enhanced crystallization in the magmas during the terminal stages of earlier eruptions that emplaced the mare. Cones like Mons Esam, Rima Parry, Isis, and Osiris are aligned along linear rules and are interpreted to result from degassing of near‐surface dikes. In contrast, the cones of Marius Hills show no linear alignment and may result from strombolian eruptions at the terminal stages of earlier effusive eruptions that emplaced the domes and mare in the complex.

Remote sensing of potential lunar resources: 2. High spatial resolution mapping of spectral reflectance ratios and implications for nearside mare TiO2 content

High spatial resolution maps illustrating variations in spectral reflectance 400/560 nm ratio values have been generated for the following mare regions: (1) the border between southern Mare Serenitatis and northern Mare Tranquillitatis (including the MS‐2 standard area and Apollo 17 landing site), (2) central Mare Tranquillitatis, (3) Oceanus Procellarum near Seleucus, and (4) southern Oceanus Procellarum around Flamsteed. We have also obtained 320–1000 nm reflectance spectra of several sites relative to MS‐2 to facilitate scaling of the images and provide additional information on surface composition. Inferred TiO2 abundances for these mare regions have been determined using an empirical calibration which relates the weight percent TiO2 in mature mare regolith to the observed 400/560 nm ratio. Mare areas with high TiO2 abundances are probably rich in ilmenite (FeTiO3) a potential lunar resource. The highest potential TiO2 concentrations we have identified in the nearside maria occur in central Mare Tranquillitatis. Inferred TiO2 contents for these areas are &gt;9 wt % and are spatially consistent with the highest‐TiO2 regions mapped previously at lower spatial resolution. We note that the morphology of surface units with high 400/560 nm ratio values increases in complexity at higher spatial resolutions. New telescopic spectra of landing sites successfully reproduce the Charette relation, although we find that the 400/560 nm value is strongly a fimction of the sample area size. The increased spectral contrast of the 400/730 nm ratio over the 400/560 nm ratio demonstrates the potential increased precision with which the 400/730 nm ratio might be used to estimate TiO2 abundances. Comparisons have been made with previously published geologic maps, Lunar Orbiter IV, and ground‐based images, and some possible morphologic correlations have been found between our mapped 400/560 nm ratio values and volcanic landforms such as lava flows, mare domes, and collapse pits.

Lunar mare domes: Classification and modes of origin

A classification of over 200 lunar mare domes shows that they have two major modes of occurrence: (1) low, flat, generally circular structures with convex shapes, slopes less than about 5°, and displaying summit craters, and (2) irregular structures often adjacent to highland regions and rarely containing summit craters. On the basis of morphologic and morphometric similarities, the first mode of occurrence appears to be analogous to small terrestrial shield volcanoes, and to represent primary volcanic constructs, while the second class of domes appears to result from secondary volcanic effects (flooding of highland material to produce kipukas and draping of lavas to produce irregular dome-like topography). Domes comparable to small shield volcanoes generally range from 3–17 km in diameter and up to several hundred meters in height and occur predominantly in groupings in the lunar equatorial region in northeast Tranquillitatis (Cauchy area), between Kepler and Copernicus (Hortensius area), and in the Marius Hills. In the Marius Hills, domes generally lack summit craters and have a rough surface texture formed in part by superposed cones and steep-sided flows. Elsewhere, domes representing volcanic sources are smooth-surfaced and usually contain a summit crater. These features are similar in general morphology to small terrestrial lava shields. They are generally intermediate in volume, slope, and height between small shields of terrestrial basaltic plains (such as the Snake River Plains) and larger Icelandic shields. Summit craters on lunar domes are considerably larger than craters on terrestrial shields of comparable diameters, apparently due to a combination of factors, including vent enlargement during extrusion, possibly higher lunar extrusion rates, different amounts of collapse, and impact erosion. Most vent-related domes appear to be associated with, and are thus approximately the same age as, surrounding lava plains, although relationships in specific areas have not yet been established. On the basis of age ranges of mare deposits established by Apollo samples, mare vent-related domes formed over an approximately one billion year period starting about 3.7 b.y. ago. Extrusion rates were apparently relatively low compared to the very high values characteristic of flows associated with major lunar sinuous rilles and terrestrial flood basalts, but may have been relatively high compared to similar terrestrial shields. Large shield volcanoes equivalent to the terrestrial Hawaiian-type or to the martian edifices such as Olympus Mons, do not occur on the Moon. Lack of these features may be due to the low viscosities and high effusion rates typical of many lunar eruptions and the lack of continuous eruptions from single sources.