Physical Geography

About one-fifth of the world's land is desert. Deserts lie mostly between 15°–30° N and S latitude (Trade Wind belts on western coasts), where descending air of subtropical high pressure belts prevails and offshore Trade Winds fail to bring moisture. Mid-latitude deserts (Gobi, Turkestan) are dry due to continental interiors, far from rain-bearing winds. **Five types of desert landscape:** 1. **Hamada** (rocky desert): Large stretches of bare, swept rock; e.g. Hamada el Homra, Libya (~52,000 sq km) 2. **Reg** (stony desert): Angular pebbles and gravels; called **serir** in Libya/Egypt; **koum** in Turkestan = sandy 3. **Erg** (sandy desert): Sea of sand dunes; e.g. Calanscio Sand Sea, Libya; most iconic desert scenery 4. **Badlands**: Gully-eroded terrain; originally South Dakota, USA; e.g. Painted Desert, Arizona 5. **Mountain deserts**: Dissected highlands with wadis (dry valleys); e.g. Ahaggar Mountains, Tibesti Mountains, Sahara **Wind erosion processes:** Deflation (lifting and blowing away loose materials), Abrasion (sand-blasting), Attrition (fragments wearing each other down) **Wind erosion landforms:** - Rock pedestals/Mushroom rocks (Gour in Sahara): Undercut near base by sand-blasting - Zeugen: Tabular masses with hard layer over soft; ridges 3–30m; horizontal strata orientation - Yardangs: Steep-sided ridges in vertical strata alignment with wind; corridors between; best in central Asia - Mesas: Flat-top ped hard-capped hills; reduced to isolated buttes (e.g. Table Mountain, Cape Town is a mesa) - Inselberg ('island-mountain'): Isolated steep-sided residual hills in old-age desert; granite/gneiss; e.g. N Nigeria, W Australia, Kalahari; Ayers Rock (Uluru) Australia - Ventifacts/Dreikanter: Wind-faceted pebbles (3 faces = dreikanter); form desert pavement - Deflation hollows: Qattara Depression, Sahara (135m below sea-level); Faiyum Depression, Egypt (40m below sea-level) **Wind deposition features:** - **Barchans**: Crescent-shaped mobile sand dunes; tips point downwind; most common - **Seif dunes**: Long, knife-edged longitudinal dunes parallel to prevailing wind - **Loess**: Fine wind-blown dust deposited beyond desert margins; extremely fertile; thick deposits in China's Loess Plateau (N China), also in Mississippi valley (prairie soils), Pampas (Argentina) **Sahara Desert facts:** 5,150 km east-west, at least 1,600 km wide; total area 9 million sq km — larger than all 50 US states combined. Atacama (Peru/Chile) is driest — less than 13mm annual rainfall. One station received 0.5mm over 17 years.

Volcanic activity creates both **intrusive** (plutonic) and **extrusive** (volcanic) landforms. Intrusive landforms form when magma solidifies within the crust; extrusive form when it reaches the surface. A volcano is a place where gases, ashes, and/or molten rock material (lava) escape to the ground. Magma refers to molten material within the upper mantle, while lava is molten material that reaches the earth's surface. **Intrusive landforms:** Sill (horizontal intrusion along bedding planes), Dyke (vertical intrusion), Laccolith (dome-shaped mound, e.g. Henry Mountains, Utah), Lopolith (saucer-shaped, e.g. Bushveld, South Africa), Phacolith (lens-shaped at crest of anticline/trough of syncline), Batholith (huge granite mass — most spectacular, e.g. Wicklow Mountains Ireland, Main Range Malaysia). Intrusive forms are created when lava cools within the crustal portions. **Types of lava:** - Basic lava: Very fluid, ~1,000°C, dark, low silica, flows fast (16–48 km/h), builds shield/dome volcanoes with gentle slopes (e.g. Hawaiian volcanoes — Mauna Loa, Kilauea) - Acid lava: Viscous, high silica, light-coloured, slow-flowing, forms steep-sided cones with violent explosions, pyroclasts/volcanic bombs; can form volcanic spine/plug (e.g. Mt. Pelee, Martinique) **Types of volcanoes:** - Shield/dome: Basic lava, gently sloping, wide (Hawaii). These are the largest volcanoes on Earth (barring basalt flows) and are characterized by low explosivity unless water enters the vent. - Cinder/ash cone: Less fluid lava, steep, small (<300m), e.g. Mt. Nuevo (Naples), Mt. Paricutin (Mexico) - Composite/strato-volcano: Most common and highest; built by alternating lava and ash layers from main conduit with parasitic cones; e.g. Mt. Vesuvius, Mt. Fuji, Mt. Etna, Mt. Stromboli, Mt. Popocatapetl. These volcanoes erupt cooler and more viscous lavas and often result in explosive eruptions, accumulating pyroclastic material and ashes in layers. - **Caldera**: Large depression (several km across) formed by violent eruption and subsidence of volcano into magma below. Can form caldera lakes (e.g. Lake Toba, Sumatra; Crater Lake, Oregon). Calderas are formed by the collapse of highly explosive volcanoes. - **Flood Basalt Provinces**: Outpour highly fluid lava that flows for long distances, covering thousands of square kilometers (e.g., Deccan Traps in India). - **Mid-Ocean Ridge Volcanoes**: Occur in oceanic areas along mid-ocean ridges, with frequent eruptions in their central portions. **Distribution:** ~500+ active volcanoes. Two-thirds in **Circum-Pacific Ring of Fire** (~100 Philippines, 70+ Indonesia, 40 Andes, 35 Japan). Mediterranean belt: Vesuvius, Etna, Stromboli. East African Rift Valley: Kilimanjaro, Kenya (extinct). **Himalayas have NO active volcanoes**. **Geysers and Hot Springs:** Geysers erupt superheated steam/water to ~45m. Three main areas: Iceland, Rotorua (New Zealand), Yellowstone Park (USA). 'Old Faithful' at Yellowstone erupts every ~63 minutes. **Earthquakes:** 50,000+ recorded annually. An earthquake is the shaking of the earth, caused by the release of energy along a fault. The point where energy is released is called the **focus** or **hypocentre**. The point on the surface nearest to the focus is the **epicentre**. Earthquake waves are of two main types: **body waves** (P-waves and S-waves) and **surface waves**. Body waves travel through the Earth's interior, while surface waves travel along the surface and are generally more destructive. Tsunamis (Japanese term) = tidal waves caused by submarine earthquakes. 70% of earthquakes in Circum-Pacific belt; 20% in Mediterranean-Himalayan belt. Measured by seismograph. Earthquakes are scaled by **magnitude** (Richter scale, 0-10) and **intensity** (Mercalli scale, 1-12).

