- The internal structure of a well-preserved lithified dune (aeolianite) is primarily composed of large-scale, high-angle cross-bedding. What specific aeolian process is directly responsible for creating these inclined layers?
Answer: Periodic avalanching of sand down the dune's steep leeward slipface.
Cross-beds are formed by the deposition of sediment on the inclined surfaces of bedforms like dunes or ripples. In a sand dune, sand is transported up the gentle windward slope and accumulates at the crest. When the pile becomes too steep (exceeding the angle of repose), a small avalanche of sand slides down the leeward slipface. This process repeats, building up successive inclined layers (foreset beds). These layers are what become the preserved cross-bedding in the rock record. The other processes occur but do not directly form the large, inclined bedding.
- A vast field of transverse dunes exists in an arid basin with a constant westerly wind and abundant sand. A long-term climate shift introduces slightly more rainfall, allowing patchy grasses to colonize the dune field. If the wind regime remains unchanged, what is the most likely morphological evolution for this dune field?
Answer: The transverse dunes will be dissected by wind erosion through gaps in the vegetation, forming a field of parabolic dunes.
The introduction of vegetation is key to transforming dune types. The vegetation will begin to anchor parts of the transverse ridges. The wind will continue to erode sand from the un-vegetated sections, creating 'blowouts'. As the blowouts enlarge, the remaining vegetated portions of the ridge form U-shaped horns that point upwind (west). This describes the formation of parabolic dunes. This transition from transverse to parabolic dunes is a common response to the introduction of stabilizing vegetation.
- Which of the following scenarios provides the optimal combination of conditions for the formation of a large, active sand sea (erg)?
Answer: An arid basin adjacent to a mountain range, with a large supply of sand and persistent unidirectional winds.
The formation of a large sand sea requires three main ingredients: (1) a large and continuous supply of sand-sized sediment, (2) a climate that is arid enough to prevent the growth of stabilizing vegetation, and (3) persistent winds with enough energy to transport the sand. An arid basin near mountains (a source of sediment via weathering) with strong, steady winds provides the ideal conditions.
- A geologist examines a cross-section of an ancient sandstone formation and observes large-scale cross-bedding where the layers consistently dip towards the east. What is the most likely conclusion about the paleoenvironment in which this sandstone formed?
Answer: The prevailing winds were from the west, causing dunes to migrate towards the east.
The dipping layers in cross-bedding represent the former slipface of a migrating dune. Sand is eroded from the windward (stoss) side and deposited on the leeward (slipface) side. Therefore, the dip direction of the cross-beds indicates the direction of dune migration. If the beds dip east, the dunes were migrating eastward, which is caused by a prevailing wind from the west.
- Saltation is the primary mode of sand transport. It is often observed that the wind velocity required to sustain saltation is lower than the threshold velocity required to initiate it. What is the best physical explanation for this observation?
Answer: Airborne saltating grains impact stationary grains, transferring kinetic energy that helps eject them into the airflow.
Initiating grain movement (fluid threshold) requires the wind's aerodynamic lift and drag to overcome gravity and interparticle forces. However, once saltation begins, the process becomes more efficient. Saltating grains return to the surface at high speeds, and their impacts transfer kinetic energy to other grains on the bed. This impact ejection (impact threshold) can launch new grains into the air even at wind speeds below the initial fluid threshold. This creates a self-sustaining cascade.
- Over a period of several millennia, persistent winds act on a desert surface composed of a heterogeneous mixture of boulders, gravel, sand, and silt. The finer particles are progressively removed, leaving behind a concentrated surface layer of closely packed, interlocking larger fragments. What is the name of this resulting landscape feature and the primary aeolian process responsible for its formation?
Answer: A desert pavement, formed by the process of deflation.
This scenario describes the formation of a desert pavement. The process by which wind selectively removes fine-grained sediment (sand, silt, clay) from a surface is called deflation. This process leaves behind the larger, heavier particles (gravel, cobbles), which become concentrated into a durable, interlocking surface layer known as desert pavement.
