chasmas

Chasma is a term used in planetary geology to describe a long, wide, and deep canyon or trough on the surface of a planet or moon. The word is typically applied to extensive, fault-bound depressions that contrast with smaller, more regularly shaped canyons. The standard plural in formal naming is chasmata (Latin), though the anglicized form chasmās or even chasmas is sometimes encountered in less formal writing.

Formation and morphology

Chasmata usually form through tectonic extension that thins and fractures the crust, creating graben-like troughs. They may widen and deepen as block subsidence continues and as later processes such as erosion, collapse, or volcanic activity modify the walls and floor. Morphologies range from relatively straight, fault-bounded troughs to complex networks that intersect other faults and form stair-stepped terraces or layered exposures along the walls.

Distribution and examples

Chasmata are prominent on several planetary bodies, most notably Mars, where they occur within large canyon systems. The Valles Marineris system comprises multiple chasmata, including Coprates Chasma, Ganges Chasma, Tithonium Chasma, and Ius Chasma, extending across a major portion of the planet’s equatorial region. Similar features are observed on other planets and moons, underscoring the role of tectonics in shaping crusts beyond Earth.

Earth usage

On Earth, chasma is rarely used in common geological language, with most large canyons and rift-related features described as canyons, gorges, or rift valleys. When applied, chasma conveys a sense of a long, tectonically controlled trough rather than a simple canyon.