Permafrost refers to the permanently frozen sub-soil found in tundra regions, primarily north of the Arctic Circle. This condition results from the extremely low mean annual temperatures characteristic of the polar climate, where the ground remains solidly frozen for most of the year. During the brief summer, the upper layer of the soil may thaw, but typically only to a depth of about 150 mm (6 inches), leaving the ground frozen to great depths underneath. The presence of permafrost significantly influences the physical environment. It contributes to poor drainage in the tundra lowlands, as the frozen sub-soil prevents water from percolating downwards, leading to the formation of ponds, marshes, and waterlogged areas in hollows. This permanently frozen sub-soil is also a crucial factor limiting plant growth, particularly trees, as it makes the ground inaccessible to deep roots for all but a few months and forces shallow roots to grow laterally near the surface.

Physical geography encompasses the study of the Earth's natural environment, including its various realms: the lithosphere, atmosphere, hydrosphere, and biosphere. The lithosphere involves the study of landforms, drainage, relief, and physiography. The atmosphere focuses on its composition, structure, elements and controls of weather and climate, such as temperature, pressure, winds, and precipitation, along with climatic types. The hydrosphere covers the realm of water on Earth's surface, including oceans, seas, lakes, and associated water features. The biosphere deals with all life forms, including human beings and macro-organisms, their sustaining mechanisms like food chains, ecological parameters, and ecological balance. Soils, formed through pedogenesis, are also a crucial part of its study, considering parent rocks, climate, biological activity, and time. The importance of physical geography stems from the fact that each of these elements is vital for human beings. Landforms provide the base for human activities, with plains used for agriculture, plateaus for forests and minerals, and mountains offering pastures, forests, tourist spots, and river sources. Climate significantly influences human aspects like housing, clothing, food habits, vegetation, cropping patterns, livestock farming, and certain industries. Precipitation, for instance, recharges groundwater aquifers essential for agriculture and domestic use. Oceans are recognized as vast storehouses of resources, including fish, seafood, and minerals. Soils are renewable resources that underpin agriculture and accommodate the biosphere. Thus, physical geography is critical for evaluating and managing natural resources, requiring an understanding of the intricate relationship between the physical environment and human activities to achieve sustainable development, especially in light of ecological imbalances caused by accelerated resource utilization.

