TROPICAL ENVIRONMENTS.pdf

TROPICAL ENVIRONMENTS 1. TROPICAL CLIMATES  TROPICAL ENVIRONMENTS OVERVIEW The tropical environment is the area between 23.5°N and 23.5°S. This area covers about 50 million km2 of land, almost half of it in Africa.  AIR MASS An air mass is a large body of air in which the horizontal gradients o...

TROPICAL ENVIRONMENTS 1. TROPICAL CLIMATES  TROPICAL ENVIRONMENTS OVERVIEW The tropical environment is the area between 23.5°N and 23.5°S. This area covers about 50 million km2 of land, almost half of it in Africa.  AIR MASS An air mass is a large body of air in which the horizontal gradients of the main physical properties, such as temperature and humidity, are fairly gentle. Air masses derive their temperature and humidity from the regions over which they lie. These regions are known as source regions. The principle ones are: i. Areas of relative calm, such as semi-permanent high-pressure regions ii. Areas where the surface is relatively uniform, including deserts, oceans and ice-fields Cont'd Air masses can be modified when they leave their sources. A maritime tropical (mT) air mass refers to one that is warm and moist. A continental tropical (cT) air mass refers to one that is warm and dry. As air masses move from their source regions they can be changed by the area over which they move. For example, a warm air mass that travels over a cold surface is cooled and becomes more stable. Hence, it may form low cloud or fog but is unlikely to produce much rain. By contrast, a cold air mass that passes over a warm surface is warmed and becomes less stable. The rising air is likely to produce more rain.  Inter-tropical convergence zone (ITCZ) The inter-tropical convergence zone is an area in the tropics in which the northeast trade winds and the southeast trade winds converge. If there is a difference in temperatures between the winds the warmer air rises over the denser, colder air and can produce rain.  The Intertropical Convergence Zone (ITCZ), also known as the equatorial trough, is a broad area near the equator where winds from the northern and southern hemispheres come together. It's typically located around 5 degrees north of the equator. This zone shifts with the seasons, following the path of the overhead sun by about 2 months. In regions like the Indian Ocean, where there's a large landmass (Asia) to the north, the movement of the ITCZ is more noticeable because it responds to the changing Cont'd seasons and temperatures. Over the eastern Atlantic and eastern Pacific Oceans, the ITCZ moves due to the influence of cold ocean currents (like the Benguela and Peru currents). Wind conditions within the ITCZ are usually light or even calm, which sailors historically called the "doldrums." Occasionally, strong westerly winds can burst through these calm conditions, creating brief periods of stronger winds.  Sub-tropical anticyclones are areas of high pressure caused by descending air at the tropopause. Centres or ridges of high pressure imply subsiding air. They tend to be found over continents, especially in winter. The subtropical high or warm anticyclone is caused by cold air at the tropopause descending. The position of the high pressure Subtropical alters in response to the seasonal drift of the ITCZ. High Pressure The subtropical high pressure belt tends to lie over the ocean, especially in summer, when there are low pressures over the Systems continents caused by heating. Highs tend to be larger than low-pressure systems, reaching up to 4000 km in width and 2000 km north–south. Therefore, smaller pressure gradients are involved and so winds are lighter. The subtropical high generally moves eastwards at speeds of 30– 50 km/hr. Hence a 4000 km system moving at an average of 40 km/hr would take about 4 days to pass over – if it kept moving.  Effects Where there is moisture at low level and air pollution, low- level stratus clouds may form, causing an anticyclonic gloom. Arid climates result from a prevalence of high pressure. Northeast Brazil is arid, even at a latitude of 8°S, because it protrudes far enough into the South Atlantic to be dominated by high pressure. Oceanic influences: 1. Ocean current The oceanic gyre (swirl of currents) explains why east coasts are usually warm and wet, because warm currents carry water polewards and raise the air temperature of maritime areas. In contrast, cold currents carry water towards the Equator and so lower the temperatures of coastal areas. West coasts are cool and dry due to advection of cold water from the poles and cold upwelling currents. Continental east coasts in the sub-tropics are humid and west coasts arid – this is mainly due to the easterly winds around the tropics. 2. Wind The temperature of the wind is determined by the area where the wind originates and by the characteristics of the surface over which it subsequently blows. A wind blowing from the sea tends to be warmer in winter, but cooler in summer, than the corresponding wind blowing from the land.  