home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
The Epic Interactive Encyclopedia 1998
/
Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso
/
M
/
Metamorphic_rock
/
INFOTEXT
< prev
Wrap
Text File
|
1992-09-03
|
1KB
|
32 lines
A rock altered in structure and composition
by pressure, heat, or chemically active
fluids after original formation. (If heat is
sufficient to melt the original rock,
technically it becomes an igneous rock upon
cooling.) The mineral assemblage present in a
metamorphic rock depends on the composition
of the starting material (which may be
sedimentary or igneous) and the temperature
and pressure conditions to which it is
subjected. For example, a clay rich in
sediment might become a slate when
metamorphozed at low temperature and
pressure, a mica-schist at a higher
temperature and pressure, or a gneiss if
temperature and pressure are very high.
thermal metamorphism involves mainly heat
changes as in rocks adjacent to an igneous
body; dynamic metamorphism occurs with
changes in stress as in a fault. Regional
metamorphism involves both heat and pressure
and is associated with rock deformation
taking place at convergent plate boundaries
(see plate tectonics). Most metamorphism
invloves little change in bulk chemistry
except for loss or gain of water and other
volatiles; chemical changes may, however,
occur due to the action of fluids. Very
high-grade metamorphism can cause a rock to
melt, and some granites appear to have been
formed in this way.