**Ocean Salinity:** Salinity is the total amount of dissolved salts in seawater, expressed in parts per thousand (ppt or ‰). The average salinity of the world's oceans is about 35 ppt. Factors affecting salinity: - Evaporation: Higher evaporation increases salinity (sub-tropical zones are most saline) - Precipitation and river inflow: Dilute salinity (equatorial zone, river mouths) - Freezing and melting of ice: Freezing leaves behind salt (increases salinity in polar waters); melting dilutes - Ocean currents: Mix waters of different salinities - Wind: Influences salinity by transferring water to other areas. Horizontal distribution: Salinity varies from about 33–37 ppt in open oceans. Sub-tropical regions (Mediterranean, Red Sea) have highest salinity. Polar regions and river mouths have lowest salinity. The Dead Sea (a landlocked lake) has extremely high salinity (~340 ppt). Other highly saline water bodies include Lake Van in Turkey and the Great Salt Lake in Utah. Salinity can reach up to 70 o/oo in hot and dry regions with high evaporation. The North Sea records higher salinity due to the North Atlantic Drift, while the Black Sea has very low salinity due to large freshwater influx. The Bay of Bengal shows a low salinity trend due to river water influx, contrasting with the Arabian Sea which shows higher salinity due to high evaporation and low freshwater influx. The Atlantic Ocean's average salinity is around 36 o/oo, with a maximum of 37 o/oo between 20° N and 30° N and 20° W - 60° W. In the Pacific Ocean, salinity decreases on the western parts of the northern hemisphere due to melted Arctic water, and similarly south of 15°-20° S. Vertical distribution: Salinity generally increases with depth up to the halocline (zone of rapid salinity change), then remains relatively constant in deeper waters. Salinity at depth is fixed, as water is not lost and salt is not added. Lower salinity water rests above higher salinity dense water. Increasing salinity causes seawater density to increase, leading to high salinity water sinking below lower salinity water and creating stratification by salinity. **Tides:** Tides are the periodic rise and fall of sea levels caused by the gravitational pull of the Moon and the Sun on Earth's oceans. Tides are mainly caused by the gravitational pull of the Sun and the Moon, and additionally by the centrifugal force which counterbalances gravity. Together, these forces create two major tidal bulges on the Earth. - Spring tides: Occur when the Sun, Moon, and Earth are aligned (new moon and full moon). Gravitational forces of Moon and Sun combine — highest high tides and lowest low tides. - Neap tides: Occur when the Sun and Moon are at right angles to each other (first and third quarter of moon). Forces partially cancel — moderate tides. The Moon's gravitational pull is approximately 2.17 times more effective than the Sun's in generating tides (despite the Sun being much more massive, the Moon is much closer). Two high tides and two low tides occur approximately every 24 hours 52 minutes in most coastal locations (semidiurnal tides), though some locations have diurnal (once daily) tides. Tidal bulges gain greater height on wide continental shelves and become lower when hitting mid-oceanic islands. The shape of bays and estuaries, particularly funnel-shaped ones, can greatly magnify tidal intensity. Tidal bores occur in funnel-shaped estuaries where the incoming tide is funnelled into a narrow channel, creating a wave that travels upstream (e.g., Hugli river in West Bengal).

Ocean currents are continuous, directed movements of seawater, often described as "river flow in oceans". They are of two types: surface currents (driven by winds, salinity differences, and temperature) and deep-water currents (thermohaline circulation, driven by differences in water density due to temperature and salinity). These movements are influenced by both primary forces that initiate the water's motion and secondary forces that shape their flow paths. **Causes of Ocean Currents:** 1. **Planetary winds**: Trade winds drive equatorial currents westward; westerlies drive mid-latitude currents eastward 2. **Temperature differences**: Warm water at equator, cold at poles — creates density differences; heating by solar energy causes water to expand, resulting in higher water levels near the equator (approx. 8 cm higher) which creates a slight gradient for flow. 3. **Salinity differences**: Higher salinity = higher density; sinks and moves as deep currents. Denser water tends to sink, while lighter water tends to rise. 4. **Earth's rotation (Coriolis effect)**: Deflects currents clockwise in Northern Hemisphere, anti-clockwise in Southern Hemisphere — creates large gyres. It causes warm currents from low latitudes to move right in the Northern Hemisphere and left in the Southern Hemisphere. 5. **Shape of coastline**: Coastlines redirect ocean currents 6. **Gravity**: Tends to pull water down piles and create gradient variation. **Warm and Cold Currents:** - Warm currents flow from tropical to polar regions; Cold currents flow from polar to tropical regions - Warm currents: North Atlantic Drift (Gulf Stream extension), Kuroshio (Japan), Brazilian Current. Warm currents are usually observed on the east coast of continents in low and middle latitudes (both hemispheres) and on the west coasts of continents in high latitudes (Northern Hemisphere). - Cold currents: Labrador Current, California Current, Benguela Current, Humboldt (Peru) Current, Canary Current. Cold currents are usually found on the west coast of continents in low and middle latitudes (both hemispheres) and on the east coast in higher latitudes (Northern Hemisphere). **Major Ocean Current Gyres:** - North Atlantic Gyre (clockwise): Gulf Stream → North Atlantic Drift → Canary Current → North Equatorial Current - South Atlantic Gyre (anti-clockwise): Brazil Current → Benguela Current → South Equatorial Current - North Pacific Gyre: Kuroshio → North Pacific Drift → California Current - Indian Ocean: Seasonal reversal due to monsoon **Significance:** - Warm currents: Moderate climates on adjacent coasts; increase precipitation; reduce fog. On west coasts of continents in middle and higher latitudes, they cause a distinct marine climate with cool summers and relatively mild winters. - Cold currents: Cool and dry coastal climates (deserts on west coasts — Namib, Atacama); create upwelling zones → nutrient-rich waters → fishing grounds. West coasts of continents in tropical and subtropical latitudes are bordered by cool waters, leading to low average temperatures, narrow diurnal/annual ranges, fog, and generally arid conditions. - Fishing grounds: Where warm and cold currents meet (Grand Banks — Labrador + Gulf Stream; Japan's Fishing Grounds — Kuroshio + Oyashio). The mixing of warm and cold currents replenishes oxygen and favors plankton growth, which are primary food for fish.

