The Physical World

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The Physical World

• Revision Notes

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The Physical World

• This unit is all about Rivers, Coasts and
  Glaciers and how these physical entities change
  the land on which we live over thousands of
  years.
• In particular, you will need to know in detail
  how various land forms are created, and the
  processes that help to create the landforms.
• A Landform is a particular shape of the land
  resulting from thousands of years of change.
• A process is a continuing force that can change
  the natural world.

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Rivers How they change from source to mouth

• Rivers change immensely on their journey from
  Source areas (where they start) to their
  finishing point at their mouths.
• Velocity increases as more water is added to
  rivers via tributary rivers. This means that
  less of the water is in contact with the bed of
  the river and the mouth so there is less energy
  used to overcome friction. Hence rivers flow
  progressively faster on their journey downstream.
• Width and depth increases as more water is added
  from tributaries.
• Gradient (the slope of the land) decreases as
  rivers flow because the river meanders across the
  land rather than erode into it and follow a
  straight path as it does in the source. This
  means it covers a decrease in height over a
  longer distance the further downstream you get.
• Stone size decreases downstream and the stones
  get rounder and smoother as rivers erode the
  rocks progressively as the stones are transported
  downstream.

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Rivers How they change from source to mouth

• The diagram on the right shows the major changes
  downstream.
• In the source area the drainage basin (an area of
  land drained by a river and its tributaries)
  contains V shaped valleys and waterfalls, and the
  dominant processes are erosional. Erosion tends
  to be vertical (straight down into the land).
• In the middle section of the drainage basin the
  river starts to erode laterally. This section
  contains meanders and Ox bow lakes, and the river
  creates a flood plain often with Levees. Here,
  Material is deposited and erosion can also occur.
• In the lower drainage basin deposition dominates
  as a river enters a sea or lake, the valley is at
  its widest and deltas and estuaries are major
  landforms and habitats.

Watershed
Interlocking spur
A diagram of the drainage basin
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Landforms in upper River Valleys - Waterfalls

• Waterfalls are created by erosion processes.
• They occur where a band of hard rock (e.g.
  granite) overlies a softer rock (e.g. sandstone).
• Erosion processes such as Hydraulic Action (the
  force of the water) and Abrasion (where the river
  rubs stones that are being transported against
  the bed of a river thereby breaking it down)
  dominate.
• The softer rock is eroded quicker than the harder
  rock.
• This creates a plunge pool where water is swilled
  around.
• Over time, the softer rock is eroded further
  creating an over hang of hard rock.
• Eventually, this hard rock collapses because it
  is unsupported and the waterfall moves back
  upstream.
• This creates Gorges, steep sided deep river
  valleys.

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Landforms in upper River Valleys V-shaped
valleys

• Vertical erosion processes wear away the rock in
  upper valleys.
• As the river erodes down over it leaves behind
  valley sides that are shaped like a letter V
• The result is that the river valley contains
  interlocking spurs, slivers of land that
  interlock. Interlocking spurs are areas of more
  resistant rock left behind because the river
  erodes softer rock quicker. The river moves
  between the interlocking spurs.

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Landforms in the middle valley Meanders and
Ox-bow lakes

• Meanders occur in the middle valley and are the
  result of erosion AND deposition processes on a
  river.
• In this section of the valley the river erodes
  laterally and migrates across the valley floor
  over time, widening the valley. Within the river
  itself, the fastest current is found on the
  outside of a bend and the slowest current on the
  inside of the bend. This is because the depth of
  the water on the outside of the bend is deeper,
  so there is less friction and hence higher
  velocities.
• Over time, this means that erosion occurs on the
  outside of meanders bends and deposition occurs
  on the inside.
• In the diagrams, erosion of the outside of the
  meander means that the neck of land becomes
  narrower and narrower over time.
• On the inside of the bend the slow flow
  encourages the deposition of beaches.
• After a long time the neck of land gets totally
  cut through by erosion processes such as
  hydraulic action and abrasion.
• This cuts off the meander bend which is totally
  isolated by deposition leaving an Ox bow lake

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Landforms in the middle valley Floodplains and
Levees

• Rivers flood on a regular basis.
• When rivers flood in the middle valley the cover
  an area of land known as the flood plain.
• When they flood velocity is slowed and deposition
  of any rocks being transported is encouraged.
• This deposition leaves a layer of sediment across
  the whole floodplain.
• After several floods there are several layers of
  sediment (rocks) deep on the flood plain.
• In addition, the largest rocks and most
  deposition occurs next to the river channel.
  This leaves a ridge of higher material next to
  the river channel on both banks of the river
  known as a levee.

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Landforms in the lower valley Estuaries and
Deltas

• Deposition dominates in the lower valley as river
  velocity falls as it enters a sea or lake.
• This results in deposition of fine sediments such
  as silt and clay.
• This sediment can build up over time forming
  estuaries and deltas.
• Deltas have distributaries where the river
  channel splits up into smaller rivers on its
  journey to the sea.
• Birds foot deltas are comprised of fine sediments
  like silt and a good example is the Mississippi
  delta seen on the top photograph.
• Arcuate deltas are composed of coarser sands and
  gravels and can be seen on the river Nile on the
  bottom photograph.

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What are Glaciers?

