geomorphology final exam

Bierman Ch. 11 Eolian Geomorphology

Bierman Ch. 11 Eolian Geomorphology

What is the predominant geomorphic process in deserts on Earth?

• Fluvial processes • Eolian processes • Mass wasting • Periglacial processes

prominant geomorphic process in Earth's deserts?

fluvial

Eolian

abundant sediment supply that can be mobilized

aka any time there is exposed sediment - i.e. not vegetated

human activity

water table drops

get big dust storms (i.e. Al Arad, Iraq)

these sands are really well-sorted (compared to alluvial)

Where would you expect to find evidence of eolian processes on Earth (i.e. what's important for eolian processes to occur)?

Eolian geomorphic features not restricted to deserts

• Common in locations with abundant sediment supply (beaches)

• Active when environment is too dry for vegetation, or when vegetation has been disturbed by human activity

• Wind is an important geomorphic agent even in humid environments, but features are not always obvious

tera rosa soils

in Mediterranean - lime stone bedrock

red because of dust

Mars

has a weak atmosphere and would not have dust in suspension

how is air different than water as a fluid and what implications does this have for sediment transport?

can transport silt and clay by suspension

can transport sand by saltation (grains bouncing off each other)

Water is >800 times denser Water has a viscosity>50 times air

• Can only transport fines(usually)

• Silt and clay carried by suspension

• Sand usually transported by saltation or rolling

• Wind can transport gravels on Earth only in very unusual circumstances (Creep or sliding, or pumice gravels)

race track playa

rocks moving when playa has thin ice film then wind blows them

Settling speed of particles in air

Stoke's Law

Ss - Settling speed r - particle radius r - density of particle or fluid g - acceleration of gravity mf - dynamic viscosity of fluid

Why was it so much dustier during LGM?

LGM was 5 to 10 times dustier

• Winds were stronger • Glaciers effective at physically eroding bedrock • Large changes in the discharge of meltwater • Not much vegetation, especially at glacial terminus

drier during LGM bc less vigorous hydro cycle (water in ice)

when was the Last Glacial Maximum?

~22,000 years ago

Spatial Distribution of wind-driven geomorphic processes

• Sediment state of geomorphic system

• Identifying the source of wind -transportable sediment

• Determining the availability of transport

• Considering the wind energy available for transport

Wind-active geomorphic environments

• Deserts • Shorelines • Margins of ice sheets • Areas of landscape disturbance • Last Ice Age (much dustier)

the first 4 relate to there being no vegetation

glacial flour

named because of silt-sized partles like flour

Sediment transport by wind...

is a threshold phenomenon with significant feedbacks

sand is hard to transport by eolian, but if it is...

it will have a frosted look from hitting other grains

Transport of Sand

Sand grains - rounded to angular and frosted

mainly quartz, but other minerals common regionally

• Transport of sand increases exponentially with wind speed

• Also true for wind damage (concern about stronger hurricanes

esker stream

flowing at the bottom of a glacier

Eolian deflation

land disturbance = Water table changes = eolian deflation

Wind Deflation

• Common in dry lakes (playas) • Common in areas where the water table has dropped and vegetation died • Often erodes down close to modern water • Erosion common where sediment is "weak, granular, or poorly cemented"

Wind Transport and Erosion in Deserts

• Stronger Winds • Greater sources of dust and sand • Wind depositional and erosional features more obvious

Earth's deserts controlled by downward part of Hadley Cell

deflation

wind removes silt and fine sand from deposits

common when sediment is dry (no rain or veg)

white deposits in desert

wetland deposits

Si-rich water with diatoms

(pic with truck)

What are the main erosional features associated with eolian deflation and eolian sediment transport?

Yardangs Eolian Grooves ventifacts desert pavement Sand Dunes and Features (asymmetrical dunes)

eolian grooves

wind erroded in linear features

ventifacts

3-sided rocks, strong winds, and sand

Ventifacts are wind eroded rocks on the scale of decimeters to meters found in a ricty of locations. They are prevalent in relatively arid regions and in regions that were arid in the past. Common in mid- and low-latitude deserts, ventifacts are also found in polar regions, particularly along valley bottoms occupied by sediment-loaded outwash streams.

