Glacial and Interglacial Periods
- the earth goes through cold periods, which last for millions of years called ice ages. during ice ages, large masses of ice cover parts of the earth's surface.
- the last ice age was the pleistocene that began around two million years ago
- during ice ages there are cooler periods called glacial periods when the ice advances to cover more of the earth's surface. each one lasts for about a hundred thousand years.
- in between the glacial periods are warmer periods called interglacial periods when the ice retreats to cover less of the earth's surface. each one lasts around 10,000 years
- the last glacial period began around 100,000 years ago and ended around 10,000 years ago
Ice Covering the Land
since the beginning of the pleistocene there have been permanent ice sheets on Greenland and Antarctica. ice has also covered other parts of the world during the colder glacial periods.
ice covered alot more of the land around 20,000 years ago (during the last glacial period) - over 30% of the earth's land surface was covered by ice. including nearly all of the UK.
we're currently in as interglacial period that began around 10,000 years ago. today about 10% of the earths land surface is covered by ice - the only ice sheets. are the ones on greenland and antarctica
El Niño Effect- a periodic 'blip' in the usual global climate characteristics caused by a short-term reduction in the intensity of the cold ocean current that normally exists off the west coast of south america. it results in unusual patterns of tempreature and rainfall can lead to droughts and floods in certain parts of the world
Ice Sheet- a large body of ice over 50,000 km² in extent
ice cap- a similar body of ice (less than 50,000 km²) usually found in moutainous regions
Glacier- a finger of ice usually extending down hill from an ice cap and occupying a valley
Ice Levels over Time
why does tempreature fluctuate?
- subtle variations in the earths orbit - nearer or further from the sun
- slight changes in the earth's tilt - towards or away from the sun
- variations in the pattern of the ocean currents - moving heat around the globe
Clime change during the pleistocene period
- geological time is divided into periods: Holocene (10,000 - present day, an intergalcial period), Pleistocene (2 million yrs - 10,000 yrs before present, a glacial period)
- within the pleistocene or ICE AGE there where fluctuations in global tempreatures
- at 18,000 yrs BP the ice reaches its maximum extent and the started to retreat
the antarctica ice holds 90% of the worlds fresh water, the greenland icesheet is 1.7km² and is slowly retreating
the glacial system
inputs (top of glacier) - snow, avalanches, rock depris. zone of accumilation
store - glacier, snow and ice
outputs (snout of glacier) - calving ice, evaporation, sublimation, melt water.zone of ablation
calving ice = the breaking off of a mass of ice from a glacier, iceburg or ice shelf
sublimation = a solid changing to gas without first becoming liquid
ablation = the ice melting and the glacier getting smaller
how ice forms
- snowfall is the first step in the formation of a glacier. for a glacier to form there must be more snowfall than melting each year.
- after a few days the delicate snowflake starts to break up.
- gradually snowflakes are squashed by the weight of more snow falling on top of it and they change into small round grains a bit like sugar. after a year the snow is now called firn
- the weight of the overlaying snow continues to squash the grains together and squeeze the air out. after about 10-20 years the firn becomes ice
- the ice becomes thicker and then due to gravity the ice starts to move down the slope. when the ice starts to move downhill it is known as a glacier.
the size and length of glaciers are always fluctuating - they advance and retreat. before 1850 glaciers were advancing rapidly in europe, today they are mostly retreating. this always depends on the glacial budget or the balance between inputs and ouputs.
in winter there will be a lot of accumilation and little ablation. in summer, when it is warmer, ablation will tend to dominate over accumilation. acc>abl = advance (posotive budget) abl>acc = retreat (negative budget)
weathering, erosion and glacial movement
freeze thaw is a type of weathering not erosion. this is because weathering is a breakdown of surface rock mainly by weather without the debris being transported away and then re-depositied. they are broken down in situ.
plucking and abrasion are types of erosion. this is because erosion is the wearing away of surface rock by material being carried by ice, water or wind which is then carried away and then re-deposited.
types of glacial movement
basal/rotational slip - melt water at the bottom and underneath the glacierr alow the glacier to float ontop and slide down the mountain (2-3m per day)
internal deformation - preassure from the glacier abouve forces to ice crystals near the bedrock to line up alowing the ice to slip downhill (1-2m per day)
glacial surge - this is rare and happens where the gradient is very steep and the ice moves rapidly downhill. 10-100 times the normal rate
freeze thaw weathering
1. during the warm day or summer melt water fills cracks, joints or pores within the rock.