In arid regions, wind acts as a significant agent of erosion, transportation, and deposition, becoming more efficient in deserts compared to humid areas due to the scarcity of vegetation and moisture that would otherwise bind loose surface materials. This absence allows the effects of wind erosion to be largely unrestrained. The primary mechanics of wind erosion include deflation, abrasion, and attrition, which collectively shape the distinctive desert landscape. Deflation is the process where wind lifts and blows away loose materials, such as unconsolidated sand and pebbles, leading to the lowering of the land surface and the formation of large depressions known as deflation hollows. Abrasion involves the sand-blasting of rock surfaces, where wind hurls sand particles against them, causing the surfaces to be scratched, polished, and worn away, most effectively near the base of rocks. Attrition occurs when wind-borne particles collide with each other, causing them to wear down, reduce in size, and become rounded. These combined processes result in a variety of characteristic desert landforms.

Plate tectonics explains the large-scale movement of Earth's lithospheric plates, leading to the formation of mountain systems, ocean trenches, volcanic arcs, and earthquake belts. The Earth's outer shell is divided into a series of rigid plates that float on the semi-molten asthenosphere beneath. The continents and ocean floors are products of the movement of these plates. Where plates move apart (divergent boundaries), new oceanic crust is formed at mid-ocean ridges. Where plates collide (convergent boundaries), one plate may subduct beneath the other, creating deep ocean trenches and volcanic mountain chains. Where plates slide past each other (transform boundaries), fault zones develop. The theory emerged from earlier ideas of continental drift proposed by Alfred Wegener. Evidence for plate tectonics includes: the matching coastlines of continents, identical fossil records on continents now separated by oceans, matching geological structures across continents, paleoclimatic evidence, and the distribution of earthquake and volcanic zones. The Himalayas formed through the collision of the Indo-Australian plate with the Eurasian plate. The Deccan Plateau represents a remnant of the ancient Gondwana supercontinent. The Andes and Rockies formed at subduction zones along the western edges of the American continents.

Geomorphic processes are the internal (endogenic) and external (exogenic) forces that continuously modify the Earth's surface. Endogenic processes originate from within the Earth — including volcanism, earthquakes, and tectonic movements — and tend to build up relief. Exogenic processes are driven by forces at or near the Earth's surface — including weathering, erosion, transportation, and deposition — and tend to wear down relief. Weathering is the process of disintegration and decomposition of rocks in situ (in place), without any transport. It is of three types: 1. **Physical (Mechanical) Weathering**: Breakdown of rocks by temperature changes, freeze-thaw cycles, and pressure release without chemical change. Block disintegration, exfoliation, and granular disintegration are major types. 2. **Chemical Weathering**: Decomposition of rocks through chemical reactions such as oxidation, hydration, carbonation, and solution. Prevalent in humid tropical climates (like much of India). 3. **Biological Weathering**: Disintegration or decomposition caused by plants, animals, and microorganisms. Erosion involves the wearing away of the land surface by running water (fluvial erosion), glaciers (glacial erosion), wind (aeolian erosion), and waves (marine erosion). Each agent creates distinctive landforms. Mass movement refers to the downslope movement of weathered material under the influence of gravity — including landslides, rockfalls, soil creep, and mudflows.

Wind depositional landforms in deserts result from the accumulation of materials eroded and transported by wind. While the finest dust can travel enormous distances, even beyond desert limits, coarser sands are too heavy and remain within the desert, forming distinct depositional features. The migration and shaping of these landforms are influenced by factors such as particle size, wind direction and velocity, the nature of the surface, and the presence of water or vegetation. The primary wind depositional landforms are dunes and loess. Dunes are hills of sand shaped by wind movement, found predominantly in sandy deserts (ergs). They can be active, constantly moving, or inactive, stabilized by vegetation. Two significant types of dunes are barchans and seifs. Barchans are crescentic, live dunes that advance steadily, exhibiting a convex, gently sloping windward side and a concave, steep leeward (slip-face) side. Seifs, or longitudinal dunes, are long, narrow sand ridges that align parallel to the prevailing winds, often extending over a hundred kilometers and reaching heights of more than 60 meters. Loess refers to the fine, wind-blown dust deposited beyond desert boundaries, typically yellow, friable, and fertile. It is a porous loam, rich in lime, where water easily infiltrates, leaving the surface dry. Extensive loess deposits can be found in regions like north-west China, where dust from the Gobi Desert has accumulated to significant depths, forming the 'Huangtu' or yellow earth. Similar deposits are also found in parts of Europe and the U.S.A.

