Geosphere Flashcards
What is the principle of Mass Conservation?
Mass cannot be created or destroyed
What is the equation of mass conservation when applied to the landscape?
Change in land surface elevation = Amount of uplift or subsistence \+ Balance between sediment supply and removal
Give an example of when it is important to know the net change in height of the landscape.
When building bridges
Height change due to erosion = what?
Volume removed / surface area (m3/m2=m)
Annual change in the height of the bed due to sediment movement =
Qs(IN) - Qs(OUT)/ Riverbed area (m3/y)/m2= m/y
What is the equation for the equilibrium of the landscape?
Sediment input - sediment output + uplift = 0
OR
Uplift= denundation
Equilibrium is a ……. dependent concept?
Scale
To identify equilibrium one must specify the time and space scale of interest
Give examples of how equilibrium concepts can be applied to the landscapes in different ways.
LONG TERM evolution of mountains —-> uplift = erosion
SHORT TERM evolution of rivers ——–> sediment in= sed out
What is Hack’s concept of dynamic equilibrium?
Dynamic equilibrium- cyclic cycle (10(4)years)
Steady state equilibrium- graded time (10(2) years)
Static equilibrium- steady time (10(-1)years)
Really important to look at the time and scale to determine if a landscape is in dynamic equilibrium or static equilibrium
What did Grove Karl Gilbert pioneer?
The concept of ‘mutual adjustment between form and process’ towards equilibrium
Describe the cycle that describes the concept of ‘ mutual adjustment between form and process’ towards equilibrium
Form shape of river–> distribution of water–> transport of sediment–> erosion and deposition
Feedback between landform and process is critical to explaining both
When was the quantitative revolution?
Mid 20th century
Led to greater focus on process- based understanding of landscape and its evolution
Arthur Strahler (1952): geomorphology should be ‘grounded in basic principles of mechanics and fluid dynamics’
What is the law of power?
Power laws plot as a straight lines on graphs that have logarithmic axes
The gradient of the line is equal to the power (K)
On graphs without logarithmic axes, power laws appear as curves
Name the two types of sediment transport processes
A) Diffusive processes:
Sed movement due to surface gradient e.g. Mass movement
B) Advective processes: sediment movement by water- driven processes e.g. Water flow
What do positive and negative feedback a do to the initial disturbances?
Positive feedbacks amplify the initial disturbances
Negative feedbacks dampen the initial feedback
Topographic changes during landscape evolution often leads to what..?
Strong negative feedbacks because..
1)
2) an imbalance between sediment supply and sediment removal at a point in the landscape leads to changes in surface height and slope
What are the two types of hillslope profiles?
Concave- gradient is declining and becoming less steep e.g. At the bottom of the slope
Convex- slope degree increases e.g. At the top of a slope
Why are the concepts of mass conservation and equilibrium so important?
They are fundamental to understanding the landscape
The concept of dynamic equilibrium reconciles the apparently contradicts the concepts of..
Evolution and balance
How can the evolution of landforms and their equilibrium shape be explained..?
In terms of the feedbacks between landform and processes
Rates of sediment transport around the landscape are strongly dependent on what?
The slope
Landscapes have a tendency to evolve towards..
Stable, equilibrium forms
What does topography define?
Drainage direction ‘water sediment (generally) move downhill
What happened near Salt Lake City where all the water runs into a closed basin (has no route to the sea)?
It runs into lakes where it is evaporated leaving behind salt
What are drainage basins?
An area of land that all drains to the same outlet
What is river discharge?
It is a flow rate measured in m3 s-1 (cumecs- cubic meters per second)
Amount of water that passes by time unit through a section of water
What is bankful discharge?
The discharge that fills a stable alluvial channel up to the elevation of the active floodplain
What is velocity?
Measured in meters per second and is dependent on a rivers slope, depth and roughness
What is river capture?
The natural diversion of the head ward waters of one stream into the channel of another, typically resulting from rapid headward erosion by the latter stream.
Leads to changes in drainage basin area, river discharge and sediment load.
Give an example of a river that has been significantly affected by river capture.
Headward said erosion of the Indus River into the Ganges changes the course of much of its flow
What is denundation?
