Phonolite Highly alkaline, leucocratic, intermediate-silica
aphanitic or glassy rock composed of feldspathoids, alkali
feldspar, and minor mafic minerals. See Figures 2.12 and 2.13.
Phreatic explosion Produced where ground-water makes
contact with hot rock, converting the water into a large
volume of steam.
Phreatomagmatic explosion See Hydromagmatic explosion.
Phreatoplinian eruption Highly explosive venting of volatile-
rich, usually silicic, magma enhanced by contact with external
water that vaporizes to steam.
Phyllite Aphanitic metamorphic rock that has a lustrous or
silky sheen on well developed foliated surfaces. Mostly
pelitic. Phyllites are transitional between slates and schists
and share associations and properties of both.
Phyllonite A type of mylonite that contains abundant
phyllosilicates; may have S–C fabric.
Picrite An olivine-rich basalt in which MgO 18 wt.%
and (Na
2
O K
2
O) 1–3 wt.%.
Pipe vesicle See Vesicle.
Pilotaxitic See Felty.
Pipe (plug) Slender, subvertical columnar or funnel-
shaped body of intrusive magmatic rock.
Pitchstone Massive, dark-colored glass that has a waxy
luster in hand sample; contains 6–10 wt.% water absorbed
at near atmospheric conditions.
Plagiogranite Phaneritic or granophyric rock composed
mostly of sodic plagioclase, lesser quartz, and minor mafic
minerals; occurs in ophiolite.
Plane strain Change in shape or size is restricted to two
dimensions while the third dimension remains unchanged.
This is the type of strain in rocks that is represented, as a mat-
ter of convenience, on a two-dimensional sheet of paper.
Plastic A behavior or response to applied stress in which
strain is nonrecoverable (irreversible) at a stress above the
yield strength of the material; below that stress no strain
occurs.
Platinum group elements (PGE) Ru, Rh, Pd, Os, Ir, Pt.
Plinian eruption Highly explosive venting of volatile-rich,
usually silicic, magma that forms a steady turbulent plinian
plume.
Plinian plume A highly buoyant volcanic plume.
Plug See Pipe.
Pluton An intrusion of magma that is not sheet-like.
Plutonic Processes and products related to magma beneath
the surface of the Earth. Contrast Volcanic.
Poikilitic Magmatic texture in which large oikocrysts
enclose smaller, randomly oriented mineral inclusions.
Poikiloblastic Metamorphic texture in which porphyro-
blasts contain numerous inclusions of other minerals that are
usually, but not always, the same minerals as in the surround-
ing finer grained matrix. The inclusion-filled porphyroblasts
are called poikiloblasts. In thin sections, high-relief poik-
iloblasts of garnet and staurolite that contain abundant
inclusions of low-relief quartz resemble a sponge or a sieve,
hence the alternate designation sieve texture.
Point defects See Crystal defects.
Polygenetic Resulting from two or more processes or hav-
ing multiple sources or originating at more than one time in
more than one place.
Polymer In a melt, chain of linked ions, usually Si and O.
Polymerization, degree of In a melt, the proportion of ions
that form polymers; the ratio of non-bridging O to network-
forming tetrahedrally coordinated cations, such as Si and Al.
Silica-rich melts are more polymerized than silica-poor melts.
Polymetamorphism Repetition of essentially unrelated
episodes of penetrative ductile deformation, or recrystalliza-
tion, or both. Typical of regional metamorphism in long-lived
orogens.
Porosity Proportion of openings in a rock that can hold
liquid relative to the whole rock volume.
Porous flow Involves movement of liquid along grain
boundaries or between connected pore spaces.
Porphyritic Inequigranular magmatic texture made of two
grain sizes; more or less euhedral larger crystals, called phe-
nocrysts, are embedded in a finer grained or glassy matrix.
Porphyroblastic Common texture in metamorphic rocks in
which larger grains, called porphyroblasts, are surrounded
by a finer grained matrix of other phases. Porphyroblasts
are typically some type of alumino-silicate, such as garnet,
staurolite, and andalusite.
Porphyroclastic Texture of mylonites where ovoidal,
strained mineral grains, called porphyroclasts, that have
mostly survived ductile deformation lie in a finer foliated
matrix.
Porphyry Granitic rock that crystallized from a crystal-
bearing magma in a shallow intrusion due to rapid reduction
in water pressure, quenching the melt; creates aphanitic
porphyritic texture.
Power law An exponential equation that relates the activa-
tion energy of ductile strain, applied nonhydrostatic stress,
strain rate, and T, permitting extrapolation to metamorphic
conditions not attainable in the laboratory.
ppb Parts per billion.
ppm Parts per million.
Precision Reproducibility; a number that indicates how
much statistical variation from the average or mean value
occurs in replicate determinations.
Pressure Force acting over an area; has units of bars or
Pascals (Pa). Confining pressure (also referred to as load
or lithostatic pressure) at some depth within the Earth is
produced by the weight of overlying rock or water. See also
hydrostatic state of stress.
Pressure ridge Elongate, commonly arcuate prominence
on the surface of a moving lava flow; wavelength and ampli-
tude of uplifts are on the order of meters.
Pressure solution As a consequence of Riecke’s principle,
ions diffuse from more soluble, more stressed grain margins
to less stressed surfaces where grain growth takes place; the
presence of an intergranular fluid phase enhances the diffu-
sive transfer especially in aggregates of calcite and quartz that
are relatively soluble under metamorphic conditions.
708 Igneous and Metamorphic Petrology