Dryland Geomorphology: Processes, Landscapes, and Coastal Dynamics
Arid Geomorphology: Processes and Landforms
Drylands cover 30% of the planet, encompassing both deserts and steppes. These areas primarily correspond to subtropical desert climates, characterized by low temperatures near the tropics, high-pressure zones, and topographic depressions where air is compressed. Large reliefs often intercept moisture, preventing cloud formation.
Geomorphic Processes in Arid Areas
There are two main types of alteration: physical weathering of rocks and minerals, and chemical weathering resulting in fine clay and metallic soils. Water is a limited shaping factor due to sparse rainfall, leading to the formation of ephemeral streams that are dry for most of the year but flow during the rainy season.
Evolution of Arid Landscapes
Arid landscapes are characterized by closed basins with internal drainage. Their evolution depends on tectonic activity (faulting can uplift blocks) and internal drainage patterns, which can produce alluvial fans, slopes, playas, and salt lakes. Erosion in the mountains surrounding deserts leads to sediment accumulation in desert basins, reducing local relief. The highest peaks protruding from these basins are called inselbergs. Extensive mountain ranges covered by sediments, eroded from the mountains themselves, lie beneath many large deserts.
Desert Wind Action
Wind in the desert plays a significant role in sediment transport, differing from water in that it primarily carries fine-grained materials. This sediment can be distributed over wide areas. Wind transports materials by dragging them across the surface, causing them to bounce (saltation, accounting for 25% of sand movement during a storm), or through suspension. While its erosive function is generally negligible, wind can shape depressions called blowouts and create desert pavements. It can also erode rocks, leaving behind flat, carved crests.
Wind-transported sediment is deposited in dunes, which are generally asymmetrical sand ridges with a gentler windward slope and a steeper leeward slope.
Dune Formation
Dunes migrate in the direction of the prevailing wind. They vary in type (transverse, longitudinal, parabolic, etc.) depending on the amount of available sediment and the prevailing wind direction. Wind also generates loess deposits, which are silt deposits found in periglacial areas and deserts.
Coastal Geomorphology: Processes and Dynamics
The world’s coasts are constantly undergoing erosion. The primary agent of coastal erosion is wave action. Waves erode by the impact of water and entrained particles against the shore. Wave energy is reflected in the erosion of sediments. Most beach sand originates from rivers, with the sea responsible for its circulation. However, larger sediments often come from cliffs eroded by waves, which are then transported to the coast by undertow.
On irregular coastlines, wave energy concentrates on projections, working to create a more uniform coastline.
Tidal Influences
Tides are caused by the moon’s gravitational pull on the Earth’s hydrosphere and atmosphere. While some latitudes experience minimal tidal activity, most global coastlines experience two to four tides per day, depending on latitude. When the Sun and Moon are aligned, the gravitational force is amplified, resulting in spring tides. Conversely, when the Sun and Moon form a 90-degree angle, neap tides occur. The timing of high tide shifts approximately one hour later each day due to the moon’s rotation.