Denundation is the long term sum of processes that cause the wearing away of the Earths surface by moving water, ice, wind and waves leading to a reduction in elevation and relief of landforms and landscapes.
What is drainage basin splitting driven by?
Uplift and denudation.
What is the hydrological cycle?
The system that controls the flow of water around the earth including controlling processes such as runoff, erosion, transport and deposition of sediment.
What is the Hjulstrom Curve and what does it show?
What’s unusually about the curve?
The Hjulstrom Curve is a graph that shows the velocity that different types of particles erode, transport and deposit at.
It takes a less velocity to erode a sand particle than a silt and clay particle.
What is the competence of the river?
The maximum particle size that a river is able to transport
What is the transport capacity of flowing water?
A measure of the total amount of sediment that it can carry.
Often sediment transport capacity is calculated as a function of what?
Shear stress exerted by the fluid
And
Critical shear stress instead of velocity
What is the sediment delivery ratio?
The fraction of sediment eroded from the slopes that reaches the drainage basin outlet:
SDR = sediment output from basin/ sediment eroded from slope
What happens to the sediment delivery ratio (SDR) downstream?
It tends to decline downstream because sediment storage potential increases as valleys become wider and less steep.
What is the source to sink concept?
Sediment moves from source to sink along a ‘jerky conveyor belt’ - concept can be applied at a range of scales
Some sediments will be stored on small scale (temporary) sediment stores
Some will be stored on large scale sediment stores (increased preservation likelihood?)
Which is bigger the Indus River fan or the Bengal? And what will happen in the future?
The Indus River is much smaller so the fan is as big as the Bengal fan however over time it will become bigger.
What is sediment accommodation space?
The space available to store sediment
The amount of sediment stored in a river depends on..?
The equilibrium long-profile shape of the land surface (river) that transports the sediment
A) Effect of sea level rise
B) Effect of tectonic uplift
What is the long river profile? And why does it adjust over time?
The long river profile represents a a change in the height of the river bed moving downstream from the headwaters to the sea
Like hillslope profiles, river long profiles adjust to transport the amount of water and sediment supplied from upstream
What is the Lane Diagram?
Explains why a slope is concave or convex
About the total amount of water available
What is aggradation?
The term used in geology for the increase in land elevation, typically in a river system.
What happens to discharge downstream?
It increases as drainage basin area increases
What controls are there on the river long profile?
Discharge- increases downstream as drainage basin area increases
Sediment load- increases downstream but at a declining rate because of increased sediment storage
The size of the sediment being transported - tends to decline (gets finer) due to abrasion and changes in sediment sources and river competence
In combination these trends lead to changes in river gradient (and associated changes in channel pattern).
What shape are river long profiles?
Concave - because discharge increases more than sediment transport rate which declines sediment size
What are the controls on channel pattern? (E.g. Straight, meandering or braided)
- Slope and discharge
- Bank strength
- controls widening
- limits lateral sediment supply
- dependent on bank materials, rivers sediment load and mechanism of floodplain construction
Long profile aggradation must happen over the whole valley floor? True or false?
True
Why are alluvial ridges dangerous?
Because over time the river is on top of a small hill and will therefore change quickly to a lower part of the landscape- very dynamic
Construction of an alluvial ridge will lead to what during flooding?
Avulsion during flooding- action of pulling or tearing away
Basin headwaters are major source areas for what?
Water and sediment
Most large basin sinks are found where?
In downstream basin areas or beyond the basin outlet
Rivers fill accommodation space by what?
Channel and floodplain aggradation which often involves multiple channel avulsions across the valley floor
Equilibrium river long profile can be explained in terms of:
- downstream changes in water and sediment load
- the relationship between discharge, river gradient, sediment size and rivers capacity to transport sediment
Accommodation space available for sediment storage is controlled by:
Base level, tectonics and equilibrium landform morphology
What are the two types of sea level change?
Eustatic controls: sea, water and ice volumes, thermal expansion of water, change in the shape of the ocean basin- change in the amount of water the ocean can hold- global effect
Isostatic adjustments: due to an increase or decrease in the height of the land- surface loading and unloading- when the height of decreases sea levels rise- local effect
Tectonic controls on Ocean basin levels
What 4 sections is the coastal zone broken into?
