11.8 Fundamentals of Aeolian Sediment Transport: Aeolian AbrasionNT Bridges, Applied Physics Laboratory, Laurel, MD, USAJE Laity, California State University, Northridge, CA, USA

r 2013 Elsevier Inc. All rights reserved.

11.8.1 Introduction 135

11.8.1.1 Grain-to-Grain Abrasion 135 11.8.1.2 Aeolian Abrasion of Landforms 135 11.8.2 Target Characteristics 137 11.8.2.1 Ventifacts 137 11.8.2.2 Yardangs 137 11.8.3 Abrader Characteristics 138 11.8.3.1 The Efficacy of Sand versus Other Materials 138 11.8.3.2 Composition 139 11.8.3.3 Size 139 11.8.3.4 Shape 139 11.8.4 Environmental Factors 139 11.8.4.1 Wind Speed and Shear Stress 139 11.8.4.2 Wind Direction 140 11.8.4.3 Particle Supply, Wind Frequency, and Integrated Flux 141 11.8.4.4 Local Topography 141 11.8.4.5 Local Rock Distribution 142 11.8.5 Planetary Comparisons 142 11.8.5.1 Mars 142 11.8.5.2 Venus 144 11.8.5.3 Titan 144 11.8.6 Conclusions 144 References 145

13

Br

ae

D.

Di

GlossaryAbrader A particle moved by the wind that abrades other

rocks and landforms.

Abrasion The mechanical wear of rock or sediments by

the impact of particles in saltation.

Deflation The removal of loose, fine material from a

geomorphic surface.

Friction speed This speed is proportional to the gradient

in wind velocity above the surface. It is also equal to the

square root of shear stress divided by air density, which is

approximately the magnitude of vertical turbulence.

Shear stress The stress exerted on the ground surface by

turbulent wind.

Threshold friction speed The friction speed at which

particles are lifted off the surface by wind.

Ventifact A wind-eroded rock, whose surface may

show facets or additional textural features such

Treatise on Geomor4

idges, N.T., Laity, J.E., 2013. Fundamentals of aeolian sediment transport:

olian abrasion. In: Shroder, J. (Editor in Chief), Lancaster, N., Sherman,

J., Baas, A.C.W. (Eds.), Treatise on Geomorphology. Academic Press, San

ego, CA, vol. 11, Aeolian Geomorphology, pp. 134–148.

as flutes, grooves, or pits that are indicative of wind

abrasion.

Wind flute An erosional mark that is common on

ventifacts, characterized by an ‘arrowhead’ form that is open

downwind and closed at the upwind end.

Wind-related inverted terrain Inverted relief that

develops when previously low areas, such as river channels,

are left standing in positive relief by wind erosion.

Yardang A ridge, generally elongate in form, which shows

clear signs of having been eroded by the wind, with a long

axis parallel to the strongest flow. Blunt edges generally

point upwind, and tapered edges point downwind.

Yardangs generally occur in small to large groupings and

have sizes of meters to several kilometers. Although

commonly composed of relatively soft rock and sediments,

they can be formed out of any rock type in areas where there

is strong wind and a sufficient sand supply.

phology, Volume 11 http://dx.doi.org/10.1016/B978-0-12-374739-6.00301-8

Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion 135

Abstract

Aeolian abrasion is the process whereby rocks and landforms are eroded by saltating particles, contributing to the for-

mation of ventifacts, yardangs, deflation basins, and inverted relief. Sand is the predominant or sole abrasive agent.Abrasion is sensitive to the highest velocity winds in a region, which controls the directionality of ventifact features. As high-

velocity winds are infrequent, many decades, centuries, or even millennia may be required for ventifact formation.

Ventifacts, yardangs, and inverted relief are also common on Mars, and although abrasion is infrequent, the process does

not require winds or atmospheric densities different than those found today.

11.8.1 Introduction

Aeolian abrasion is generally examined in two different con-

texts: first, in a sedimentological sense, wherein grain-to-grain

impacts of quartz sands in an airstream progressively chip the

edges and corners to produce a more rounded form, thereby

releasing fine particles into the atmosphere, and second, in a

larger geomorphic sense, as a process whereby the surfaces of

rocks and landforms are eroded and modified by the impact

of windblown particles. After introducing grain-to-grain col-

lisions, this chapter will focus principally on the latter process,

which plays a major role in molding the surface of both Earth

and Mars.

11.8.1.1 Grain-to-Grain Abrasion

Although the effect of grain–grain collisions does not affect

landscape modification through abrasion of rocks and land-

forms, it does result in the breakdown of grains, which, in

turn, plays a role in the supply and behavior of particles. These

grains can subsequently participate in abrasion as described

in other parts of this chapter. Experiments have shown

that collisions of sand onto sandy surfaces, such as dunes, and

between grains in saltation, can cause abrasion of angular

protrusions on grains, resulting in rounding and production

of silt-sized particles that can contribute to loess (Whalley

et al., 1982; Wright et al., 1998). For example, calculations

show that in a four-day dust storm, 287 g of silt can be pro-

duced for every kilogram of saltating sand (Wright et al.,

1998). Such a process has been proposed to explain why loess

is located downwind of sand seas in the Middle East (Crouvi

et al., 2008, 2010). Over the long term, the quantity of silt

produced from grain abrasion is outweighed by glacial

grinding and rock weathering, but abrasion can cause spikes in

production, given suitable conditions (Wright et al., 1998).

Although field studies show that the effects of clay coatings on

sand complicate the relationship between silt/dust production

and sand grain parameters (Bullard et al., 2007), it seems clear

that grain abrasion is an important geologic process.

11.8.1.2 Aeolian Abrasion of Landforms

Abrasion is an erosional process akin to sandblasting. It is

distinct from deflation, in which loose material is removed

from soils or sediments or from weathered rock surfaces owing

to the force of the wind alone. The properties of the rock, the

abrading particles, and the local environment all affect the

abrasion process.

Abraded surfaces range from individual rocks, called

ventifacts, to larger and more complex landforms – yardangs,

inverted relief, and deflation basins. Ventifacts form on a

diversity of rock types and deposits. They are commonly

characterized by one of more of the following: (1) a facet or

facets; (2) polish; (3) abrasion features that are rock-

dependent due to variations in primary texture or composition

(etching, fretting, and knobs); and (4) textural features that

are universal among rock types, such as flutes, grooves, and

elongated pits (see examples in Figure 1) (Laity, 1994, 2009).