The Earth is one of nine planets revolving around the Sun in elliptical orbits. The solar system is part of the Milky Way galaxy, which alone contains about 100,000 million stars. The Earth is not a perfect sphere — it is slightly flattened at the poles and bulges at the equator, making it a **geoid**. Its equatorial diameter (12,761 km) is 42 km more than its polar diameter. The Earth is about 4,500 million years old. The Earth completes one rotation on its axis in approximately 24 hours (solar day) and one revolution around the Sun in 365¼ days. The Earth's axis is tilted at 23½° to the plane of its orbit, which causes the seasons and variation in day length throughout the year. Key consequences of Earth's motions: - **Rotation**: Causes day and night, deflection of winds (Coriolis effect), difference in time between meridians - **Revolution + Tilt**: Causes seasons, variation in the length of day and night, and the apparent movement of the sun

Lakes formed by volcanic activity arise from several distinct processes related to eruptions and subsequent geological changes. One primary method involves the formation of **crater and caldera lakes**. During a volcanic explosion, the summit of a cone may be ejected, leaving a natural depression known as a crater. This crater can subsequently enlarge through subsidence, forming a caldera. In dormant or extinct volcanoes, these depressions, which are typically dry, steep-sided, and roughly circular, accumulate rainwater due to the absence of superficial outlets, thus forming a lake. Another way volcanic lakes are created is through **lava-blocked lakes**. In active volcanic areas, molten lava flowing across a valley can solidify, creating a natural dam. This dam then obstructs the flow of a river, leading to the accumulation of water behind it and the formation of an elongated lake. Finally, lakes can also form due to the **subsidence of a volcanic land surface**. This occurs when the crust of a hollow lava flow collapses, resulting in a wide and shallow depression that can subsequently fill with water to become a lake.

Man-made lakes, also known as artificial lakes, are created by human activities, primarily through the construction of concrete dams across river valleys. These dams hold back river water to form reservoirs. A prominent example is Lake Mead, formed by the Hoover Dam on the Colorado River in the U.S.A. Beyond damming, human activities such as mining (e.g., tin mining in Peninsular Malaysia) and the creation of ornamental or fishing lakes also contribute to the formation of man-made lakes. Biological lakes are those formed due to the activities of animals. Beavers, for instance, are known to construct dams across rivers using timber. These beaver dams create relatively permanent lakes, such as Beaver Lake in Yellowstone National Park, U.S.A. Both natural and artificial lakes serve vital human purposes. Man-made lakes, specifically, are crucial for domestic water supply to towns and industrial cities, especially in regions lacking natural lakes. They are also utilized for generating hydro-electric power, often as 'man-made reservoirs' in mountainous districts. Multi-purpose dams that form these lakes further supply water for irrigation and are constructed to control floods by absorbing excess water during heavy rains. Additionally, artificial lakes are created for inland fish breeding in many countries.

Islands are classified into **Continental islands** (once part of mainland, separated by rise in sea-level or land subsidence) and **Oceanic islands** (rise from ocean floor with no continental connection). **Continental islands:** - Individual: Newfoundland (Strait of Belle Isle), Malagasy (Mozambique Channel), Sri Lanka (Palk Strait), Tasmania (Bass Strait), Taiwan (Formosa Strait) - Archipelagos: British Isles, Balearic Islands, Aegean islands - Festoons/Island arcs: Continuation of mountain ranges — East Indies, Aleutian Islands, Ryukyu Islands, Kurile Islands **Oceanic islands:** - Volcanic islands: Built up from ocean floor volcanic cones; e.g. Mauna Loa (Hawaii, 4,169m above sea-level but 5,490m from ocean floor); Azores, Ascension, St. Helena, Madeira, Canary Islands (Atlantic); Mauritius and Reunion (Indian Ocean) - Coral islands: Very low, just above water; Marshall Islands, Gilbert and Ellice Islands (Pacific); Bermuda (Atlantic); Laccadives and Maldives (Indian Ocean) **Coral Reefs:** Reef-building corals are polyps that secrete calcium carbonate. They thrive only under: 1. Water temperature NOT below 20°C (68°F) — limited to tropical/subtropical zones 2. Water depth NOT exceeding 30 fathoms (~55m/180 feet) — for photosynthesis by algae 3. Clear, saline, sediment-free, well-oxygenated water — hence absent near muddy river mouths 4. Absent on western coasts of continents where cold currents upwell Pacific and Indian Oceans have the most coral reefs. Gulf Stream extends coral range far north of West Indies. **Three types of coral reef:** 1. **Fringing reef**: Platform close to shore, sometimes separated by shallow lagoon; widest at headlands; absent at stream mouths; ~1 km wide; e.g. common in Red Sea, Indian Ocean 2. **Barrier reef**: Separated from coast by wide, deep channel/lagoon; partially submerged; has navigable gaps; **Great Barrier Reef** (Queensland, Australia): 1,930 km long, channel up to 160 km wide, >60m deep 3. **Atoll**: Ring/oval coral reef enclosing a central lagoon; island/land has sunk (Darwin's theory); e.g. Marshall Islands, Maldives, Laccadives **Darwin's Subsidence Theory**: Fringing reef → Barrier reef → Atoll as volcanic island subsides. Coral keeps pace with subsidence by growing upward, eventually forming ring around submerged island.

