Lexikon
The metamorphic rock is the result of the transformation of an existing rock type in a process called metamorphism, which means "change in form". The metamorphic rock is any rock derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing stress, and chemical environment .The metamorphic rocks can be derived from sedimentary, igneous or another older metamorphic rock after having been subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change. Metamorphic rocks make up a large part of the Earth's crust and are classified and named by texture, by chemical and mineral assemblage and by the characteristics of the preexisting rock. The mineral compositition of the metamorphic rocks depends on the composition of the preexisting rock, according to which the metamorphic rocks are grouped as: ultramafic, mafic, pellitic (clayee), carbonate bearing, quartz and quartz-feldspar containing rocks. Most metamorphic rocks are named based on their texture and structural features, and their mineralogy. The first step in naming a metamorphic rock is to always identify whether the rock is foliated or non-foliated. Among the foliated textures, metamorphic rocks are further classified based on how the minerals within the parent rock are affected by the changes in temperature and pressure. In general terms, increases in temperature and pressure result in different, more complex textures, and different foliated metamorphic rocks: foliated (slate, phyllite, schist, gneiss) and non-foliated. Non-foliated metamorphic rocks are usually named exclusively on the basis of mineral composition (hornfels, amphibolite, quartite). A close examination of the non-foliated rock will reveal the dominant one or two minerals within the rock. In some cases one may choose some prefix-type modifiers to attach to names to stress some important or unusual textural or mineralogical aspects. For example an “ortho”- prefix indicates an igneous parent, and a “para”- prefix indicates a sedimentary parent (orthogneiss, paragneiss). Metamorphic rocks are characterised by typical mineral assemblages according to the preexisting rock and the intensity of metamorphism. The intensity of metamorphism and the relevant metamorphic rocks are shown below:
- Very low intensity metamorphism (shale, metabasalt). Typical minerals: clay minerals, laumontite (zeolit), prehnite (zeolit).
- Low intensity metamorphism (serpentinite, chlorite shale, sericite shale). Typical minerals: sericit, pyrophyllite, chlorite.
- Medium intensity metamorphism (mica schist, marble). Typical minerals: mica, quartz, plagioclase, garnite, andalusite.
- High intensity metamorphism (eclogite, gneiss). Typical minerals: quartz, muscovite, biotite, plagioclase, kaliumfeldspar, sillimanite, staurolite.
plutonic rocks (also called intrusive igneous rocks) resulted from magmas solidified below ground. When magmas crystallize deep underground they look different from volcanic rocks because they cool more slowly and, therefore, have larger crystals. Igneous rocks cooled beneath the Earth's surface are called intrusive rocks. The intrusive equivalents of basalt, andesite, and rhyolite are gabbro, diorite, and granite, respectively. In Hungary the Velencei mountain is composed of intrusive igneous rocks such as, granodiorite and diorite and the granite block in Mórágy was formed in similar conditions.
The underground crystallization stages of the magma are the following:
A. Preliminary crystallization stage (approx. 1100–1000 °C)
During the preliminary crsystallization stage ultrabasic and basic rocks are formed. The temperature decrease results separation of the silicate and sulphide melts. The preliminary crystallization gives economically important ore deposits: chromite, magnetite, ilmenite, platina, diamond and apatite.
B. Main crystallization stage (approx. 1000–700 °C)
In the main crystallization stage the magma solidification occurs. The olivine, pyroxene, amphiboles and the feldspars crystallize in parallel and finally the quartz.
C. Post magmatic stage (from approx. 700 °C)
The volatile containing residual magma is crystallised in this phase. The post magmatic stage includes three phases:
Pegmatite phase (approx. 700–550 °C): The mineral composition of the pegmatites crystallized in this phase is identical with that of the main crystallization phase however the pegmatites contain much larger crystals. The pegmatites in general occur in veins and are rich in rare elements such as stanium, uranium, thorium, boron, lithium, berillium, zirconium, titanium, tanthal.
Pneumatolitic phases (approx. 550–375 °C): The halogene rich solutions are chemically very active and thus are able to considerably modify the solidified rocks. This phase results various minerals such as quartz, fluorite, wolframite, turmaline.
Hydrothermal phase (from approx. 375 °C): The water diluted, solutions of the residual magma penetrated the cracks, voids of the rocks forming hydrothermal veins. During the hydrothermal phase mainly the following metals are concentrated: gold, silver, copper, lead, zinc, mercury and the iron, cobalt and nickel remained still in the residual solution.
in geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids. The Earth's outer solid layer, the lithosphere, is made of rock. The scientific study of rocks is called petrology. Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Rocks are classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary and metamorphic. igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic igneous rocks and volcanic rocks. sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation). Petrographic descriptions start with the field notes at the outcrop and include megascopic description of hand specimens. The macroscopic characters of rocks, those visible in hand-specimens without the aid of the microscope, are very varied. The geologist in the field depends principally on them and they usually serve for a preliminary classification. However, the most important tool for the petrographer is the petrographic microscope. The detailed analysis of minerals by optical mineralogy in thin section and the micro-texture and structure are critical to understanding the origin of the rock. Characteristics observed under the microscope include colour, colour variation under plane polarised light, fracture characteristics of the grains, refractive index and optical symmetry. The more difficult and skilful part of optical petrography is identifying the interrelationships between grains and relating them to features seen in hand specimen, at outcrop, or in mapping. (http://en.wikipedia.org)
extrusive igneous rocks are formed at the Earth crust's surface as a result of the partial melting of rocks within the mantle and crust. The melt, with or without suspended crystals and gas bubbles, is called magma. When it REACHes the surface, magma extruded onto the surface either beneath water or air, is called lava. The lava cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture. In terms of the composition the volcanic rocks are classified in three large groups: basalts (greyish black coloured), andesites (greyish, redbrown coloured), rhyolite (white coloured). In Hungary basalt rocks are to be found in the Balaton and Salgótarján area, while andesite rocks extend from the North Danube area to the Mátra. The Zemplén mountains are mostly made up of rhyolites and partly by andesites. See also volcanic rock, magmatic rock