- A geologist compares a sample of aeolian sandstone from a dune with a sample of fluvial sandstone from a river channel. Which of the following is the most reliable characteristic to differentiate the two depositional environments?
Answer: The aeolian sandstone will be better sorted, with a higher concentration of well-rounded, frosted grains.
Wind is a highly selective transport agent. It can only move a narrow range of particle sizes effectively, leading to very well-sorted deposits. In contrast, rivers can carry a much wider range of sizes, leading to moderately or poorly sorted deposits. Additionally, the constant impacts between grains during aeolian saltation result in highly rounded and characteristically 'frosted' (pitted) grain surfaces. This combination of excellent sorting and frosted grains is a hallmark of aeolian deposits.
- Two isolated barchan dunes are located in a region with identical, constant wind conditions. Dune A is 5 meters high, and Dune B is 20 meters high. Assuming both are actively migrating, what is the most likely relationship between their migration rates?
Answer: Dune A will migrate faster because it has a smaller volume of sand to transport.
For a given wind condition, there is a certain flux (volume per unit width per time) of sand being transported. For a dune to migrate one full dune length, its entire volume must be moved from the stoss side to the slipface. Because Dune B is much larger and contains significantly more sand than Dune A, it will take much longer for its volume to be transported. Therefore, smaller dunes migrate faster than larger dunes under the same wind conditions.
- It is counterintuitively observed that slightly higher wind speeds are needed to initiate movement of 0.1 mm diameter silt particles than 0.25 mm diameter fine sand particles. Which statement best explains this phenomenon?
Answer: Stronger interparticle cohesive forces (e.g., electrostatic) are more significant for the smaller silt grains.
While it seems that lighter particles should move more easily, this is not always the case for very fine grains. For particles like silt and clay, cohesive forces (electrostatic charges, moisture bonds) that cause them to stick together become very significant relative to the mass of the particles. These forces make the surface layer resistant to erosion, requiring a higher wind velocity (a higher fluid threshold) to break the particles free compared to slightly larger sand grains where gravity is the dominant force to overcome.
- During the migration of an active sand dune, sand is deposited on the leeward slipface. This face maintains a remarkably consistent maximum slope angle as the dune moves. This maximum angle is primarily determined by the:
Answer: Internal friction between sand grains, known as the angle of repose.
The slipface of an active dune builds up as sand is deposited at the crest. The slope becomes progressively steeper until it reaches a critical angle, at which point it becomes unstable and a small avalanche occurs, returning the slope to a stable configuration. This maximum angle of stability, determined by the frictional properties of the granular material, is called the angle of repose. For dry sand, this is typically around 34 degrees.
- An aerial photograph of a coastal region shows a field of crescent-shaped sand dunes. The tips, or horns, of these dunes point inland, towards the west. Sparse vegetation is observed to be stabilizing the horns of many dunes. If the prevailing winds in this area are known to blow onshore from the east, what is the correct classification for these dunes?
Answer: They are parabolic dunes, because their horns are anchored by vegetation and point upwind.
Parabolic dunes are U-shaped or crescent-shaped dunes whose horns point upwind (against the wind). They often form in coastal areas where vegetation anchors parts of the sand, allowing the central part to be 'blown out' by the wind. Barchan dunes are also crescent-shaped, but their horns point downwind. Since the horns point west (upwind) and vegetation is present, they are parabolic dunes formed by the onshore wind from the east.
- A geologist discovers a collection of rocks on a wind-swept desert plain. The rocks are faceted, with several smooth, polished surfaces that intersect at sharp edges. These polished surfaces are consistently oriented in the direction of the prevailing wind. These rocks are best identified as:
Answer: Ventifacts formed by aeolian abrasion.
A ventifact is any rock that has been shaped, abraded, or polished by wind-driven sand. The process is called abrasion, similar to sandblasting. The smooth, faceted surfaces (and sharp edges between them) are characteristic features created as the wind-blown sand scours the rock surface over long periods. Yardangs are much larger, streamlined ridges of rock, not individual stones.