While deserts are typically associated with aridity, very few are entirely devoid of rain. Water action plays a significant role in shaping desert landscapes, despite the low and irregular annual precipitation, which typically ranges from 127 to 254 mm (5 to 10 inches). Rain often occurs as torrential thunderstorms and downpours, leading to devastating flash-floods. The scarcity of vegetation in deserts means that the surface soil is unprotected, allowing large quantities of rock wastes to be transported by these sudden torrents. The erosional work of water includes the cutting of deep gullies and ravines, resulting in "badland topography." Subsequent downpours widen and deepen these features by washing away more soft rocks. Larger dry channels, known as wadis, are deepened by vertical corrasion during cloudbursts and remain dry for most of the time. In places like Algeria, such gorges are termed chebka. Some desert streams, called exotic streams, originate from melting snow in distant mountains outside the desert regions. Depositional landforms created by water include alluvial cones, fans, or "dry deltas," which form when masses of debris carried by flash-floods are deposited at the base of hills or valley mouths. In arid and semi-arid regions, intermittent streams drain into lower depressions, creating temporary lakes with high salt content that appear glistening white when dry. These lakes and their associated alluvial plains are known as playas, salinas, or salars in the United States and Mexico, and shotts in northern Africa. The floor of such depressions may also feature a bajada, which is a depositional accumulation of alluvial material, and a pediment, an erosional plain at the base of surrounding mountain scarps.

Glaciation is a significant geological process that leads to the formation of various types of lakes. These lakes originate from the erosional and depositional activities of glaciers and ice sheets. Key types include cirque lakes, also known as tarns, which form in the armchair-shaped hollows at the heads of mountain valleys and can develop into long, deep ribbon lakes if they occupy glacial troughs. Kettle lakes are depressions created in outwash plains by the melting of stagnant ice masses, typically irregular, small, and shallow due to the uneven morainic surface. Rock-hollow lakes result from ice-scouring, where the intense abrasive action of valley glaciers or ice sheets scoops out hollows on the land surface. Beyond erosion, glacial deposition also contributes to lake formation. Lakes can be formed through the morainic damming of valleys, where debris deposited by valley glaciers blocks a valley, causing water to accumulate behind the barrier. Both lateral and terminal moraines are capable of forming such dams. Furthermore, in glaciated lowlands characterized by a drumlin landscape and poor drainage, small lakes can form in the waterlogged depressions created by the deposition of glacial drifts. Finland, for example, is renowned for its abundance of glacial lakes, often referred to as "Suomi – the Land of Lakes."

