how can a rock change during metamorphism?

metamorphic rock, any of a class of rocks that result from the amending of preexisting rocks in response to changing environmental conditions, such as variations in temperature, pressure, and mechanical stress, and the improver or subtraction of chemical components. The preexisting rocks may exist igneous, sedimentary, or other metamorphic rocks.

The give-and-take metamorphism is taken from the Greek for "change of form"; metamorphic rocks are derived from igneous or sedimentary rocks that accept altered their form (recrystallized) as a result of changes in their physical environment. Metamorphism comprises changes both in mineralogy and in the material of the original stone. In general, these alterations are brought about either past the intrusion of hot magma into libation surrounding rocks (contact metamorphism) or by large-scale tectonic movements of Earth'due south lithospheric plates that modify the pressure-temperature weather condition of the rocks (regional metamorphism; see also plate tectonics). Minerals within the original rock, or protolith, reply to the changing atmospheric condition by reacting with one another to produce a new mineral assemblage that is thermodynamically stable nether the new pressure-temperature weather. These reactions occur in the solid state but may be facilitated by the presence of a fluid phase lining the grain boundaries of the minerals. In contrast to the germination of igneous rocks, metamorphic rocks do non crystallize from a silicate melt, although high-temperature metamorphism tin can lead to partial melting of the host rock.

metamorphism of banded gneiss
Banded gneiss produced by metamorphism of quartzofeldspathic sediments in Scotland. White bands have undergone partial melting and recrystallized into granite. Dark bands represent textile composed predominantly of biotite and minor hornblende.
Jane Selverstone

Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.

Britannica Quiz

(Bed) Rocks and (Flint) Stones

Diamonds may be a girl's best friend, simply what is that mineral'south closest relative? Test your cognition of rocks, minerals, and all things "yabba dabba doo" in this quiz.

Because metamorphism represents a response to changing physical conditions, those regions of Earth's surface where dynamic processes are about active volition also exist regions where metamorphic processes are most intense and easily observed. The vast region of the Pacific margin, for example, with its seismic and volcanic activity, is also an area in which materials are being buried and metamorphosed intensely. In general, the margins of continents and regions of mountain edifice are the regions where metamorphic processes proceed with intensity. But in relatively quiet places, where sediments accumulate at slow rates, less spectacular changes besides occur in response to changes in pressure and temperature conditions. Metamorphic rocks are therefore distributed throughout the geologic column.

Because most of Earth's mantle is solid, metamorphic processes may also occur in that location. Drapery rocks are seldom observed at the surface considering they are too dense to rise, but occasionally a glimpse is presented past their inclusion in volcanic materials. Such rocks may represent samples from a depth of a few hundred kilometres, where pressures of nearly 100 kilobars (3 one thousand thousand inches of mercury) may be operative. Experiments at high pressure take shown that few of the common minerals that occur at the surface will survive at depth within the mantle without changing to new, loftier-density phases, in which atoms are packed more closely together. Thus, the common class of SiO₂, quartz, with a density of ii.65 grams per cubic cm (1.53 ounces per cubic inch), transforms to a new phase, stishovite, with a density of 4.29 grams per cubic centimetre (ii.48 ounces per cubic inch). Such changes are of disquisitional significance in the geophysical interpretation of Earth's interior.

In full general, temperatures increase with depth within World along curves referred to as geotherms. The specific shape of the geotherm beneath whatever location on World is a part of its respective local tectonic authorities. Metamorphism can occur either when a stone moves from one position to another along a single geotherm or when the geotherm itself changes form. The former can accept identify when a stone is buried or uplifted at a rate that permits it to maintain thermal equilibrium with its environs. This blazon of metamorphism occurs below slowly subsiding sedimentary basins and also in the descending oceanic plate in some subduction zones. The latter procedure occurs either when hot magma intrudes and alters the thermal land of a stationary stone or when the stone is rapidly transported by tectonic processes (e.g., thrust faulting or big-calibration folding) into a new depth-temperature regime in, for example, areas of standoff betwixt two continents (see also mistake and fold). Regardless of which process occurs, the result is that a drove of minerals that are thermodynamically stable at the initial weather condition are placed under a new ready of conditions at which they may or may not be stable. If they are no longer in equilibrium with one another under the new conditions, the minerals will react in such a way as to approach a new equilibrium land. This may involve a complete modify in mineral assemblage or simply a shift in the compositions of the preexisting mineral phases. The resultant mineral assemblage volition reflect the chemic composition of the original rock and the new pressure-temperature weather condition to which the stone was subjected.

Because protolith compositions and the pressure-temperature conditions nether which they may be placed vary widely, the multifariousness of metamorphic stone types is large. Many of these varieties are repeatedly associated with one another in space and time, however, reflecting a uniformity of geologic processes over hundreds of millions of years. For example, the metamorphic stone associations that adult in the Appalachian Mountains of eastern N America in response to the collision between the North American and African lithospheric plates during the Paleozoic Era (541 million to 252 million years ago) are very like to those that developed in the Alps of s-cardinal Europe during the collision between the European and African plates that occurred during the Mesozoic and Cenozoic eras (252 1000000 years ago to the present). Too, the metamorphic rocks exposed in the Alps are grossly similar to metamorphic rocks of the aforementioned age in the Himalayas of Asia, which formed during the continental collision between the Indian and Eurasian plates. Metamorphic rocks produced during collisions between oceanic and continental plates from different localities effectually the world also show striking similarities to each other (run into below Regional metamorphism) nonetheless are markedly different from metamorphic rocks produced during continent-continent collisions. Thus, it is oft possible to reconstruct tectonic events of the by on the ground of metamorphic stone associations currently exposed at Globe's surface.