Desert Geology
by Owen Borville
January 30, 2021
Learning, Geology, Science
A desert is an arid or dry region with very little precipitation. Wind and water-based deposits define arid desert regions. Aeolian deposits refer to wind transported deposits of sediment which can be transported as suspended load or bed load. Suspended load is the wind transported deposits which include silt and clay size particles. Bed load includes the larger sand size particles which are moved across the ground by wind.
Saltation by wind is the process of of particles transported as bed load such as sand grains, which stay close to the ground but are not in contact continuously. These particles move along in a series of steps. First the particle jumps into the flow. Then the particle is pulled forward by the flow, hitting the bottom and bouncing up again or ejecting another particle from the ground, which jumps into the flow.
Abrasion is the wearing away of the Earth’s surface by agents of erosion such as wind, water, or ice. Ventifacts are rocks which have been cut, and sometimes polished, by the abrasive action of wind. Yardangs are larger landforms up to tens of meters in height and kilometers in length and are formed and streamlined by desert wind erosion.
Deflation is the lowering of the land surface due to the removal of fine-grained particles by the wind. Deflation concentrates the coarser grained particles at the surface, eventually resulting in a surface composed only of the coarser grained fragments that cannot be transported by the wind. Such a surface is called desert pavement. Desert pavement is a layer of pebble and cobble sized rock which forms at the surface in a desert environment. Desert pavements likely develop through the removal of the smaller and lighter sands, silts, and clay by wind action. Desert pavements help protect the surface from further erosion.
A blowout or deflation hollow is a depression created by the wind in easily eroded sediment such as sand or silt deposits with little or no moisture or vegetation content. Vegetation present in sediment at the surface stops the deflation process.
Desert varnish is a gray or reddish-brown layer on rock surfaces that darkens and thickens with increasing age and contains thin skins of clay and manganese oxide minerals. Durable crust is a mineral cemented outer crust or layer formed when rock chemically reacts with water and possibly atmospheric dust, producing a hard outer surface that resists weathering. Silcrete is a hard soil common in arid environments such as Australia formed when silica is dissolved in solution and precipitates or solidifies as cement.
Dune Types
A sand dune is a hill or ridge of sand forming where environmental conditions provide sand, steady wind and a location for the sand to collect. Dunes may occur in both coastal and more inland settings. Dunes migrate in the direction of the wind. Ergs are sand seas or regions covered by sand dunes. A draa is a large dune formed in the desert. Barchan dunes are the simplest and the most well-known dune. The barchan dune is a crescent-shaped dune usually located in desert climates. The arms or horns of the dune point downwind, and move faster than the center of the dune.
Longitudinal or sief dunes are long, straight, or slightly sinuous sand ridges that can be more than 160 kilometers long and may occur as isolated ridges. However, longitudinal dunes generally form sets of parallel ridges separated by miles of sand, gravel, or rocky inter-dune corridors. Some longitudinal dunes merge to form Y-shaped compound dunes. Many longitudinal dunes form in bidirectional wind regimes. The long axes of these dunes extend in the resultant direction of sand movement.
Barchanoid dunes are wave-like dunes that form from barchan dunes when more sand becomes available. When sand continues to accumulate, the barchanoid dunes grade into transverse dunes.
Transverse dunes are forward marching ridges that form when barchan dunes become aligned together along a plane perpendicular to the wind and the dunes becomes relatively straight.
Parabolic dunes occur in sandy vegetated areas when strong winds erode a section of the vegetated sand (commonly referred to as a blowout). Downwind motion occurs if sand from the blowout is deposited on the opposite slope of the parabolic dune and vegetation holds the arms of the dune in place as the windward nose of the dune migrates forward toward the main dune field.
Star dunes are star-shaped dunes which form in basin confined dune fields that experience seasonal variations in wind patterns. Opposing crosswinds cause the sand to pile up with one dune typically rising high above other dunes in a field. Some of the tallest sand dunes in the world are star dunes.
Loess is believed to be a thick wind-blown (or possibly water transported) deposit of sediment made mostly of silt-sized grains. The composition of loess is a gritty, lightweight, porous material composed of tightly packed grains of quartz, feldspar, mica, and other minerals. Loess can originate from glacial deposits, deserts, dune fields, playa lakes, or volcanic ash. During the Ice Age, glaciers advanced down into the mid-continent of North America, grinding underlying rock into fine powder-like sediment called glacial flour. As temperatures warmed, the glaciers melted and enormous amounts of water and sediment rushed down the Missouri River valley. The sediment was eventually deposited on flood plains downstream, creating huge mud flats.
During the winter season the melt waters would recede, leaving the mud flats exposed. After drying, the fine-grained mud material or silt was picked up and carried by strong winds. These large dust clouds were moved eastward by prevailing westerly winds and were re-deposited over broad areas. Heavier and coarser silt deposited closest to its Missouri River flood plain source formed sharp, high bluffs on the western margin of the loess hills. Finer, lighter silt deposited farther east created gently sloping hills on the eastern margin. This process repeated for thousands of years, building layer upon layer until the loess reached thicknesses of 60 feet or more and became the dominant feature of the terrain.