Yardangs are larger scale landforms that have experienced

abrasion, in some cases combined with deflation. They com-

monly have an inverted keel shape in cross-section and, in

plan view, a blunt upwind face and tapered downwind tail (see

Chapter 11.14). Yardangs are formed in both moderately co-

hesive sediments, including loess, lacustrine deposits, and ash

materials, and harder rocks such as sandstones, basalts, and

dolomites. Inverted relief can also result from abrasion, most

notably when ancient river channels, made resistant by

chemical induration of sediments or a pavement of stream-

deposited rocks, are left standing as the softer surrounding

materials are eroded and lowered by the passage of saltating

sand grains. Finally, abrasion contributes to the formation of

desert depressions, sometimes in association with yardang

formation. An understanding of the abrasion process is nee-

ded to properly interpret the conditions, wind directions, and

time scales required for wind-eroded landforms. Because the

larger of these features can take thousands of years or longer to

form, and many preserved today are not actively forming,

insight is gained on past environments and climates through

their study.

Aeolian abrasion occurs where there is sufficient wind to

mobilize particles, an ample particle supply, little or no

vegetation, and, integrated over time, exposed target materials

that can be abraded. Thus, on Earth, ventifacts occur in des-

erts, and in periglacial, paraglacial, and beach settings. Abra-

sion has occurred in the geologic past, with fossil ventifacts

found not only from the Pleistocene, but also from the more

ancient geologic record (Blackwelder, 1929; Smith, 1967,

1984; Laity, 1992). On Mars, ventifacts and yardangs are

widespread as well, indicating that abrasion has significance

on a planetary scale (Scott and Tanaka, 1982; Malin et al.,

1998; Bridges et al., 1999; Bradley et al., 2002; Greeley et al.,

2006; Mandt et al., 2008; de Silva et al., 2010; Zimbelman and

Griffin, 2010). There is limited evidence for abrasion on the

two other solid bodies in the Solar System with atmospheres:

Venus and Titan. This may be due to their lower wind speeds,

although the smaller amount of surface data compared with

Mars hinders our understanding.

(a) (b)

(c) (d) (e)

Figure 1 Rock abrasion styles. (a) Marble in the Little Cowhole Mountains, Mojave Desert, CA, displaying faceted windward edge. (b) Basaltin the Cady Mountains, Mojave Desert CA. Flutes and grooves are abundant on the entire rock face, reflective of elevated abrasion heights andmultiple wind regimes. (c) Diorite near Silver Lake, Mojave Desert, California, showing a sharp keel separating two facets that result frombidirectional sand transport. (d) Abraded Bishop Tuff in the Aeolian Buttes area, near Mono Lake, California, with a prominent notch that maycorrespond to the zone of maximum abrasion. The very high levels of erosion probably resulted from fluctuating sand base levels in the past.(e) The 1st author next to a Campo Piedra Pomez ignimbrite yardang in the Argentinean Puna. The height of the notch, and its similar orientationto that of the surrounding ripples, indicates that it formed from aeolian abrasion.

136 Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion

The study of ventifacts goes back to the initial geological

surveys of the American West in the nineteenth century (Blake,

1855). For the next hundred years or so the literature was

dominated largely by qualitative studies (Travers, 1870; Gilbert,

1875; Woodworth, 1894; Evans, 1911; Blackwelder, 1929;

Bryan, 1931; Powers, 1936; Needham, 1937; Maxson, 1940).

These investigations established, through geological arguments,

that the facets and textures on ventifacts are due to wind ero-

sion, with most papers pointing to impacting sand as the main

abrasive agent. More quantitative experimental studies began in

the mid-twentieth century. These included long-term (up to

16 years) field investigations of abrasion of various target ma-

terials by sand (Sharp, 1964, 1980) and the use of simple ex-

perimental chambers (Kuenen, 1960; Suzuki and Takahashi,

1981; Greeley et al., 1982). On Mars, ventifacts were tentatively

identified in Viking Lander images in the 1970s and un-

equivocally from the much clearer Pathfinder (Bridges et al.,

1999) and later Mars Exploration Rover (MER) images

(Sullivan et al., 2005; Greeley et al., 2006, 2008; Thomson

et al., 2008). The presence of ventifacts on two planets with very

different gravities, atmospheric densities, and typical wind

speeds spurred further field, wind tunnel, and theoretical re-

search on fundamental formative mechanisms (Greeley et al.,

2002; Bridges et al., 2004, 2005; Laity and Bridges, 2009).

Yardang studies began with field research in Africa and

western China in the late nineteenth and early twentieth

centuries (Stapff, 1887; Walther, 1891, 1912; Kozlov, 1899;

Hedin, 1903). These, and subsequent investigations, were

largely descriptive in nature, but several made reference to the

roles of water and wind (including deflatation vs. abrasion) in

yardang formation. Martian yardangs were first identified in

the early 1970s from Mariner 9 orbital images (McCauley,

1973), prompting the first wind-tunnel studies (Ward and

Greeley, 1984). Today’s higher resolution images, such as from

HiRISE (McEwen et al., 2007), have stimulated renewed field

investigations (de Silva et al., 2010). However, to date, there is

much less process-based research on yardangs than ventifacts.

Studies of abrasion in desert depressions have largely focused

on dust generation rather than surface lowering, and the for-

mation of inverted relief by wind erosion has received limited

attention. As a result, this chapter will focus largely on abra-

sion studies based on ventifact research (and, to a lesser ex-

tent, yardangs), with the assumption that much of this work

has applications to other landforms.

All particles blown by the wind contain kinetic energy

(KE), which is equal to 0.5 mv2, where m is the mass and v is

the velocity. When these particles collide with a rock or other

surface, some of this energy is transferred into the surface, with

Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion 137

the exact value dependent on the restitution coefficient (the

ratio of outgoing to incoming particle velocity). Outgoing grain

velocities also affect the efficiency of secondary impacts. The

energy imparted into the rock can be dissipated in a number of

ways, including plastic and brittle deformation, heating, and

removal of material (aeolian abrasion). This chapter reviews

those aspects of the process that are presently understood,

focusing on aeolian abrasion. The morphology, texture, and

geographic distribution of abraded rocks and landforms are

briefly discussed in order to understand the main erosional

outcomes, with more detailed overviews reserved for the

companion chapters in this volume, notably Chapters 11.14

and 11.15. Together, these three chapters, as well as the other

contributions to this volume, should give the reader a complete

overview of aeolian abrasion.

