Glaciers Flashcards
Glacial period
A period of ice advance associated with falling temperatures eg: Pleistocene
Interglacial
A period of ice retreat associated with rising temperatures eg:Holocene
Ice sheet
A large body of ice over 50,000 km2 in extent.
Ice cap
A smaller body of ice (less than 50,000 km2)usually found in mountainous regions.
Glacier
A finger of ice usually extending downhill from an ice cap and occupying a valley.
Last Glacial Maximum
This refers to the time of maximum extent of the ice sheets during the last glacial period, approximately 22 to 18000 years ago.
What is geological time divided into?
Epochs
What period are we currently in?
The Quaternary Period
What epoch are we currently in?
The Holocene epoch- an inter-glacial period
When was the last time the UK was covered in ice?
During the Pleistocene epoch
Describe the process of freeze-thaw weathering
1, Water enters cracks in the rock
2, When temperatures drop, the water freezes and expands causing the crack to widen
3, The ice melts and water makes it way deeper into the cracks
4, The process repeats itself until the rock splits entirely.
What happened during the last ice age?
Snow and ice covered much of the UK. Glaciers in the north and west carved deep valleys and troughs. Further south and east land was permanently frozen.
What is the main process in glacial environments?
Freeze-thaw weathering
Freeze-thaw weathering is
seasonal (freezes in winter and thaws in summer)
What does freeze-thaw weathering do?
- helps create a jagged landscape of frost-shattered rock
- weakens rocks so they are more easily eroded
- creates scree which acts as a powerful erosion tool when trapped under moving glaciers.
What are the two man types of glacial erosion?
abrasion and plucking
Describe the process of abrasion
A ‘sandpaper’ effect caused by ice scouring the valley floor. It leads to striations beneath the ice.
Plucking
It occurs when meltwater beneath a glacier freezes around rock. Loose rock is ‘plucked’ away as the glacier moves over it.
What are striations?
Scratches caused by large rocks below the ice
By how much does the volume of water expand by when it freezes?
By 9%
How do glaciers move?
- In summer, meltwater lubricates the glacier, causing it to slide downhill. This movement can be quite sudden and it is called basal slip.
- In hollows high up on the valley sides, the movement may be more curved and it is called rotational slip
Describe the process of internal deformation
- In winter, the glacier becomes frozen to the rocky surface.
- The sheer weight of the ice and the influence of gravity cause the individual ice crystals to change shape in a plastic-like way.
What does internal deformation cause the glacier to do?
To slowly move downhill
How do glaciers transport material?
- Rock fragments resulting from freeze-thaw and eroded by the ice are transported by the glacier. This sediment-called moraine-can be transported on and in and below the ice.
- As the glacier moves forward, it pushes loose debris ahead of it effectively transporting it downhill. This is called bulldozing.
Why does deposition occur?
-It occurs when ice melts. As most melting occurs at the front (the snout) of a glacier (ablation > accumulation) this is where the most deposition happens.
How does till form?
As a glacier slowly retreats it leaves behind a bed of broken rock fragments called till. Due to the lack of water to transport it, till is poorly sorted, with jagged rock fragments of all sizes.
Till is also known as
boulder clay
What happens to rocks ahead of the glacier?
Meltwater rivers will carry sediment away so by the process of attrition, rock fragments will become smaller and more rounded. The sediment is well sorted, with larger rocks deposited close to the ice and finer material carried many kilometres away. This sandy and gravel material is called outwash.
What are corries also known as?
cirques and cwms
What are corries?
Large, hollowed-out depressions found on the upper slopes of glaciated valleys.
How are corries characterised?
By a steep back wall and a raised ‘lip’ at the front.
What may a corrie contain?
A lake called a tarn.
Describe the formation of a corrie
1,Snow accumulates in a sheltered hollow on a hillside. 2,Nivation enlarges the hollow enabling more snow to collect.
3,Gradually, the snow turns to ice and a small corrie glacier is formed.
4,Through rotational slip, the glacier abrades an over-deepened hollow.
5,There is reduced erosion at the front of the corrie, due to the ice being thinner and less erosive, forming a raised lip.
6, Sometimes moraine may be deposited here. A tarn may form in the bottom of the corrie.
What is nivation?
Snow-related processes, such as freeze-thaw weathering, meltwater and slumping
What is an arête?
An arête is a knife-edge ridge often found at the back of a corrie or separating two glaciated valleys.
Arête characteristics
They are often extremely narrow and popular with hill walkers, although strong winds can make then very dangerous.
Describe the formation of an arête
A typical one forms when erosion in two back-to-back corries causes the land in-between to become narrower.
How does a pyramidal peak form?
If 3 or more corries have formed on a mountain, erosion may lead to the formation of a single peak instead of a ridge. This is called a pyramidal peak.
Where do most glaciers flow?
Along pre-xisting river valleys
What is a glacial trough?
- A steep-sided, wide and flat-bottomed valley, formed by abrasion
- most glacial troughs start out as V-shaped river valleys
- when the landscape becomes glaciated, individual glaciers occupy the river valleys, eroding them through the process of abrasion to form U-shaped glacial troughs
How is a truncated spur formed?
