Permafrost 3 Flashcards
T/F
frost heaves can be explained just by volumetric explanion
false
the magnitude of the heave exceeds what’s explained by volumetric expansion alone
Explain the water/ ice dynamics in forming frost heaves
- pore water migrates to the freezing front, which can happen up (against gravity) as well as down
- @ freezing front, water freezes into an ice lens –> it’s this migration that causes such a dramatic heave in periglacial enviros
How does the water move in frost heaves?
cryosuction! = neg pressure because of temp-dependent differences
- capillary: water moving up confined spaces by surface tension and adhesion
- films of unfrozen water will also migrate from warmer to colder ground. Migration happens according to grain size (primarily) and rate of freezing front movement
- slow cooling/ low rate of freezing front movement= thicker lens
excess ice=
occurs when the ice volume is greater than the pore space volume
Why is excess ice important?
Thaw consolidating
- in silt, upon thawing, excess ice volume is lost by settling
- doesn’t happen so much in gravel
implications for building!
describe the process of frost pulling
clasts adfreeze (adhesion by freezing) to frozen soil, which gets heaved up.
the void beneath gets partially filled by sediment, so that clast cannot go back down to original position
= clast has been pulled up by being stuck to a heaving soil!
What process causes farmers to have to pick rocks out of the field every year, even when they got it rock-free the year before?
frost pulling due to frost heaving!
pulls rocks up
What makes a body of sediment frost susceptible? (2 factors)
- grain size: any sediment w/ >3% silt is termed frost susceptible
- silt combines high unfrozen water content and mobility of pore water, that’s why it’s so special - duration and rate of freezing
- long and slow freezing promotes growth of thick ice lenses
Give 2 geomorphic consequences (landforms) of ice segregation and heave
- sorted stone circles
- donut ring of coarser sediment with fine grain at center - frost boil/ mud boil
- fine grain in center, coarse grain on outside
____ through processes of frost heave and frost pulling can form sorted stone circles and frost boils
convection
specifically, active layer convection
Imagine you have a flat landscape with sorted circles on it. It gets steeper as you go downslope. What landforms might you expect to see downslope?
stone stripes!
sorted circles –> oblong nets –> stone stripes
get ‘stretched out’ as you go downslope steeper
seasonal frost mounds=
ice-cored landforms at the base of hill; ephemeral (lasting short time) from year to year
earth hummocks stay longer than seasonal frost mounds. Why?
because they’re insulated by vegetation
pingo=
cone-shaped ice-cored hills
2 ways pingos form:
- open systems (feature of discontinuous permafrost zone)
- closed systems (found where there is continuous permafrost)
Explain how open systems pingos form
water below the base of the permafrost migrated downslope
It pools at the bottom of the slope, forming a talik –> growth of large segregated ice lenses
they can be ~10m high, but that relates to the amount of water available
*usually in discontinuous permafrost
Explain how closed system pingos form
They grow by the same process (ice segregation) as open system pingos, but the water supply is internal (it doesn’t come from upslope like in open systems)
- common in formed lake basins that have drained
- found where there is continuous permafrost
Explain the timeline of a closed system pingo developing
t1: lake in the landscape with permafrost under/ surrounding it
t2: lake drains and permafrost degrades (or ice moves into the talik)
t3: unfrozen water in drained lake basin continues to freeze –> continues to raise the talik up & forms a pingo
Describe the life cycle of a pingo
ie how do they die :(
happy pingo= good cover of vegetation, closed top, peat/ organic-rich soils
dying pingo= lots of exposed mineral soil (b/c less veg) –> less insulated from warming, esp. summer air temp
* this becomes a + feedback once a bit of ice is exposed
dead pingo= open top, ice is thawed into a little lake :(
Frost cracking can causes polygonal ground. What other process, not related to ice could form this polygonal shape?
cooling of magma
T/F
there is frost cracking present on mars
true!
thermal contraction crack=
opening of a thin crack through a medium (eg permafrost/ lava) as it cools and contracts
Describe how an ice wedge forms
A thermal contraction crack opens up. In the summer, this crack fills with water and refreezes below the active layer. This ice persists because it’s in the permafrost. Each winter, the crack grows as more water gets in and expands when it freezes= forms an ice wedge evetually!
at the edges of polygons in the ground, you might see ____ ___
ice wedges