11.8.2 Target Characteristics

11.8.2.1 Ventifacts

Ventifact morphology and the rate of abrasion are dependent

on several parameters of the target rock. The orientation

of the rock face relative to the wind determines the impact

angle, the imparted kinetic energy, and concentration of

impacting grains. The greatest mass loss seems to occur

at facet angles of 901, based on theoretical considerations

(Bridges et al., 2005). At these steep angles, abrasion occurs

mainly through parallel erosion across the entire facet (i.e., the

facet angle does not change much). Wind tunnel studies show

that facet angle changes the most at intermediate angles

(Bridges et al., 2004), in agreement with earlier work that

showed the greatest change in windward profiles at a facet

angle of 601 (Schoewe, 1932). As the facet angle decreases

further, the kinetic energy flux reduces considerably, lowering

the abrasion rate (Bridges et al., 2005). Eventually, an equi-

librium angle may be reached at which little or no further

angle changes occur.

Including the probabilistic exponential distribution of

saltating grains and their component kinetic energies shows

that the KE flux is greatest at heights of 10–40 cm (Anderson,

1986), with the exact value dependent on factors such as

particle size, friction speed, and ground hardness, in agree-

ment with field and wind tunnel studies (Sharp, 1964, 1980;

Wilshire et al., 1981; Liu et al., 2003). At these heights, particle

trajectory angles are nearly horizontal, with orthogonal rock

faces abrading at the greatest rate. This causes rocks with

heights up to the maximum KE point to evolve a ramped facet

sloping away from the direction of maximum windblown

particle transport. In taller rocks, notches can develop in some

places, as wind erosion lessens with height above the max-

imum kinetic energy point (Figure 1(d) and (e)) (Bridges

et al., 2005). Field conditions are much more complex than

those observed in the wind tunnel and may promote higher

levels of wind abrasion. For example, in distributed rock fields,

sand can bounce from one rock to another to heights of many

meters (Laity and Bridges, 2009). Additionally, ramp and

moat structures are common in front of the upwind faces of

ventifacts, with the ramps propelling saltating grains to higher

elevations on the rock.

Rocks of all sizes are susceptible to abrasion. However, field

evidence shows that small stones, with diameters of a few

centimeters, are more apt to develop facets at the expense of

abrasion textures such as flutes and grooves (Schoewe, 1932;

Wentworth and Dickey, 1935; King, 1936; Needham, 1937;

Maxson, 1940; Glennie, 1970; Czajka, 1972; Whitney and

Dietrich, 1973; Babikir and Jackson, 1985; Nero, 1988; Laity,

1994). Intermediate-sized rocks generally show both facets

and features/textures, with very large rocks (4B1 m) com-

monly lacking facets (Laity, 1994). The reasons for these dif-

ferences have not been well studied, but may simply be that

small rocks are sized at a scale smaller than that of the typical

primary textural variation that gives rise to flutes and grooves,

and the great mass loss needed to form a facet on a large rock

exceeds the duration of abrasion.

Rock hardness and heterogeneity affect the rate of abrasion

and the production of ventifact textures. Most extant ventifacts

are igneous or metamorphic. Sedimentary rocks abrade

rapidly because they are generally softer (e.g., shales and

siltstones) or have component grains that are easily disaggre-

gated (e.g., sandstones). Furthermore, such properties mean

that they are also susceptible to other forms of physical

and chemical weathering; thus, they are less well preserved.

Weathering of igneous and metamorphic rocks in deserts,

particularly along grain boundaries, also affects the preser-

vation of ventifact features (Selby, 1977; Lancaster, 1984;

Laity, 1992). Heterogeneous hardnesses and rough or pitted

primary surfaces promote the formation of ventifact textures.

In the first case, softer minerals abrade at greater rates than

harder ones, with the differential abrasion producing inden-

tations and protrusions, respectively. Primary textures on

rocks, such as vesicles in basalt, promote abrasion because

they present multiple geometries to impacting particles,

including those angles most favorable to abrasion, and can

cause particles to rebound off multiple surfaces. Over time,

these can develop into elongated pits, flutes, and grooves

(see Chapter 11.14).

11.8.2.2 Yardangs

Wind erosion is central to the genesis of yardangs from ini-

tially flat-lying or topography-draping materials, to their sub-

sequent evolution as three-dimensional forms. Preliminary

erosion is generally focused on preexisting surface lineaments,

such as fluvial channels or joints, which are exploited by the

wind in the corrasion process. Because abrasion is confined to

the saltation cloud in the lower 10s to 100s of centimeters,

this process dominates the lateral abrasion of yardangs and

downcutting of corridors to produce the basic initial form. The

upwind faces of yardangs are commonly marked by flutes and

grooves up to heights of 1–2 m (Hobbs, 1917; Hagedorn,

1971; Grolier et al., 1980), showing the current or the most

recent zone of enhanced abrasion. Undercutting is common

on windward and lateral yardang slopes (Figure 1(e)) (Laity,

1994). There is some debate on the relative roles of abrasion

and deflation in yardang evolution, but the consensus is

that only very soft yardang materials experience significant

erosion via deflation (Laity, 1994, 2009), although highly

weathered material may be deflated from harder rocks. Sec-

ondary processes in yardang formation are fluvial erosion

138 Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion

(including gully formation and lateral erosion), chemical

weathering and solution, and mass movement (Krinsley, 1970;

Mainguet, 1972; Laity, 1994, 2009).

Yardangs show considerable variation in form, with ma-

terial properties, including internal variations in resistance to

erosion and weathering, playing an important role (Grolier

et al., 1980). Soft yardangs erode rapidly and are therefore

generally smaller than their harder counterparts (de Silva

et al., 2010). Horizontally bedded primary forms erode into

tabular yardangs with protruding shelves (Laity, 1994, 2009),

in contrast to inclined or vertical layers that erode into

ridges (Blackwelder, 1934). Many ignimbrite yardangs contain

indurated layers formed from heat sintering and volatile-

induced cementation. These act as protective caps for under-

lying softer materials, such that these types have relatively flat

tops, with undermining of the indurated layers producing

blocks that fall to the yardang bases (de Silva et al., 2010).

11.8.3 Abrader Characteristics

11.8.3.1 The Efficacy of Sand versus Other Materials

The wind transports particles via suspension and saltation.

Grains on the order of o10 mm in diameter have terminal

fall velocities that are less than the vertical component of

turbulence (which is equivalent to the friction speed, u�)

and therefore remain suspended over long time scales. This

material is classified as dust. Larger particles (B100 to a few

hundred microns, depending on conditions) saltate (hop)

near the ground, being launched on ballistic trajectories from

the force of impact from upwind saltating grains or lifted off

the surface from shear stress (t) in the near-surface boundary

layer. Such particles include silicate sand, as well as snow, ice

crystals, and dust aggregates. Heavier particles are confined to

sliding or rolling along the surface from saltation-induced

impact (Greeley and Iversen, 1985) and, because velocities are

low, play no role in abrasion.

