Simply put, undercooling is the rate of cooling below the
boiling point or true crystallization temperature of a mineral
within a system. Undercooling does not only affect the rate of
crystal growth but also affects nucleation and diffusion rates. The
degree of undercooling produces different textures that are a
result of the interplay among the rates of crystal growth,
nucleation, and diffusion.
Low degree of undercooling: moderate crystal growth, low
nucleation, moderate diffusion. Produces a phaneritic texture.
Moderate degree of undercooling: low crystal growth, high
nucleation, low to moderate diffusion. Produces an aphanitic
texture.
High degrees of undercooling: very low crystal growth,
nucleation, and diffusion. Rock is quenced and very little, if any,
crystals form. Produces a glassy rock, or holohyaline texture.
If you want a more in-depth understanding, keep reading. I will
describe, in general, how the rate of undercooling relates to the
formation of some common igneous textures. First some theory
though.
Undercooling is the temperature below the theoretical
equillibrium crystallization temperature of a mineral at which
crystallization actually initiates. The theoretical temperature of
crystallization is always higher than what happens in reality due
to the fact that geologic systems are rarely, if ever, homogenous
and it is difficult to account for all the variabilites.
Prior to crystallization, the prerequisite nucleation must first
be satisfied. Even at saturation temperatures (boiling point) where
all other conditions are favorable for the crystallization of a
certain mineral, instabilities within the nucleus can result in
spontaneous separation. This is because initial crystals are small
and consequently have a high surface area/volume ratio. This
results in higher surface energies making the initial crystal
unstable. If a critical size of an "embryonic cluster" is not
obtained, the high surface energies are able to overcome the
internal bonding strengths causing the nucleus to spontaneously
separate. The critical size can be achieved by either some degree
of undercooling or supersaturation allowing enough stable ions to
cluster together to form a nucleus.
Initially undercooling enhances growth and nucleation rates. As
undercooling increases (more rapid cooling, higher change resulting
in lower temperatures), the opposite effect occurs. Growth and
nucleation rates begin to decrease significantly as kinematics
decrease and viscosity increases.
Lower degrees of undercooling (the system is cooling slowly...
think of a plutonic magma body at depth), the nucleation rate is
low and the growth rate is moderate. This produces a phaneritic
texture (coarse-grained crystals easily observable with the unaided
eye). The higher temperature of the system supplies more energy
which hinders crystal nucleation while facilitating element
diffusion. At elevated temperatures, it is less likely that enough
compatible ions will be close enough to form an embryonic cluster
because of the overall higher kinetic energy of the system. This
means it is harder for crystals to begin forming, but once they do,
it is easier to move (diffuse) the necessary elements to the
nucleation site to continue crystal growth. Since there are fewer
nucleation points, the crystals are allowed to grow to larger sizes
before running into each other.
Moderate degrees of undercooling (rapid cooling) produce an
aphanitic texture (fine-grained rock, individual grains cannot be
observed by the unaided eye... think of a lava flow without
cyrstals). Crystal growth is lower than nucleation so the crystals
cannot grow as large because they run into each other hindering
growth. The sudden drop in temperature decreases the energy of the
system. Lower temperatures diminish the rate of diffusion (increase
in viscosity) while increasing the possibility of ions forming
nucleation sites. It is now easier for ions to move closer to one
another than diffuse away to find more compatible partners. This
could be one explanation why more mafic rocks have less complex
mineral formulas while felsic rocks are more complex.
Large degrees of undercooling (extremely rapid cooling, think
obsidian or other holohyaline rocks) produce a rock composed
entirely of glass. This is because both nucleation and crystal
growth are very low; there is no time for either to occur because
the rate of temperature decrease is so high.
Two stages of cooling can also occur. For example, a porphyritic
rock, defined as a bimodal grain size distribution, forms from an
eariler phase of slower cooling followed by a phase of more rapid
cooling. Bimodal grain size distribution means larger crystals
(phenocrysts) are set in a fine-grained groundmass (matrix).
The degree of undercooling (cooling rate) aids geologists in
determining how rocks form (petrogenetic studies). As stated above,
a porphyritic texture indicates two stages of cooling. A cooling
magma body at relatively shallow depths that experiences recharge
(or some other eruption trigger such as tectonic activity or an
increase in volatile content) may erupt. Since the magma body was
experiencing lower degrees of undercooling for some amount of time,
it will have large crystals floating in the remaining melt. Upon
eruption, the system experiences higher degrees of undercooling
freezing the larger crystals in a fine-grained groundmass.
Hopefully this helps. Please understand that the above is a
generalized explanation, but conceptually, it should do just fine.
Never say never and never say always in geology! There is ALWAYS an
exception.