3.3 How Do Dryland Landforms Evolve Over Time as Climate Changes? Flashcards
(45 cards)
How have dryland landforms evolved over time due to climate changes?
Dryland landforms have evolved through oscillations between wet and dry conditions over the past 500,000 years.
The impact of these changes is clear in desert landscapes many present-day landforms are relict features, left over from a period when the climate was much wetter than today
What are pluvial periods?
Wetter periods in dryland environments lasting for thousands of years, known as pluvials.
How did pluvial conditions affect geomorphic processes in drylands?
Pluvial conditions transformed the geomorphic environment in drylands, intensifying weathering and mass movement and increasing the importance of fluvial action, Indeed many modern desert landforms are known to be fossil features- the legacy of past. wetter climatic phases
What is the impact of pluvial conditions on weathering processes?
Most weathering processes depend on water. While past pluvial episodes were associated with warmer conditions reducing the effectiveness of freeze-thaw weathering, rates of hydration and chemical weathering would have accelerated. Today on the Colorado Plateau rockfalls often occur after heavy rain when sandstones exposed on steep slopes absorb water and increase their mass. Thus rates of recession of inselbergs would have been more rapid during pluvial periods. Surface streams and rivers flowing from plateaux and inselbergs also undercut steep slopes by erosion, further increasing recession rates. These streams and rivers transported large sediment loads, abraded pediments and spread rock debris across pediment surfaces. In the Mojave Desert in California and northwest Utah extensive systems of surface drainage and lakes developed during the last pluvial.
What happened to Lake Bonneville during the last pluvial?
. At its maximum extent, lake Boneville in Utah covered thousands of square kilometres and was 300 metres deep. Today the great salt lake is just a tiny remnant of lake Boneville. The old shorelines of lake Boneville are clearly visible on hillsides west of salt lake city and part of the former lake bed includes the famous Bonneville salt flats. To the southwest, Lake Manly extended almost the entire length of Death Valley. Similar lakes existed to the west below the eastern slopes of the Sierra Nevada. Modern salt flats and playas are the remains of these lakes.
A more humid climate sustained permanent rivers, and more vigorous fluvial erosion, transport and deposition. It is therefore reasonable to assume that prominent landforms in present-day dryland landscapes, such as alluvial fans, bajadas and cany owe much of their development to wetter climate conditions in the past.
How have recent climate changes affected dryland landscapes?
As dryland environments have become more and more arid in the past 6000 years, rates of weathering, mass movement and fluvial erosion in deserts have slowed. However, with less vegetation and soil cover, run-off may have increased in intensity. More extreme events and powerful (but sporadic) flash floods may have to some degree offset lower rainfall. Meanwhile, the drier conditions will have increased the effectiveness of aeolian erosion and transport.
However, the permanent rivers that existed in the last pluvial have disappeared, replaced by intermittent drainage. Pluvial lakes have either shrunk or disappeared with high rates of evaporation they have been replaced by old lake beds, and waters, where they survive, which are extremely saline.
What is the forecast for future climate change in hot arid environments?
Future projections of climate change in drylands are geographically variable. In hot arid environments such as Africa, Arabia and Australia, further decline in rainfall (and therefore vegetation cover) is forecasted and rates of landscape change associated with sub- aerial processes might slow in these regions. However aeolian erosion and transport, with deflation, and the advance of dunes, could become more widespread. And if rainfall declines, but becomes more extreme, fluvial erosion might even increase. This scenario does not extend to mid-latitude drylands, such as the Great Plains and central Asia, where rainfall is expected to increase slightly.
How have drylands been characterised by colder climatic conditions?
Climate changes in a previous time period and the resultant colder conditions The global climate has fluctuated between colder and warmer periods on numerous occasions in the past 500,000 years. The most recent Pleistocene glacial reached its maximum around 20.000 years ago. Global temperatures at that time were 3-5 C lower than at present and large parts of Eurasia and North America were submerged by ice sheets and glaciers. However, middle latitudes were largely ice-free. There, extreme cold with average air temperatures around 6 C lower than the present day, created periglacial conditions Their defining feature was perennially frozen ground or permafrost.
Today some areas that experienced periglacial conditions in the late Pleistocene are deserts and drylands They include mid-latitude continental interiors such as central Asia and the Great Plains. In lower latitudes mountains and plateaux including the Ethiopian Highlands the Saharan uplands and the southern Rockies supported periglacial environments. Then between 13.000 and 10,000 BP the dimute warmed and periglacial environments receded north to high latitudes.
What are periglacial conditions?
Periglacial environments are free of ice sheets and glaciers, but apart from a shallow surface layer, the ground is permanently frozen. In these conditions a number of geomorphic processes operate such as freeze-thaw weathering frost heave and gelifluction.
What is freeze-thaw weathering, frost heave and gelifluction?
