Volcanism on Mars

More than 15 years of planetary exploration of Mars have given insight into the geologic processes that have shaped its surface. The newly acquired Viking data have shown that volcanism is one of the most important geologic processes operating on Mars throughout its history. In situ chemical analyses of Martian soil by the Viking lander spacecraft indicate mafic to ultramafic source rocks. This is consistent both with available remote sensing data, which indicate the presence of mafic minerals such as pyroxene and olivine, and with petrologic modeling, based on available geophysical data which suggest that Martian lavas are probably iron rich and ultramafic. These data strongly suggest that basaltic volcanism is widespread on Mars, and much of the photogeological data may be studied in this context. Photogeological analysis of the Martian surface has shown two main types of volcanic morphologies: the first type is central volcanoes, which are volcanic landforms developed by continued and prolonged eruption from a point source vent. This category includes (1) shields, the classic low‐profile volcanic mountains of which Olympus Mons is the most spectacular example, (2) domes, steep‐sided constructs, such as Tharsis Tholus, that may represent lower rates of eruption than the shields or, possibly, more silicic lava compositions, (3) highland patera, radially textured low‐profile volcanoes that occur in the cratered terrain and are interpreted as ash shields, (4) Alba Patera, an apparently unique volcanic landform consisting of a vast volcanic center over 1500 km across with flank slopes of less than a tenth of a degree, and (5) various small features such as cinder cones. The second major category is volcanic plains, which are units recognized by several criteria, of which the presence of mare ridges and flow lobes are the most useful. Volcanic plains are subdivided into four main groups: (1) simple flows, broad, smooth to rolling plains that contain numerous mare‐type ridges but no flow lobes, interpreted as being composed of thick, single‐cooling units, (2) complex flows, displaying multiple overlapping flow lobes interpreted to be indicative of thin, multiple‐cooling units, (3) undifierentiated flows, plains that typically lack any morphologic identifying feature but are considered to be volcanic partly on the basis of their association with large volcanic centers, and (4) questionable plains, volcanic(?) units heavily modified by other processes (erosion, tectonism, etc.) so that their origins are uncertain. When these categories of volcanic morphologies are combined with relative age data provided by crater statistics, a volcanic history for Mars can be derived as follows: Early heavy bombardment of Mars was accompanied and followed by small‐scale fluvial channeling, extensive flood volcanism (the plateau plains), and ash shield volcanism in the cratered terrain. Shortly after this time, less extensive flood volcanism continued to resurface the planet during formation of the northern/southern hemisphere dichotomy. Central volcanism became more prominent with the development of the Alba Patera center as well as the older shields and domes of the northern hemisphere (early Tharsis and Elysium regions). The development of the Tharsis and Elysium uplifts may have triggered the release of large‐scale catastrophic floods, producing large channels. Continued uplift and lithospheric thinning concentrated volcanic activity in the Tharsis region, producing large shield volcanoes and extensive lava plains. Both central vent and plains volcanism have been active throughout Martian history, but the volumes of extrusion have gradually decreased with time. This is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time, gradually ‘turning off’ the volcanism. Although many questions remain regarding Martian volcanism, the Viking data have provided a remarkable, detailed overview of the probable nature of the volcanic history of Mars.