15. Volcanic Activity on Mars Martian volcanism, preserved at the surface, composition), (2) domes and composite cones, is extensive but not uniformly distributed (Fig. (3) highland paterae, and related (4) volcano- 15.1). It includes a diversity of volcanic land- tectonic features. Many plains units like Lu- forms such as central volcanoes, tholi, paterae, nae Planum and Hesperia Planum are thought small domes as well as vast volcanic plains. to be of volcanic origin, fed by clearly defined This diversity implies different eruption styles volcanoes or by huge fissure volcanism. Many and possible changes in the style of volcanism small volcanic cone fields in the northern plains with time as well as the interaction with the are interpreted as cinder cones (Wood, 1979), Martian cryosphere and atmosphere during the formed by lava and ice interaction (Allen, evolution of Mars. Many volcanic constructs 1979), or as the product of phreatic eruptions are associated with regional tectonic or local (Frey et al., 1979). deformational features. An overview of the temporal distribution of Two topographically dominating and mor- processes, including the volcanic activity as phologically distinct volcanic provinces on Mars well as the erosional processes manifested by are the Tharsis and Elysium regions. Both are large outflow channels ending in the northern situated close to the equator on the dichotomy lowlands and sculpting large units of the vol- boundary between the cratered (older) high- canic flood plains has been given by Neukum lands and the northern lowlands and are ap- and Hiller (1981). This will be discussed in proximately 120◦ apart. They are characterized this work together with new findings. Both by volcanoes, whose morphologies are strongly Plescia and Saunders (1979) and Neukum and analogous to basaltic volcanic landforms on Hiller (1981) gave a chronological classifica- Earth. The huge volcanoes in the Tharsis re- tion of the volcanic features on Mars. The gion (Olympus Mons, Ascraeus Mons, Pavonis difference between these two attempts is the Mons, and Arsia Mons) share many character- applied reference crater production function. istics with Hawaiian basaltic shield volcanoes Plescia and Saunders (1979) based their refer- (Carr, 1973). They are constructed from mul- ence crater production function on counts in tiple lobate flows of lava, generally show com- the Lunae Planum region. As has been demon- plex nested coalescing summit calderas of vary- strated by Neukum and Wise (1976) and Wise ing age, and gently slope on the order of a few et al. (1979a), strong resurfacing obscured the degrees. Their eruption style is effusive and of crater size–frequency distribution of this region. a relatively non–explosive nature. The main Therefore, such an ambiguous calibration curve differences between the Martian and terrestrial as used by Plescia and Saunders (1979) might volcanoes are the greater sizes and lengths of result in wrong stratigraphic relationships, if flows of the Martian volcanoes, mainly due to applied on a more global scale. higher eruption rates, the ”stationary” charac- Based on Mars Orbiter Laser Altimeter ter of the source (no plate tectonics) and the (MOLA) topographic data, Plescia (2004) reex- lower gravity. amined the dimensions and volumes of all ma- jor volcanic constructs of Mars and could cor- Plescia and Saunders (1979) have summa- rect the earlier findings of the post–Viking era. rized the chronology and classification of Mar- During the first two years of the ESA Mars Ex- tian volcanic activity based on the Viking im- press mission, the High Resolution Stereo Cam- agery data. They grouped the volcanic land- era imagery data provides the first opportu– forms into (1) shield volcanoes (to be of basaltic 121 122 Volcanic Activity on Mars Figure 15.1.: The volcanic provinces of Mars are indicated in yellow. Known central vent constructs are displayed inred. The individual volcanoes are assigned in separate maps. The topographic dichotomy is shown in blue for the lowland parts and green for the highland parts. Large impact structures are shown in white. The background topography is given as a MOLA shaded–relief map. 123 nity for detailed insights into the morphology canic episodes (e.g. highland paterae) is man- and topography of these volcanoes and their ifested in the observation of phreatomagmatic chronostratigraphic evolution. interaction e.g. at Tyrrhena Patera or at the Following the mapping of Spudis and Greeley flank base of western Elysium Mons (Wilson (1977) and by Scott and Carr (1978), a synthe- and Mouginis-Mark, 2003). The coincidence sis of their results indicate that as much as 60% of the Hellas basin formation and the accumu- of the surface is covered with volcanic materi- lation of highland paterae has been noted by als. The primary focus outlined in the following