Freeze-thaw weathering occurs when water, confined in rock joints, pores and crevices freezes, expands and causes the mechanical breakdown of rocks, Frost heave is the upward swelling of the ground surface due to the growth of ice crystals in soils and regolith, On slopes, expansion is at right angles to the ground surface, but on melting, soil particles settle vertically. This results in a slow downslope displacement. Frost heave is key to understanding mass movements known as solifluction or gelifluction. Solifluction is defined as the gravitational flow of the saturated regolith. Where regolith rests on a layer of permafrost, the process is called gelifluction.
What is the evidence for periglacial processes?
Periglacial processes remained active in some mid- and low-latitude drylands up to 10,000 years ago. The evidence for this is preserved in a range of periglacial landforms. In the mountains of central Arizona and New Mexico extensive talus slopes developed by frost shattering above 2400 m. These fossil features, inherited from cold climatic conditions, fed rock glaciers on their lower slopes. Rock glaciers are linear accumulations of angular rocks which have moved downslope as a result of the formation and melting of interstitial ice (ie. ice formed between rock particles). Further north at lower altitude in Hettinger County, North Dakota, large ognips up to 2.6 km in diameter, are the remains of ice-cored hills or pingos that collapsed as the climate warmed.
In the same area, micro-scale features, known as cryoturbation structures (soils and sediments churned by frost heave) and ice wedge casts (the infilled casts of vertical triangular sheets of ground ice) formed. Similar features also occur in the northern Gobi Desert in southern Mongolia.
In the semi-arid Semien Highlands of Ethiopia frost shattering of bedrock is only active today at elevations above 4250 m. However, frost-shattered rubble can be found between 3100 and 3750 m indicating colder conditions in the past. This region has other periglacial landforms: extensive boulder fields or blockfields, formed by frost weathering of massively bedded rocks talus slopes and gelifluction deposits down to 3000 m. Nivation hollows are found where physical weathering and frost have occurred beneath long-established snow patches The resulting debris is then removed by flowing meltwater Once formed, nivation hollows are self- generating, trapping snow and prolonging the action of freeze-thaw, frost heave and surface wash.
What is the modification of periglacial landforms by more recent and future processes associated with climate change?
The periglacial landforms in dryland landscapes are for the most part relict features, inherited from an earlier colder climatic phase. Thus the processes that formed them are no longer operational Present-day climate and related weathering, mass movement, fluvial and aeolian processes will modify and eventually destroy these fossil features
In low latitudes, large boulders forming blockfields are exposed to desert weathering processes such as insolation and salt weathering. Exfoliation, ass rock layers peel away, slowly results in gr disintegration. Rock glaciers which under periglacial conditions moved by the action of interstitial ice, become inactive. But located at the foot of slopes floods transport rock glacier particles which are in and often at the head of valleys, occasional as coarse sediment to dry river beds and was slopes may continue to develop at high altitudes where frost weathering occurs, but at lower levels rockfall from free faces is mainly caused by heavy rain saturating rocks which then fail along, major joints.
In drier conditions, solifluction and gelifluction processes no longer take place. As features such a sheets, lobes and terraces dry out, these landforms are eroded and dissected by ephemeral streams rivers, while aeolian activity removes finer particles. Meanwhile, features like nivation hollows are buried beneath fluvial- and aeolian-transported gravels, sands and silts.
What features indicate past periglacial activity in the Semien Highlands of Ethiopia?
Frost-shattered rubble, boulder fields, and nivation hollows.
What modifications occur to periglacial landforms due to climate change?
Periglacial landforms are modified by present-day weathering and erosion processes.
How do desert weathering processes affect large boulders in low latitudes?
They are exposed to insolation and salt weathering, leading to disintegration.
What happens to features like nivation hollows in drier conditions?
They are buried beneath fluvial- and aeolian-transported materials.
How have dryland landforms evolved over time due to climate changes?
Dryland landforms have evolved through oscillations between wet and dry conditions over the past 500,000 years, leading to the formation of relict features from wetter periods.
What are pluvial periods?
Wetter periods in dryland environments lasting for thousands of years, known as pluvials, have significantly influenced landform development.
How did pluvial conditions affect geomorphic processes?
Pluvial conditions intensified weathering and mass movement, increased fluvial action, and resulted in many modern desert landforms being fossil features.
What changes occurred in dryland environments over the past 6,000 years?
As dryland environments became more arid, rates of weathering and fluvial erosion slowed, but increased runoff and extreme flash floods emerged.
What are the future projections for climate change in drylands?
Future projections vary; in hot arid regions, rainfall decline is expected, while mid-latitude drylands may see slight increases in rainfall.
What climatic conditions influenced dryland landscapes in the past?
Colder climatic conditions during the last Pleistocene glacial period led to periglacial environments in many current dryland areas.
What are periglacial processes?
Periglacial processes include freeze-thaw weathering, frost heave, and gelifluction, which shape landforms in cold environments.
What evidence of periglacial activity exists in Arizona and New Mexico?
Extensive talus slopes and rock glaciers developed from frost shattering above 2400 m, indicating past periglacial conditions.