Monsoon What is a Monsoon? A monsoon is a seasonal wind pattern that brings significant changes in precipitation to a region. It is characterized by a pronounced seasonal reversal of wind direction. The basic cause is the seasonal difference in heating of land and sea on a continental scale. Seasonal Difference in Heating: During summer, land surfaces heat up more quickly than oceans due to lower heat capacity. This creates a temperature difference between land and sea. Warm air rises over the heated land, creating a low-pressure area. Cool, moist air from the ocean flows in to fill this low-pressure zone, bringing moisture and causing rainfall. Winter Conditions: In winter, the situation reverses: land cools faster than oceans. The temperature difference now causes cooler, denser air to sink over the land, creating a high-pressure area. Dry air from land flows towards the warmer ocean, resulting in dry conditions and little precipitation. Practical Impacts of Monsoons Examples of Monsoons Indian Monsoon: One of the most well-known monsoon systems, affecting the Indian subcontinent. It brings heavy rainfall from June to September, vital for agriculture and water supply. East Asian Monsoon: Affects countries like China, Japan, and Korea, bringing summer rains and influencing regional climates. A video on Köppen climate classification  KOPPEN'S WORLD CLIMATES Köppen's climate classification system is one of the most widely used methods for categorizing the world's climates. It divides climates into five major groups, each with several subtypes based on temperature and precipitation patterns. Major Climate Groups 1. Tropical Climates (A) : Tropical Rainforest (Af), Tropical Monsoon (Am), Tropical Savanna (Aw) 2.Dry Climates (B): Desert (BWh, BWk):, Steppe (BSh, BSk) 3.Temperate Climates (C) : Humid Subtropical (Cfa, Cwa), Mediterranean (Csa, Csb), Marine West Coast (Cfb, Cfc) 4.Continental Climates (D): Humid Continental (Dfa, Dfb, Dwa, Dwb), Subarctic (Dfc, Dfd, Dwc, Dwd) 5. Polar Climates (E): Tundra (ET), Ice Cap (EF) Additional Subcategories: Highland Climates (H) NB: Subtypes often include additional letters to indicate specific seasonal precipitation patterns (e.g., "s" for dry summer, "w" for dry winter, "f" for no dry season)  Tropical climates According to the climatologist Koppen, a Tropical climate (A) is where the coldest month is always above 18°C, which is the critical temperature for tropical forests. There are three main types of tropical climates: Rainforest (Af): No dry season, consistently wet all year. Monsoon (Am): Short dry season with heavy rains during the wet season. Savanna (Aw): Dry winter season with most rain during the summer. Tropical humid climates (Af) refer to areas in the tropics that are wet all year round.They typically have over 50 mm of rain per month for 8 to 12 months. These are usually found within 5–10° of the Equator. Despite the high midday sun, high humidity and cloud cover keep temperatures from getting too hot. Some months, like April and October, may be wetter due to the Intertropical Convergence Zone (ITCZ). Seasonally humid climates (Aw) refer to areas within the tropics where there is a distinct dry season as well as a wet season. They have a noticeable dry season, which gets longer the further from the Equator you go. Rainfall decreases from the moist low latitudes to near-desert areas. When the midday sun is at its highest, temperatures rise, air pressure drops, and thunderstorms occur. As the sun’s angle decreases, the rains stop, and dry conditions return. The tropical wet monsoon (Am) climate is similar to the Aw climate but with a distinct wet season. Winters are dry and hot, reaching their peak just before the monsoon season. During the monsoon, temperatures drop slightly due to cloud cover and heavy rains brought by tropical maritime air,  Tropical landforms Tropical landforms are diverse and complex. They are the result of many interrelated factors including climate, rock type, tectonics, time, vegetation, drainage, topography and, increasingly, human impact. 1. Climate Temperature and Precipitation: Tropical regions are characterized by warm temperatures and high levels of precipitation, which promote intense weathering processes. High rainfall leads to strong chemical weathering of rocks and the development of deep soil profiles. Humidity: High humidity accelerates the decomposition of organic matter and the formation of humus, influencing soil characteristics and vegetation. 2. Rock Type Lithology: Different rock types weather at different rates. For example, limestone is susceptible to chemical weathering (karst formation), while granite weathers more slowly. Mineral Composition: Rocks rich in easily weatherable minerals break down faster, influencing the landform development Cont'd 3. Tectonics Plate Movements: Tectonic activity, such as the movement of tectonic plates, leads to the uplift and formation of mountain ranges, volcanic activity, and earthquakes, which shape the landscape. Faulting and Folding: Tectonic forces can create faults and folds in the Earth's crust, leading to the development of valleys, ridges, and escarpments. 4. Time Geological Time Scale: Over long geological timescales, weathering, erosion, and deposition processes gradually shape the landforms. Older landscapes tend to be more eroded and have more mature landforms compared to younger ones.  5. Vegetation Root Systems: Vegetation stabilizes soil through root systems, reducing erosion. Dense vegetation, like that in tropical rainforests, protects the soil from heavy rainfall. Organic Matter: Decomposing plant material adds organic matter to the soil, influencing its fertility and structure. 