Geography (9635) | Arid landscape development in contrasting settings

Welcome to Section 3.1.2.3: Arid Landscape Development in Contrasting Settings!

Hey there, future geographer! This chapter is all about discovering the amazing ways deserts are shaped. When most people think of deserts, they only think of sand and wind, but we are going to learn that water plays a surprisingly important role—even if it only appears very rarely!

Understanding these processes and the unique landforms they create is crucial for mastering the "Hot desert systems and landscapes" unit. Don't worry if some of the terms seem new; we'll break them down into simple, manageable pieces.

---

1. The Mighty Wind: Aeolian Landforms

Aeolian processes are those related to the action of wind. Since deserts often have very little vegetation to protect the ground, the wind is highly effective at eroding, transporting, and depositing material.

Processes of Aeolian Erosion

The wind shapes the landscape through two main mechanisms:

1. Deflation: This is like picking up dirt with a giant vacuum cleaner. Wind removes fine, loose surface particles (like silt and clay). Continuous deflation can lead to a lowering of the land surface, creating a Deflation Hollow (a shallow depression).

2. Abrasion: This is like sandblasting a rock. Particles carried by the wind (especially sand) hit exposed rock surfaces, wearing them down physically. Abrasion is most effective close to the ground, where the heavier sand particles are concentrated.

Key Aeolian Erosional Landforms

Desert Pavements:

After deflation removes the fine material, the remaining surface is covered by a layer of interlocking coarse pebbles, gravel, and stones. This armour-like surface protects the underlying ground from further erosion.

Ventifacts:

These are rocks that have been shaped, polished, or pitted by wind abrasion. The side facing the prevailing wind is often smoothed, sometimes creating multiple flat facets (sides).

Yardangs and Zeugen:

These are spectacular rock formations sculpted by persistent wind abrasion.

• Yardangs: Long, stream-lined, irregular ridges of rock separated by parallel troughs. They are aligned parallel to the prevailing wind direction. (Think of them as long, skinny ships carved by the wind.)
• Zeugen: Table-shaped, mushroom-like rock masses. They form when wind attacks a horizontal layer of softer rock protected by a cap of harder, more resistant rock.

Memory Tip: A YarDANG is DANGerously long, while a ZEUgen looks like a ZED (Z) or table shape when viewed from the side.

Processes of Aeolian Deposition (Sand Dunes)

When wind loses energy (perhaps due to an obstacle or a decrease in velocity), it deposits the transported sand, forming dunes. These dunes migrate (move) over time.

Barchans:

These are crescent (or C-shaped) dunes. They form in areas with a limited supply of sand and consistent wind direction. The 'horns' or tips of the crescent point downwind.

Seif Dunes (or Linear Dunes):

These are long, narrow ridges of sand, running parallel to the direction of the wind. They can be hundreds of kilometres long and form where winds vary slightly in direction (bimodal wind regimes).

Key Takeaway for Section 1: Wind is a powerful sculptor in deserts. Deflation removes fines to create pavements and hollows, while abrasion creates streamlined features like ventifacts, yardangs, and zeugen. Deposition creates barchans and seif dunes.

---

2. The Surprise Element: Fluvial Landforms (Water)

Although deserts are defined by aridity, water is often the most significant erosional and depositional force over the long term, because when rain does fall, it is typically episodic (rare, but intense and short-lived).

The Episodic Role of Water

Because desert soils are often impermeable (they cannot absorb water quickly), intense rainfall leads to very rapid, powerful overland flow called sheet flooding and channel flash flooding. This water quickly carves features before rapidly disappearing.

Sources of Water in Deserts:

• Exogenous: Water originating outside the desert, often from distant humid areas (e.g., the Nile River flowing through the Sahara).
• Endorheic: Water draining internally into basins or depressions, never reaching the sea (e.g., Great Salt Lake Basin).
• Ephemeral: Short-lived surface water, usually after a heavy, isolated storm.

Key Fluvial Landforms

Wadis (Arroyos or Dry Riverbeds):

These are steep-sided, flat-bottomed channels found in deserts. They are typically dry but fill rapidly with violent flash floods after rain. They show evidence of large-scale water erosion, proving that water has been active, even if rarely.

