Duricrust

Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few millimeters or centimeters to several meters.

Duricrust
A duricrust inselberg near Dori, Burkina Faso
Key minerals	Soluble minerals
Climate	Arid
Primary	Various
Secondary	Various

It is a general term (not to be confused with duripan) for a zone of chemical precipitation and hardening formed at or near the surface of sedimentary bodies through pedogenic or non-pedogenic processes. It is typically formed by the accumulation of soluble minerals deposited by mineral-bearing waters that move upward, downward, or laterally by capillary action. It is commonly assisted in arid settings by evaporation.^[1][2] There are different types of duricrusts, each distinguished by a dominant mineralogy. For example, ferricrete (laterite) is dominated by sesquioxides of iron; alcrete (bauxite) is dominated by sesquioxides of aluminum; silcrete by silica; calcrete (caliche) by calcium carbonate, and gypcrete (gypcrust) by gypsum.^[1]

Duricrust is often studied during missions to Mars because it may help prove that the planet once had more water. Duricrust was found on Mars at the Viking 2 landing site, and a similar structure, nicknamed "Snow Queen", was found under the Phoenix landing site.^[3] Phoenix's duricrust was later confirmed to be water-based.^[4]