Snow Pack Metamorphosis
Diurnal Temperature Profile in Snowpack
M. Williams, CU Boulder
M. Williams, CU Boulder
Snowcrystals.com; Libbrecht 2005
Snowcrystals.com
Types of Snow Metamorphism in Snow pack
Equilibrium-Temperature-Metamorphism
The rounding process builds rounded grains
(rounds) which bond well to one another creating a
snowpack (or layer) that is generally increasingly
strong.
In weak temperature gradients(<1 degree / 10cm)
sublimation typically moves ice from convex
surfaces (points) to concave surfaces (hollow)in
2 stages:
"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association
Equilibrium-Temperature-Metamorphism
Stage 1: In the initial stage of rounding
the sharp ends of new crystals and the points of
faceted grains sublimate and the resulting water
vapor is deposited in concave areas.
At high temperatures, molecules also glide along
the grain surface from convexities to concavities.
As well, large grains with broad curvatures grow
at the expense of small grains with sharp
curvatures.
The result is a concentration of mass with a
minimum surface area.
"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association
Equilibrium-Temperature-Metamorphism
Stage Two:
Under weak temperature gradients, water
vapor moves from warm areas to cold, but the
rate of movement is much slower than in
strong temperature gradient environments.
Slow moving vapor is deposited on the colder
surfaces in a more homogenous manner and
the faceted, stepped pattern associated with
a strong temperature gradient does not occur.
"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association
Equilibrium-Temperature-Metamorphism
The following conditions promote rounding:
• A weak temperature gradient generally less
than 1 degree C per 10 cm (which moves
water vapor slowly from warm areas to cold)
• Dense, tightly packed snow
• Small grains (which produce denser snow)
• A high snow temperature, typically
above -10 degrees C (which promotes
weaker temperature gradients)
"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association
Equilibrium-Temperature-Metamorphism
Equilibrium-Temperature-Metamorphism
During the rounding process the particles bond together by
forming necks. Sintering is the process by which two particles
weld together without a liquid present. Notice that there are
bonds or "necks" between the rounded crystals in the right
hand image.
Temperature-Gradient Metamorphism
The faceting process builds angular
grains (facets) which bond relatively
poorly to one another and other grains
creating a snowpack (or layer) that is
generally increasingly weak.
Also called Constructive metamorphism
Synonyms:
constructive metamorphism
temperature-gradient metamorphism
kinetic growth
depth hoar
sugar snow
"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche
Association
Temperature-Gradient Metamorphism
When the temperature gradient is strong (> 1 degree / 10cm) water
vapour moves rapidly from warm grain surfaces to colder surfaces.
Because the snowpack usually is warm (at or near 0 degrees C) at the
ground and colder at the surface, ice sublimates from lower, warmer
grains and is deposited onto colder grains higher up in the snowpack.
These colder grains first develop sharp corners, then stepped facets.
If the faceting process continues, large, six - sided hollow or filled cup
shaped grains called depth hoar are formed.
Depth Hoar is common in Rocky Mountain climates, around large
rocks and high shrubs, and where the snowpack is thin.
"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche
Association
Temperature-Gradient Metamorphism
The following conditions promote faceting:
A strong temperature gradient, generally greater than
1 degree / 10cm (which quickly drives water vapor
from warm areas to cold)
Loose, low density snow (which facilitates the free
movement of water vapor between grains)
Presence of crusts (which concentrate water vapor,
promoting vapor transfer in the concentrated area)
Moderate snow temperature (which maximizes the
amount of vapor the snowpack can hold but does not
reduce the overall temperature gradient significantly)
"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association
Temperature-Gradient Metamorphosis
TG metamorphosis occurs in response to a strong temperature gradient (> 10°C /m).
As water vapor is deposited on the grains, they grow larger, and can eventually form the
large crystals known as depth hoar.
Depth hoar normally forms near the base of the snow pack, where the vapour pressure
gradient is strongest and most persistent.
Sublimation: The transition of a substance from the solid phase directly to the vapor
phase, or vice versa, without passing through an intermediate liquid phase. Sublimation
is a phase transition that occurs at temperatures and pressures below the triple point
Temperature-Gradient Metamorphosis
Well rounded crystals
Rounded crystals with
developing facets
Melt-Freeze Metamorphism
Melt-freeze metamorphism occurs when the sun melts the upper
layers of the snowpack during the day, but freezing still takes place
at night. With MF Metamorphism, larger grains grow at the expense
of smaller ones, helping to strengthen the snowpack in most cases.
With MF metamorphosis, the density of the snow pack can increase
to 0.6 g cm-3.
Melt cluster
(clustered rounded grains)
All photographs by S. Colbeck
Rounded poly-crystals
Slush (snow grains
completely surrounded
by liquid water)
PROCESS-ORIENTED
CLASSIFICATION
ADDITIONAL INFORMATION ON PHYSICAL
MORPHOLOGICAL CLASSIFICATION
PROCESSES AND STRENGTH
Dependence on
Basic
most
Common
Classificatio
Place of
Physical
important
effect on
n
Subclass
Shape
Formation Classification Processes
parameters
strength
Wet Grains Clustered
Clustered rounded Wet Snow
Grain clusters Wet snow at Meltwater can Ice-to-ice
rounded grains crystals held
without melt- low water
drain; too much bonds give
together by large
freeze cycle content,
water leads to strength
ice-to-ice bonds;
pendular
slush; freezing
water in internal
regime;
leads to meltveins among three
clusters form freeze particles
crystals or twoto minimize
grain boundaries
surface free
energy
Rounded poly- Individual crystals Wet Snow
crystals
are frozen into
solid
polycrystaline
grain; may be seen
either wet or
frozen
Poorly
bonded;
rounded
single
crystals
High liquid Water drainage Little
content;
blocked by
strength due
equilibrium impermeable to decaying
form of ice in layer or ground; bonds
water
high energy
input to snow
cover by solar
radiation, high
air temperature
or water input
Shear Density Relationship, Snoqualmie Pass
Casson et al. 2008
Determining Shear in Snow Pack
Fig. 4. A snow wedge vertically driven into the
backside of a snow column to find failure
planes within a snow-pack. The
slope angle is 8 and the wedge angle is Beta.
(La Chappelle and Ferguson 1980)