Pediments:

These are gently sloping, smooth rock surfaces found at the base of steep desert mountains. They are formed primarily by the action of sheet flooding and rills (small channels) carrying material away from the uplifted areas.

Bahadas (Alluvial Fans):

When high-velocity water leaves the steep mountain canyons and enters the flat plain, it loses energy rapidly and deposits its coarse sediment load. This creates fan-shaped mounds of debris called Alluvial Fans. A Bahada is formed when several alluvial fans merge together along the foot of a mountain range.

Analogy: Think of a Bahada like a gigantic apron of sediment draped around the mountains.

Playas (Salt Pans or Salinas):

These are flat, typically saline (salty) floors of ephemeral lakes in internal drainage basins (endorheic). When the flash flood water evaporates, it leaves behind evaporite minerals like salt and gypsum.

Inselbergs:

These are dramatic, isolated, steep-sided hills or rock formations rising abruptly from a gentle plain. They are essentially leftover, resistant rock masses that have not been eroded away by the pediment-forming processes. (The most famous example is Uluru/Ayers Rock in Australia.)

Key Takeaway for Section 2: Water, despite its rarity, controls the largest landforms in many deserts. The high energy of flash floods creates wadis, while the subsequent deposition builds bahadas and pediments, leaving behind inselbergs.

---

3. Arid Landscape Development in Contrasting Settings

The development of desert landscapes depends entirely on the climate (which affects weathering and water), geology (which affects resistance), and, crucially, latitude. We need to compare Low Latitude (Tropical) and Mid Latitude (Temperate) deserts.

The Low Latitude Desert Setting

These are tropical deserts (like the Sahara or Arabian Desert) located near the tropics, typically resulting from the global circulation of air (descending air around 30° N/S).

• Climate/Processes: Extremely high temperatures, very low precipitation (hyper-arid), and strong, persistent winds.
• Dominant Process: Aeolian processes dominate. Wind erosion and transport are constant.
• Characteristic Landscapes:
  • Vast areas of continuous sand dunes (Ergs), including extensive barchans and seif dunes.
  • Significant surface deflation leading to large desert pavements.
  • Large, wind-eroded features like yardangs are common, formed by continuous abrasion.
• Role of Time: Due to the stable, hyper-arid climate, these landscapes have often developed over extremely long periods, allowing wind action to dominate and create massive features.

Did you know? Despite the image, only about 20% of the Sahara is covered by sand dunes (ergs); the rest is rock and gravel pavement.

The Mid Latitude Desert Setting

These are often temperate or continental deserts (like the Gobi or parts of the Great Basin Desert in the USA). They are usually far from the ocean and often result from being in the rain shadow of mountain ranges.

• Climate/Processes: Wider temperature range (very hot summers, cold winters). Precipitation is often higher than in low-latitude deserts but still sparse, usually falling as rain or snow in short bursts.
• Dominant Process: Fluvial processes are more significant (episodic). While wind is active, the greater variability in moisture means water plays a greater role in carving out large-scale relief.
• Characteristic Landscapes:
  • Landscapes are often dominated by mountains, alluvial fans (bahadas), and expansive pediments.
  • Large, flat play areas (playas) where internal drainage collects.
  • Inselbergs are highly characteristic, demonstrating the long-term lowering of the surrounding plain by water action.
• Role of Time: These landscapes often reflect a complex history, including influences from wetter periods (pluvials) in the recent past, enhancing the role of water-eroded features.

Summary of Contrasting Landscape Development

The interaction between process (wind vs. water), time (how long processes have been stable), and landforms determines the final desert landscape:

Low Latitude Deserts (E.g., Sahara)

• Key Process: Steady, dominant Aeolian action.
• Landscape: Dune fields (barchans, seifs) and rock surfaces shaped by abrasion (yardangs, ventifacts).
• Result: A landscape defined by the shifting movement and deposition of sand.

Mid Latitude Deserts (E.g., Gobi)

• Key Process: Strong, episodic Fluvial action (flash floods).
• Landscape: Extensive pediments, merged alluvial fans (bahadas), and isolated residual hills (inselbergs).
• Result: A landscape defined by mountains and basins created by the powerful, albeit rare, action of water.

You’ve conquered the desert landforms! Remember to always link the landform back to the process (aeolian or fluvial) and the setting (low vs. mid latitude) for high marks.