Presumably, katabatic winds from nearby glaciers suspended outwash sand and silt that abraded the ventifacts. Ventifacts are often polished, faceted, and may have pits or flutes (elongated pits) on their surfaces. These distinctive surface features suggest that ventifacts formed through abrasion of their immobile surfaces by saltating or suspended material (sand and silt). Most ventifacts are low to the ground, consistent with high sediment concentrations found in air near the ground surface.

yardangs

bedrock eroded by wind abrasion

eroded from the dominate wind direction

Yardangs are stream lined, positive-relief abrasional forms cut into bedrock or other cohesive earth materials by wind-driven sediment

they are much larger than ventifacts, ranging in size from meters to hundreds of meters in length and have a blunt upwind side and taper downwind like an inverted ship's hull. Yardangs are most likely formed where the prevailing wind is unidirectional. The long axas of a yardang is oriented in the same direction as the wind that eroded the feature. Fields of yardangs are found in man mid- and low-latitude deserts such as the Sahara.

sphynx

could be a natural yardang that was carved

blow outs/deflation hollows

Blowouts and deflation hollows are areas where sediment has been removed by wind, forming a shallow pit or depression. They usually extend no deeper than the water. table, where apparent cohesion of the sediment prevents further erosion. Blowouts are most common in areas where sediment is weak, granular, or poorly cemented. For example, blowouts are common in dry lakebeds and in coastal dune fields between vegetated areas, Blowouts are also common in vegetarion-stabilized sand sheers and dunes. Sometimes, small dunes composed of fine-grained sediment (termed lunettes form immediately downwind of blowouts

pinkdunes picture with girl

eolian deposit is striped middle and alluvial stream deposits top and bottom/poorly sorted - desert stream

coppice dunes

small dunes form behind wind break (ex. a bush)

sand ramps

dunes anchored against enscarpment or mountain and clime upwards

pavements

interlocking gravels

silt eroded at surface leaving large clasts

below clasts is silt layer

Av horizon bc pores/vesicles appear if gets wet

Pavements, concentrations of interlocking class on desert surfaces, were once thought to form by deflation, or the selective winnowing and removal of fine sediment by wind. They are common on gently sloping surfaces in arid regions and are especially well-developed on the low-gradient distal sections of alluvial fans. Although deflation can leave a residual lag of clasts, evidence collected since the 1980s suggests that many pavements are born at the surface and prob ably result from the gradual incorporation of wind-deposited silt over time. Well-developed desert pavements have a single layer of interlocking, heavily rock- varnished clasts underlain by a stone-free, columnar, fine grained Av horizon and a reddened B horizon.

What are the main deposits of eolian sediments on Earth?

Ergs anchored dunes (coppice dunes

sand ramps/climbing dunes) free dunes (transverse or linear) ripples loess

freed dunes

• Develop independent of topography • Form depends on wind direction and sediment supply • Sometimes parts of free dunes are anchored by vegetation (e.g., parabolic dunes)

not anchored to anything so move a bit

anchored dunes

Examples of anchored dunes include those downwind of topographic obstructions and vegetation, as well as climbing dunes or sand ramps attached do cliffs and steep slopes. The orientation of active dunes reflects todav's wind direction(s)

ripples

• Super imposed on larger dunes • Generally cm in height, cm to m in wavelength • Adjust rapidly to changes in wind direction

change frequently in response to wind direction

Superimposed on these larger dunes are smaller ripples, ubiquitous asymmetric bedforms that cover dune surfaces. The upwind sides of ripples are gently sloping and the downwind slopes are generally steeper, like dunes. Ripples are ubiquitous in sandy areas without vegetation. Individually, ripples tend to be short-lived, their orientation adapting rapidly to changing wind direction. Sediment grain size appears to control the wavelength and height of ripples. Most ripples have wavelengths of centimeters to meters and heights of centimeters, and they are oriented perpendicular to the wind flow. Low slope angles for ripples, 2 to 7 degrees for stoss slopes and 2 to 10 degrees for lee slopes suggest that suspension rather than saltation and avalanching are the dominant sand-transport processes on these fine-scale features.

Ergs

• Largest Eolian Features(thousands of km2) • Most active Ergs in sub-tropical deserts • Many relict Ergs as well

large areas of sand dunes (ex. Nebraska sand)

Ergs, which can cover thousands to hundreds of thousands of sauare kilo- meters, contain sand derived from ether longshore dritt Alone che coastior (rom direce deposition be rivers. Mose active ergs are in arid and semi-arid regions.

