MARBLE ROCKS

Marble is metamorphosed limestone, composed of very pure calcium carbonate, CaCO3. The softness of marble and its relative isotropy and homogeneity make marble very desirable for sculpture and building stone. Although marble comes from limestone, the temperatures and pressures necessary to form marble usually destroy any fossils that may have been present in the limestone.

Kinds of marble:

* Carrara (Italy)
* Pentelicus (Greece)
* Proconnesus (Turkey)

Carrara marble is prized for sculpture.

In the building trades, the term "marble" is used for any massive, crystalline calcitic rock (and some non-calcitic rocks) useful for building stone. For example, Tennessee Marble is really a massive, highly fossiliferous gray to pink to maroon Ordovician dolostone, known as the Holston Formation by geologists.

Marble derives its name from the Greek marmaros, shining stone. [source: OED2]. This stem is also the basis for the English word marmoreal meaning "marble-like".

In folklore, marble is associated with the astrological sign of Gemini. Pure white marble is an emblem of purity. It is also an emblem of immortality, and an ensurer of success in education.

METAMORPHIC ROCKS GENERAL

Metamorphic rocks result from mineralogical and structural adjustments of solid rocks to physical and chemical conditions differing from those under which the rocks originally formed. Changes produced by surface conditions such as compaction are usually excluded. The most important agents of metamorphism are temperature, and pressure. Equally as significant are changes in chemical environment that result in chemical recrystallization where a mineral assemblage becomes out of equilibrium due to temperature and pressure changes and a new mineral assemblage forms.

Three types of metamorphism may occur depending on the relative effect of mechanical and chemical changes. Dynamic metamorphism, or cataclasis, results mainly from mechanical deformation with little long-term temperature change. Textures produced by such adjustments range from breccias composed of angular, shattered rock fragments to very fine-grained, granulated or powdered rocks with obvious foliation and lineation termed mylonites. Contact metamorphism occurs primarily as a consequence of increases in temperature where differential stress is minor. A common phenomenon is the effect produced adjacent to igneous intrusions where several metamorphic zones represented by changing mineral assemblages reflect the temperature gradient from the high-temperature intrusion to the low-temperature host rocks; these zones are concentric to the intrusion. Because the volume affected is small, the pressure is near constant. Resulting rocks have equidimensional grains because of a lack of stress and are usually fine-grained due to the short duration of metamorphism. Regional metamorphism results from the general increase of temperature and pressure over a large area. Grades or intensities of metamorphism are represented by different mineral assemblages. Regional metamorphism can be subdivided into different pressure-temperature conditions based on observed sequences of mineral assemblages. It may include an extreme condition, where partial melting occurs, called anatexis.

Other types of metamorphism can occur. They are retrograde metamorphism, the response of mineral assemblages to decreasing temperature and pressure; metasomatism, the metamorphism that includes the addition or subtraction of components from the original assemblage; poly-metamorphism, the effect of more than one metamorphic event; and hydrothermal metamorphism, the changes that occur in the presence of water at high temperature and pressure which affect the resulting mineralogy and rate of reaction.

METAMORPHIC ROCKS MORE DETAILS

Metamorphic rock are the result of the transformation of a pre-existing rock type, the protolith, in a process is called metamorphism. The nature of the protolith can be either sedimentary, igneous or older metamorphic rocks.

Metamorphic rocks can be classified according texture or mineral assembly (metamorphic facies).

Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i.e. without melting, in response to new conditions of pressure and/or temperature, and/or introduction of fluids.

Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.

Limits of metamorphism

The temperature lower limit of metamorphism is considered to be between 100-150°C, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. There is no agreement as for a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic. But, see below, some types of metamorphism can occur at extremely lower pressures.

The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The temperature interval is between 700-900°C, with pressures that depend on the composition of the rock. Migmatites are rocks formed on this borderline. They present both melting and solid-state features.

Kinds of metamorphism

Regional metamorphism

This type of metamorphism occurs over broad areas of the Earth's crust. Regionally metamorphosed rocks are originated in the core of mountain belts, formed during an orogenic event. These mountains are later eroded exposing the metamorphic rocks. Regional metamorphic rocks are usually strongly deformed. Structural geology is largely based in studies of these rocks because they contain useful information. Regional metamorphism can be described metamorphic zones. It can also be separated into Barrovian- or Buchan-type metamorphism, according to the pressure-temperature gradients recorded in the rocks.

Contact metamorphism

Contact metamorphism occurs typically around igneous intrusive rocks, as a result of the temperature increased caused by the igneous body. Pressures are usually low because the contrasting temperature effect is more effective at shallow crustal depths. The area surrounding the igneous rock where the contact metamorphism effects are present is called metamorphic aureole. As expected, the contact metamorphism effects are greater in the vicinity of the intrusive rock and fade away to the exterior of the aureole. Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Rocks formed by contact metamorphism do not present signs of strong deformation and are usually fine grained. Contact metamorphic rocks are usually known as hornfels. Skarns are another example of contact metamorphism and can have great economic interest.

Hydrothermal metamorphism

Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of distinct composition. The difference in composition between protolith and fluid triggers a set of methamorphic reactions. This kind of metamorphism is responsible for many economic metal deposits. Convection circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism.

Impact metamorphism

This kind of metamorphism occurs when an extraterrestrial object (a meteorite for instance) collides with the Earth's surface, or, during an extremely violent volcanic eruption. Impact metamorphism is, therefore, characterized by ultrahigh pressures conditions and low temperature. The resulting minerals (such as SiO2 polymorphs coesite and stishovite) and textures are characteristic of these conditions.

CLASSIFICATION OF ROCKS