Coastal plain, shoreface, continental shelf (water depth 100-200m), continental slope
What is morphodynamics?
The dynamics of beach morphology- the study of the interaction and adjustment of the sea floor topography and fluid hydrodynamic processes
How are waves generated?
By wind and seismic activity
Deep water wave height is determined by…?
Wind speed and duration
What happens as a wave approaches the shore?
It is influenced by the frictional drag of the sea bed
They steepen before breaking
Some of the waves energy is dissipated, providing energy to drive sediment transport
How are tides produced?
By the attraction of the sun and the moon and are influenced by the shape and size of the the ocean basins and the coriolis force
Actual near shore water levels can be increased by storm surges (e.g. Dawlish 2014)
What is the tidal range controlled by?
Bathymetry- the topography of the sea floor
Width of continental shelf
Coastal configuration
Distance from amphidromic point ( a point of zero amplitude of one harmonic constituent of the tide)
What determine the extent of the intertidal zone?
The tidal range and coastal gradient
Microtidal 4m
Erosion all coasts have:
High energy systems with low sediment inputs
What are the controls on coastal erosion?
Wave environment (wave height, storm event frequency)
Coastal lithology (susceptibility to weathering and erosion, structure, availability of erosion all tools
Coastal morphology (platform configuration, cliff height and angle, bathymetry)
Tidal action, climate, biological activity
Give examples of erosional landforms:
Cliffs Caves Arch Wave cut notch Platform
De positional coasts have..?
Low energy systems or high sediment inputs
Beach profiles depend on?
Beach gradient depends on sediment size
Beach form varies seasonally
Give examples of planform coastal features..
Barrier islands
- linear, parallel to land and separated by lagoon
- low gradient coasts with low tidal range
Estuaries
- valleys and lowland areas drowned by post glacial sea level rises
- zone of salt and freshwater mixing
- sediment derived from local, fluvial and coastal sources
- deposition controlled by sediment size, water energy and salinity
Salt marshes and mud flats
- salt marshes occur on upper parts of the intertidal zone
- dissected by tidal creeks that supply/ remove water sediment
- salt tolerate vegetation helps to trap sediment
- sensitive to changes in sea level and sediment supply
What controls are there on sediment supply to the coast?
Catchment characteristics (size, relief, lithology, tectonics)
Climate, soil and vegetation
Human activity in the catchment
What are the controls on sediment dispersal at the coast?
River flow characteristics (e.g. Flood frequency)
River sediment load (amount and size of sediment)
Strength of estuarine tidal currents
Strength or absence of offshore currents
Buoyancy of river water (determined by density contrast)
Wave environment at the coast
What are the different types of delta?
Fluvial dominated- elongate
Tide dominated- estuarine
Wave dominated- cuspate
Mississippi has a what kind of delta?
‘Bird foot’
Delta growth reflects the balance between..
Sedimentation (due to sediment supply)
Subsidence and compaction
Sediment reworking by waves, tides and currents
Sea level change
South Louisiana is the product of how many years of accumulation?
200 million
How have changes occurred in sediment delivery to the ocean?
Dam constructions have reduced sediment supplies to the coast
Give an example of a mega delta.
A mega delta is a generic term given to very large Asian deltas such as the Ganges- Brahmaputra, Indus ect
What impacts are there expected to be on deltas and mega deltas by 2050?
Changing sediment input Precipitation change Sea level change Land and shelf subsistence Coastal erosion Subsurface abstraction oil, water, gas Increased population pressure (500 people per km2 on average)
Coastal environments are influenced by:
Water and sediment supply from rivers
Wave environment and tidal range
Ocean basin and Coastal configuration
Shoreline morphology and lithologg
Sediment fluxes to the oceans have changed significantly due to
human activities such as dam construction, agriculture and deforestation
How many people in the future will be at risk due to sea level rise?
Up to half a billion people will be at risk from flooding in the near future due to sea level rise, coastal subsidence, declining sediment supply from rivers and climate change
What does vegetation influence?
Hydrology Slope stability Soil quality Surface roughness Atmospheric composition Sediment trapping
How can vegetation effect landslides?
