Case hardening is a

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...

phenomenon of rock surface

induration A skin condition, also known as cutaneous condition, is any medical condition that affects the integumentary system—the organ system that encloses the body and includes skin, nails, and related muscle and glands. The major function of this ...

. It is observed commonly in: felsic alkaline rocks, such as

nepheline syenite Nepheline syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are al ...

, phonolite and

trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...

;

pyroclastic rock Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

s, as

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

deposit, fine air-fall deposits and vent-filling pyroclastic deposits;

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s, as

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

and mudstone.

Weathering process

Chemical weathering alters the minerals constituent of rock surface. Decomposition of mafic and opaque minerals releases ions and colloids of iron, magnesium, calcium and sulphur. Alteration of

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

s and

feldspathoid The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing prim ...

s releases silica colloid. These materials are reached and transported by surface water. The remnant materials are highly aluminous and siliceous. They could have certain mechanical firmness of own minerals, however no cohesion. Therefore, physical disintegration of the rock takes place to form the surface. In certain cases, the weathered surface obtains mechanical firmness higher than subsurface. The reached materials dissolved in the surface infiltrates in the weathered surface and cement the silica-aluminous remnant materials. The surface induration by means of this process is named case hardening. The physical weakness of the subsurface in comparison with the surface is called core softening as exemplified by the

tonalite Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total ...

at Catavina, Baja California.

Natural occurrence

Case hardening takes place in various types of rocks, being notable in porous ones, for example sandstone, quartzite and volcanic ash. The cement effect by silica observed in sandstone and quartzite is prominent. Sandstone from the

Valley of Fire Valley of Fire State Park is a public recreation and nature preservation area covering nearly located south of Overton, Nevada. The state park derives its name from red sandstone formations, the Aztec Sandstone, which formed from shifting sa ...

, Nevada, is usually cemented by calcite but the calcium borate mineral, colemanite, has also been found in some cases to act as a cement. A similar phenomenon is observed in felsic

alkaline rock In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a so ...

s, such as nepheline syenite, alkaline syenite, phonolite, and trachyte, because of weathering vulnerability of nepheline and alkaline feldspar.Motoki, A., Soares, R., Lobato, M., Sichel, S.E., Aires, J.R. 2007. Feições intempéricas em rochas alcalinas félsicas de Nova Iguaçu, RJ. Revista Escola de Minas, 60-3, 451-548 The case hardening on trachytic clasts of volcanic breccia shows peculiar fabrics. The Mars scientific exploring machine Spirit has observed case hardening of the rock surface at Gusev meteorite Crater. This phenomenon is attributed to the weathering by means of surface water, being considered to be an evidence of liquid water in a far past on that planet.Farmer, J.D. 2005. Case-hardening of rocks on Mars: evidence for water-mediated weathering processes. Abstracts of annual meeting of the Geological Society of America, Salt Lake City, paper 223-5, CD.

Mineral dissociation

On the rock surface in the case hardening of alkaline felsic rocks, the weathering selectively affects certain minerals. Nepheline is very sensitive to weathering and is altered to natrolita and cancrinite. On the outcropping surface of a nepheline syenite, phonolite or nepheline syenite gneiss, the minerals formed by nepheline alteration appear white in colour on the dark-coloured weathered background. After the nepheline alteration products are leached, small holes are formed on the rock. A similar phenomenon takes place also in selective alteration of alkaline feldspar. This mineral is more resistant than nepheline. However, water infiltrates along cleavage planes and the weathering advances easily up to the mineral core. Because of this phenomenon, physical disintegration of alkaline feldspar takes place. The selective elimination of nepheline and alkaline feldspar is named mineral dissociation. When this phenomenon is highly advanced, the altered rock surface show the fabric similar to vesicular texture, called pseudovesicular structure.

References

{{DEFAULTSORT:Case Hardening Of Rocks Weathering Geological processes