Physical Environment of Mountains: An Overview

Transcription

1 1 Climate and Mountains Physical Environment of Mountains: An Overview Key influences on the nature and rates of geomorphological processes. The processes operates at different space and time scales. Mountains can modify global atmospheric processes and generate their own climatic conditions thus affecting the climates of adjacent regions. Influences include latitude, altitude, continentality, and topography. Latitude Latitude affects solar radiation receipts, temperature, seasonality, and modifies the influence of altitude, causing treeline and snowline altitudes, and the occurrence of permanent snow and ice to descend polewards. Pressure systems: Equatorial low pressure (0-20 degrees N&S Latitude), Subtropical High Pressure (20-40 degrees Latitude), Subpolar Low pressure (40-70 degrees Latitude), and Polar high pressure (70-90 degrees Latitude). High pressure zones tend to be drier, whereas low pressure zones tend to be wetter. Seasonality and the day length vary from the equator to the pole. The impact of latitude on solar radiation concerns the height of the sun and the angle at which its rays hit the surface of the earth. Slope angle and slope aspect are therefore important. The influence of global circulation also contributes to precipitation. Altitude The general trend with increasing altitude is a reduction in temperature, air density and pressure, proportions of carbon dioxide, water vapor and concentrations of impurities such as dust. The intensity of solar radiation, and especially the UV component, increases with altitude. Continentality The distribution of land and sea in relation to the location of mountains is important. Oceans have the effect of moderating climate. Coastal mountains tend to be wetter and cloudier due to the effect of humid air blowing onshore and being forces up, thus giving rise to precipitation. Continental interiors tend to be more extreme with greater temperature fluctuations occurring more rapidly, drier conditions, less cloud and consequently higher solar radiation receipts. Topographic and Barrier Effects Barriers and the importance of relief and mountain mass affecting air circulation.

2 2 Temperature Maybe the most important environmental variable, and is closely related to solar radiation wet (3.2 F/1000ft or 0.3C/100m) and dry adiabatic (5.5F/1000ft or 1C/100m) lapse rates. Lapse rates vary with latitude and season, reaching their highest in tropical deserts during summer. Turbulence, differential heating and global atmospheric circulation also influence lapse rates. Clouds are another important factor. Greater cloud cover of mountains compared with lowlands as a result of uplift and condensation enhances the effect of thinner air and reduced heat retention. Precipitation Two main sources convectional and synoptic. The type and amount of precipitation depends on the moisture content of the air, the rate of ascent, wind speed, and degree of uplift. Forms of precipitation -- snow, dew, fog, frost, and rime. Orographic precipitation is also associated with the deposition of pollutants. Where industrial activity in adjacent lowlands produces airborne particles, these drift up valleys and act as condensation nuclei that are washed out of the air. Solar radiation The thinner air at higher elevations causes rapid fluctuations in the response of temperature to changes in solar radiation receipts. Two issues are important the quantity and the quality of the radiation receipts. Affected by cloudiness, aspect and topography. Snow cover increases surface albedo, and therefore reduces the absorption of energy and so covers duration affects total radiation receipts. Rapid cooling of air and surfaces causes an increase in relative humidity and therefore less evaporation but more condensation, and hence fog. Local winds effects cause turbulence and mixing and give rise to aridity on many summits. Winds Winds occur at different scales ranging from the disturbance of upper air circulation due to peaks penetrating these levels, through regional scale effects of winds such as bora, fohn, and mistral, to local valley circulation. Winds are either synoptic or thermally induced. Chinook winds on the Great Plains as air clears over the Rockies. Thermally induced winds comprise slope winds, mountain and valley winds, and other more locally modified winds such as glacier winds. The combination of slope and mountain-valley winds creates a general daytime wind up slopes and valley floor, with anti-winds blowing down valley and towards the centers of structures. The combined effect of aspect and angle significantly modify climate on a local scale.

