Glaciated Landscapes Flashcards
(61 cards)
Fluvioglacial landforms- Eskers and Varves
The formation of eskers
-These are sinuous (winding) ridges of sediment that occur due to
the flow of subglacial meltwater within tunnels. They can be from a kilometer (like the esker found by the river Tweed in Wark) to hundreds of kilometers in length (Munro Esker, Canada is 400km in length) but are more often found as discontinuous sections and 20-50m in height.
Eskers are often sorted with coarser gravels towards the edge and sands or silt towards the centre.
- The ice tunnels that eskers are found in normally contain water under pressure so the velocity is too high for deposition. However, in the autumn and winter there will be less meltwater resulting in a slower velocity. At this time some sediment can be deposited. Coarser sediment is deposited first at the slowest point (by the tunnel walls as there is most friction) with the finer sediment only deposited towards the centre at times of very low meltwater flow. Eskers are not exposed until the glacier has retreated. The ice will have to be stagnant so the deposited sediment is not moved.
It is also suggested that Eskers could form if the subglacial tunnel becomes blocked.
Additional esker info
There are instances of beaded and discontinuous eskers forming. These have thinner and less tall areas between wider and taller sections, resembling a bead necklace. These are thought to have formed through:
The subglacial tunnel changing in size
Variations in the flow of the meltwater.
Blocks of ice fall into the subglacial tunnel.
Varves
Varves can form in proglacial lakes (lakes in front of or close to the front of a glacier created from melt water).
As with all lakes there is deposition as the water enters the lake due to the decrease in velocity. However, the variation in water flow into the lake will vary more greatly than in non-glacial lakes.
In the late spring and summer there will be higher levels of melt water meaning that larger sediment can be carried (as shown in the Hjulstrom curve) before being deposited in the lake. This will leave a layer of lighter coloured (larger) sand particles.
During the late autumn and winter there will be less melt water which will result in melt water having a lower velocity. This will mean that only finer silts and clays can be carried. When they reach the lake they will deposit in a layer on top of the sand.
This repeats, creating a layered effect.
Where are periglacial environments found
- Periglacial’ is a term that literally means ‘around’ (peri) and ‘ice’ (glacial), and therefore was originally used to name the locations, climate and landscape type found surrounding ice sheets. However, now the term is used more generally to refer to cold environments and processes that occur in these cold but non-glaciated locations.
-These range from high latitude polar locations (today only really found in the northern hemisphere) and high-altitude areas. The majority of periglacial areas today are found in Canada, Alaska (USA) and Russia.
-Periglacial environments today cover around 20% to 25% of the Earth’s (land) surface. During the Pleistocene this was up to 20% greater.
Characteristics of the periglacial environments
Daily temperatures below 0oC for at least 9 months of the year.
At least 6 months of the year below -10oC.
Temperatures that rarely rise above 18oC at the warmest.
Overall average annual temperatures between 1oC and -4oC.
Low precipitation, typically less than 600mm per year (with less than 100mm in winter and less than 500mm in the summer).
Permafrost
Permafrost is the term given to a layer of soil that is permanently frozen because this layer of soil remains below 0oC throughout the year, we call this cryotic conditions.
Above the permafrost will be a layer of soil that freezes in the winter and melts in the warmer summer months, this is called the active layer. The active layer can be up to 3m depending on the length of time the locations temperature is above 0oC (as well as other conditions such as precipitation levels having an effect).
continous permafrost
No breaks in the permafrost layers, found in locations with a mean annual air temperature below 6C.
discontinous permafrost
The majority of the area has permafrost but there are common breaks in it.
sporadic permafrost
Most of the soil is unfrozen at points during the year with only some fragmented areas having permafrost (normally located in areas of continual shade) and it is only a few meters thick.
Influences on permafrost
-Proximity to water - water has a higher specific heat capacity and will therefore not alter temperature as quickly, meaning that it remains warmer than the land for much of the winter.
-The angle and facing (orientation) of any slope - this will impact the concentration of sunlight and the solar radiation received that could melt the permafrost.
-The characteristics of the soil - how compacted the soil is its colour and the presence of rocks within the soil will impact both the amount of solar radiation reflected/absorbed (albedo) and the specific heat capacity.
-Vegetation - This can insulate the ground from the cold.
-Snow cover - This can slow the freezing process in the winter but also delay the thawing of the active layer in the spring.
Periglacial Landforms
Ice wedge Polygons
Ice wedges are caused by the repeated freezing and thawing of water in cracks in the ground.
1)In summers the active layer thaws allowing water (meltwater or from precipitation) to flow into cracks.
2)In the winter the water will freeze and expand the crack.
3)This process will continue each year with the interstitial ice (ice within cracks) melting creating larger and larger cracks.
These wedges can be up to 1-2 metres wide at the top and from 8 to 10 metres deep. This means it can extend into the permafrost layer where the ice in the ice wedge may not fully melt in the summer.
As with other ground cracks these tend to occur naturally in polygon shapes with raised rims along the sides the wedges. The polygons tend to be 5m to 30m across between the ice wedges and can take up to 100 years to form.