Based on field, theoretical, and experimental studies, dust

has been proposed to polish and even abrade rocks (Maxson,

1940; Sharp, 1949; Higgins, 1956; Dietrich, 1977a; Whitney,

1978; McCauley et al., 1979; Lancaster, 1984; Breed et al.,

1989). In cold climates, abrasion by blowing snow and ice has

been advocated (Teichert, 1939; Dietrich, 1977b; Schlyter,

1994). Critically examining this body of work, and with ref-

erence to recent laboratory and field studies and theoretical

considerations, silicate sand is considered as the major or sole

abrasive agent for the following reasons (Laity, 1994; Laity and

Bridges, 2009):

1. Sand is 10–100 times larger in diameter than dust, and

therefore has B103–106x the volume. The bulk density

is about three times that of typical dust, such that the mass

is B3�103–3�106x greater. Dust, being suspended, is

strongly coupled to the airflow, and therefore tends to flow

around rocks. Any impacts that do occur are generally

oblique (Anderson, 1986). In contrast, sand is largely

decoupled from the wind and hits rocks on ballistic

trajectories, including nearly orthogonal impacts that impart

considerable kinetic energy (see Section 11.8.2.1). Model-

ing shows that 10 mm dust impacts rocks about 10% as

often as 100 mm sand (Anderson, 1986). With the thinner

atmosphere of Mars, dust should be less effectively coupled

than on Earth, but the increased probability of impact is

probably counteracted by the generally smaller size of

Martian dust (on the order of 1–2 mm) (Tomasko et al.,

1999; Wolff et al., 2006) compared with that on Earth.

Considering all of these factors, integrated kinetic energy

from sand collisions is B104–107 or more times greater

than from dust.

2. The failure criterion for abrasion is not simply the mo-

lecular bonding strength of the target materials. Rather,

brittle substances are characterized by microflaws as

described by Griffith theory (Lawn, 1995). When a particle

collides with a rock, a stress field is applied and, if it

intersects a microflaw, a fracture is produced and failure

(abrasion) can result. Because the kinetic energy of sand

collisions is orders of magnitude greater than that of dust,

the deformation zones are much more likely to intersect

fractures, thereby causing abrasion.

3. Ventifacts with prominent polish – a characteristic that has

been proposed to result from dust abrasion (Sharp, 1949;

Lancaster, 1984) – exhibit cleavage fractures and micro-

gouges when examined at high resolution using scanning

electron microscopy (Laity and Bridges, 2009). This texture

is consistent with sand, not dust, impact.

4. Sand and dust exhibit distinctly different kinetic energy flux

profiles with height (z). As discussed in Section 11.8.2,

saltating sand has a prominent kinetic energy maximum a

few 10s of centimeters above the surface, where the com-

bination of particle concentration and individual KEs

peaks. In contrast, dust concentration is relatively constant

in the lower atmosphere and the velocity increases in

proportion to the logarithmic wind speed profile (ln(z/z0),

where z0 is the roughness height). Therefore, the kinetic

energy increases in proportion to ln(z/z0)2. The kinetic

energy flux for dust near the surface is negligible (Ander-

son, 1986), as opposed to sand, where saltating grains

make a significant contribution (Bridges et al., 2005). Were

dust to dominate abrasion, rocks would show very modest

slope retreat, with erosion continuing steadily to heights of

many meters above the surface. Rocks would lack the dis-

tinctive notch seen in some ventifacts and many yardang

field examples (Figure 1(d) and (e)) – a feature consistent

with the maximum zone of abrasion predicted by the sal-

tation model (Sharp, 1964, 1980; Wilshire et al., 1981;

Laity and Bridges, 2009).

5. In all places where ventifacts occur, sand is either present

today or was in the past. Even a cursory sampling of the

literature shows sand as the major abrasive agent in deserts

(Sharp, 1964, 1980; Sugden, 1964; Selby, 1977; Smith,

1984; Laity, 1992, 1995), along coastlines (King, 1936;

Knight, 2008), and in periglacial regions (Powers, 1936;

Tremblay, 1961; Miotke, 1982; McKenna-Neumann and

Gilbert, 1986; Nero, 1988).

6. Dusty areas without current or past sand do not have

ventifacts. For example, the 7.5 Ma Cima volcanic field,

California, lacks sand, but has more than a meter of dust-

derived silt deposited in some areas (Dohrenwend, 1987).

The rocks show no evidence of abrasion. This is in marked

contrast to the B18 ka Pisgah flow field, near Barstow,

Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion 139

California, which is in a sandy area, and contains abundant

ventifacts (Laity and Bridges, 2009).

7. Some literature suggests that dust entrained in vortices

causes leeside abrasion (McCauley et al., 1979). Such

models are questionable owing to the arguments presented

above in this section. Rather, ‘leeside’ abrasion features

actually represent windward abrasion, which may be ex-

plained by seasonally or within-storm reversing sand-laden

winds (Greeley et al., 2002).

8. In cold environments, saltating snow and ice are abundant

and it is has been proposed that these materials contribute

to rock abrasion (Teichert, 1939; Dietrich, 1977b; Schlyter,

1994). However, other studies have shown sand as the

main abrader in cold climates (Blackwelder, 1929; Trem-

blay, 1961; Miotke, 1982; Nero, 1988). Field experiments

in Canada demonstrate that sand rapidly abraded through

eight layers of painted enamel on poles, yet blowing snow

and ice had no effect (McKenna-Neumann and Gilbert,

1986). Snow, although hard at cold temperatures, has half

the density, and therefore 4x less the kinetic energy, than

quartz (McKenna-Neuman, 1993). Snow crystals are fra-

gile, being made of hexagonal dendrites, needles, and other

shapes (Libbrecht, 2005), all of which should easily frac-

ture upon impact, in contrast to quartz, whose round

shape, in addition to high density and hardness, make it a

good abrader. More than a century of inclement weather

aviation demonstrates that ice collisions, at much higher

velocities and kinetic energies than in the natural en-

vironment, do not cause mass loss from airplane wings.

For all of these reasons, sand, rather than other materials

such as dust, snow, and ice, can be considered the dominant

or sole contributor to rock abrasion. This is assumed in the

discussion of other parameters that follow.

11.8.3.2 Composition

With regard to grain composition, two factors – density and

hardness – affect abrasion. The density determines the mass

contribution to kinetic energy and the hardness determines

the ability of the grains to transfer this KE into the target (as

opposed to the grains getting deformed themselves). Most

sand on Earth is quartz and on Mars is basalt (Christensen

et al., 2004; Morris, 2004; Squyres et al., 2004; Soderblom

et al., 2004). The typical densities of these materials are

2650 kg m�3 and 3000 kg m�3, respectively. Therefore, as-

suming the same velocities and other conditions, the kinetic

energy imparted upon impact is about 10% greater for grains

composed of basalt compared with quartz.