Coastlines are continuously modified by waves, tides, and currents. Waves are the most powerful agent of marine erosion, and coastal processes are highly dynamic and destructive. Normal ocean waves: 6m high, 120m long (wavelength). On approaching shore, waves slow, crest curls and breaks as **breakers**; water rushing up beach = **swash**; retreating water = **backwash**. Storm waves and tsunami waves can cause far-reaching changes in a short period. **Marine erosion processes:** Corrasion/Abrasion (rock debris battering cliffs), Attrition (fragments grinding each other), Hydraulic action (compressed air in joints exploding outward), Solvent action (dissolution of limestone coasts). **Coastal erosion landforms:** - **Capes and bays**: Hard rocks persist as headlands/capes; soft rocks eroded into bays/coves - **Cliffs**: Steep rock face adjoining coast; wave-cut notch at base undermines cliff; **wave-cut platform** (gently sloping rocky shelf at base, exposed at low tide) and **wave-cut terrace** (flat or gently sloping platform at the foot of cliffs, covered by rock debris, occurring above average wave height). - **Cave**: Excavated hole in cliff at zone of weakness due to lashing waves and rock debris; e.g. Flamborough Head, England - **Arch**: Two caves from opposite sides of headland join (e.g. Durdle Door) - **Stack**: Roof of arch collapses, leaving isolated rock pillar, or resistant rock masses left as remnants when cliffs retreat - **Stump**: Stack eroded to low rock exposed only at low tide - **Blow-hole**: Vertical chimney from cave roof to cliff top through which water/spray jets **Coastal deposition landforms:** - **Beach**: Accumulation of sand/shingle/pebbles deposited by waves; formed by swash depositing and backwash transporting. Beaches are temporary features, with sediment from land or wave erosion. Can be sandy or shingle. - **Spit**: Elongated ridge of sand/shingle extending from headland; can be a barrier bar keyed up to one end of a bay, or attached to headlands/hills. Hooked/recurved end due to refracted waves; e.g. Chesil Beach (Dorset), Dungeness - **Bar**: Spit that extends completely across a bay; can enclose a lagoon. Includes **off-shore bar** (ridge of sand/shingle parallel to coast in off-shore zone) and **barrier bar** (an off-shore bar exposed due to further sand addition). - **Tombolo**: Bar connecting an island to mainland; e.g. several Mediterranean examples - **Sand dune**: Wind-blown sand inland from beach; held by marram grass, forming long ridges parallel to coastline. - **Lagoon**: Body of water enclosed behind bar/spit/barrier bars. They eventually get filled by sediments to form coastal plains. - **Wave-built terrace**: Develops in front of a wave-cut terrace in the offshore, with addition of material, after considerable cliff development and retreat. **Types of coastline:** - **Drowned (Concordant/Dalmatian)**: Submergence of river valleys → rias; glacial troughs → fiords; ridge-parallel to coast → Dalmatian coast (Adriatic). Also referred to as **high, rocky coasts** where erosion features dominate. - **Emergent (Discordant)**: Raised beaches; marine platforms now above sea-level. Also referred to as **low, smooth and gently sloping sedimentary coasts** where depositional features dominate. - **Haff coastline**: Long bars/sandbars separating lagoons (haffe) from sea; e.g. Baltic coast **Strandflat** (wave-cut platform): Off western Norway, width up to 48 km. The west coast of India is a high rocky retreating coast, dominated by erosional forms, while the east coast of India is a low sedimentary coast, dominated by depositional forms.

Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO₃). It is soluble in rainwater mixed with CO₂ (forming weak carbonic acid). A region of extensive limestone with distinctive solutional topography is called a **karst region**, named after the Karst district of Yugoslavia (now Slovenia/Croatia) in the Balkans adjacent to the Adriatic Sea, where such features are best developed. Dolomite = limestone with magnesium also present. Chalk = very pure, soft form of limestone. **Karst characteristics:** - General absence of surface drainage (most water goes underground) - Dry surface valleys - Prominent jointed/fractured limestone surface **Surface karst features:** - **Limestone pavement**: Flat bare rock; enlarged joints called **grikes**; rectangular blocks called **clints** - **Swallow holes/Sink holes**: Small depressions where rainwater sinks into rock; e.g. Gaping Ghyll, Yorkshire. Sinkholes can vary in area from a few square meters to a hectare, and in depth from less than half a meter to thirty meters or more. - **Doline**: A single enlarged swallow hole (also called dolina). The term doline is sometimes used to refer to collapse sinks, where the bottom of a sinkhole forms a cave roof that collapses. - **Uvala**: Several dolinas merged by subsidence; also called valley sinks. - **Polje**: Very large depression (up to 100 sq km) partly due to faulting; e.g. in Yugoslavia; floors can be temporary lakes seasonally - **Lapies**: Extremely irregular surface with a maze of points, grooves, and ridges formed due to differential solution activity along parallel to sub-parallel joints. **Underground karst features:** - **Caves and caverns**: Carved by underground streams along joints/bedding planes. Prominent where limestones are dense, massive, and occur as thick beds, or where alternating beds of rocks (shales, sandstones, quartzites) are present. - **Stalactites**: Downward-hanging calcium carbonate pinnacles from cave roof, broad at their bases and tapering towards the free ends. - **Stalagmites**: Upward-growing calcium carbonate deposits from cave floor, forming due to dripping water from stalactites or the surface. They can take the shape of a column, a disc, or have a smooth, rounded bulging end or a miniature crater-like depression. - **Pillar/Column**: Stalactite and stalagmite joined together. - **Gorge**: Formed when roof of underground tunnel collapses (e.g. Cheddar Gorge) - **Resurgence (Vauclusian spring)**: Underground river re-emerges when limestone meets impermeable rock - **Tunnels**: Caves having openings at both ends. **Notable limestone caves:** Batu Caves (Kuala Lumpur), Mammoth Caves (Kentucky), Carlsbad Cave (New Mexico), Postojna Caves (Yugoslavia). **Major limestone regions:** NW Yugoslavia (Karst district), Causses (S France), Pennines (Britain), Kentucky (USA), Yucatan Peninsula (Mexico), Cockpit Country (Jamaica). **Chalk landforms:** Less dramatic than limestone; dry valleys (coombes); low rounded hills ('downs' in S England, N France); short turf pasture.

