metamorphism due to indirect weather effects

Metamorphism Due To Indirect Weather Effects

Learning Outcomes

• Understand temperature gradients.

• Understand rounding and its effect on the snowpack.

• Know the conditions that are conducive to rounding.

• Understand sintering and its effect on the snowpack.

• Know the conditions that are conducive to sintering.

• Understand faceting and its effect on the snowpack.

• Know the conditions that are conducive to faceting

Once snow grains are buried in the snowpack, they become insulated from direct weather effects.

Metamorphism still occurs in these deeper layers, however, and the weather still plays a role, but the effects of weather are indirect.


When indirect weather is playing a role, weather factors influence the environment in the snowpack rather than the grains themselves.

In differing environments, snow grains metamorphose differently and the overall effect on the grains and the layers varies.

Metamorphism due to indirect weather generally occurs more slowly than metamorphism due to direct weather factors.


The primary engine that drives metamorphic processes deep in the snowpack is the temperature gradient. Temperature GradientTemperature gradient is defined as: “The change in temperature over height.”


Metamorphism

Temperature gradients are measured in ºC per 10 cm.

Measure the gradient directly at regular 10 cm intervals.

If the change in temperature was 3 ºC per 10 cm,

The temperature gradient is 3 ºC per 10 cm.

American Institute for Avalanche Research & EducationLevel II Avalanche Course

In reality, temperature gradients are present in many snowpacks much of the time and they do not always have a detrimental effect.

To obtain a “calculated” temperature gradient we can use the following formula: T10 – Tgnd

= cTG HS/10 Where:T10 is temperature of the snow 10 cm below the surface

Tgnd is temperature of the ground

HS is the height of snow in centimeters

cTG is the calculated temperature gradient

Metamorphism (indirect)

Metamorphism:Temperature gradients

Lets try a few examples and calculate the temperature gradient.

1.T10 = -20ºC, Tgnd = 0, HS = 100

2.T10 = -5ºC, Tgnd = 0, HS = 100

3.T10 = -10ºC, Tgnd = 0, HS = 50

4.T10 = -10ºC, Tgnd = 0, HS = 200

T10 – Tgnd

HS/10 = cTG

TemperatureGradients

0º

-20º

HS = 100

Tº C

Calculate the temperature gradient:

T10 = -20, Tgnd = 0, HS = 100, TG = ______

-10º

HS

, Dep

th (c

m)

T10 – Tgnd

HS/10 = cTG


0º

-5º

HS = 100

Tº C


T10 = -5, Tgnd = 0, HS = 100, TG = ______

-10º

HS

, Dep

th (c

m)

T10 – Tgnd

HS/10 = cTG

20º


0º

-10º

HS = 50

Tº C


T10 = -10, Tgnd = 0, HS = 50, TG = ______

-10º

HS

, Dep

th (c

m)

T10 – Tgnd

HS/10 = cTG

-20º


0º

-10º

HS = 200

Tº C


T10 = -10, Tgnd = 0, HS = 200, TG = ______

-10º

HS

, Dep

th (c

m)

T10 – Tgnd

HS/10 = cTG

-20º

Metamorphism

Primary factors:

Air temperature

Ground temperature

Snow height

TG < 1º C per 10 cm = weak TG > 1 º C per 10 cm = strong

MetamorphismWhen the temperature gradient is less that 1ºC per 10 cm (e.g., TG is weak), snow grains are constantly trying to attain a state of equilibrium.

To do this, they reduce their surface area.

Initially, the original grain decomposes into fragments.

Decomposition & Fragmentation

/ The symbol for DF grains is:

When the original snow grains no longer exist but their original form can still be inferred from the fragments, the snow grains are defined as “decomposed and fragmented”.

MetamorphismRounding/Sintering

If the temperature gradient remains weak (<1oC/10 cm) the snow grains continue to decrease in size as they attempt to reach and equilibrium state.

The ice that makes up the grain sublimates and becomes water vapor.

Stellar crystal begins to round

MetamorphismRounding/Sintering Vapor pressure or

concentration is higher over convex areas of the grain and lower in concave areas.

The vapor tends to move from convex areas of the grain (regions of high vapor pressure) to concave areas (regions of low vapor pressure).

Stellar crystal begins to round


As this process continues, the convexities are reduced and concavities are filled in resulting in a rounded grain of smaller size than the original from which it was created.

This process is called rounding.

