Permafrost 3

Question 1

T/F
frost heaves can be explained just by volumetric explanion

Answer 1

false
the magnitude of the heave exceeds what’s explained by volumetric expansion alone

Question 2

Explain the water/ ice dynamics in forming frost heaves

Answer 2

1. pore water migrates to the freezing front, which can happen up (against gravity) as well as down
2. @ freezing front, water freezes into an ice lens –> it’s this migration that causes such a dramatic heave in periglacial enviros

Question 3

How does the water move in frost heaves?

Answer 3

cryosuction! = neg pressure because of temp-dependent differences

1. capillary: water moving up confined spaces by surface tension and adhesion
2. 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

Question 4

excess ice=

Answer 4

occurs when the ice volume is greater than the pore space volume

Question 5

Why is excess ice important?

Answer 5

Thaw consolidating
- in silt, upon thawing, excess ice volume is lost by settling
- doesn’t happen so much in gravel

implications for building!

Question 6

describe the process of frost pulling

Answer 6

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!

Question 7

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?

Answer 7

frost pulling due to frost heaving!

pulls rocks up

Question 8

What makes a body of sediment frost susceptible? (2 factors)

Answer 8

1. 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
2. duration and rate of freezing
   - long and slow freezing promotes growth of thick ice lenses

Question 9

Give 2 geomorphic consequences (landforms) of ice segregation and heave

Answer 9

1. sorted stone circles
   - donut ring of coarser sediment with fine grain at center
2. frost boil/ mud boil
   - fine grain in center, coarse grain on outside

Question 10

____ through processes of frost heave and frost pulling can form sorted stone circles and frost boils

Answer 10

convection

specifically, active layer convection

Question 11

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?

Answer 11

stone stripes!

sorted circles –> oblong nets –> stone stripes

get ‘stretched out’ as you go downslope steeper

Question 12

seasonal frost mounds=

Answer 12

ice-cored landforms at the base of hill; ephemeral (lasting short time) from year to year

Question 13

earth hummocks stay longer than seasonal frost mounds. Why?

Answer 13

because they’re insulated by vegetation

Question 14

pingo=

Answer 14

cone-shaped ice-cored hills

Question 15

2 ways pingos form:

Answer 15

1. open systems (feature of discontinuous permafrost zone)
2. closed systems (found where there is continuous permafrost)

Question 16

Explain how open systems pingos form

Answer 16

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

Question 17

Explain how closed system pingos form

Answer 17

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

Question 18

Explain the timeline of a closed system pingo developing

Answer 18

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

Question 19

Describe the life cycle of a pingo

ie how do they die :(

Answer 19

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 :(

Question 20

Frost cracking can causes polygonal ground. What other process, not related to ice could form this polygonal shape?

Answer 20

cooling of magma

Question 21

T/F
there is frost cracking present on mars

Answer 21

true!

Question 22

thermal contraction crack=

Answer 22

opening of a thin crack through a medium (eg permafrost/ lava) as it cools and contracts

Question 23

Describe how an ice wedge forms

Answer 23

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!

Question 24

at the edges of polygons in the ground, you might see ____ ___

Answer 24

ice wedges