Relict ergs are common in areas that were either drier (less vegetation) or had greater sediment supplies in the past when climate conditions were different. For example, the Sand Hills of Nebraska in central North America have sufficient moisture in today's climate regime to retain a cover of stabilizing vegetation, but were an active erg during drier times in the Pleistocene and Holocene epochs. Erg reactivation does occur, either from disturbance, such as the removal of vegetation, or from climate

know the relation between sediment supply, wind direction, and types of sand dunes (took a screen shot of diagram 12/7)

parabolic dunes star dunes transverse dunes linear dunes barchan dunes

parabolic dunes

Parabolic dunes have their arms stabilized by vegetation. They form where winds blow in primarily one direction.

limited sand and mainly unimodal

star dunes

Star dunes form in large sand seas where sand supply is ample and wind blows from several directions over the course of the year.

unlimited and multiple directions = octopus looking dune

transverse dunes

Transverse dunes form where wind direction is nearly constant but sand supply varies widely.

crescent dunes

either barchan or parabolic

barchans on sides

linear dunes

Linear dunes form over a range of sand supplies. Wind direction varies bimodally in directions <120° apart.

barchan dunes

Barchans are sand-starved dunes that form in places where the wind blows primarily from one direction.

loess deposits

Wind-blown sediments— sand to dust (loess)

typically fine and very well sorted, but not rounded

allowed by stronger winds due to big temperature contrasts between glaciers and tropics

glacial vs inter glacial periods form...

glacial periods = form loess inter glacial periods = forms soils

tropic ecosystems

constant nutrient loss because extreme chemical weathering and ground water movement

nutrients is brought by dust

How can we use loess deposits to reconstruct past climate?

Deposition of silt-sized wind-blown dust

Large spatial distribution

Fertile agricultural soils

Long, continuous records of climate

Deposited mainly during glacial periods when Earth was much dustier Loess

deposited during glacial periods

Red paleosols form during interglacial periods and associated with stronger summer monsoon in Asia

glacial dust sorces

glacier kettle lake esker outwash fan or delta

Western Pacific Dust Flux

Loess Plateau and Pacific Ocean

FIGURE TO NOW

linear dunes also called longitudinal

size of dust

Granules

• 2mm Sand

• 67 micrometers Silt

• 2 micrometers Clay < 2 micrometers

3 bullet points - wind as a geomorphic agent

a. most eolian sediments are rich in quartz b. biological activity greatly reduces the effectiveness of Aeolian processes c. eolian processes are most easily detected where wind-transportable sediment is abundant

3 bullet points - transport of sediments by wind

a. water and air are both fluids that transport sediment on Earth b. particles settle much more rapidly in air than in water d. in many ways sediment transport in air is much more similar to transport by groundwater than by surface water

3 bullet points - How would you reduce eolian erosion on an agricultural field?

a. Increase your Z0 (roughness length) b. plant agricultural crops of different heights c. plant trees between fields

If you were starting a wind energy company in the US, where do you think this company could be most profitable?

midwest

particles about 1 mm in size are generally found in

sand sheets

Identify the continent that is not a significant source of dust on Earth.

South America

4 erosional features

a. ventifact b. yardang c. deflation hollow d. grooves

What kind of dunes would form in Oxford (a sand-starved environment) if winds are generally from the south?

barchans with their arms pointing north

3 bullet points - loess deposits

a. most loess deposits were laid down during glacial periods and derived from glacial outwash plains

b. they are an important parent material for agricultural soils

c. soil development generally occurred on loess deposits during warmer, wetter interglacial periods

What is the main source of surface clasts in a desert pavement according to the textbook?

the surface clasts originally at the surface stay at the surface

Bierman Ch 13 Glacial and Periglacial Geomorphology

Importance of Glaciers and Glacial Processes

• Play a key role in the climate system

• Created much of landscape today in high and mid-latitudes

• Landforms influence movement of water and pollutants

• Most efficient erosional agent on Earth

• Global Warming - Sea level Rise

Hydrologic Budget of Ice

• Today 10% • LGM 25-30% • Permian Glaciation • Neoproterzoic 80%

Ice Melts at higher pressures

it is a percent of the landsurface

North American Glacial Stages

1. Wisconsin- Sangamon, (Ill)

2. Illinoian- Yarmouth (Iowa)

3. Kansan- Aftonian (Afton Junction Ill)

4. Nebraskan

Never Kiss In Winter After You Sneeze

T.C. Chamberlain wrong?

• Nebraskan/Kansan moved further even though less cold because was not frozen to bedrock and lubricated

How do Glaciers Form?

Snow - Density g/cc = 0.05-0.07

Granular Ice - Density g/cc = 0.1-0.4

Firn - Density g/cc = 0.4-0.8

Glacial - Density g/cc = Ice 0.85-0.9

grams/cubic cm

Where is the ELA?

Equilibrium Line Elevation / Equilibrium line altitude

dictates when you can have a glacier (zone of accumulation and zone of ablation

How Fast do Glaciers Move?

• Average = 3-300 m/yr • Steep Ice Falls = 1-2 km/yr

World Record: 15 km/yr (120 ft./day)!

How do glaciers move?

