Environmental Engineering Reference
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and is particularly important in oceanic crust. Migmatization represents the extreme
range of metamorphism at the boundary with magmatization. All are further
distinguishable from rock deformation , which is mostly a mechanical effect on the
geometry of entire rock structures rather than on their internal lithological nature.
Metamorphism takes two forms and is graded in intensity. Contact metamorphism
bakes or recrystallizes rock in a localized metamorphic aureole around a magma
intrusion. Regional metamorphism deforms and recrystallizes rock on a large scale by
compression and heating in crustal shortening/subduction zones. Magmas reveal that
temperature/pressure effects are difficult to separate, but our familiarity with some forms
of metamorphism may help. Wrought iron is hardened by hammering (compression).
Simultaneous heating greatly assists by making the minerals more ductile, capable of
adjusting to the required shape such as a sword blade. When hot, it can also be bent
without losing strength. Similarly, fragile snowflakes may assume a dense, crystalline
form (ice) by compression, accompanied by pressure melting at crystal edges. This does
not break the rule about not melting! Tiny films of water are essential catalysts and
permit realignment of constituent minerals in all forms of metamorphism. The
snowflakes have not melted and refrozen; like the iron, their original constituents merely
recrystallize into a new, stronger configuration.
Proceeding beyond diagenesis, in which a given rock will have reached textural and
chemical equilibrium, regional metamorphism may first compress the material. This
causes shortening in the direction of compressive stress by squeezing any remaining
plate-shaped grains or minerals into line, enhancing its foliated or planar structure. Poorly
cemented clastic (granular) sediments deform most readily, and their grain-size and
relative abundance of phyllo- or sheet silicates determine the quality of foliation (see
Figure 12.2). Thus mudstones with weak bedding - horizontal diagenetic structures
acquired during deposition - are converted into shale and then slate by progressive
compression. Increasing foliation or cleavage converts flagstone suitable for pavement
into strong slate suitable for roofs. Coarser silty clays and sands are transformed into
phyllites and schists respectively, the latter developing coarser schistosity rather than
closely spaced cleavage. The role of phyllosilicates in pressure metamorphism is a vital
clue to an important function of thermal metamorphism. Parallel rise of temperature with
pressure in regional metamorphism converts minerals species stable in a less compressed
state to mineral species more comfortable in a more compressed state by solid-state
recrystallization . Foliation is conditioned therefore not by initial textures alone but by
the extent to which new crystalline textures can develop. This depends largely on the
available minerals and is exemplified by granite and its metamorphic 'twin', gneiss.
Random quartz-feldspar- biotite crystal assemblages in granite contrast with marked
foliations in gneiss in which the phyllosilicate mineral biotite forms bands. Absence of
phyllosilicates leads to massive rather than foliated structure. Marble, quartzite and
amphibolite represent metamorphic forms of carbonate, quartz and basalt.
As with magmas, high temperature allows recrystallizing minerals to grow with time
and the right mineral assemblage can replicate fine or coarse textures over short or long
time scales. Small amounts of pore fluids enhance recrys-tallization by diffusing
dissolved minerals through the rock, or depositing others as they are driven off at higher
temperatures, often as vein minerals in fractures. Some minerals depend on particular
temperature/pressure environments exclusive to metamorphic rocks. They form facies or
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