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43rd Lunar and Conference (2012) 1075.pdf

Significance of Maars on : Terrestrial Analogs to Monogenic . R. A. De Hon, Department of , Texas State University, San Marcus, Texas, 78666, [email protected]

Introduction. Improved resolution on the initial rim deposits; cinder eruptions of the provides improved building intercrater cones; or eruptions images of volcanic features at scales filling the crater. Still further eruptions may comparable to terrestrial imaging (1, 2). completely engulf the earlier formed crater Studies of terrestrial monogenic volcanic as the grows to completely fields in arid regions on Earth provide engulf the maar. insights into the significance of cinder Vent opening contains cones, maars, and pseudo-craters in blocks and smaller fragments of lithics interpreting Martian ground-water or entrained in the gases rising to the surface. environment and subjacent This material provides a unique sampling of . material traversed by the gases on their way Attributes of maars and cinder to the surface (6). Although most maar cones. Terrestrial monogenic volcanic eruptive material is made up of accidentals, fields are typically composed of an some maars and their underlying diatremes assortment of eruptive landforms ranging and pipes contain material derived from pseudocraters, minor rimless from the mantle. Prolonged venting is depressions, cinder cones and associated accompanied by slumping of the walls of the lava flows (3). Maar-type craters are initial narrow crater and re-entrainment of common features in many volcanic fields. slumped materials into the escaping gases. They range from a few hundred meters to a Material erupted during this phase tends to few kilometers in diameter. Preservation of be shallow materials immediately beneath the original vent morphology is a function of the surface that slumps into the actively surface processes of degradation which are outgasing vent. This material is commonly largely related to the climate and age of the deposited by as base surges flowing away feature as well as a function of continuing from the vent, and is preserved as trough, eruptions at the vent (Fig. 1). Maars are crossed-beds radial to the vent (7). Before often restricted to the edges of volcanic maar-volcanism was completely fields because they are buried by more understood, these epiclastic deposits were abundant tephras and where sometimes mistaken as fluvial in origin. eruptions are concentrated. Base surge cross-bedding and graded Maars are generally attributed to bedding from air fall mimic fluvial deposition. pheatomagmatic eruptions in which a rising Clues to the volcanic origin include such encounters (4, 5). The things as flow direction indicators in bedded initial eruption creates a cone-shaped crater tuffs, bomb sag structures, and accretionary surrounded by a low, raised ring of vent- lapilli. opening-breccia. If no further gas venting Significance. Pseudocraters attest occurs the initial small crater may be to a water- or ice-saturated surface over modified by slumping of the walls to widen which the lava flows are moving (8). Maar- the initial crater and reduce the overall type craters are common products of an relief. Continued eruption may proceed as initial eruptive phase that may be followed renewed gas-venting laden with accidental by more voluminous phases as cinder or ejecta; cinder or scoraceous cinder material lava eruptions. Maar craters are evidence derived from the rising magma. Eventually of a water or ice saturated zone in the lava may reach the surface. The initial subsurface. Rim deposits of maars and crater may be modified by renewed rings preserve earliest, pyroclastic base- explosive eruption resulting in a crater-in- surge and fall-materials representing the crater form; less explosive venting building initial vent-opening breccia. These breccias 43rd Lunar and Planetary Science Conference (2012) 1075.pdf

include a mixture of mantle xenoliths, lithic cinder cones. Cinder cones on Mars ash, and tuff breccia composed of provide evidence of magma water fragments of subcrater lithology traversed interaction. Cinder cone fields on Mars by the initial venting of gases from the provide fertile ground for sampling sites of underlying magma. Vent-opening breccias underlying lithologies. may be present as basal layers of many

Fig.1. Progression of landforms in monogenic volcanic fields as a result of renewed volcanic activity or modification by .

References. [1] Borz, P. and E. Hauber Conf. 133-138. [7] Fisher, R.V. (1966) (2011) LPSC 42, Abs 1379. [2] Lanz J. And Amer. J. Sci. 264, 287-298. [8] Fagents S.A. others. (2010) JGR, 115, E12019, (2007) in Geology of Mars Cambridge doi:10.1029/2010JE003578. [3] C.A. Wood University Press, 151-177. and J. Kienle (1992) Volcanoes of North America; United States and . Cambidge University Press, 354p. [4] Lorenz, V., 1973, Bulletin Volcanologique, v. 37, p. 183-204. [5] Ort, M.,and others (1998) in Geologic excursions in northern and central : Flagstaff, Northern Arizona University, p. 35-58. [6] Semken, S. (2003) Soc, Guidebook, 54th Field