Reduce the risks of landslides by draining the soil, and the binding of the roots together stops the soil from slipping
However if canopy vegetation is well evolved the weight of the vegetation can increase the risk of landslide
Vegetation can catch the wind increasing the chance of landslides
Is vegetation key in the formation of sand dunes?
Yes
Do some landscapes not have vegetation?
Yes, deserts, poles
What are the controls on rainfall?
- Atmospheric circulation
- Continental isolation (lowest amounts of rainfall occur in central Australia due to the continents isolation)
- Rain shadow effect (warm air evaporated off the ocean, rises,
cools and rains over the mountains, land behind mountains is dry) - Ocean circulation (when cold water streams pass through warmer areas, much less condensation so drier conditions prevail)
What is Aeolian sediment transport?
Lack of vegetation means that soil is subject to wind erosion
Lack of moisture (in arid regions) will cause an additional trigger for wind transport as there is less cohesion between soil particles and the soil particle size is smaller than with moisture present making them lighter and more susceptible to wind erosion
Explain the aeolian sediment transport mechanisms:
Suspension- finest particles, carried by turbulent eddies, transported over large distances
Saltation- coarser (sand) particles, move in long low hoops, pass momentum by grain impacts
Creep- larger particles slide along the surface
Sediment transport air vs water.
Larger particles transported in water
Saltation dominates air flow
Suspension dominates water flow
What are desert pavements/ regs?
Loose particles are blown away (eroded) from the surface
Sediment sorting by selective entrainment produces a pavement
Erosion is limited by surface coarsening
What are ventifacts (Driekanters) and how are they formed?
Large boulders
Produced by abrasion (sand blasting) by wind
Changing wind directions and ventifacts orientation lead to formation of multiple facets that meet at sharp ridges
What are yardangs?
Wind abraided ridges orientated with the prevailing winds and separated by abraded chutes that conduct wind blown sand
The ridges are orientated along the prevailing wind direction
What are inselbergs?
Isolated mountains of resistant rock
Rounded slowly by abrasion over long periods of time
E.g. Ayers Rock
What are sand seas/ ergs?
Large regions of sand deposition, induced by changes in wind speed (due to barriers or transitions between climate zones)
Most deserts don’t look like this- occurs when the wind suddenly drops
When wind transports quite a lot of material and then stops the material is suddenly deposited
What are Barchans (crescentic dunes):
Solitary, crescent showed dunes whose horns point down wind
Form where sand supply is limited and wind is strong and constant
What are transverse dunes?
Form in sand rich environments with string steady prevailing winds
Barchans merge to form long wavy ridges perpendicular to wind
What are longitudinal (seif) dunes:
Long straight ridges more or less parallel to the wind
Moderate to low sand supply and strong prevailing wind with minor variations in direction
What are the parabolic dunes:
Crescent shaped but tips point up to wind
Common in coastal areas with moderate to abundant sand and winds and some vegetation cover
Vegetation anchors parts of dune while other parts migrate
Name the different types of sand dune:
A) Barchans
B) Transverse dunes
C) longitudinal (seif) dunes
D) Parabolic dunes
What controls dune formation?
Sand supply, wind regime, vegetation
What impacts does agriculture have on vegetation?
Loss of natural vegetation
Increased runoff and erosion
Soil organic matter decline
Soil compaction
Downward spiral in soil quality and fertility
Soil thickness depends on erosion vs weathering rate
Given an example of an area where agriculture has had a significantly impact on agriculture.
Loess Plateau in China
Impacts of deforestation:
Slope instability Soil erosion Loss of soil productivity Increased river sediment load Destruction of coral reefs Climate impacts (loss of carbon storage is soil and vegetation
How are rivers without vegetation effected?
Vegetation influences external (water and sediment supply) and internal process form feedbacks (bank strength, floodplain formation) that control river form and functioning
What are ephemeral rivers?
Rivers characterised by flashy flow regimes and unarmoured river beds
Promotes high rates of sediment transport during floods
What are arroyos?
Deep channels found in semi-arid environments
Formation driven by land use and climate change (leading to vegetation change)m occurrence of intense storms and positive feedbacks
What is the influence of vegetation on the river channel pattern?
Mechanisms driving channel adjustment:
- Vegetation colonises bars that are dry at high flow
- Vegetation prevents significant floodplain flow
- Vegetation slows bank erosion and prevents bend cut off
These mechanisms reduce Chanel width and force the thalweg regions (pools) to link up
In an abiotic world how would the earth look?
