In geology, metamorphic rock is formed when existing rock formations are exposed to great heat, pressure or chemical interaction for an extended period. Because its elemental composition remains essentially the same, the type of metamorphic rock that forms will depend on the rock from which it was formed and the temperature to which it was exposed.
Metamorphic rock formations can often be found adjacent to intrusive igneous rock, where the cooling magma that formed the intrusive rock heated rock that had already existed in the area.
- Some graphite formations formed from coal, though this is unlikely to have happened on Mars. (Igneous and meteoric graphite should be present.)
- Marble forms from limestone, though some limestones are from biological sources and unlikely to exist on Mars.
- Gneiss forms from granite.
- B. Cairncross - Field guide to rocks and minerals of Southern Africa 2004. ISBN 978-1-86872-985-2 p. 260-261