6 - Ice Wedges

Página 1

Ice Wedges:

25-30% of total ice volume, present in the uppermost 5-10m of permafrost.

Develop in unconsolidated sediment, best form in lowland poorly drained tundra atop continuous permafrost.

Involves deformation of both ice and ground, often due to the ice length exceeding the crack length and ice being less dense than surrounding permafrost.

can be active, inactive or relict and filled by ice or icy silt.

epigenetic: ice wedges that for in pre existing permafrost and therefore are younger than the surroundind sediment.

syngenetic: ice wedges that for in an area of aggrading sediment, the ice migrates upwards with the sediment accumulation.

anti-syngenetic: ice wedges that form as sediment is beign eroded so the ice wedge migrates downward with erosion.

Evolution of polygonal terrain netowrks:

Surface view of ice wedge networks, ice wedges grow sub surface forcing the surrounding sediment to push upwards, therefore directly above ice wedges troughs form. Many connected troughs constitute surface polygons.

Generally acknowledged that primary troughs are divided by secondary and tertiary. Furthermore it is suggested that networks become more regular with age.

However patterned ground can exist in dry areas from solely thermal regimes causing cracking each rapid winter freeze, if moisture is introduced to the system then there will be a tendency for cracks to fill with moisture during the thaw and then freeze in the following winter resulting in ice wedges.

Polygon Types:orthogonal or hexagonal

The study concluded that regularity increased in terrain networks noticibly.

Types of thermal-crack-contraction polygons:

Ice wedge

Sand Wedge

Composite wedge

Sublimation wedge

Significant conclusions:

Ice wedges initiate polygons

Polygons evolve during the harshest winters with the most rapid freeze.

Number 2 means that recently drained thaw lakes tend not to exhibit polygons for a while after drainage because of the talik present beneath the lake bed and the latent heat released by freezing or the talik.

Growing ice wedged result in deepening troughs and increased snow depth.

Relict polygon patterns in Svalbard:

Previously active polygonal patterns are left on the ground surface made visible by differential moisture content OR vegetation colonization possible as the area is no longer periglacial.

In Svalbard cracking occured when air temperatures of -20 persisted for a considerable perdiod in anti-cyclonic conditions.

French

Haltigin et al. 2012

Burn 2004

Svensson 2007

Christiansen 2005