Current Research in EEPS: Dr. Hap McSween, University of Tennessee at Knoxville
Rocks on Mars: Igneous, Sedimentary, and Metamorphic
The compositions of rocks on Mars have been inferred from orbital remote-sensing measurements, chemical and mineralogical analyses by rovers, and laboratory study of martian meteorites. The picture that emerges is admittedly fragmentary and chronologically biased, but it provides enough clarity to recognize familiar geologic processes as well as some surprises. Some basic conclusions from these studies are:
- Nearly ubiquitous basaltic magmas, which evolved from alkaline to sub-alkaline compositions over time, formed by decompression melting in a secular convectively cooling, compositionally heterogeneous mantle.
- Felsic/feldspathic igneous rocks, formed by fractionation of mafic plume magmas, are uncommon, but may have been more abundant in early Mars.
- Clastic sedimentary rocks formed from mostly basaltic protoliths and were weathered under acidic, low-water conditions resulting in geochemical trajectories that differed from weathering on Earth.
- Clays vary in composition, depending on whether they formed by surface weathering or subsurface hydrothermal processes, and evaporites are mostly sulfates.
- Metamorphic rocks are primarily rocks altered by impact shock, or thermally metamorphosed rocks formed in low-pressure hydrothermal systems, likely associated with large impact craters.
- Without plate tectonics, Mars has not had flux melting or high-pressure thermal metamorphism associated with subduction, nor deposition in subsided basins and rapid erosion from tectonic uplift; in more recent times the martian rock cycle was truncated by global desiccation and a lower geothermal gradient.