Soils would be shallow or absent
Hillslopes would be rocky
Rivers would be steeper and rarely meander
Would landforms in an abiotic world look different? Where can we look for evidence?
Landscape on Mars
The topographic signature of life is not yet detectable
Despite the fact that life (vegetation) exerts a critical influence on landscape processes
E.g. Characteristic topography that cannot be found on other planets that lack life
The development of tree like plants how many years ago led to significant changes in the landscape?
400 million years ago
What is uniformitarianism?
The idea that the present is key to understanding the past
The principle that landscape forming processes that operated in the past were the same as those that operate today
What is gradualism?
The earth and its landscape has evolved slowly over a very long period of time
What is catastrophism?
The idea that the landforms which shape the Earth are a result of a few extreme events such as volcanic eruptions, landslides ect
What is geomorphic work?
The total amount of sediment eroded from a hillslope or the total amount transported by a river during a year
Landslide sediment flux =….
Magnitude X frequency
What is the height and length of Dry Falls in Washington?
Length = 3.5 miles Height = 121m
What is the Missoula Flood?
A huge flood in northern USA
What has happened to the Columbia River Basalt?
It has been stripped of loess by flooding
What is palaeo-competence studies?
A way of calculating how big a flood is from the size of the sediment left behind e.g. The boulders left behind from the Missoula flood
How big and what was the impact of the chi-chi earthquake in Taiwan 1999?
Mw 7.6- earthquakes have a big impact on sedimentary delivery
What is the difference between post and pre earthquake river sediment loads?
Sediment loads in rivers increase after earthquakes due to landslides ect which deposit sediment into the rivers
E.g. The amount of sediment increases in the North Fork Toutle River after the Mount St Helens eruption 1980
What was the response of the North Fork Toutle River after Mount St Helens?
Key channel responses:
- initial aggradation
- subsequent incision and widening
- bed armouring
What happens after an event which has caused the river bed level to increase?
Over time a new equilibrium level will be reached then another event may occur disturbing the bed level again
Which events dominate landscape evolution?
Those that have long term impacts on landscape form and functioning
A landslide up stream will cause what to happen downstream?
Will create a dynamic river downstream through braiding ect
Evidence found within landslides today maybe used for what?
To reconstruct the characteristics of past geomorphic events
In combination with dating techniques this allows magnitude frequency relationships to be derived
How has global vegetation changed?
Vegetation cover has gradually changed from more extreme dessert to more forest
This drives changes in the hydrological cycle, rates of erosion and the global distribution of landforms
What happened to post glacial sea levels?
They dramatically increased
What is post glacial rebound?
The rebounding of the earth (lithosphere) after the ice sheet which was on top melts
How do we have evidence of glaciation?
Through erosional and deposition all glacial landforms
Name erosional landforms and how they are formed.
U- shaped valley
- made through abrasion
- plucking
- meltwater erosion
- crushing/fracturing
Hanging valley
Cirque
Horn and arete
Stoss and lee forms
Name the different moraine types.
Moraine (glacial deposits)
Terminal moraine- marks furthest extent of glacier terminus
Lateral moraine- marks lateral margins of ice
Recessional moraine- marks retreat position
Push moraine- advancing ice pushed older deposits in front of it
Often referred to as ice contact features
Name the different depositional landforms.
Drumlins
- Streamlined landforms may occur in swarms
- higher and wider at upglacier end
- debate over mechanism of formation
Kames
- mound like hills of layered sand and gravel
- material deposited in an ice supported cavity
- ice melts leaving heat of sediment
Eskers
- long sinuous ridges of stratified sand and gravel
- maybe formed by meltwater stream flowing underneath stagnant ice glacier
- aligned parallel to ice movement
- from 10s meters up to hundred km long
Outwash Plains (Sandur) - extensive deposits of fluvial sediment
What impacts of glaciation are there?
Paraglacial landscapes:
Deposition of easily eroded fluvio- glacial deposits and till/drift
Disequilibrium between slope erosion and river sediment loads during Holocene
Modern river loads
Sediment delivery ratio increases downstream
Timescale for recovery > 20,000 years
What are river terraces?