When the concentration of vapor pressure becomes great enough in the concave areas, the vapor condenses or deposits in the hollow by shifting phase from vapor to ice.

Field symbolRounding

Rounding

In latter stages, the entire grain becomes rounded (convex) and the process begins to deposit ice in the joints between the grains.

A neck grows and increases in size over time which greatly strengthens the bonds between grains.

This process is called sintering.

The smaller and more rounded the grains become, the closer they pack together and bond, and the more sintering is present the stronger the snow will be.


sintering

Rounded grains bond together into simple chains.

The polarized light results in different grains generally having a different color.

Rounded grains

Round snow grains. Red color is pore space between snow grains.

Note the sintering (bonding) that occurs where two different grains touch as a result of "neck" growth by vapor deposition to the concavity.

Rounded grains

Bonding between two different grains touch as a result of "neck" growth by vapor deposition to the concavity.

Rounded grains

In weak temperature gradients(<1°/ 10cm) sublimation typically moves ice from convex surfaces (points) to concave surfaces in stages:

Rounded grains

If the temperature gradient is strong (>1oC/10 cm) there is a strong heat flux in the snowpack.

That simply means that heat is moving through the snowpack from warmer regions to cooler regions as nature tries to equalize the temperature of the ground and the air.

This heat flux changes the metamorphic process. The ice that makes up the grains continues to sublimate.

MetamorphismFaceting

Strong TG heat flux & sublimation

The strong movement of heat through the snowpack causes the vapor to move from warmer regions to cooler.

Vapor sublimates from grains in the warmer area (usually nearer the ground) and condenses into ice on grains in the cooler area (generally higher in the snowpack).


Early in the faceting process, the grains become angular in shape; flat sides and corners are readily observed. Bonds between grains weaken and the layer becomes softer.

This vapor tends to condense on convexities and larger grains are created at the expense of smaller ones.

This process is called faceting.


Early in the faceting process, the grains become angular in shape; flat sides and corners are readily observed. Bonds between grains weaken and the layer becomes softer.

MetamorphismFaceting & Depth Hoar In latter stages, visible lines

(striations) form on the angular grains.

The striations represent subsequent deposits of vapor.

When grains are fully angular and have more than two striations, they are referred to as depth hoar.

It is not uncommon to see 10 mm depth hoar which is visible to the naked eye, especially early in the season.

MetamorphismFaceting & Depth Hoar In latter stages, visible

lines (striations) form on the angular grains.

The striations represent subsequent deposits of vapor.

Depth hoar grains can become quite large when the faceting process is strong.It is not uncommon to see 10 mm

depth hoar which is visible to the naked eye, especially early in the season.

MetamorphismFaceting & Depth Hoar

As the grains become larger and more faceted, they become less packed and bond more poorly, resulting in weaker snow. Faceted grains have a dense molecular structure and as a result, once formed, they resist rounding even if temperature gradients become weak.

Field symbols

^

Faceted grains

Depth hoar

Depth hoar crystal

Water vapor is moving upwards, from the bottom of the image towards the top of the image.

Depth hoar grains grow downwards and into the source of water vapor.

As each wave of water vapor condenses on the depth hoar grain, the grain becomes larger.

The result is a very unstable grain that acts like a lever

Depth Hoar - facets

Image is about 5 cm. Note that each grain is pointed towards the top of the image and widest towards the bottom of the image.

Another example of depth hoar.

Again, the depth hoar grain is growing from the top of the screen towards the bottom of the screen.

Depth Hoar - facets

Angular grains with poor sintering.

Each different color is a different facet within the depth hoar grain.

Each facet represents a wave of water vapor that depositied as a single unit onto the existing grain.

Depth Hoar - facets

A depth hoar grain.photograph using polarized light.

Water vapor is moving upwards, from the bottom of the image towards the top of the image.

Hence the depth hoar grains are growing downwards and into the source of water vapor.

As each wave of water vapor condenses on the depth hoar grain, the grain becomes larger.

The result is a very unstable grain that acts like a lever

Depth Hoar - facets

Image is about 5 cm. Note that each grain is pointed towards the top of the image and widest towards the bottom of the image.

New SnowDF Faceting & Mixed Forms

If new snow or DF grains are exposed to a strong temperature gradient, they may become faceted without becoming a rounded grain.

This generally happens at or near the surface of the snowpack.

DF = decomposition and fragmentation

What is a strong temp. gradient?

