51st Lunar and Planetary Science Conference (2020) 1066.pdf SMALL VOLCANIC CRATERS AND PITS. R. A. De Hon, Department of Geography, Texas State University, San Marcos, TX, 78666, [email protected] Planetary Volcanism: Mafic lava Maar. Maars are steep-sided flows dominate on rocky planetary bodies [1, volcanic craters formed by venting magma- 2]. Most lavas were emplaced as very fluid, derived volatiles or magma-heated flood basalts. They make up the surface that groundwater (phreatomagmatic eruption) was pummeled by early saturated cratering, surrounded by a raised rim of volcaniclastic and they constitute large areas of plains- material [3, 4, 5]. The rim may be composed forming material which mask underlying of mostly accidental material, or it may be cratered surfaces. As planetary datasets mixture of accidental and magmatic-derived increasingly contain higher resolution detail, material. Maars may have cinder cones it becomes possible to examine increasingly situated in their floor by later eruptions. smaller landforms. In addition to the large Tuff ring: Tuff rings consist of a raised craters on volcanic cones and shields, a ring of volcanic tuff around a shallow crater. large variety of craterform features and pits Tuff ring by this usage is the raised rim of the are found in monogenetic volcanic fields and maar crater. Some tuff rings may be superposed on lava flows. composed of volcanic cinder, and the floor Three mechanisms are largely may not be significantly depressed below the responsible for these volcanic landforms. surrounding surface. They are 1) craters and pits formed over Craterform structures: Crater-like volcanic vents; 2) craters formed by features may be formed by that are not interaction of surface flows and underlying associated with volcanic vents. water or ice saturated ground; and 3) crater Pseudocrater. Pseudocraters, also like structures and pits formed by collapse. A called rootless cones, are tephra-rimmed compendium of volcanic craterform craters produced by a hot lava flow that structures and pits— as seen in terrestrial crosses a water/ice saturated surface [6, 7, volcanic fields—is provided for comparison 8]. The resulting explosive steam that with similar appearing planetary features. escaped through the overlying basalt Craters: The term crater as used for produces one or more shallow craters in the volcanic structures has a rather loose flow. definition. Any depression atop an Rootless shield. Rootless shields are accumulation of volcanic material is called a small, shield-like domes of elastoviscous crater. As such it may be formed by crust on active lava flows that are beached explosion or collapse. In contrast, meteor and allow underlying fluid lava to escape to craters are formed by explosive release of the surface [9]. After lava has escaped, the energy at the surface by hypervelocity shield is preserved as a small rise as a impact of a meteorite or comet. blocky lined pit surrounded by radiating lava Caldera: A caldera is volcanic channels on the surface of the flow. depression formed by the collapse of the Deflated Inflation Plateau. Deflated ground above a magma chamber, which inflation plateaus are raised rings of blocky empties during very large volcanic eruptions. and slabby lavas surrounding wide, shallow The diameter of a caldera may be many pits [10]. The inner walls are blocky or slabby times larger than the size of the individual (like the outer rim), but the interior floors are vents. The caldera is part of a volcanic relatively smooth lava like the lava flows on complex which may include many vents and which the pits are found. The floor consists smaller volcanic structures superposed both of the original lava crust that was inflated to inside and outside of the caldera. form a lava plateau that receded to lower levels as supporting lava escaped. A raised 51st Lunar and Planetary Science Conference (2020) 1066.pdf rim of stretched and fractured brittle crust Significance: Small impact craters remains. The floor is often cut by large are typically bowl-shaped depressions fissures. These crater-like structures can be surrounded by a raised rim of fragmented easily misinterpreted as explosion craters target material. It may be difficult to because of their blocky rims. They may have distinguish between small impact craters and highly irregular outlines in contrast to the some volcanic explosion craters. Meteor circular or elliptical outlines of explosion crater placement is random, and rim craters, and they are surrounded by lava components are limited to brecciated, local channels produced by escaping lava. surface material. In contrast, rim materials of Pits: Rimless depressions are maar craters contain samples of subsurface common in volcanic fields. They overlie materials which are traversed by the vents, or they may form by other ascending gases. Some mare eruptions mechanisms. bring materials from the mantle. Correct Lava subsidence pit (drain-back identification of volcanic landforms is craters). Some volcanic vents consist of pits important in interpreting planetary thermal that originally erupted lava to the surface, but history. Eruptive vents often reflect structural in later stages of activity the lava drained control whereas faux craters have no such back into the subsurface leaving a pit. control. Pseudocraters indicate the Fumarole. A fumarole is a vent or presence of water or ice at the time of opening through which issues steam or other formation [11]. Collapse craters suggest the volcanically derived gases. Fumaroles many possibility of subsurface voids [12] that might be small cracks, elongate fissures, or small provide might have provided habitats for holes. primitive lifeforms [13] or shelter for future Collapse pit. Collapse depressions astronauts. form on basalt flows as fluid lava drains from References: [1] Greely, R., 2013), beneath a solid crust. Variations range from Planetary Geomorphology. [2] Melosh, H. J., a viscoelastic crust sagging into a void to (2011), Planetary Surface Processes. [3] blocky remnants of the overlying crust lining Lorenz, V. (1973) Bull. Volcanol. 37,183-20. the depression. Collapse pits may be [4] Zeitner, M.E. (1981) Third International randomly scattered on a flow or aligned over Colloquium on Mars, 290. [5] De Hon, R. A. lava tubes. (2012) Lunar Planet. Sci. Conf., abs1075. [6] Inflation pit. Inflation pits are formed Thorarinsson, S. (1953) Bull. Volcanol. 14, 3- when thin, fluid lava flows encircle local high spots in the underlying surface. Subsequent 44. [7] Greeley, R. and S.A. Fagents (2001) thickening of the lava flow by inflation J. Geophys. Res., 106, 20527-20546. [8] produces a steep-sided depression in the Frey, H, et al. (1979) J. Geophys. Res. 84, flow [9]. Inward facing slopes may be only 8075-8086. [9] Walker, G.P.L. (1991) Bull. slightly fractured if the lava crust is elastic Volcanol. 53, 546-558. [10] De Hon, R.A. and and can stretch, or it may be highly fractured R. A. Earl (2018) New Mexico Geology 40, or broken onto slabs and blocks if the lava 17-26, [11] Fagents, S.A. (2007) in Geology crust is brittle. Many pits on lava flows which of Mars, 151-177. [12] Wyrick, D. et al. were identified as collapse pits are, in fact, (2004) J. Geophys. Res.: Planets 109, inflation pits. E06006. [13] Boston, P.J. (1992) Icarus 95, 30. .