• Glaciers are large sheets of ice that sit on the
  surface of the land.
• How do glaciers form?
• Glaciers form in regions where there is enough
  snow fall. Over several years when enough snow
  accumulates the snow is compacted into ice. This
  happens because the weight of the snow on the top
  layers forces the air out of the snow in the
  bottom\layers closest to the rock surface.
• Why do glaciers form?
• Glaciers form where ever temperature is low
  enough to allow permanent ice and snow cover that
  doesnt all melt over summer. High latitudes
  (e.g The polar regions) and areas of high
  altitude (e.g. Mountains) are the best places to
  find glaciers. North facing slopes, that receive
  less sunlight due to the angle that the sun hits
  the Earth, also get glaciers.

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How do Glaciers Move?

• There are 4 factors responsible for glacier
  movement
• Weight (Mass) Glaciers can be huge, kilometres
  long and hundreds of meters deep. They therefore
  weigh a lot.
• Slope Glaciers that occur in mountainous areas
  are found on steep slopes, this allows quicker
  movement.
• Gravity -this is the force responsible for
  pulling these huge slabs of ice down the
  mountainside.
• Water at the base helps to allow some glaciers to
  move

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How do glaciers erode?

• Plucking
• First, ice has the property of being able to
  freeze to rock. The glacier uses this to adhere
  to part of the ground. Then, as the glacier
  forces the ice frozen to the ground to continue
  moving down slope, the rock may be pulled out of
  the ground and moved down slope as well.

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How do Glaciers erode?

• Abrasion
• Second, once glaciers have rock and sediment at
  their bases they can push this sediment against
  the ground and use it like sandpaper. This
  abrasion of rock against rock can scour the
  landscape and leave large gouges, small
  striations, or even a finely polished surface.

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How Corries Form

• Corries form in hollows where snow can
  accumulate. The snow compacts into ice. This
  moves down hill, plucking and abrading the
  hollow. This creates a steep back wall and a
  hollow known as a corrie or cirque. When the ice
  melts a corrie lake forms.

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How Corries form
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Landforms Created by Glaciers.

• U Shaped Valleys.
• These are steep sided, wide valleys in the shape
  of the letter U.
• They are formed by a valley glacier that moves
  down the valley because of gravity.
• As the glacier moves down the valley it plucks
  the rock from beneath and those rocks then rub
  against the bed of the valley, eroding it
  further.
• This deepens and widens the valley.

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Glacial Landforms

• Truncated Spurs.
• These are interlocking spurs without the land
  that interlocks!
• As a river erodes the upper valley it cuts down
  into the rock and meanders in and out of the
  surrounding rock.
• When glaciers form as the temperature decreases
  they move down the valley.
• As the glacier moves it plucks the rock from the
  floor and it abrades the valley sides and floor.
  
• At the front edge of the glacier material is
  bulldozed forward.
• The end result is that the interlocking spurs are
  eroded, creating truncated (cut off) spurs.

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Glacial Landforms

• Ribbon Lakes.
• These are Long and thin lakes that form after the
  glacier has melted. During glaciation the
  glacier erodes some parts of the valley floor
  more than others. When the glacier melts water
  fills the depressions (holes) where the valley
  floor was eroded most.

Hanging Valleys These are the valleys of
tributary (smaller) glaciers that are left high
above the floor of the main glacier. You often
get waterfalls here.
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The Changes Glaciers make to River Valleys
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Additional Diagrams
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The coastal system

• Coasts are dynamic environments that have inputs
  of sediment and losses of sediment.
• The major processes are
• Erosion- Hydraulic action, corrosion, abrasion,
  attrition.
• Weathering Chemical, biological and physical.

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Coastal Landforms - Erosion

• Cliffs retreat inland as a result of erosion
  processes.
• Waves pound the base of cliffs, hydraulic action
  (the sheer force of the water) and abrasion
  (rocks hurled against the base of the cliff)
  erode a wave cut notch.
• Eventually, the material on the cliff face,
  weakened by weathering, collapses because it is
  unsupported.
• This material is washed away by the sea and the
  cliff retreats in land.
• This may leave a very flat smooth area of rock in
  front of the cliff known as a wave cut platform.

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Coastal Landforms - Erosion

• Stacks, stumps and caves are formed on cliffs.
• Waves attack vertical lines of weakness in the
  rock known as Faults.
• Processes such as hydraulic action and abrasion
  widen the cracks into caves.
• Over time, the cave will be eroded into an arch,
  accessible to the sea on both sides. Finally,
  the erosion continues and the arch collapses
  leaving behind a stack (a vertical column of rock)

Stacks
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Coastal Landforms - Erosion

• Coves and bays form where rock runs in bands
  horizontal to the direction of wave attack.
  There is a band of resistant rock closest to the
  sea and a band of less resistant rock inland.
• The waves seek out faults in the hard rock and
  erode through to the soft rock.
• Wave processes erode the softer rock faster and
  this leaves a circular cove with a narrow
  entrance where the sea enters.
• A good example of this is Lulworth cove. Which
  can be seen opposite.

Softer rock
Harder Rock
Lulworth Cove
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Coastal landforms - Deposition

• Beaches are formed by wave processes.
• Gently sloping beaches are formed by strong
  destructive waves that backwash more material
  away from the beach that they swash up the beach.
• Steeply sloping beaches occur by constructive
  waves that swash more material up the beach than
  they backwash away, building up a steep beach
  gradient.

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Coastal Landforms -Deposition

• Spits are created by the process of Long shore
  drift.
• Material is carried along the shore in a zigzag
  fashion by waves as they swash material up the
  beach at an angle and backwash material down the
  beach at a right angle.
• Wherever there is a break in the coastline (e.g.
  across a river or a change in coastline
  direction) then material is deposited closest to
  the shore.
• Eventually this material builds up out into sea
  to form a spit. As seen in the picture opposite
  of Spurn Head
• Spits often have salt marshes build up behind
  them.