New SnowDF Faceting & Mixed Forms Since temperatures in the

upper layers of the snowpack often fluctuate with day/night temperature swings ----

Gradients in the upper regions of the snowpack are often not sustained for long periods.

It is unusual to see depth hoar type grains grow on the surface.

DF = decomposition and fragmentation

New SnowDF Faceting & Mixed forms Often new snow and DF

grains retain some of their original form and faceting takes the form of angular or striated additions to the original grain form.

Since a faceted grain is reluctant to round, facets often persist in the upper part of the snowpack even though they may never become well defined or large.

New SnowDF Faceting & Mixed forms

The process of constant change in the surface temperature gradient.

If maintained over time, often results in a mixture of grain types including DFs, poorly developed rounded grains, and early stage facets.

These layers are often referred to as mixed forms.

Metamorphic CombinationsRounds to Facets

Early stage rounds that are well developed and not yet very hard or dense are highly susceptible to faceting if the temperature gradient shifts from weak to strong.

If rounding has been occurring due to a weak temperature gradient and the gradient changes to strong, the rounding process will be arrested and faceting will begin to occur.

Layers that consist of small rounded grains, are hard, and have a high density resist faceting because there are few pore spaces for the heat flux to move through the snowpack.

Metamorphic ReversalFacets to Rounds

If faceting has been occurring due to a strong temperature gradient and the gradient changes to weak, the faceting process will be arrested and rounding will begin to occur.

The process of changing a facet to a round is slower than the process of changing a round to a facet.


Faceted grains and depth hoar can and do become rounded and gain hardness and strength but, due to the molecular density of faceted grains, the process of changing a facet to a round is slower than the process of changing a round to a facet.

The more well developed a facet or depth hoar grain is, the slower it is to revert to a rounded form.


Where well developed depth hoar has formed (e.g. continental climate, shallow snowpack) facets and depth hoar persist for long periods until the snowpack melts.

Is this true in Maritime climates?


•Facets and depth hoar do gain hardness if not density over time under the influence of weak temperature gradients.

•The grains lose their sharp corners, striations become blurred or disappear, and eventually bonds between grains do form.

•Bonding and hardening in well-developed facet layers may be due to sintering between the grains (highly angular grains are composed of largely convex features so vapor may be moving to the joints between the grains).

Metamorphic Reversal

Facets to Rounds•Faceted/depth hoar layers probably also gain strength later in the season due to the pressure of the overlying snowpack and moderating temperatures both of which are conducive to settlement and bonding.

• •Metamorphic reversals often result in layers that contain mixed forms including rounded, partly rounded, faceted, and/or partly faceted grains.

•These mixed forms may eventually become one or the other if the reversal in metamorphic processes occurs before a grain type has become well developed and/or if the reversal remains in effect for a long period or permanently.

Metamorphic Combinations It is possible that faceting will occur in one part of the snowpack and rounding in another if the temperature gradients fluctuate from one area to another.

Temperature gradients may vary on the vertical scale due to air temperature fluctuations (e.g. strong near the surface, weak near the base).

0º

HS

Tº C

Strong TG

Weak TG

Weak TG

Temperature gradients may vary horizontally due to variation in spx depth (e.g. a shallow area on a windswept rise, a deep area in a windloaded hollow).

Temperature RegimeReview

In a 200 cm snowpack with a temperature of 0º at the bottom and -10º at the top, we have a TG of 0.5o/10cm.

A weak temperature gradient.

If we shift that so the temperature is -10º at the bottom and -20º at the top, the TG remains 0.5º /10cm.

The former is a warm temperature regime and latter is a cold temperature regime.

In both cases, rounding will be the dominant metamorphic process.

0º

HS

Tº C -10º

-10º -20ºHS = 200 cm


Rounding will occur more quickly in the warm regime (0º to -10º).

Warm air can carry more vapor than cold and physical processes are faster in warmer temperatures.

It is possible that faceting will occur in one part of the snowpack and rounding in another if the temperature gradients fluctuate from one area to another.

0º

HS

Tº C

HS = 200 cm

-10º

-10º -20º

Temperature RegimeReview Temperature gradients may

vary on the vertical scale due to air temperature fluctuations (e.g. strong near the surface, weak near the base).

Temperature gradients may vary horizontally due to variation in snowpack depth (e.g. a shallow area on a windswept rise, a deep area in a windloaded hollow).