6. Drainage Cont'd River Systems: Rivers and streams carve valleys, create floodplains, and deposit sediments. In tropical regions, intense rainfall can lead to significant river erosion and sediment transport. Wetlands: Areas with poor drainage can develop into swamps and marshes, which play a role in the landscape's hydrology and ecology.  Cont'd 7. Topography Elevation and Slope: The slope of the land affects the movement of water and sediments. Steeper slopes tend to have more rapid erosion, while flatter areas may accumulate sediments. Relief: The overall relief (difference in elevation) of an area influences microclimates and water flow patterns, impacting landform development. 8. Human Impact Deforestation: Clearing forests for agriculture or urban development increases soil erosion and alters natural water flow. Agriculture: Farming practices, especially on slopes, can lead to soil degradation and changes in landform stability. Urbanization: Construction and infrastructure development modify natural landforms, alter drainage patterns, and contribute to erosion and sedimentation issues. ThePhoto by PhotoAuthor is licensed under CCYYSA.  Examples of Tropical Landforms Karst Landscapes: Formed in limestone regions through chemical weathering and erosion, resulting in features like caves, sinkholes, and limestone pavements. Volcanic Landforms: Including volcanic cones, calderas, and lava plateaus, created by volcanic activity common in tectonically active tropical regions. Coastal Landforms: Such as mangrove swamps, coral reefs, and sandy beaches, shaped by the interaction of marine  Weathering Mechanical and chemical weathering occur widely in the tropics. In the humid tropics, the availability of water and the consistently high temperatures maximize the efficiency of chemical reactions, and in the oldest part of the tropics these have been operating for a very long period. In contrast, in many savanna areas, where there is less moisture, exfoliation or disintegration occurs.  Cont'd In many regions, weathering is complete and the weathering profile is very deep. The warm temperatures and heavy rainfall cause rocks to break down deeply into loose soil, creating thick weathered layers. As this weathered material accumulates, the slopes become less stable, making them prone to landslides and mudslides, especially during periods of intense rain. This instability occurs in a cyclical pattern: after a landslide, the slope may temporarily stabilize, but ongoing weathering continues to weaken it, leading  WEATHERING PROFILE A weathering profile is a vertical section of soil and rock that shows the various layers, or horizons, that have formed as a result of the weathering process. This profile typically includes: Unweathered Bedrock: The solid rock at the base that remains relatively unaffected by weathering. Partially Weathered Rock: The layer above the bedrock where the rock has started to break down into smaller pieces but still retains some of its original structure. Saprolite: A layer of thoroughly weathered rock that has lost its original structure and is often soft and friable. Soil Horizons: Layers of soil that have formed from the weathered material, often categorized into different ThePhoto by PhotoAuthor is licensed under CCYYSA. horizons (O, A, B, C) based on organic content, mineral composition, and other factors. The weathering profile reflects the extent and intensity of chemical, physical, and biological weathering processes that have acted on the rock over time.  Weathering cont'd Weathering profiles vary widely. The idealised weathering profile has three zones – residual soil, weathered rock and relatively unweathered bedrock. Weathered rock is also known as saprolite. In the weathered zone, at least 10% of the rock is unweathered corestones. This zone is typically highly permeable, especially in the upper sections, and contains minerals in a wide range of weathering stages. The ‘weathering front’ or ‘basal surface of weathering’ between solid rock and saprolite (weathered rock) can be very irregular. Typically, deep weathering occurs to depths of 30–60 m, but because of variations in jointing density and rock composition, the depth varies widely over short distances.  Tors are natural formations made up of large, rounded boulders that sit directly on solid bedrock. These boulders have been shaped by weathering, which has smoothed their edges. Surrounding the tors, you can often find loose, weathered rocks and soil that have broken off from the main structure over time. Tors, Location: Most tors are found in strongly jointed rock. Height: Tors vary in height from 20 m to 35 m. Inselbergs, Core Stones: Core stones can be up to 8 m in pediplains diameter. and Formation: Tors are formed by chemical weathering of the rock along joints and bedding planes beneath the etchplains surface. Joint Spacing and Core Stone Size: Widely Spaced Joints: Large core stones. Closely Spaced Joints: Increased weathering and smaller core stones.  Inselbergs Inselbergs are isolated residual hills that stand prominently over a level surface. Inselbergs are best developed on volcanic materials, especially granite and gneiss, with widely spaced joints and a high potassium content. These residual hills are the result of stripping weathered regolith from a differentially weathered surface. The main debate is whether hills need deep weathering to form. One idea, the two-stage model, suggests that hills form first from a large amount of weathered material underground that is later removed. Another ThePhoto by PhotoAuthor is licensed under CCYYSA. idea is that weathering and erosion happen at the same time. The variety of hills we see suggests that both processes might work together.  