SCHIST
Schist (pronounced /ʃɪst/ SHIST) is a medium-grade metamorphic rock[1] with medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. These lamellar (flat, planar) minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite. Geological foliation (metamorphic arrangement in layers) with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.
The names of various schists are derived from their mineral constituents. For example, schists rich in mica are called mica schists and include biotite or muscovite. Most schists are mica schists, but graphite and chlorite schists are also common. Schists are also named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, and glaucophane schist.
The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen with the naked eye. Schist is characteristically foliated, meaning that the individual mineral grains split off easily into flakes or slabs. The word schist is derived ultimately from the Greek word σχίζειν schízein meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie.
Most schists are derived from clays and muds that have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists are derived from fine-grained igneous rocks such as basalts and tuffs.
METAMORPHIC PROCESSES
Metamorphism refers to all the transformations undergone by a rock (sedimentary, magmatic or metamorphic) under the effect of changes in temperature, pressure, the nature of the fluids and sometimes the chemical composition of the rock. These transformations, which can be mineralogical, textural, chemical or structural, lead to a reorganization of the elements in the rock and to a recrystallization of the minerals in the solid state.
The upper limit of metamorphism, situated under still relatively low temperature and pressure conditions, separates the latter from the diagenesis. This limit being ill-defined, the fields of metamorphism and diagenesis are differentiated according to crystallographic criteria, notably by the use of the crystallinity of illite. The lower limit of metamorphism separates it from magmatism and is located in high temperature conditions. Metamorphism is distinguished in principle from magmatism by the fact that it is characterized by solid state recrystallization processes, whereas magmatism involves the participation of a silicate liquid, requiring higher temperatures. However, some areas of metamorphism under conditions of high temperatures cause the partial melting of certain minerals and the creation of silicated liquids of often granite composition, then called anatexia. In the case where these liquids crystallize within the rock that gave birth to them, the result is the appearance of migmatites which belong to the domain of metamorphism.
One of the most direct characterizations of metamorphism is the mineralogical transformation of the rock (sometimes with a change in the chemical composition, in particular by the addition or departure of fluids). Moreover, deformations commonly accompany metamorphism, in particular regional metamorphism, insofar as conditions bearing rocks at high pressures and / or temperatures are commonly associated with important constraints, as in the case of collision chains. This is the reason why the study of deformations (schistosity, foliation, lineation) is commonly included in the study of metamorphism.
IGNEOUS ROCK
Igneous rock (derived from the Latin word ignis meaning fire), 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. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses.
You will find the answers at low tide.
QUESTIONS:
1- Observe the rocks and indicate the cracks orientation?
2- Explain the formation of these rocks on site?
3- Describe to touch the texture?
Found it and send me your answers proposals either via my profile or via geocaching.com messaging (Message Center), and I will contact you in case of problems.