1. Creep - Plastic deformation (Individual ice crystals slip over microscopicdistances over short periods of time)

2. Sliding - Glaciers slip along bedrock/ice contact (Basal Slip/Sliding: Movement of a rigid ice slab along a base lubricated by water) - water under can lift up glacier - water that comes out/floods at bottom of glacier = Jokulhlaup

ice at bed rock does not move but ice within a glacier flows (ductily/plastic

fractures)

warm ice is easier to be ductile

extending and compressing flow

pulled apart and pushed together

crevasse

glacial ice inside glaciers deforms ductiley and snow on inside is brittle and breaks to form crevasse (~50 km)

glacial surge

when lots of water at glacier base - can block streams and cause flooding

know the main differences between wet-based and cold-based glaciers (velocity profile and process)

took picture 12/8

Wet: • Wet-based glaciers with bedrock contact • If under enough hydrostatic pressure, water may offset weight of overlying glacier • Dependant on distribution and pressure of water at base of glacier Warm-based ice is not frozen to the bed and thus can slide over the bed in addition to deforming internally. Ice can move quickly by sliding, especially in summer when there is more meltwater.

cold: creep and sliding Base frozen to landmass Cold-based ice is frozen to the bed. Ice movement is driven by deformation (creep) of the ice driven by the ice surface slope, O. Creep rates of ice are slow.

Sub-Glacial Processes

• Processes occurring at the base of glaciers are difficult to study!

• Sub-glacial drainage, hydrostatic pressure, friction

• Regelation - Melting and refreezing due to changes in pressure from irregular surface

know the main processes of glacial erosion/Main Processes that cause Periglacial Features

• Permafrost - (permanently frozen ground) and its influence on hydrology • Frost action - weathering and heaving of rock and soils • Mass Wasting - especially common at permafrost interface

Importance of Periglacial Processes

• Active over a wide area of the Earth's surface today, and was more extensive during the Pleistocene (Relict periglacial landforms widespread

Appalachia, France)

• Engineering is difficult in regions influenced by Periglacial Processes and Permafrost

• Processes believed to be widespread on Mars

note: Periglacial processes largely neglected inAmerican Geomorphology until theALCAN Highway was built in WWII

American Geologist Muhler translated all ofthe Russian manuals and reports onpermafrost and periglacial processes

building on permafrost

Key is to not melt the permafrost!

rock glaciers

• Large, tongue-shaped or lobate features composed of angular boulders. Range from true debris-covered glaciers with plastic flow to lobes of rocky material cemented with ice that creeps downslope

• Origin can be either glacial or purely peri-glacial

How thick are most perma frost layers?

a few tens of meters

What controls the thickness of permafrost?

A. Surface temperatures below zero B. How long surface temperatures have been below zero C. Geothermal gradient D. Thermal conductivity of bedrock and unconsolidated sediment E. Nature of groundwater flow - it can flow any direction

glacial erosion landform - abrasion

Glacial abrasion produces some of the most distinctive and characteristic evidence of glaciation including striations, grooves, glacial polish, loess, and rock flour. Rocks of all sizes form tools in the ice that abrade the rock below as they are dragged by moving ice across a glacier's bed. One result of this abrasion is rock flour, the finely ground, silt-sized, rock fragments that color streams issuing from glacial margins a distinc tive milky blue-green

glacial erosion landform - striations

Glacial striations are the fine grooves and scratches left on polished rock surfaces by the movement of debris-rich basal glacial ice or deforming basal sediment.

glacial erosion landform - plucking/quarrying

Moving ice can entrain and remove fractured rock it flows over and around and drags away loosened material. Some debris may also freeze onto the base of the glacier. Field evidence for bedrock quarrying is clear -large blocks of rock are conspicuously removed along joint planes from outcrops. Most quarrying occurs in low-pressure regions like the down-ice side of hills because large cavities form most readily in such places.

Regelation

the triple point of water is impacted by pressure, so it melts and refreezes under glacier

this is important in plucking

glacial erosion landform - grooves

Glacial grooves of varying scales can be found on many outcrops aligned with the paleo ice-flow direction. Grooves, striations, and gouges are routinely used as flow direction indicators for now-vanished ice sheets. These erosive forms are distinctive

some have survived burial over hundreds of millions of years to document ancient glaciations

know the main glacial depositional environments

alpine glacier - mountain valley ice caps - mountain range ice sheets - continental scale

alpine galciers

Alpine glaciers are topographically constrained by the cirques in which they originate and the valley walls that confine them Lower reaches of alpine glaciers are often bordered by moraines.

are in a valley

ice caps

Ice caps occupy highlands and in many places bury existing topography. They are drained by outlet glaciers that transport ice to lower elevations where it melts and deposits moraines.