Palagonite is a form of volcanic glass similar to obsidian but with a chemical composition more closely related to basalt. Palagonite results from the interaction between water and basalt melt. The water flashes to steam on contact with the hot lava and the small fragments of lava react with the steam to form the light colored palagonite tuff cones common in areas of basaltic eruptions in contact with water. An example is found in the pyroclastic cones of the Galapagos Islands. Charles Darwin recognized the origin of these cones during his visit to the islands.
Palagonite has also been observed in the regolith of Mars. The spectroscopic signature of palagonite alteration on Mars is used as evidence for the existence of water on Mars.
Palagonite tuff is a tuff composed of sideromelane fragments and coarser pieces of basaltic rock, embedded in a palagonite matrix. A composite of sideromelane aggregate in palagonite matrix is called hyaloclastite.