- The glacier is unable to flow around existing interlocking spurs so cuts straight through the, forming steep-edged truncated spurs.
How is a hanging valley formed?
- they are smaller tributary valleys above the main glacial trough.
- smaller glaciers in these valleys were unable to erode down to the same level as the main glacier
- marked by waterfalls
How is a ribbon lake formed?
- they are long,narrow lakes often tens of metres deep
- most result from severe erosion of the glacial trough
- this happens when the ice becomes thicker after a tributary glacier has joined, or when a weaker band of rock is eroded more easily.
- a shallow ribbon lake may sometimes form in a glacial trough behind a dam of deposited moraine.
What is the source?
The start of a river
Tributary
a small stream that joins a larger river
Confluence
where a tributary joins a larger river
mouth
the end of a river, usually where a river joins the sea
Watershed
the edge of a river basin
Drainage basin
an area of land drained by a river and its tributaries
How does the long profile of a river change downstream?
- in the mountains, velocity would vary considerably
- where the water is shallow and turbulent there is friction with the bed and banks, slowing the rate of flow.
- If you encounter rapids, where the channel narrows and the river becomes deeper, you would move much faster.
- further downstream, the river’s channel is much deeper due to the tributaries bringing additional water.
- less water is now in contact with the bed and banks and the velocity increases, even though the gradient is less steep than in the mountains. Would be floating faster.
Ideal river long profile
- steep gradient in upper course
- much gentler gradient in lower course
- in most cases, a river’s long profile will vary because of the river crossing bands of tough and weak rock. A waterfall creates a step in the long profile of a river.
What is a cross profile?
It is an imaginary ‘slice’ across a river channel and its valley at a particular point.
What can alter the cross profile of a river?
River management can alter the shape of a river channel, and different types of rock or human activities such as quarrying
Why does the cross profile change downstream?
- As tributaries bring water from other parts of the drainage basin, the river becomes bigger
- with more water and more energy, it is able to erode its channel, making it wider and deeper.
What are the main causes of the changes to the valley cross profile?
- mainly due to channel erosion broadening and flattening the base of the valley
- together with weathering and mass movement, these processes make the sides of the valley less steep.
What are the two types of erosion?
- vertical erosion
- lateral erosion
What landforms are in the upper course of the river?
- mostly erosion landforms eg: waterfalls
What landforms are there in the middle course?
Mostly erosion and deposition landforms eg: meanders and transportation
What landforms are there in the lower course?
Mostly deposition landforms eg: levees
What are the four types of transportation?
1, Solution
2, Suspension
3, Traction
4, Saltation
Solution (transportation)
dissolved load
Suspension
small sediment held in the river
traction
large particles rolled on the riverbed
saltation
‘bouncing’ of particles too heavy to be suspended
What are the four processes of erosion that take place in a river?
1, hydraulic action
2, Abrasion
3, Attrition
4, Solution
What process occurs in transportation and erosion?
solution
Hydraulic action
The force of the water hitting the river bed and banks. This is most effective when the water is moving fast and when there is a lot of it.
Abrasion
When the load carried by the river repeatedly hits the bed or banks dislodging particles into the flow of the river.
Attrition
When stones carried by the river knock against each other, gradually making the stones smaller and more rounded.
Solution (erosion)
When the river flows over limestone or chalk, the rock is slowly dissolved. This is because it is soluble in mildly acidic river water.
What is the material transported by a river called?
Its load
What are the four main types of transporation that occur in a river?
- traction
- saltation
- suspension
- solution
What does the size and total amount of load carried and deposited depend on?
It depends on the river’s rate of flow- it’s velocity
What do rivers look like after a rainstorm and why?
- They look very muddy
- This is because they are flowing fast and transporting a large amount of sediment.
What do rivers look like at low flow?
They usually look clearer as very little sediment is being transported.
When does deposition occur?
Deposition occurs when the velocity of a river decreases.
Why does deposition occur as the velocity of a river decreases?
It no longer has enough energy to transport its sediment so it is deposited.
Larger rocks
- tend to be deposited in upper course of river
- transported very short distances, mostly by traction, during periods of high flow
Finer sediment
- carried further downstream
- mostly held in suspension
- material will be deposited on the river bed or banks, where velocity is slowed by friction
Where does a large amount of deposition occur and why?
- It occurs at the river mouth where the interaction with tides and gentle gradient greatly reduces the river’s velocity.
How can rocks be carried in a glacier?
On top of ice, within the ice (buried by layers of ice and snow) or beneath the ice where they carry out the process of abrasion
What is moraine?
Angular and poorly sorted deposits.
When deposited by the melting ice, what can the sediment form?
A blanket of material up to several metres thick. This is known as till or boulder clay, due to the range of sizes of the sediment.
Where is till found along?
Much of the east coast of England.
Characteristics of till
- It forms fertile soils
- it is weak and easily eroded when exposed on the coast
Ground moraine
- material dragged underneath the glacier and it is left behind when the ice melts
- often forms uneven hilly ground.
