Page 216 - Laboratory Manual in Physical Geology

FIGURE 7.4 Folded and foliated (layered) gneiss. The dark FIGURE 7.5 Hydrothermal mineral deposits. The dark part
minerals are muscovite, and the white minerals are quartz. Some of of this rock is chromite (chromium ore) that was precipitated from
the quartz has been stained brown by iron. Regional metamorphism hydrothermal fluids (watery hot fluids). The light-colored minerals
caused this normally rigid and brittle rock to be bent into folds without form a vein of zeolites (a group of light-colored hydrous aluminum
breaking. Pressure applied to the flat mica mineral grains has caused silicates formed by low-grade metamorphism). The vein formed
them to shear (slide parallel to and past one another) into layers called when directed pressure fractured the chromite deposit, hydrothermal
foliations. Metamorphic rocks with a layered appearance or texture are fluids intruded the fracture, and the zeolites precipitated from the
foliated metamorphic rocks. FIGURE 7.5 is a nonfoliated metamorphic hydrothermal fluids as they cooled (making a healed fracture and a
rock because it lacks layering. vein of zeolites).

Recrystallization is a process whereby unequal-sized crys- sometimes has a spotted appearance caused by the partial
tals of one mineral slowly convert to equal-sized crystals of decomposition of just some of its minerals. In still other
the same mineral, without melting of the rock. The longer cases, one mineral may decompose (leaving only cavities
the process continues, the larger the crystals become. For or molds where its crystals formerly existed) and be simul-
example, microscopic calcite crystals in chemical limestone taneously replaced by a new mineral of slightly or wholly
(travertine, as in a cave stalactite) can recrystallize to form different composition. When the hydrothermal fluids
a mass of visible calcite crystals in metamorphic marble. cool, minerals precipitate in the fractures and “heal” them
Mineral composition of the rock stays the same, but tex- ( FIGURE 7.5 ).
ture of the rock changes.

Neomorphism is a process whereby mineral crystals not Types of Metamorphism
only recrystallize but also form different minerals from Metamorphism can occur at different scales and in
the same chemical elements. This happens when bonds different types of environments.
of the original minerals break, and the chemical elements
rearrange themselves into different crystalline structures Burial metamorphism is the most common type of
and/or different molecules. For example, shales consisting metamorphism and occurs on a regional scale as rocks
mainly of clay minerals, quartz grains, and feldspar grains form and get buried. The metamorphism is caused by
may change to a metamorphic rock consisting mainly of confining pressure ( FIGURE 7.2 ).
muscovite and garnet.
Regional metamorphism, as the name implies, occurs
Hydrothermal Fluid Effects on Rocks on a regional scale, but the term now refers specifically to
Just as hot water can cook vegetables and remove their large-scale metamorphism at convergent plate boundaries,
color by breaking down molecules within them, it can where there is directed pressure (differential stress) and high
also change the composition and form of rocks. Thus, temperature that causes folding and foliation of the rocks.
water is an important agent of metasomatism , the loss or It is also called dynamothermal (pressure-temperature)
addition of new chemicals during metamorphism. Hornfels metamorphism.

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