However, the absolute hardness of quartz (820 kgf mm�2

on the Knoop scale (Shackelford and Alexander, 1991)) is

about 30% greater than the typical feldspars and pyroxenes

that make up basalt. This probably counteracts the slightly

lower quartz density, consistent with the similar abrasion

susceptibilities found for quartz and basalt particles (Greeley

et al., 1982, 1984). Some sand on Mars consists of coarse

hematite grains (B5100 kg m�3; hardness of 750 kgf mm�2)

(Squyres et al., 2004; Weitz et al., 2006) and probably gypsum

(B2300 kg m�3; 32 kgf mm�2) (Langevin et al., 2005).

If hematite saltates on Mars, it should be a more effective

abrader than quartz and basalt, all other conditions being

equal, whereas gypsum will be relatively ineffective.

11.8.3.3 Size

Sand mass scales with the cube of the diameter, such that

doubling the average size results in nearly an order of magni-

tude increase in kinetic energy for a given velocity. At threshold,

the minimum speed needed to move sand, velocities are low

and little abrasion takes place. Rather, as demonstrated in cor-

relations between the trends of winds and ventifact features, it is

at wind speeds significantly above threshold that most abrasion

takes place (Bridges et al., 2004), and, at any given wind speed,

grains will reach different velocities upon collision as a function

of their mass (Bridges et al., 2005).

For example, a 25 m s�1 wind at 1.2 m height will accelerate

150 mm and 600 mm quartz grains to B9 and 3 m s�1, re-

spectively (Bridges et al., 2005). The lower velocity of the

600 mm grains is due to their greater weight, therefore requiring

higher threshold speeds to induce motion and higher speeds

above threshold to reach velocities equivalent to that of smaller

grains. However, the resulting KE of the 600 mm grain is about

20x greater because it is 64x heavier. Therefore, an area with

coarse sand may experience less abrasion simply because winds

are insufficient to accelerate the particles to sufficient velocities.

Considering these factors, a general rule is that for winds suf-

ficiently above threshold, larger particles abrade more, but as

wind velocities decrease, smaller particles will do more of the

work. This may be why in coastal areas and desert fluvial

channels, where sand may be more poorly sorted and coarser,

ventifacts abrade more rapidly (see Chapter 11.14).

11.8.3.4 Shape

Most aeolian sand on Earth and Mars, whether quartz or

basalt, is well rounded due to multiple saltation collisions that

have smoothed the grains into spheroids. Therefore, shape is

generally not a factor in abrasion. However, if grains are an-

gular, there is evidence that they are able to cut and gouge the

target rocks more effectively, enhancing abrasion (Miotke,

1982). Therefore, in environments with relatively fresh grains,

such as recent tephra, alluvial, or impact deposits, abrasion

may be greater than in regions with mature, rounded sand.

11.8.4 Environmental Factors

11.8.4.1 Wind Speed and Shear Stress

Under ideal turbulent conditions, the wind speed (u) at a

given height (z) is:

uðzÞ ¼ ðu�=kÞlnðz=z0Þ

where k is the von Karman constant (B0.4), z0 is the height at

which the wind velocity is zero due to the influence of local

roughness elements (sand grains, rocks, etc.), and

u� ¼ ðt=rÞ0:5

where r is the air density. The threshold friction speed, u�t, is

reached when the shear stress (t) exceeds the critical value at

140 Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion

which grains are lifted from the surface. For particles with low

cohesion (generally those 4B100 mm in diameter (Greeley

and Iversen, 1985), this velocity is proportional to the square

root of the particle size. Unfortunately, determining the

velocities that grains reach upon collision with a rock, which is

needed to assess abrasion potential, is complex, depending

on a number of factors that must be modeled numerically

and that cannot be represented in a simple analytic ex-

pression. As introduced in Section 11.8.3.2, higher speed

winds can saltate larger particles, but, depending on the con-

ditions, not necessarily at greater speeds than smaller particles.

For example, as shown in Figure 2, the kinetic energy

of 150 mm sand exceeds that of 600 mm grains up to wind

speeds (at 1.2 m) of 14 and 42 m s�1 on Earth and Mars,

respectively (from data used in Bridges et al., 2005). At higher

velocities, the KE of the 600 mm grains is greater than that

of 150 mm sand. The flux of saltating grains is proportional

to u�3 for friction speeds above threshold (Bagnold, 1941).

Thus, a doubling of the wind speed from 10 to 20 m s�1, in

addition to causing a B10x increase in kinetic energy per

grain (Figure 2), also increases the flux by a factor of

8 (23), such that the integrated kinetic energy increases by

almost two orders of magnitude. This is consistent with field

data that show alignment of ventifact features with the

highest speed winds, not winds at threshold (Bridges et al.,

2004). Therefore, some general rules are that, (1) when

comparing settings with different grain sizes, conditions

must be carefully considered, as increasing wind speed does

10−4

10−5

10−6

10−7

10−8

Kin

etic

ene

rgy

per

grai

n (J

)

10−9

10 15 20 25Wind speed

Kinetic energy of grains

Figure 2 Calculations of grain kinetic energies as a function of wind speed

not necessarily result in faster grain collisions, and (2) for a

given grain size, a critical wind speed must be reached for

significant abrasion to occur, above which rates increase

dramatically.

11.8.4.2 Wind Direction

Clearly and obviously, the sides of ventifacts and yardangs

subjected to the greatest flux of high energy sand impacts will

be the most abraded. Diagnostic abrasion textures and facet-

ing on the windward side of ventifacts (Blackwelder, 1929;

Sharp, 1964; McKenna-Neumann and Gilbert, 1986) allow

them to serve as vanes of current and past high-speed winds,

both on Earth and Mars (Laity, 1992; Bridges et al., 1999;

Greeley et al., 2006, 2008; Thomson et al., 2008). Unlike the

trends of dunes and ripples, which are reflective of all winds

above threshold, ventifacts record wind speeds equal to or

greater than that needed to accelerate particles to velocities at

which abrasion takes place (Bridges et al., 2004). Diurnal and

seasonal reversing wind flow is common in many settings and

it is here that ventifacts with bidirectional facets and flutes

occur (e.g., Figure 1c). Because ventifacts form by abrading

into hard rock, they represent a record of high-speed winds

spanning over decades, centuries, or millennia, as opposed to

bedforms, or preserved aeolian crossbeds, which reflect only

the most recent or last activity. Yardangs can also record

changes in wind activity. For example, in one case on Mars,

at 1.2 m (m s–1)30 35 40 45 50

150 µm quartz (Earth)600 µm quartz (Earth)

600 µm basalt (Mars)150 µm basalt (Mars)

at end of saltation trajectory

on Earth and Mars.

Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion 141

stacked yardangs exhibit morphologies consistent with chan-

ges in wind direction over time (Wells and Zimbelman, 1997;

Zimbelman and Griffin, 2010).

Figure 3 Large ventifact at top of hill, near the Mojave River sink,Mojave Desert, California. Enhanced abrasion has occurred becauseof accelerated wind flow over the topographic crest. The particlesupply available for abrasion is related to fluvial processes and torainfall in the distant Los Angeles area. Scale is a 1 foot(30.5 cm) ruler.

11.8.4.3 Particle Supply, Wind Frequency, and IntegratedFlux

Aeolian abrasion requires strong and frequent winds and a

sufficient supply of mobile sand. Thus, on one end of the

scale, the Moon and other airless bodies may have grain-rich

regoliths, but the absence of wind precludes aeolian abrasion.

Similarly, windy areas that lack loose sand, are heavily vege-

tated, or are covered in stabilized sand, have few, if any,

actively forming ventifacts. Abrasion generally ceases when

winnowing of fine sediments leaves behind a coarse lag, and

therefore, renewal of the sand supply or a surface distur-

bance is necessary to continue erosion (Sharp, 1964, 1980;

Ballantyne, 2002). Fossil ventifacts occur in regions that once

had strong winds or a supply of mobile particles, but currently

do not (Laity, 1992, 1994). These end-member cases describe

conditions in which the particle supply or flux is essentially

nil. When the particle supply is low, but not zero, little abra-

sion is expected, as it takes many sand impacts to form a

ventifact. Moreover, the retention of the ventifact form repre-

sents climatic conditions in which abrasion is more effective

than other weathering processes, such as solution. For ex-

ample, the partial abrasion of varnish on older ventifacts in

the Mojave Desert suggests that morphologic development

may occur over multiple cycles associated with changing

particle fluxes. Where the particle size is ample, but the fre-

quency of winds above threshold is infrequent, abrasion may

occur, given sufficient time and low (non-abrasion-related)

weathering rates.

The interplay of factors discussed above can be approxi-

mated. Abrasion susceptibility, Sa, the ratio of the mass of

sand expended to the mass of rock eroded, has been measured

in the laboratory over a range of conditions, and is on the

order of 10�4 (Greeley et al., 1982). From this, the abrasion

rate per unit area, R, can be computed by:

R¼ Saqf

where q is the flux and f is the frequency of winds required for

abrasion. The mass of a 150 mm quartz grain is 4.7�10�9 kg.

To abrade 1 cm3 of a rock of the same density with an Sa of

10�4 requires 5.7 billion sand collisions. Typical sand fluxes in

high wind events at a height of 10 cm are on the order of

0.01 kg m�2 s�1 (Anderson and Hallet, 1986) or about 200

grains cm�2 s�1. At this rate and susceptibility, abrasion of a

1 cm2 indentation to 1 cm depth would take only about

10 months. However, the frequency of winds at or above that

needed to cause abrasion is not one. A typical desert en-

vironment has frequencies of winds with velocities close to or

greater than 10 m s�1 on the order of 0.01–0.2 (Fryberger,

1979; Bridges et al., 2004; Lancaster, 2004), implying a period

of about a decade to a century to form observable ventifact

features. This is consistent with field-based mass loss rates

from rocks and other targets of B30–500 mm yr�1 (Sharp,

1980; Malin, 1985). If the wind frequency diminishes, or the

sand supply is reduced, formation rates will be significantly

lowered. Our calculations are rough, as the frequency of winds

above threshold, and how this relates to abrasion, has not

been explored in detail. This topic should be considered an

avenue of research for future investigators. Furthermore, many

other variables, including rock size and lithology, seasonal and

long-term changes in wind direction and sand supply, and the

effects of local rock distribution and topography, influence

abrasion rates.

11.8.4.4 Local Topography

Local topography influences the direction and speed of winds,

as well as constraining the location of sand, and therefore, the

potential for abrasion. Topographic notches act to funnel and

accelerate wind flow, as reflected in the location, directionality,

and degree of development of ventifacts (Laity, 1987, 1994).

Ridges orthogonal to flow compress streamlines and, via

Bernoulli’s principle, increase the wind speed and shear

stress over their summits, and therefore the velocity and flux

of impacting sand (Jackson and Hunt, 1975; Lancaster,

1984). This accounts for well-developed ventifacts located on

hillcrests (Figure 3) (Laity, 1994). Katabatic winds, prevalent

on mountain glaciers and in the ablation zone of ice caps,

are strong downglacial flows that, where there is an ample

particle supply, erode ventifacts at glacier margins. Examples

include active ventifacts found in the Antarctic dry valleys

(Hall, 1989; Lancaster, 2004; Gillies et al., 2009) and fossil

ventifacts in regions once dominated by Pleistocene glaciers

(Blackwelder, 1929; Powers, 1936), such as the ‘Aeolian

Buttes’ region near Mono Lake, California (Figure 1(d)). On

Mars, the alignment of ventifact features in topographically

complex regions has been tied to wind controlled by the

topography of local hills (Greeley et al., 2008) and large

ripples (Thomson et al., 2008).

142 Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion

11.8.4.5 Local Rock Distribution

Within any given rocks, there are strong and complicated

interactions among the boulders, the wind regime, and sedi-

ment. As a result, there is considerable variation in the form of

rocks (Figure 4). Furthermore, many rocks experience a com-

plex history of development as they are alternately buried and

exposed by shifting sand, both on a diurnal and on a seasonal

basis. In addition to their role as abrasion targets, rocks can act

as ‘shadows,’ shielding other nearby downwind boulders from

impacts, or can serve as high-energy rebound surfaces, with the

sand bouncing high into the wind gradient and accelerated

further downwind (Laity and Bridges, 2009). In addition, the

interaction of sand deposits and rocks influences the saltation

flow. For example, sand ramps developed on the upwind side of

rocks provide paths for elevated sand transport, resulting in

downwind abrasion heights greater than that predicted by

standard saltation theory (Laity and Bridges, 2009).