3 3 Geomorphology Most of the major mountain ranges are associated with plate boundaries and are uplifted as a result of plate collisions. Volcanic and seismic activity is also closely connected with mountain ranges. The process of mountain building arises from the movement of the earth s crustal plates. The convergence of two or more plates causes buckling up of the intervening sediments and up thrusting to form upstanding relief. For example, the Himalayan arc, that was formed primarily around 30 millions years ago hard core mountains beneath softer marine sediments and one the latter are stripped off by erosion I is these hard igneous masses which form the highest peaks. The landscape of mountains, consisting of dissected and differentially eroded surfaces with abundant steep slopes and high absolute and relative relief, creates an environment of high energy that gives rise to high rates of erosion. Mountains are characterized by a preponderance of high magnitude, low frequency major landslides, earthquakes and floods. The main processes consist of frost action, glaciers, fluvial and mass movements. Weathering is important, particularly in humid tropical environments with high temperatures and abundant water, and Aeolian processes tend to be restricted to summits and arid zones where vegetation cover is restricted. Processes High mountains are dominated by freeze-thaw, periglacial and glacial activity, intermediate slopes by erosion and deposition processes of glaciers and rivers as will as mass movements. Glaciers They reached their last maximum during the Quaternary period. The last glacial maximum ended around 14,000 years ago. Fluvial Action Closely related to precipitation and to snow and ice melt and thus is variable in time and space. Mass Movements Soil creep, mudflows, snow avalanches, debris flows, slumps Mountain Soils

4 4 Soils on summits develop from sediment collected in pockets and on slopes, anchored by vegetation and protected from high winds. They tend to be thin, stony and often low in nutrients and in need of improvement. Finer, deeper material accumulates in alluvial fans and in valley bottoms with a better developed A-horizon. Glacial tills provide the basis for many soils. In high alpine meadows, the development of dens meadow turf protects the surface as long as it remains intact. Excessive trampling or other surface disturbance can quickly lead to deep gullies once the turf mantle is broken. Bedrock and surface deposits significantly influence the nature of soils. Soils on alluvial fans tend to de freer draining, whereas valley bottom soils are subject to regular water logging. Soils formed on lava and volcanic ash tends to be very rich and fertile, and often free draining. Soils tend to be deeper on south-facing slopes where greater insolation results in deeper weathering and more active micro-organism activity. Classification of soils can be broadly aligned to vegetation zones. Formation of soils is dependent on climatic conditions and substrate. At the highest altitudes, the slow rate of weathering primarily by physical breakdown and the low temperatures restrict chemical and biological processes and the high wind speed contribute to deflation and desiccation. Geoecology Zonation of mountain flora is important. Microclimate and habitat niches play an important part in the survival of many species. The interdependence of vegetation and soils may sustain patches of distinct vegetation. The complex superimposition of different effects and influences from climate, relief, soils, and vegetation and fauna operate within a dynamic time-frame. Distributions of mountains and their orientation affect the source of flora and the potential for migration during periods of environmental change. Importance of island biogeography has been applied to mountains. Specialization and adaptation to the environmental conditions of high mountains has produced distinctive floras and associations of high rates of endemism. Colonization of mountain surfaces occurs by succession. Rates of soil formation, snow cover, and seasonal or diurnal climatic conditions can slow the rate of succession, and in addition the sources of plants may be distant and slope to migrate. The main components of mountain vegetation are forests, alpine meadows, and tundra. Three main types of mountain forests evergreen conifer (pines), evergreen broadleaf (magnolia and Banyan), and broadleaf deciduous (oaks). As altitude increases, ecological complexity reduces as does the density of forest stands. Broadleaf deciduous trees tend to occur in lower altitudinal belts than the conifers in the same region. Cloud Forests Characterized by very high rates of diversity and endemism, and have a specific and important hydrological function, gathering their water supplies directly from the rising

5 5 air. They have a distinctive flora and structure, and occur in a locally narrow altitudinal zone. Treeline The most dramatic ecotone krummholz (stunted tree). As a general rule, timberlines globally correspond with the 10 degreesc July isotherm Meadows and Tundra Above the treeline the open meadows and tundra comprise a treeless plain, dominated by herbs and grasses and in some cases shrubs. Wildlife In poorly developed ecosystems, it s advantageous for species to be more generalist in their habitat preferences to maximize options for survival. Faunas tend to be dominated by rodents, scavengers, and insects. Life resolves around an alternating life of feast or famine. Some larger mammals are well able to cope with hypoxia lamas and alpaca. Human Physiology One of the most important parameters for determining stress to human biology is the fact that atmospheric pressure reduces with altitude and limits the oxygen-absorbing capacity of the blood. With the decline in pressure the process by which oxygen is bound into the hemoglobin in the bloodstream does not work so efficiently. This is known as hypoxia, normally encountered at 2500 meters at which oxygen stress can be clearly identified.