0º

HS

Tº C

HS = 200 cm

-10º

-10º -20º


In a 50 cm, snowpack with a temperature of 0º at the bottom and -10º at the top, has a TG of 2º/10cm.

A strong temperature gradient.

If we shift that so the temperature is -10º at the bottom and -20º at the top, the TG remains 2º/10cm.

In both cases, faceting will be the dominant metamorphic process.

Faceting will occur more quickly in the warm regime (0º to -10º).

0º

HS

Tº C

HS = 50 cm

-10º

-10º -20º

Temperature Gradient Review

Temperature gradient is:

A weak temperature gradient is: __________

A strong temperature gradient is: __________


Ice sublimates into water vapor and vapor pressure moves vapor from: ________________

Vapor condenses as ice in: ________________

As a result, grains break down into: ________________

Eventually, the grains become: ________________

If this goes on long enough: ________________

The result is: ________________

When we have a weak temperature gradient (<1oC/10cm)


Ice sublimates into water vapor and vapor pressure moves vapor from: convex regions to concave regions.

Vapor condenses as ice in: concave regions.

As a result, grains break down into: smaller pieces (decomposed & fragmented forms).

Eventually, the grains become: smaller and rounded.

If this goes on long enough: sintering occurs.

The result is: stronger snow.

When we have a weak temperature gradient (<1oC/10cm)

Review: Rounding and Temperature Gradients

The factors that promote rounding are:

Warm climate

Deep snowpack

Weak temperature gradient

Warm temperature regime

High density snow

Metamorphic Combinations

It is possible that faceting will occur in one part of the snowpack and rounding in another if the temperature gradients fluctuate from one area to another.

This is true both in the vertical and horizontal scales.

Temperature gradients may vary on the vertical scale due to air temperature fluctuations (e.g. strong near the surface, weak near the base).

0º

HS

Tº C

Strong TG

Weak TG

Weak TG

Temperature gradients may vary horizontally due to variation in spx depth (e.g. a shallow area on a windswept rise, a deep area in a windloaded hollow).

Ice sublimates into: __________

Vapor condenses as: __________

As a result, grains: __________

Eventually, the grains become: __________

If this goes on long enough: __________

The result is: __________


When we have a strong temperature gradient (>1oC/10cm)

Ice sublimates into: water vapor and the heat flux moves water vapor from warmer regions to cooler

Vapor condenses as: ice on convexities

As a result, grains: increase in size and become angular

Eventually, the grains become: faceted

If this goes on long enough: depth hoar develops

The result is: weaker snow


When we have a strong temperature gradient (>1oC/10cm)

Review: Rounding and Temperature Gradients

The factors that promote faceting are:

Cold climate

Shallow snowpack

Strong temperature gradient

Warm temperature regime

Low density snow

DF grain, some riming

Sintering – vapor transport

Meta-rounding

Sintering

Fig. 1. (a) A SEM image of a sintering snow grain, three grain boundaries are visible linking the grain via necks to its neighbors; (b) the same sintered snow grain after etching for 8 min under vacuum at –70 °C.

Slide (00)SC21: Early stage faceted grains

Slide (00)SC22: Faceted grains

Slide (00)SC23: Depth hoar grains

Slide (00)SC24: Depth hoar grains

Slide (00)SC25: Depth hoar grain

DiscussionFaceting and rounding, the resulting grains, and strong or weak snow layers in and of themselves are not necessarily good or bad.

Snow stability analysis needs to take into account not only individual factors but also combinations of factors (big picture).

In general, weak or weakening snow is not desirable. In certain combinations and over certain time spans, it may not be bad and, in fact, it may have a positive effect on stability (in the short term anyway).

Conversely, strong/strengthening snow is usually preferred in the long run but it may not necessarily be good for stability in the short term.

0º

HS

Tº C

vs TG

w TG

-1º

-21º301 300

TG = ___ºC

Temperature Gradient Exercise

1)TG = ____º C

2) What are the conditions that promote such TG?

3) What can form during such TG metamorphism?

4) What next?

To obtain a “calculated” temperature gradient we can use the following formula: T10 – Tgnd

= cTG HS/10 Where:T10 is temperature of the snow 10 cm below the surface

Tgnd is temperature of the ground

HS is the height of snow in centimetres

cTG is the calculated temperature gradient

Metamorphism (indirect)

metamorphism due to indirect weather effects

Documents

t10 tgnd hs10

temperature gradientslets

effects of weather

snow grains

direct weather factors

height of snow

overall effect

detrimental effect