Loess deposits contain valuable records of paleoclimates, or climates of the past. Loess deposits cover approximately 10 percent of the Earth's surface. Loess deposits can yield valuable information about past wind direction, past wind strength, moisture balance, and degree and type of vegetation cover. Between loess deposits, ancient buried soils are often found that mark periods when loess was not being deposited. These buried soils, called paleosols, provide valuable information about past climate and vegetation in addition to being rich in agricultural value. Loess distribution in North America is found in the Mississippi River Valley, the Great Plains of the United States, and Alaska. Shaanxi, China is also known for loess deposits in a region called the Loess Plateau.
Rock varnish is a thin shiny coating that is found on many desert rocks and increases with age. The varnish, composed of dark manganese oxide and clay minerals, is likely caused by weathering, evaporation, and precipitation. Caliche or calcrete is a hard, white soil horizon rich in calcium carbonate that commonly forms in arid and semi-arid areas by cementation when calcium is added to the soil by precipitation and drainage.
Tafoni are small cavity features on rock that resemble honeycomb and are formed by a variety of weathering and erosion processes commonly in desert environments.
Playas (or salt pans) are shallow, short-lived lakes that are often saline and form where water drains into basins with no outlet to the sea and quickly evaporates. Playas are common features in arid or desert regions. A sabkha (also known as a salt flat) is an environment located along arid coastlines and consists of evaporite, carbonate, and clastic sediment deposits. Sabkhas are common in the Persian Gulf region.
Alluvial fans are fan-shaped piles of sediment that form where a rapidly flowing mountain stream enters a relatively flat valley. As water slows down, sediment (alluvium) is deposited that gradually builds a fan shape. Bajadas are formed by the lateral merging and blending of a series of alluvial fans along a mountain front.
An arroyo is a stream channel found in deserts with almost vertical walls and a flat base that forms in fine grained sediment which is very erosion prone. An ephemeral stream is a stream that is usually dry and fills with water only during brief episodes of rainfall. Many desert streams are ephemeral including arroyos.
Pediments are gently sloping erosional bedrock surfaces at the base of a mountain. An inselberg is an isolated hill or small mountain that has steep sides and is surrounded by a flat plain. Inselbergs usually have a volcanic origin and are made of erosion resistant rock.
Mesas are isolated, flat-topped hills or mountains with steep sides that are smaller in area than a plateau. Mesas are commonly longer in width than height.
Buttes are erosional landforms in arid climates which are taller than wide and can be a hill shape or tower shape. Buttes were originally mesas but eroded to become smaller and can eventually erode into thin columns.
by Owen Borville
January 30, 2021
Learning, Geology, Science
A desert is an arid or dry region with very little precipitation. Wind and water-based deposits define arid desert regions. Aeolian deposits refer to wind transported deposits of sediment which can be transported as suspended load or bed load. Suspended load is the wind transported deposits which include silt and clay size particles. Bed load includes the larger sand size particles which are moved across the ground by wind.
Saltation by wind is the process of of particles transported as bed load such as sand grains, which stay close to the ground but are not in contact continuously. These particles move along in a series of steps. First the particle jumps into the flow. Then the particle is pulled forward by the flow, hitting the bottom and bouncing up again or ejecting another particle from the ground, which jumps into the flow.
Abrasion is the wearing away of the Earth’s surface by agents of erosion such as wind, water, or ice. Ventifacts are rocks which have been cut, and sometimes polished, by the abrasive action of wind. Yardangs are larger landforms up to tens of meters in height and kilometers in length and are formed and streamlined by desert wind erosion.
Deflation is the lowering of the land surface due to the removal of fine-grained particles by the wind. Deflation concentrates the coarser grained particles at the surface, eventually resulting in a surface composed only of the coarser grained fragments that cannot be transported by the wind. Such a surface is called desert pavement. Desert pavement is a layer of pebble and cobble sized rock which forms at the surface in a desert environment. Desert pavements likely develop through the removal of the smaller and lighter sands, silts, and clay by wind action. Desert pavements help protect the surface from further erosion.
A blowout or deflation hollow is a depression created by the wind in easily eroded sediment such as sand or silt deposits with little or no moisture or vegetation content. Vegetation present in sediment at the surface stops the deflation process.
Desert varnish is a gray or reddish-brown layer on rock surfaces that darkens and thickens with increasing age and contains thin skins of clay and manganese oxide minerals. Durable crust is a mineral cemented outer crust or layer formed when rock chemically reacts with water and possibly atmospheric dust, producing a hard outer surface that resists weathering. Silcrete is a hard soil common in arid environments such as Australia formed when silica is dissolved in solution and precipitates or solidifies as cement.
Dune Types
A sand dune is a hill or ridge of sand forming where environmental conditions provide sand, steady wind and a location for the sand to collect. Dunes may occur in both coastal and more inland settings. Dunes migrate in the direction of the wind. Ergs are sand seas or regions covered by sand dunes. A draa is a large dune formed in the desert. Barchan dunes are the simplest and the most well-known dune. The barchan dune is a crescent-shaped dune usually located in desert climates. The arms or horns of the dune point downwind, and move faster than the center of the dune.