Greeley and Spudis (1981), but we find that at sections is on the evolution of the large volcanic least the later–stage patera activities were not provinces Tharsis and Elysium, accompanied triggered by the impact event itself. by most other volcanic regions on Mars. An Following the interpretation of the volcanic additional goal is to understand the interacting history outlined by Greeley and Spudis (1981) processes of erosion and deposition (related to based on the Viking imagery, the plateau plains volcanic and fluvial processes) with respect to activity is followed by massive flood volcanism, new and old findings. which resurfaced large areas such as Lunae or Based on the observation of superposition, Hesperia Plana and huge amounts of the Mar- cross–cutting relations, and, if available, on the tian lowland areas. Massive volcanic constructs number of superposed impact craters, Gree- such as the Tharsis rise, notably Olympus Mons ley and Spudis (1981) first described the vol- and Alba Patera, cover the dichotomy start- canic history of Mars. To understand the vol- ing at least 3.8 Ga ago (the age of the aureole canic evolution, caution must be given to the has been recalculated from Neukum and Hiller fact that the amount of volcanics represented (1981)) and continuing to about 3.5 Ga (Alba by the enveloping youngest layer on top of Patera, see Section 15.1.1). The presence of the the stratigraphic sequence and sometimes the Aureole around Olympus Mons and the absence crater size–frequency distribution reveal an ear- of such a feature around Alba Patera might in- lier phase(s) by an embayed or flooded crater dicate a changed environmental situation. The population. timing and existence of a Martian ocean in the The oldest unit considered in this investiga- northern lowlands is discussed (Chapter 14.3) tion are the highland plateau units (e.g. Wil- and constrained temporally by the surface ages helms, 1974). We found the oldest highland– of the two differently appearing flank bases. plains units to be about 4.1 Ga old (Noachis Both Martian volcanic centers are domi- Terra, see Chapter 17), which is roughly the nated by central vent volcanoes and surround- time of emplacement of the largest Martian ing plains–forming flows. Fracturing (graben basins followed by the emplacement of so–called formation) is related to the early structural inter–crater plains 3.9 Ga ago. Most of the uplift of the volcanic rises, although could be highland units range between 4.1 Ga and 3.9 caused by younger volcanic activity (see be- Ga (see e.g. Chapter 17 or 13.2), roughly the low). While Greeley and Spudis (1981) found decaying end of the heavy bombardment pe- an agreement with a moon–like thermal history, riod. At that time, the erosional scarp of the di- we will show a more diverging evolutionary his- chotomy boundary between the highlands and tory of Mars. lowlands most likely formed as has been sug- All major volcanic constructs including Pa- gested already by Zuber et al. (2000). Never- terae and Tholi have been imaged in the first theless, whatever caused the formation of the period of the ESA Mars Express mission. The dichotomy escarpment, subsequent resurfacing ability to image simultaneously in color and acted differently along the boundary (see Chap- stereo gives us the new opportunity to better ter 14.2). The temporal overlap of extensive characterize the geomorphology and chronos- fluvial activity (e. g. valley networks) and vol- tratigraphy of most volcanoes in the Tharsis 124 Volcanic Activity on Mars and Elysium region and most highland volca- noes. We have remapped major parts of the volcanic shields and calderas on the basis of the high–resolution HRSC imagery in color and stereo, in combination with nested MOC im- agery and the Super Resolution Channel (SRC) (as good as 2.5 m/pixel) of the HRSC. 15.1. The Tharsis Volcanic Province The Tharsis region is the most dominating fea- ture of the Martian topography and shows nu- merous volcanic constructs of different age and morphology: 15.1.1. Alba Patera Figure 15.2.: The MOLA shaded–relief map of Al- bor Tholus. The northernmost is the fascinating volcano Alba Patera. It is an enormous volcanic shield with a base diameter of roughly 1100 km, more than the gigantic Olympus Mons. Compared to they found ages indicating at least four episodes Olympus Mons, it is a wide but low–relief con- of activity: about 3.4 Ga, 2 Ga, 800 Ma ago struct of about 6 km in height, with flank slopes and as recent as 250 Ma ago. In our mea- of about 1◦, and one of the largest calderas surements based on Viking and HRSC data, found on Mars (diameter of about 120 km). we could confirm the maximum age of about The surrounding flank grabens (Tantalus and 3.5 Ga (see Chapter 14.1). These results were Alba Fossae) extend in a North–South direc- based on Viking imagery measurements taken tion.