11.8.5 Planetary Comparisons

We now apply the preceding discussion to abrasion on solid

planetary bodies with atmospheres – Earth, Mars, Venus, and

Titan. This discussion emphasizes a comparison between Mars

and Earth, two planets where ventifacts, yardangs, dunes, ripples,

and sand are abundant. Venus and Titan have thick atmospheres,

low-speed winds, and bedforms, but ventifacts have not been

found and only limited possible yardangs exist on Venus; thus,

our coverage is correspondingly brief. An overview of the specific

aeolian landforms in the Solar System can be found in the two

other chapters (Chapters 11.14 and 11.15) in this volume. This

section focuses on the similarities and differences of the abrasion

processes on these worlds and how this may influence the re-

sulting geomorphology.

11.8.5.1 Mars

Mars has lower gravity than Earth (about 1/3), a thinner

atmosphere (B1/100), and a basaltic particle supply (in

Figure 4 Field of basaltic ventifacts, Cady Mountains, MojaveDesert, CA. Note the diverse distribution of flutes and groovetextures, caused by the interaction of rocks and the local sand flow.

contrast to the dominantly quartz-feldspathic terrestrial

sands). Despite these differences, the size, morphology, and

local distribution of ventifacts are not appreciably different

(Figure 5) (Bridges et al., 1999; Greeley et al., 2000, 2006,

2008; Sullivan et al., 2005; Thomson et al., 2008). In contrast,

yardangs are much larger and more extensive on Mars (see

Chapter 11.15), probably due to the process(es) that formed

the voluminous yardang material, rather than differences in

abrasion style. In addition, the absence or reduced rates of

competing erosional mechanisms on Mars, such as fluvial

processes, result in yardangs being better preserved than is

typically the case on Earth.

Gravity and particle composition are less important than

atmospheric conditions in affecting abrasion. With B1/3 the

terrestrial gravitational acceleration on Mars, saltating particles

can, with sufficient velocities above threshold and under

average conditions, remain aloft longer and have flatter

trajectories than on Earth (White et al., 1976; White, 1979).

However, given the many possible particle sizes and wind

speeds on the two planets, typical saltation trajectories are not

significantly different on the two planets (Bridges et al., 2005).

Gravity also determines the particle size at which friction

speed and terminal fall velocity balance, commonly con-

sidered the boundary between suspension and saltation

(Greeley and Iversen, 1985). The predicted value is about

175 mm for basalt on Mars, compared with 55 mm for quartz

on Earth (Bridges et al., 2010). However, at least on Mars,

grains smaller than the predicted value clearly saltate because

sand o100 mm in size occurs in ripples (Sullivan et al., 2008).

Therefore, the difference in gravity seems to have little or no

effect in sequestering certain particle sizes from the abrasion

system. As discussed in Section 11.8.3.3, the slight differences

in density and hardness between quartz and basalt make their

contribution to abrasion nearly the same.

Over time, Martian gravity has not changed and the

basaltic particle supply has probably remained relatively

constant, but the atmospheric conditions have varied. Given

the similarity of ventifacts and yardangs on Earth and Mars,

an open question is therefore whether a past climate with

a thicker atmosphere (more like our own) is required.

The main effect of the low atmospheric density on Mars is

that the frequency of winds above threshold (ft) is low, be-

cause threshold friction speeds are about 10x greater than on

Earth (Figure 6). At the Viking 1 landing site, ft is estimated at

about 10�4 (Greeley et al., 1982), about 2–3 orders of mag-

nitude less than that of common desert environments on

Earth (Fryberger, 1979; Bridges et al., 2004). This is consistent

with a lack of bedform migration at scales of several meters

over the last few decades (Edgett and Malin, 2000; Zimbel-

man, 2000; Malin and Edgett, 2001) and evidence that craters

formed within the last B100 thousand years postdate the

most recent significant episodes of ripple migration (Golom-

bek et al., 2010). Such observations imply that saltation-in-

duced changes occur over much longer time scales than on

Earth. However, because threshold friction speeds are higher

on Mars, winds exceeding u�t result in grain kinetic energies

about 10x greater than on Earth (Bridges et al., 2005).

Therefore, the integrated kinetic energy of grains resulting

from the product ftu�t should be about 1/100–1/10 the

terrestrial value.

(a)

(d)

(b)

(c)(e)

(h)

(f)

(g)

Figure 5 Ventifacts and abrasion textures on Mars: (a) The scalloped rock ‘Mazatzal’ at the Spirit site (Pancam composite color). (b) Flutedrocks at the Spirit site (Pancam composite color, Sol 584). (c) Bedded rocks that have preferentially abraded along weak layers, Columbia Hills,Spirit site (Pancam, Sol 754). (d) ‘Tails’ in the lee of resistant nodules within sulfate-rich soft rock at the Opportunity site (Hazcam, Sol 142). (e)Faceted and fluted rocks at the Spirit site (Pancam, Sol 585). (f) Faceted rocks near bedforms at the Spirit site (Pancam, Sol 620). (g) Facetedrocks at summit in Columbia Hills, Spirit site (Pancam, Sol 1344). (h) Microscopic image of rock texture in the vicinity of (c) (image width is3 cm, Sol 753). Reproduced from Laity, J.E., Bridges, N.T., 2009. Ventifacts on Earth and Mars: analytical, field, and laboratory studiessupporting sand abrasion and windward feature development. Geomorphology 105, 202–217.

Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion 143

These observations imply that although abrasion is pos-

sible on Mars, it will take at least 10 to 100 times longer than

on Earth. Indeed, ancient rocks exposed at the Pathfinder

site in Ares Vallis have average integrated erosion rates of

0.01–0.04 nm yr�1 (Golombek and Bridges, 2000). These

rates are not likely continuous, as it would imply annual

shedding of rock thicknesses equivalent to that of atoms.

Rather, greater rates probably occurred in pulses as sand

passed through the region or may have mostly operated

early in the history of the site when the particle supply was

greater and high speed winds perhaps more frequent. Low

apparent abrasion rates elsewhere on Mars may be explained

by at least one, and probably multiple, episodes of surface

burial and exhumation (Malin and Edgett, 2001; Hynek

et al., 2002; Edgett, 2005). Material such as the Medusae

Fossae Formation probably shielded underlying rocks for long

time periods, such that rock exposure ages may be consider-

ably less than their formation ages. Therefore, the 1/10 average

KE value on Mars compared with Earth, derived above,

seems at most an upper bound and may be considerably

lower, perhaps spiking during higher obliquity periods when

wind speeds were probably greater (Armstrong and Leovy,

2005). In any case, a vastly different Martian climate, with a

thicker atmosphere or higher speed winds, is not required

to produce the population of observed ventifacts and

yardangs.