Lateral moraine
- forms at the edges of the glacier
- mostly scree material that has fallen off the valley sides due to freeze-thaw weathering
- when the ice melts, the moraine forms a low ridge on the valley side
Medial moraine
- when a tributary glacier joins the main glacier, two lateral moraines will merge to produce a single line of sediment that runs down the centre of the main glacier
- on melting, the medial moraine forms a ridge down the centre of the valley.
Terminal moraine
- represents the furthest extent of the glacier’s advance
- Huge amounts of material pile up at the snout of a glacier to form a high ridge often tens of metres in height across the valley
Why are there a few moraines evident in the UK?
As the ice melts and retreats up the valley, many of these features are eroded away by meltwater so there a few landforms.
Drumlins
- smooth egg-shaped hills
- about 10 m high
- several hundred metres long
- found in clusters on the floor of a glacial trough
- made of moraine that has been streamlined and shaped by the moving ice
- usually have a blunt end which faces up-valley and a more pointed end facing down-valley–> indicates the direction of movement of a glacier
Erratic
- a large boulder that is out of place, resting on a different type of rock
- they can be used to trace the history of glacation in areas like the UK where the ice has long since disappeared
- By studying the geology of an area, it is possible to work out where the erratic rocks came from and what route they followed as they were carried by glaciers
- most large erratics were probably transported on or in glaciers
What was the last glacial period in Britain?
The Late Devensian glaciation
When did the Late Devensian glaciation begin?
About 33000 years ago
When was the peak of the ice age?
About 22000 years ago
What happened 22000 years ago?
Vast ice sheets spread over the UK from the north and northeast to cover all northern Wales and northern England
What are huge tongues of ice called?
glaciers
What did unglaciated areas in the south experience?
Permafrost (frozen conditions)
Freeze-thaw weathering definition
An active process of weathering in glacial environments where rocks are exposed to changing temperatures
Where would freeze-thaw weathering be very active?
- In periglacial (edge of ice) conditions
- before and after ice covered an area
- on mountain peaks exposed above the ice
Describe the process of freeze-thaw weathering
1, water enters the crack in the rock
2, On freezing, the ice expands by 9 percent
3, On thawing, water seeps deeper before refreezing
4, After many repeated cycles, the rock fragment breaks away
5, Rocks collect as scree.
What does freeze-thaw weathering require?
- Frequent temperature changes above and below freezing to enable freezing and thawing to occur
- presence of liquid water
- presence of rocks with cracks/holes
Abrasion
Angular rocks beneath the glacier which scratch and smooth the underlying bedrock
Why is meltwater important?
It helps to lubricate the ice, enabling it to grind its way forward
Plucking
- meltwater beneath the ice freezes and bonds pieces of loose bedrock to the glacier
- When the glacier moves forward, these loose pieces of rock are plucked away from the bedrock, leaving behind a very jagged and angular surface.
How many metres thick can glaciers be?
over 1000 metres
How does a glacier move forward?
By the force of gravity
Internal deformation
- also known as plastic flow
- It involves the slipping and deformation of individual ice crystals within the glacier
When does glacial advance occur?
- When accumulation (additional snow and ice) exceeds ablation
When does glacial retreat occur?
- When the amount of ablation (melting) exceeds accumulation (additional snow and ice).
How may smaller glaciers in mountainside depressions move?
By rotational slip, a curved movement which results in the formation of a corrie.
Where does meltwater deposit sediment?
On an outwash plain in front of the glacier
What do ribbon lakes result from?
A localised increase in vertical erosion, often involving rotational slip scooping out the bedrock
Where may increased erosion occur?
- where a band of more weaker, easily eroded rock crosses the valley
- where a tributary glacier joins the valley, increasing the mass of the ice and resulting in greater erosion
- where the valley sides become narrower, increasing the depth and power of the glacier.
How are moraines formed?
From the deposition of poorly sorted, angular till deposits carried by the ice, and then dumped when the ice melts.
What can build up when glaciers retreat and why?
smaller recessional moraines build up as when glaciers retreat, they may remain stationary for some time, or even advance slightly.
State sources of evidence for long-term climate change during the Quaternary Period
- Ice cores (show methane and CO2 concentrations)
- evidence from ocean sediments help to show temperature changes
- pollen analysis
Late Devensian Glaciation
- Massive ice sheet that covers the entirety of the UK apart from the south
Freeze-thaw weathering is an
- active process
How do
Smaller glaciers in mountainside depressions my move by rotational slip, a curved movement that results in the formation of a corrie.
Outwash plain and till differences
- -outwash plain- sediment more sorted, particles are more rounded to attrition
- meltwater pouring out from the snout of the glacier , depositing it as a vast outwash plain in front of the glacier
side deposition
Medial moraine
. When two tributary glaciers meet, two lateral moraines join together to form a single ridge in the centre of the main glacier.
Different types of moraine
= ground moraine: anything along the base
- lateral moraine- right up along the mountain ridge
- Medial moraine - two glaciers carrying sediment form a ridge in the middle