Longitudinal or sief dunes are long, straight, or slightly sinuous sand ridges that can be more than 160 kilometers long and may occur as isolated ridges. However, longitudinal dunes generally form sets of parallel ridges separated by miles of sand, gravel, or rocky inter-dune corridors. Some longitudinal dunes merge to form Y-shaped compound dunes. Many longitudinal dunes form in bidirectional wind regimes. The long axes of these dunes extend in the resultant direction of sand movement.
Barchanoid dunes are wave-like dunes that form from barchan dunes when more sand becomes available. When sand continues to accumulate, the barchanoid dunes grade into transverse dunes.
Transverse dunes are forward marching ridges that form when barchan dunes become aligned together along a plane perpendicular to the wind and the dunes becomes relatively straight.
Parabolic dunes occur in sandy vegetated areas when strong winds erode a section of the vegetated sand (commonly referred to as a blowout). Downwind motion occurs if sand from the blowout is deposited on the opposite slope of the parabolic dune and vegetation holds the arms of the dune in place as the windward nose of the dune migrates forward toward the main dune field.
Star dunes are star-shaped dunes which form in basin confined dune fields that experience seasonal variations in wind patterns. Opposing crosswinds cause the sand to pile up with one dune typically rising high above other dunes in a field. Some of the tallest sand dunes in the world are star dunes.
Loess is believed to be a thick wind-blown (or possibly water transported) deposit of sediment made mostly of silt-sized grains. The composition of loess is a gritty, lightweight, porous material composed of tightly packed grains of quartz, feldspar, mica, and other minerals. Loess can originate from glacial deposits, deserts, dune fields, playa lakes, or volcanic ash. During the Ice Age, glaciers advanced down into the mid-continent of North America, grinding underlying rock into fine powder-like sediment called glacial flour. As temperatures warmed, the glaciers melted and enormous amounts of water and sediment rushed down the Missouri River valley. The sediment was eventually deposited on flood plains downstream, creating huge mud flats.
During the winter season the melt waters would recede, leaving the mud flats exposed. After drying, the fine-grained mud material or silt was picked up and carried by strong winds. These large dust clouds were moved eastward by prevailing westerly winds and were re-deposited over broad areas. Heavier and coarser silt deposited closest to its Missouri River flood plain source formed sharp, high bluffs on the western margin of the loess hills. Finer, lighter silt deposited farther east created gently sloping hills on the eastern margin. This process repeated for thousands of years, building layer upon layer until the loess reached thicknesses of 60 feet or more and became the dominant feature of the terrain.
Loess deposits contain valuable records of paleoclimates, or climates of the past. Loess deposits cover approximately 10 percent of the Earth's surface. Loess deposits can yield valuable information about past wind direction, past wind strength, moisture balance, and degree and type of vegetation cover. Between loess deposits, ancient buried soils are often found that mark periods when loess was not being deposited. These buried soils, called paleosols, provide valuable information about past climate and vegetation in addition to being rich in agricultural value. Loess distribution in North America is found in the Mississippi River Valley, the Great Plains of the United States, and Alaska. Shaanxi, China is also known for loess deposits in a region called the Loess Plateau.
Rock varnish is a thin shiny coating that is found on many desert rocks and increases with age. The varnish, composed of dark manganese oxide and clay minerals, is likely caused by weathering, evaporation, and precipitation. Caliche or calcrete is a hard, white soil horizon rich in calcium carbonate that commonly forms in arid and semi-arid areas by cementation when calcium is added to the soil by precipitation and drainage.
Tafoni are small cavity features on rock that resemble honeycomb and are formed by a variety of weathering and erosion processes commonly in desert environments.
Playas (or salt pans) are shallow, short-lived lakes that are often saline and form where water drains into basins with no outlet to the sea and quickly evaporates. Playas are common features in arid or desert regions. A sabkha (also known as a salt flat) is an environment located along arid coastlines and consists of evaporite, carbonate, and clastic sediment deposits. Sabkhas are common in the Persian Gulf region.
Alluvial fans are fan-shaped piles of sediment that form where a rapidly flowing mountain stream enters a relatively flat valley. As water slows down, sediment (alluvium) is deposited that gradually builds a fan shape. Bajadas are formed by the lateral merging and blending of a series of alluvial fans along a mountain front.
An arroyo is a stream channel found in deserts with almost vertical walls and a flat base that forms in fine grained sediment which is very erosion prone. An ephemeral stream is a stream that is usually dry and fills with water only during brief episodes of rainfall. Many desert streams are ephemeral including arroyos.
Pediments are gently sloping erosional bedrock surfaces at the base of a mountain. An inselberg is an isolated hill or small mountain that has steep sides and is surrounded by a flat plain. Inselbergs usually have a volcanic origin and are made of erosion resistant rock.
Mesas are isolated, flat-topped hills or mountains with steep sides that are smaller in area than a plateau. Mesas are commonly longer in width than height.
Buttes are erosional landforms in arid climates which are taller than wide and can be a hill shape or tower shape. Buttes were originally mesas but eroded to become smaller and can eventually erode into thin columns.
