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)
marble is prized for sculpture.
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.
derives its name from the Greek marmaros, shining stone. [source:
OED2]. This stem is also the basis for the English word marmoreal
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
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.
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.
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
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.
rocks can be classified according texture or mineral assembly
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
produced with increasing pressure and temperature conditions is
known as prograde metamorphism. Conversely, decreasing temperatures
and pressure characterize retrograde metamorphism.
Limits of metamorphism
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.
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
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
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.
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.
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
CLASSIFICATION OF ROCKS