- A desert area with a poorly sorted mixture of clay, silt, sand, and small pebbles experiences a significant, sustained increase in wind velocity from a gentle breeze to a strong gale. Which of the following describes the most likely response of the sediment to this change?
Answer: Saltation of sand will become the dominant transport process, and finer silt and clay will be carried long distances in suspension.
Different particle sizes are transported by wind differently. A strong gale has enough energy to transport all listed sizes, but via different mechanisms. Fine clay and silt are light enough to be carried high in the air in suspension. Sand-sized particles are primarily moved by saltation (bouncing along the surface). Small pebbles, being heaviest, are moved by surface creep (traction), pushed along by impacting saltating sand grains. Therefore, the increased wind velocity would initiate widespread saltation of sand and suspension of finer particles.
- Vast, thick deposits of homogeneous, unstratified silt, known as loess, are found in regions like the midwestern United States and northern China. Which of the following best describes the origin and transport of this material?
Answer: Fine-grained rock flour, produced by the grinding action of continental glaciers, was picked up by winds from barren outwash plains.
Major loess deposits are strongly associated with past glaciations. Continental ice sheets grind and pulverize bedrock, creating vast quantities of fine-grained sediment called rock flour (silt). As the glaciers melt, this material is deposited in large, barren outwash plains by meltwater streams. Strong winds blowing over these plains can then pick up the silt, transport it in suspension over great distances, and deposit it as a thick blanket of loess.
- The diagram shows a cross-section of a migrating sand dune. Which statement accurately describes the primary aeolian processes occurring at points X, Y, and Z?
Answer: Erosion by saltation is dominant at X, and deposition by grainfall and avalanching is dominant at Z.
The diagram shows a typical migrating dune. The wind blows from left to right. Point X is on the windward (stoss) side, where wind velocity is high, causing sand to be eroded and transported up the slope via saltation. Point Y is the crest. As wind passes the crest, it slows, and some sand is deposited. Point Z is on the leeward (slipface), which is sheltered from the wind. Sand that was blown over the crest accumulates here until the slope becomes unstable (exceeds the angle of repose), leading to deposition via avalanching or grainfall.
- A geologist maps two adjacent zones within a large sand sea. Zone A consists of long, sinuous ridges oriented perpendicular to the prevailing wind direction. Zone B consists of long, linear ridges oriented parallel to the net sand transport direction, which results from two seasonal wind directions. What are the correct classifications for the dunes in these zones?
Answer: Zone A has transverse dunes, and Zone B has longitudinal (seif) dunes.
Transverse dunes form as long ridges oriented perpendicular to a consistent, unidirectional wind, typically where sand supply is abundant. This matches the description of Zone A. Longitudinal (or seif) dunes form as long ridges parallel to the net sand transport direction in areas with a bidirectional wind regime where the two wind directions are not directly opposite. This matches the description of Zone B.
- A large boulder rests on a flat, sandy plain experiencing a persistent wind from the west. Which of the following best describes the initial pattern of sand deposition that will occur around this obstacle?
Answer: A small mound will form directly against the west face, and a larger, elongated 'sand shadow' will form on the east side.
Obstacles disrupt wind flow. As wind approaches the boulder from the west, it compresses and slows slightly, causing some deposition on the windward (west) side. The wind then accelerates as it flows around the sides. In the immediate lee (east) of the boulder, a zone of low-velocity, turbulent air exists. Sand being transported by the wind gets deposited in this 'wind shadow' or 'sand shadow', forming a larger, often elongated drift.
- In a desert valley, meteorological data show that strong winds blow from the north for six months of the year and from the south for the other six months. Assuming a sufficient sand supply, which type of dune is most likely to form in the center of this valley?
Answer: Reversing dunes, with two slipfaces on opposite sides of a sharp crest.
A wind regime dominated by two opposing directions (e.g., north and south) creates reversing dunes. During the winter, the north wind builds a slipface on the south side of the dune. During the summer, the south wind reworks the dune crest and builds a new slipface on the north side. The result is a dune with a sharp crest and two slipfaces that are active at different times of the year, leading to limited net migration.