Monolithic domed inselbergs called bornhardts are characteristic landforms of granite plateaus of the African savanna, but can also be found in tropical humid regions. Bornhardts: They are characterised by steep slopes and a convex upper slope. Bornhardts are a case study eventually broken down into residual hills called castle kopjes. Bornhardts occur in of an igneous and metamorphic rocks. Granite, inselberg an igneous rock, develops joints, up to 35 m below the surface, during the process of pressure release. Vertical jointing in granite is responsible for the formation of castle kopjes.  Cont'd The two main theories for the formation of bornhardts include: the stripping or exhumation theory – increased removal of regolith occurs so that unweathered rocks beneath the surface are revealed parallel retreat, which states that the retreat of valley sides occurs until only remnant inselbergs are left Classic examples of bornhardts include Mt Hora, in the Mzimba District of Malawi, and Mt Abuja in northern Nigeria.  Pediplains are gently sloping plains that have rocky hills Pediplains called kopjes scattered across them. They form through a process where slopes move back evenly over time. This process, called pediplanation, starts with the land being pushed up by tectonic forces. This uplift causes rivers to erode the land quickly, creating steep drops, waterfalls, rapids, and gorges. Once the rivers erode down to a stable level, they start to erode sideways, spreading out and forming the flat pediplains.  Etchplain An etchplain is an area of stripped and exposed unweathered bedrock (Figure 7.6). Etchplains form in old, stable areas of the Earth's crust and are linked to deep weathering processes. During times when weathering was intense, thick layers of weathered material built up. In other periods, erosion outpaced weathering, removing the weathered material and revealing the fresh, unweathered rock beneath.  Tropical karst A tropical karst is a type of landscape found in tropical regions, characterized by unique landforms created by the dissolution of soluble rocks such as limestone, dolomite, and gypsum. There are two major landform features associated with tropical karst : Polygonal or cockpit karst is a landscape pitted with smooth-sided, soilcovered depressions and conical hills. Tower karst is a landscape characterised by upstanding rounded blocks set in a region of low relief. Other features include; Sinkholes: Depressions or holes in the ground caused by the collapse of a cave roof or the dissolution of surface rock. Caves: Underground voids formed by the dissolution of rock, often with intricate systems of tunnels and chambers. Disappearing Streams: Streams that vanish into the ground, flowing into cave systems or underground channels.  Solution holes The surface is broken up by many small solution holes but the overall surface remains generally level. Cockpit karst Cockpit karst is usually a hilly area in which many deep solution holes have developed to give it an ’eggbox’ appearance. Tower karst: The widening and deepening of the cockpits has destroyed much of the limestone above the water table. Only a few limestone towers remain, sticking up from a flat plain of sediments that have filled in the cockpits at a level just above the water table. Eventually the towers will be entirely eroded, and disappear. Cont'd Polygonal or cockpit karst is characterised by groups of hills, fairly uniform in height. These can be up to 160 m high in Jamaica, with a base of up to 300 m. They develop mainly as a result of solution. Polygonal karst tends to occur in areas that have been subjected to: High rates of tectonic uplift Intense vertical erosion by rivers The spacing of the hills may be related to the original stream network. Concentrated solution along preferred routes, such as wider joints, leads to accelerated weathering of certain sections of the limestone, especially during times of high flow. Water will continue to weather the limestone as far down as the water table. This creates closed depressions and dolines. Once the water table is reached, water will flow laterally rather than vertically, developing a flat plain Cont'd By contrast, tower karst is much more variable in size than the conical hills of cockpit karst, with towers ranging from just a few metres to over 150 m in height in Sarawak. Other areas of tower karst include southern China, Malaysia, Indonesia and the Caribbean. The towers are characterised by steep sides, with cliffs and overhangs, and with caves and solution notches at their base. The steepest towers are found on massive, gently tilted limestone. Where the water table is close to the surface, rivers will be able to maintain their flow over limestone, erode the surface and leave residual blocks set in a river plain. Other important processes include: differential solution along lines of weakness the retreat of cockpit karst slopes to produce isolated tower karst l lateral erosion  Explain why tors can be described as ‘joint- controlled’. Outline the two theories for the formation of bornhardts. What is the difference between cockpit karst and tower karst?