Similar to Earth and Moon, Mars demonstrates a dichotomy. This dichotomy is based on compositional and geologic discrepancies as mafic rocks differ in the northern lowlands and southern highlands [Bandfield et al., 2001]. These mafic rocks are analogous to Terrestrial andesites and basalts, which are common Terrestrial volcanic rocks. This analogy implies the same bulk volcanic compositions and conditions existed on both planets.
Andesite is an intermediate volcanic rock (Fig. 1, 2) that is common at subduction zones such as the Andes. It is mainly composed of plagioclase, pyroxene, and amphibole [Matthes, 2001]. Contrary to basalt, it bears higher silica- but a lower mafic-content. Its melting temperature is lower and the viscosity is higher due to increased silica content. Basalt is a basic volcanic rock that consists of 40-70 Vol. % mafics, which is mainly pyroxene (in parts olivine) and felsic minerals like anorthitic plagioclase. It is present at subsurface and submarine lava flows (Island arc basalts IAB, mid-ocean-ridge basalts MORB, continent margins), rift zones, as plateau basalts, and at the Lunar surface.
Bandfield et al. [2000] classified the Martian surface into two general spectral classes ofvolcanic rocks (Fig. 1):
Surface Type 1 dominates in the ancient Martian southern highlands whereas Surface Type 2 is concentrated in the younger northern lowlands. This potentially indicates that basaltic volcanism is restricted to the past and andesitic volcanism to times that are more recent. Considering the basaltic Surface Type 1 detected by TES, there is a general correlation with pyroxene-rich regions found by OMEGA [Poulet et al., 2007].
Both surface types represent two distinct compositions that are common in hot-spot, flood volcanism, and rifting environments on Earth. These compositions are different to those of Lunar basalts or Martian meteorites [Bandfield et al., 2000]. Furthermore, as there is no evidence for Martian plate tectonics, andesite must have formed differently than is common on Earth along subduction zones.
References:
Last update: 31/05/2010 15:56