Venus

EarthMarsTitan

1

0.1

10

Thr

esho

ld fr

ictio

n sp

eed

(m s

–1)

100

Particle size (µm)

1000

Figure 6 Theoretical threshold friction speeds on the four solid planetary bodies in the Solar System with substantial atmosphere, using theparameterization of Shao and Lu (2000). Basaltic particles (r¼3000 kg m� 3) are assumed for Venus and Mars, quartz (2650 kg m� 3) for Earth,and organic materials (950 kg m� 3) for Titan. Typical temperature, atmospheric density, and gravity conditions for these bodies are used in thecalculations.

144 Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion

11.8.5.2 Venus

Ventifacts have not been found on Venus and radar data from

Magellan with a resolution up to 120 m show possible yardangs

in just one location, near Mead Crater (Greeley et al., 1992).

Bedforms were seen in Magellan data, indicating that saltation

occurs (Greeley et al., 1992), but these were limited in distri-

bution. However, the high density of the Venusian atmosphere

(at B65 kg m�3, more than 50x that on Earth) and the slightly

lower gravity mean that threshold friction speeds should be

about 7x less for basalt on Venus than quartz on Earth (i.e.,

(50�(9.8/8.9)�(2650/3000))0.5 (Figure 6). Surface winds are

low, on the order of 1–2 m s�1 (Couselman et al., 1979), so that

any particles set in motion will have very low kinetic energies.

Therefore, abrasion seems unlikely or rare on Venus.

11.8.5.3 Titan

Yardangs and ventifacts appear to be absent from Titan based

on surface images and radar data. Surface wind speeds are low,

on the order of 1 m s�1 (Bird et al., 2005), and the dominant

‘sand composition’ is probably soft, frozen organics (Barnes

et al., 2007), conditions that are not conducive for abrasion.

11.8.6 Conclusions

This chapter shows that aeolian abrasion is a process in-

corporating many factors, some of which are well understood

and others that require further investigation. The most im-

portant conclusions are:

1. Abrasion occurs both in grain-to-grain collisions, round-

ing sand grains and producing silt-sized particles, and

in grain–surface interactions, which produce important

landforms including ventifacts, yardangs, and inverted

relief.

2. Sand is necessary for ventifact formation. The efficacy of

dust, ice, and snow particles has not been demonstrated.

3. Abrasion operates close to the surface, usually within the

lower 1–2 m. Kinetic energy increases with distance from

the ground, peaks at a distinct elevation (usually in the

lower 10s of cm), and then declines. This relationship is

reflected in the evolution of facets on low-lying ventifacts

and in the development of a ‘notch’ on higher standing

yardangs. Under conditions where sand rebounds off

rocks and sand ramps, the saltation cloud and abrasion

zone can be pushed to greater heights.

Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion 145

4. Yardangs formed from relatively soft rocks are abraded in

the lower 1–2 m above the surface, and may experience

deflation at higher elevations.

5. On ventifacts, mass loss from abrasion is proportional to

the facet angle. As abrasion proceeds, this angle evolves to

an equilibrium slope, after which mass loss is reduced.

6. Preexisting pits and soft heterogeneities in rocks are

nucleation sites for enhanced abrasion, evolving to form

elongated pits, flutes, grooves, and related features.

7. Yardang abrasion is affected by the hardness of the

component material, surface crusts, and the nature and

orientation of original bedding planes. Although other

chemical and physical processes influence yardang devel-

opment, abrasion is the predominant control on the

overall orientation and morphology of the landform.

8. There is little difference in the abrasion potential of quartz

or basalt grains.

9. Ventifact features tend to align with the highest wind speeds

above threshold. This is probably due to the increased in-

tegrated kinetic energy flux as wind speed increases, with

winds below some critical value insufficient to move

enough sand at velocities to cause significant abrasion.

10. Abrasion on Mars can occur under present atmospheric

conditions, but the integrated rates are much slower

than on Earth due to the low frequency of high-speed

wind events, limited particle supply, and, in some areas,

burial. Low wind speeds on Venus and Titan suggest that

aeolian abrasion is very limited in occurrence on these

bodies.

The study of abrasion is immature compared with other

subfields in aeolian geology, such as bedform or dust storm

formation. Migrating sand and dust have received greater at-

tention because they are more apparent and have significant

effects on society. However, the discovery of abundant venti-

facts and yardangs on Mars has spurred interest in aeolian

abrasion. An improved understanding of the abrasion process

increases our ability to make reasonable interpretations of

modern and paleoenvironmental conditions at remote sites.

At present, the fundamental processes that form ventifacts and

yardangs are fairly well understood. However, many questions

await further investigation. These include the formative

mechanisms associated with universally occurring textures,

such as flutes and grooves; the relationship between lithology

and feature formation; and the degree to which features pro-

vide information on the specific environment (atmospheric

density, lithology, wind speed, the nature of the abradant,

particle supply, etc.). One of the greatest uncertainties for all

environments is the rate of abrasion. This is especially the case

on Mars, where our understanding of the frequency of high

wind events, the exposure ages of surfaces, and the availability

of sand, integrated over time, is immature. For terrestrial and

general studies, research focused on quantifying the mass loss

of rocks for given saltation conditions and back calculating

abrasion rates based on ventifact morphology is needed.

However, this knowledge must be integrated into a greater

understanding of local rock field interactions, single storm

dynamics (such as ongoing rock exposure and burial

and fluctuations in wind speed and direction), and short-

and long-term changes in climate, vegetation cover, rock

weathering, and particle supply. Therefore, even for Earth, we

have an insufficient understanding of both modern- and

paleoprocesses. For Mars, advances will be greatest if we can

obtain radiogenic ages of rocks from multiple units, thereby

constraining the times and rates of abrasion. From its initial

beginnings as topical studies, aeolian abrasion research has

advanced to a broad field of endeavor covering at least two

planets. Future investigations will surely be interesting.

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Biographical Sketch

Nathan Bridges’ research focuses on surface processes in the Solar System, principally wind erosion and transport.

His work incorporates field studies, wind tunnel investigations, theoretical treatments, and planetary data an-

alysis. He is a coinvestigator on the ‘Mars Reconnaissance Orbiter’ and ‘Mars Science Laboratory’ missions. Dr.

Bridges worked at the Jet Propulsion Laboratory from 1997–2009 and is currently a Senior Scientist at the Applied

Physics Laboratory.

Julie Laity’s principal areas of research involve wind erosion, desert geomorphology, Martian terrestrial analog

studies (the role of groundwater in channel formation and aeolian processes), and groundwater geomorphology.

She is the author of ‘Deserts and Desert Environments,’ published by Wiley-Blackwell in 2008.

148 Fundamentals of Aeolian Sediment Transport: Aeolian Abrasion