Relict fossil ice wedges can be found in the UK in East Anglia from the periglacial area during the Pleistocene. Here the ice wedges when finally melted were infilled with sand and silt.
Patterened ground
-Patterned ground includes a range of landforms that are created by ice lenses.
-Ice lenses form in the active layer due to stones in the soil; as they have a lower specific heat capacity than the soil around it. This causes the water below the stones to freeze into lenses and expand (with water expanding by 9% when becoming ice).
-Each time this occurs the ice lenses expanding mean that the stones are forced to the surface and into cracks by processes called frost push (upwards) and frost heave (outwards).
-These occur on normally domed areas and as these processes force stones into each other they form a range of shapes including stone polygons (1-5m in diameter), stone nets and stone stripes depending on the slope they are on.
Pingos
Pingos are large domes found in areas of tundra. They are 100-500m in diameter and typically 30-70m in height.
Importantly there are two types of pingo based on how the water that forms their ice core is sourced; open-system (also called East Greenland type) pingos and closed-system (also called Mackenzie Delta type) pingos.
The open system pingos have a tendency to not reach the larger sizes.
more pingos info
Pingos are intra-permafrost ice-cored hills, 3–70 m high and 30–1,000 m in diameter. They are typically conical in shape and grow and persist only in permafrost environments, such as the Arctic and subarctic.
A pingo is a periglacial landform, which is defined as a non-glacial landform or process linked to colder climates. It is estimated that there are more than 11,000 pingos on Earth.
The Tuktoyaktuk peninsula area has the greatest concentration of pingos in the world with a total of 1,350 pingos. There is currently remarkably limited data on pingos.
Pingos can only form in a permafrost environment. Evidence of collapsed pingos in an area suggests that there was once permafrost. Pingos can collapse due to the melting of the supporting ice and give rise to a depression in the landscape showing an inverse shape (horizontal mirror).
Thermokarst landscape
Thermokarst landscapes are the landscapes created by melting permafrost found in the areas surrounding the arctic circle permafrost we saw last lesson (sometimes called the circumpolar permafrost region).
As the permafrost melts at different rates small pits and valleys are formed as the ground settles unevenly, subsiding, and hummocks can be created by refreezing ice lenses. The pits and valleys will often fill with meltwater as can be seen in image b (taken in Sweden) and c (Taymyr Peninsula Russia).
The development of these areas is often studied as part of cryology (the study of ice and snow) to examine the impact of human activity on cold landscapes both as evidence and out of concern for the CO2 and CH4 being released from the melting ice.
Freeze thaw- frost weathering
Water is trapped in cracks and joints in rocks. When the water freezes it expands in volume by 9%, exerting pressure on the surrounding rock and widening the crack. This process will repeat as the temperature fluctuates between above 0oC and below 0oC.
solifluction/Gelifluction- frost weathering
Solifluction is when sediment moves under its own weight due to being heavily saturated. This is not limited to cold environments. Gelifluction is specific to cold environments and is the process of solifluction created by frozen ice melting within the active layer.
carbonation- chemical weathering
At low temperatures carbon dioxide is more soluble so more readily combines with water to make carbonic acid. This will react with calcium carbonate [CaCO3] rich rocks like limestone to produce calcium bicarbonate [Ca(HCO3)2], which is then removed by solution.
Hydrolysis- chemical weathering
In the summer months the abundance of water from the melted ice and some of the acids released by vegetation reacts with feldspar in some rocks, like granite (which has orthoclase [pink] and plagioclase [white] feldspar). This produces potassium hydroxide with the orthoclase feldspar[KHO], which is removed by solution.
Blockfields
Blockfields are created by the process of freeze thaw on flat ground (plateau). The frost shattered rocks do not fall as the surface does not have an adequate angles so stay in place. This means that they are considered in-situ.
Because of the lack of movement there will not be any erosion (attrition) so the blocks will remain angular and will (on average) be larger (often boulder sized - greater than 256mm) than the material found on scree slopes.
They are also known as “felsenmeer”, which means “rock seas” in german.
scree slopes
Scree slopes occur due to freeze thaw acting on an angled surface. When the weight of the weathered/ separated rock overcomes friction then rockfalls (rapid mass movement) will occur.
The sediment will fall down the slope until it comes to rest towards the base of the slope, called the accumulation slope. This will typically have an angle of around 32o, called the rest angle.
Towards the top of the accumulation slope will be typically very unstable and prone to additional periods of mass movement if disturbed. This can lead to the material being more eroded than seen in block fields.
pro-talus ramparts
Pro-talus ramparts occur where a prolonged bank of snow is present below a rock cliff. This means that the material that falls due to freeze thaw, will land on the snowbank and slide, roll or avalanche down the snow to accumulate at the bottom. This creates a “rampart” (ridge) at the base of the snowbank of rock fragments that shows little erosion.
As the snowbank melts this can cause multiple ramparts to form.
These can be found on the north face of Ben Nevis (Scotland) and in the Brecon Beacons (Wales).
