45th Lunar and Planetary Science Conference (2014) 2326.pdf

CAN YOU MISS WHAT YOU DON’T SEE? EROSION PATTERNS OF LAVAS AND IGNIMBRITES ON EARTH AND MARS. Tracy K.P. Gregg,1 Elisabetta Panza1 and Brandy Buford,1 Dept. of Geology, 411 Cooke Hall, University at Buffalo, Buffalo NY 14260-3050 ([email protected]).

Introduction: Tyrrhenus Mons, found in the cir- Terrestrial ignimbrites are characterized by low- cum-Hellas region of Mars, is a low-relief, central-vent relief surfaces that may contain fumerolic mounds [8] volcano whose flanks are dissected by radially oriented and may be dissected by channels [e.g., 9] and modi- channels [e.g., 1]. Its low (<2°) flank slopes have been fied by aeolian processes. De Silva and others [9] re- interpreted to have been generated by fluid lavas and veal that although jointing within Andean ignimbrites pyroclastic materials (most likely pyroclastic density plays a role in the formation of yardangs, a strong, currents [1-3]). The evidence supporting a pyroclastic persistent wind is more important. “Incipiant origin include: visible layering (revealed in the ero- yardangs” are evident in the tightly crenulated margins sional scarps bounding the volcanoes’ deposits); a fria- of ignimbrite deposits in the Central Andes [9]. In ble nature (inferred from the erosional channels dis- contrast, the margins of the Bishop Tuff locally display secting the volcanoes’ flanks); and the likelihood that relatively smooth margins that are festooned with car- the radial channels dissecting Tyrrhenus Mons have sized, rounded boulders (giving an appearance of cren- been modified by groundwater sapping processes. ulations at great distances). Modeling of possible pyroclastic flows from Tyrrhenus There is no clear evidence for fumerolic mounds Mons revealed that pyroclastic flows could likely trav- (or indurated mounds of any kind) within the Tyr- el the observed distances on Mars [1, 3]. rhenus Mons flank materials, nor do the mons materi- Hesperia Planum, which surrounds and embays the als contain jointing (Figure 1). It is important to note, eastern flank of Tyrrhenus Mons [1] is deformed by however, that a thick (>1 m) layer of dust covers the intersecting sets of mare-type wrinkle ridges [4, 5]. region (as observed in THEMIS daytime and night- The presence of these ridges has been used to interpret time images), and could be masking these fea- the Hesperia Planum materials as being composed of tures.Although Tyrrhenus Mons is dissected by flat- multiple layers of fluid lavas [e.g., 2, 6]. However, the floored radiating channels with amphitheater-shaped presence of wrinkle ridges only requires layered mate- heads, the channel walls are smooth and arcuate in rials. A comprehensive search of Hesperia Planum [7] planform—not crenulated. using 100 m/px Thermal Emission Imaging Spectrom- Fluid Lava Morphologies: There is an Amazoni- eter (THEMIS) images reveals no primaly volcanic an-aged lava flow field that emanates from the summit vents, but does reveal rare channels that have mor- caldera complex at Tyrrhenus Mons and extends for phologies more consistent with a lava-flow origin than ~1000 km to the southwest [1]. Lobate flow margins a fluvial genesis. and lava channels are easily identified in this flow However, it is frustrating to realize that for neither field, even though the flow field is covered with at Tyrrhenus Mons nor Hesperia Planum can we deter- least as much dust as are the Tyrrhenus Mons flank mine unequivocally the origin of their materials. Alt- materials. For comparison, the mons materials display hough the consensus is that Tyrrhenus Mons is com- no such features. Terrestrial flood basalts (such as the posed primarily of pyroclastic materials, the other pos- Columbia River and Deccan Traps) are characterized sibilities include fluid lavas or mudflow deposits. Sim- by a stair-step appearance on their eroded margins, ilarly, the consensus for the origin of Hesperia Planum revealing the alternating weak and strong layers of materials is that they are composed of fluid lavas, but it flow tops and flow interiors, respectively. On Earth, is difficult to definitively disprove layered sediments. individual flow layers are on the order of tens of me- Here, we present the preliminary results of our investi- ters thick [10] rather than the hundreds of meters re- gations into characterizing the erosional behaviors of vealed in the stair-step layers seen on the flanks of layered lava flow deposits and ignimbrite deposits on Tyrrhenus Mons. Earth for comparison with martian deposits. Thus, the materials composing the Tyrrhenus Mons Ignimbrite Morphologies: As of this writing, we materials do not appear to share obvious morphologic have focused our studies on the Bishop Tuff (Long similarities with terrestrial flood basalt provinces. Valley, California, USA) and various ignimbrites with- Discussion: The materials comprising Tyrrhenus in the Central Andes [e.g., 9 and references therein]. Mons are layered, with individual layers being tens to We have used Google Earth combined with the exist- hundreds of meters thick [e.g., 3, 11]. They extend for ing literature to characterize these deposits. almost 800 km from the summit caldera complex [1]. Investigation of THEMIS images, and images from the 45th Lunar and Planetary Science Conference (2014) 2326.pdf