River terraces are formed by alternating periods of erosion and deposition
The width of the trench is equal to the width of the channel belt
What are glacial cycles?
During glacial periods upland glaciers scour the landscape and supply abundant coarse material to pro- glacial braided rivers
This sediment entered storage within braid plain deposits- and is then re- mobilised during interglacials
Remobilisation involves the cutting of a trench - sediment cut from the trench is deposited downstream as a fan
What is a fluvial knickpoint?
When the gradient if the river suddenly changes due to a change in sea level normally shown by a waterfall
How can floodplains be archives of environmental change?
Sediment deposits provide a record of past environmental conditions
To make use of such deposits we must understand the relationships between environmental change and landscape response
What did a 18000 year simulation of fan evolution show?
No environmental change at all
Short term fluctuations in water sediment supply but no long term trend
What are causes of fan entrenchment?
- Paraglacial decline in sediment decline
- Product of difference between slope and river sediment transport regimes
- Process- forms feedbacks (autogenic behaviour)
What is equifinality?
When different processes or environmental conditions can produce the same landform
What are the future impacts of hydrology changes in Europe?
Increased flood risk (including flash flooding)
Change in timing of snowmelt floods
Enhanced soil erosion potential in both northern and Southern Europe
Why could sea levels be expected to rise by 5m in the future?
Melting of the Greenland Icesheet
Expansion and contraction of glaciers and ice sheets create what?
Erosional and depositional landforms
Landscape rejuvenation can be driven by..?
Tectonic uplift and sea level change
What are landscapes formed by running water called?
Fluvial landscapes
Give an example of a conceptual model of landscape evolution.
WM Davis ‘cycle of erosion’ - evolutionary approach borrowed from darwism
Over time the landscape becomes less mountainous and flatter
What are the key concepts of the Davis model of landscape evolution?
Potential energy- potential to do work (erode)
Determined by the land surface height above a reference level
Base level- the level to which the landscape erodes- equivalent to sea level
Peneplanation- the decline in surface elevation, gradient and relief over time as the landscape erodes
What modifications have been made to Davis’ simple model?
Renewed uplift- leads to landscape rejuvenation and creation of a polycyclic landscape (mix of young and old landforms)
Climate- landscape evolution depends on the ‘intensity’ of geomorphic processes which varies between humid, arid and glacial climates
Geology- lithology and structure influence the evolution of drainage patterns
What are the alternative models of landscape evolution to the Davis model?
Penck and King
What are the mechanisms and rates of uplift?
Orogenic uplift- uplift by horizontal compression and folding of the Earths crust 4-10mm per year
Epeirogenic uplift- uplift by vertical elevation of large blocks of earths crust- less than 1mm per year
What is the isostatic response to erosion?
80-85% of the height lost due to erosion is regained by isostatic adjustment of crust
Surface uplift = rock mass uplift - exhumation
What is the isostatic responses to deglaciation?
Short term rates- (for a few thousand years) immediately after deglaciation may approach up to 100mm of uplift per year
Mechanisms of denudation: weathering
Physical (frost thawing alternation)
Chemical (dissolution CaCo3)
Biological (roots)
Temperature and moisture dependent (plant growth, presence of ice vs water)
Mechanisms of denudation:
Water
Mass movement
Wind
Glaciers
How can denudation rates be estimated from the rivers load?
The total amount of material transported by a river is called its 'load'. The load has 3 components Dissolved load (carried in solution) 20% Suspended load (sand silt and clay) 70% Bed load (sand and gravel) 10%
Where will eroded sediments be re deposited?
At footslope positions e.g. Before reaching (permanent) rivers: colluvium
In river floodplains: alluvium
Intertidal zones, building up estuaries or deltas
Reach open sea, building up fans at sea/ocean basin floor
What controls are there on denudation rates?
Basin relief Climate and vegetation Lithology (physical characteristics of rocks), tectonics and storm frequency River incision rates Glaciers Seismic activity
What controls the rate of mountains?
Glacial erosion
Mountain surface area is concentrated at the snowline
What happens at tributary junctions?
Major rivers set the base level of their smaller tributaries
Isostatic compensation is a key control on:
Global topography
Mountain height and net denudation
