Case hardening is a weathering phenomenon of rock surface induration. It is observed commonly in: felsic alkaline rocks, such as nepheline syenite, phonolite and trachyte; pyroclastic rocks, as pyroclastic flow deposit, fine air-fall deposits and vent-filling pyroclastic deposits; sedimentary rocks, as sandstone and mudstone.
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 feldspars and feldspathoids 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. [1] The physical weakness of the subsurface in comparison with the surface is called core softening as exemplified by the tonalite at Catavina, Baja California. [2]
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. [3] Sandstone from the Valley of Fire, Nevada, is usually cemented by calcite but the calcium borate mineral, colemanite, has also been found in some cases to act as a cement. [4] A similar phenomenon is observed in felsic alkaline rocks, such as nepheline syenite, alkaline syenite, phonolite, and trachyte, because of weathering vulnerability of nepheline and alkaline feldspar. [5] 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. [6]
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. [5]
In geology, felsic is an adjective describing igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.
Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations. Some syenites contain larger proportions of mafic components and smaller amounts of felsic material than most granites; those are classed as being of intermediate composition. The volcanic equivalent of syenite is 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 alkali metals. It is the volcanic equivalent of syenite.
Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic. Its intrusive equivalent is nepheline syenite.
Basanite is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and plagioclase and forms from magma low in silica and enriched in alkali metal oxides that solidifies rapidly close to the Earth's surface.
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 primary quartz. A notable exception where feldspathoids and quartz-bearing rocks are found together is the Red Hill Syenite.
Volcanic rock is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.
Nepheline, also called nephelite (from Ancient Greek νεφέλη (nephélē) 'cloud'), is a rock-forming mineral in the feldspathoid group – a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites. It is used in glass and ceramic manufacturing and other industries, and has been investigated as an ore of aluminium.
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 also known. Phonolite is the fine-grained extrusive equivalent.
Theralite is, in petrology, the name given to calcic foidal gabbro, a plutonic hylocrystalline rock consisting of augite, olivine, calcic plagioclase (labradorite), and nepheline, along with accessories including biotite, magnetite, ilmenite and analcime.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.
Litchfieldite is a rare igneous rock. It is a coarse-grained, foliated variety of nepheline syenite, sometimes called nepheline syenite gneiss or gneissic nepeheline syenite. Litchfieldite is composed of two varieties of feldspar, with nepheline, sodalite, cancrinite and calcite. The mafic minerals, when present, are magnetite and an iron-rich variety of biotite (lepidomelane).
Leucitite or leucite rock is an igneous rock containing leucite. It is scarce, many countries such as England being entirely without them. However, they are of wide distribution, occurring in every quarter of the globe. Taken collectively, they exhibit a considerable variety of types and are of great interest petrographically. For the presence of this mineral it is necessary that the silica percentage of the rock should be low, since leucite is incompatible with free quartz and reacts with it to form potassium feldspar. Because it weathers rapidly, leucite is most common in lavas of recent and Tertiary age, which have a fair amount of potassium, or at any rate have potassium equal to or greater than sodium; if sodium is abundant nepheline occurs rather than leucite.
This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava.
Monte Muambe is volcanic caldera located south-east of Moatize in Tete Province of Mozambique
Rutan Hill is the local name for a hill on the United States Geological Survey Branchville 7.5-minute map. It is located about 2.46 miles (3.96 km) south-southwest of Colesville, New Jersey in the Wantage Township, of Sussex County, New Jersey in the United States. Rutan Hill rises about 270 feet (82 m) above the adjacent creek valley to an elevation of just over 1,020 feet (310 m). This hill lies entirely within private, posted property. This nondescript hill is the surface expression of a diatreme that is the northern part of the Late Ordovician Beemerville Alkaline Complex.
São Tomé and Príncipe both formed within the past 30 million years due to volcanic activity in deep water along the Cameroon line. Long-running interactions with seawater and different eruption periods have generated a wide variety of different igneous and volcanic rocks on the islands with complex mineral assemblages.
The Red Hill Syenite is a layered igneous rock complex in central New Hampshire, about 20 mi (32 km) east of Plymouth. The Red Hill Syenite is part of the White Mountain magma series, which underlays the White Mountains of New Hampshire. Red Hill is roughly oval-shaped, covers just under 7.7 square miles (20 km2), and has a summit elevation of 2,028 feet (618 m).