2. during the cold night or winter temperature drops to freezing, it expands by 9%. withing the confined space expansion exerts pressure on the surrounding rock, widening the crack. when temperature rises above freezing, the ice melts, pressure is released and the water seeps deeper into the rock along the newly enlarged crack
3.repeated freeze thaw action increases to size of the crack until fragments break off and collect and fall to the foot of the rock face to collect as a slpe of loose blocks called scree. this process is more efficient when: a. there are lots of cracks withing the rocks, b. temperatures fluctuate above and below freezing
abrasion and plucking
material carried by the glacier is held rigid, embedded in the ice base & sides. these are used as tools for abrasion as the glacier moves forwards over the bedrock. sharp edged rock fragments abrade the valley sides and floor (wearing it away like sandpaper). small rock particles polish the surface, while large rocks make deep grooved striations
melt water from the ice enters the joints in the rock. water re-freezes sticking to the base of the glasier and bedrock together. as the glacier moves forwards and advances, it 'plucks' or rips the rocks out of the ground that it is frozen to
More Key Words
- Rotational slip-slipping of ice along a curved surface
- morraine-sediment carried and deposited by the ice. lateral morraine morraine at the side of the glacier. medial morraine morraine between two glacers. terminal morraine morraine at the furthest point of the glacier. recessional moraine morraine deposited by the glacier as it retreates
- Bulldozing-the pushing of deposited sediment at the snot by the glacier as it advances.
- hummock-a small area of raised land rather like a big mole hill. also known as a drumlin
glacial transport and deposition
rock fragments from freeze thaw weathering and errosion by the ice that are transported by the glacer, like a giant conveyer belt. this can be rocks ontop, inside that have been burried by snowfall or underneath the glacier
most rocks are deposeted when the ice melts. most of the melting takes place at the snout of the glacier so this is where most rock gets deposited. the advancing glacier then bulldozes the rock further down the mountain. when the glacier is retreating then a sheet of fragmented rock and rock flour wick can form hummocks or just be washed away by melt water rivers.
Corries and Corrie lakes
corries have, deep rounded hollow, semi-circular shape, steep rocky backwall, open front with rock lip, are high up in the mountain, often filled with a lake called a tarn.
1. snow accumilates in hollows on mountain sides, especially less sunny north facing slopes. over time further snow collects in the hollow, extra weight compresses the snow to ice. the corrie glacier moves downhill under the weight of gravity
2. freeze-thaw acts above the hollow, forcing the rock to fall into the crevasses. plucking takes place, where rock that is frozen to the ice is pulled away as the glacier starts to move under gravity - creating a steep backwall. moraine is dragged along under the ice by rotational slip and the main hollow is deepened and widened and deepened by abrasion.
3. a rock lip is left where the rate of erosion decreases. whene the ice melts, a deep round lake is left on the corrie. this is called a tarn. the lip is often heightened by the deposition of moraine that acts as a natural dam to meltwater. scree is added to the bank of the corrie by continued freeze-thaw weathering
Arêtes and Pyramidal Peaks
high long narrow knife-edge shaped ridge with steep sides. arêtes are formed between corries. where two corrie side walls erroding towards eachother.
a pyramidal peak is found where three or more arêtes meet. the corrie backwalls erode backwards towards eachother.
truncated spurs are interlocking spurs that have been straightened by the solid ice.
a hanging valley is where a smaller glacier meets a larger deaper glacier. often has a waterfall.
a ribbon lake is a long narrow lake that fills a glaciated valley from meltwater
a misfit stream is an uncharacteristically small river compared to a large glaciated valley.
Loose snow and slab avalanches
loose snow avalanches - it has a single starting point. cone shaped. occurs with loose powdery snow causes less damage than slab avalanches.
slab avalanches - large starting area, faster and more deadly. started by a large slab of snow and carries every thing in its path.
factors that contribute to the risk of avalanches.
- heavy snowfall or snow layers
- steep slopes
- tree removal, trees are a natural barrier
- tempreature rise, layers of the snow melt alowing others to slide over
- heavy rainfall, lubricates the slope
- human factors, off piste skiing or increased numbers of people on slopes
prevention of avalanches
- deflection wedges: alows avalanches to slip around pylons and houses
- afforestation: conifer trees reduce risk of avalanches
- avalanche shed: alows avalanches to slide over roads and railways without blocking them
- retardant breaks: slow down the avalanche as it travels over the bumps
- houses built outside the avalanche zone: the avalanche stops before it reaches the houses
- avalanche gully: a gully dug to deflect the avalanches away from towns