Mars Reconnaissance Orbiter Context Camera [5] Goudy, C.L. et al., 2005, J. Geophys. Res. (http://www.msss.com/mro/ctx/ctx_description.html) 110(E10), doi: 10.1029/2004JE002293. [6] Greeley, and images from the High Resolution Imaging Science R. and Guest, J.E., 1987, USGS IMap 1802-B, Experiment (HiRISE) (http://hirise.lpl.arizona.edu/) 1:15,000,000. [7] Gregg, T.K.P., and Roberts, C., reveal no primary lava flow features (such as lobate 2011, GSA Abstracts with Programs 43(5):675. [A] [8] flow margins or lava channels) nor ignimbrite features Sheridan, M.F., 1973, Geol. Soc. Amer. Bull. (such as thermal contraction joints, fumerolic mounds, 81(3):851-868. [9] deSilva, S.L. et al., 2010, Planet. or crenulated erosion margins). Space Sci. 58:459-471. [10] Self, S. et al., 1997, in Am. In contrast, lava flows are readily identified in the Geophys. Union Mongraph 100:381-410. [11] Gregg, young lava flow field that emanates from the summit T.K.P. et al., 1998, USGS IMap -20252, 1:500,000. caldera complex of Tyrrhenus Mons. Thus, there is no a prior reason to think that lava flow fields on Mars should look significantly different from lava flow fields on Earth—even if they are significantly eroded (as are, for example, Earth’s Deccan Traps). Ignimbrites on Earth, however, are the result of an intimate mixture of eruption processes (magma com- position, eruption temperature, and mass eruption rate) and ambient conditions (interaction with non- magmatic water and ingestion of the ambient atmos- phere. Sheridan [8], for example, suggested that the fumerolic mounds observed on Long Valley’s Bishop Tuff were a product of internal ignimbrite degassing during cooling rather than escape of pre-existing vola- tiles heated by the overlying pyroclastic deposit. Giv- en Mars’ thinner atmosphere as compared with Earth, pyroclastic flows would not injest as much gas during emplacement [3] as would similar terrestrial pyroclas- tic flows. Without the ingested atmosphere, Mars’ pyroclastc density currents might not generate fum- erolic mounds or even suffer the same degree of con- traction as the ignimbrites cool and degas. Thus, there may be physical reasons why a martian ignimbrite might be less indurated, or modified differetly, from a similar pyroclastic density current on Earth. Pyroclas- tic deposits may therefore be much more common on Mars than has been previously recognized—including the materials comprising Hesperia Planum. Conclusions: We will continue to characterize the erosional morphologies of terrestrial ignimbrites and fluid lava flow fields on Earth as analogs to Mars. In the future, we will examine the erosional morphologies for mudflows as well. To date, our observations are consistent with the flank materials of Tyrrhenus Mons being composed primarily of pyroclastic deposits ra- ther than weathered, stacked lava flows, but have sig- nificant morphologic distinctions. References: [1] Greeley, R. and D.A. Crown, Figure 1. HiRISE image 1990, J. Geophys. Res. 95(B5):7133-7149, doi: EXP_017352_1590_RED.abrowse.jpg. Image width ~ 10.1029/JB095iB05p07133. [2] Greeley, R. and P.D. 5 km, centered on the northeast flank of Tyrrhenus Spudis, 1980, Rev. Geophys. Space Phys. 19(1):13-41. Mons at 107.3°E, 20.5°S. “Rilles” are volcanic or vol- [3] Gregg, T.K.P. and M.A. Farley, 2006, J. Volcanol. cano-tectonic features cross-cutting the flank (or mons) Geotherm. Res. 155(1-2):81-89. [4] Goudy, C.L. and materials of Tyrrhenus Mons. Image courtesy Gregg, T.K.P., 2002 LPSC XXXIII, Abstract #1135. NASA/PJL/LPL.