Wind erosional landforms in deserts emerge from the combined processes of abrasion, deflation, and attrition, which are particularly effective in arid regions due to sparse vegetation and moisture. Abrasion involves wind-blown sand particles scratching, polishing, and wearing away rock surfaces, being most effective at the base of rocks. Deflation is the lifting and blowing away of loose materials, leading to the lowering of land surface and formation of depressions. Attrition refers to wind-borne particles colliding and wearing each other down, reducing their size and rounding them into shapes like millet seed sand. These processes sculpt various characteristic landforms. Rock pedestals, also known as mushroom rocks or gour, form when winds abrade softer layers of projecting rock masses, especially near the base, creating an undercut mushroom shape. Zeugen are tabular masses with a resistant surface layer over softer rocks, carved into a "ridge and furrow" landscape by wind abrasion, with hard rocks forming ridges. Yardangs are similar but feature vertical bands of hard and soft rocks aligned with prevailing winds, where softer bands are excavated into corridors and hard rocks form steep-sided ridges. Mesas are flat, table-like landmasses with a resistant horizontal top layer and steep sides, which can be reduced in area by denudation to form isolated flat-topped hills called buttes. Inselbergs are isolated residual hills with steep slopes and rounded tops, often composed of granite or gneiss. Ventifacts (or dreikanter) are pebbles faceted and polished by sand-blasting, often with multiple flattened surfaces and sharp edges. Deflation hollows are depressions formed by winds blowing away unconsolidated materials, sometimes reaching the water-table to create oases or swamps.

Lakes are bodies of water occupying hollows of the land surface. They vary enormously in size, depth, mode of origin, and water chemistry. Lakes are temporary features in geological time — eventually eliminated by silting up or draining. Salt lakes form in regions of internal drainage where evaporation > inflow. **Major lake types by origin:** **1. Earth Movement Lakes:** - **Tectonic lakes**: Formed by warping/sagging/fracturing of crust; e.g. Lake Titicaca (highest lake — 3,810m, Andes); Caspian Sea (largest lake — 373,230 sq km) - **Rift valley lakes**: Formed by faulting; deep, narrow, elongated; floors often below sea-level; East African Rift Valley lakes: Tanganyika (1,430m deep — world's deepest lake), Malawi, Rudolf, Edward, Albert; Dead Sea (392m below sea-level — world's lowest point) **2. Glacial Lakes:** - **Cirque lakes/Tarns**: In over-deepened corrie floors; e.g. Red Tarn, English Lake District - **Ribbon lakes**: Long, narrow lakes in glacial troughs; e.g. Lake Ullswater - **Kettle lakes**: In outwash plain depressions from melted stagnant ice; e.g. meres of Shropshire - **Rock-hollow lakes**: Ice-scoured hollows; abundant in Finland (35,000+ lakes) - **Moraine-dammed lakes**: Terminal/lateral moraine dam; e.g. Lake Windermere, Lake District - **Drumlin landscape lakes**: Depressions between drumlins; e.g. County Down, N Ireland **3. Volcanic Lakes:** - **Crater/Caldera lakes**: In dormant/extinct volcanic craters; e.g. Crater Lake, Oregon (USA); Lake Toba, N Sumatra; Lake Avernus, near Naples - **Lava-blocked lakes**: Lava flow dams valley; e.g. Sea of Galilee (Jordan Valley, lava-blocked) - **Subsidence lakes**: Hollow lava flow collapses; e.g. Myvatn, Iceland **4. Erosional Lakes:** - **Karst lakes**: Solution hollows in limestone; e.g. Lac de Chaillexon, Jura; Lake Scutari, Yugoslavia - **Wind-deflated lakes**: Deflation hollows reach groundwater; e.g. Qattara Depression, Egypt; Great Basin, Utah — become salt lakes/playas **5. Depositional Lakes:** - **Oxbow lakes**: Cut-off meander loops in river's lower course - **Floodplain lakes**: Back-swamps and waterlogged areas of floodplain - **Delta lakes**: In deltaic regions; e.g. Lake Chilika (India) - **Coastal lakes**: Bar/lagoon enclosure; e.g. Chilika (salty lagoon), Chilka Lake **Salinity facts:** Dead Sea: 250 parts per thousand; Great Salt Lake, Utah: 220 ppt; Black Sea (many large rivers): only 17 ppt; Normal ocean: ~35 ppt.

During the Pleistocene Ice Ages (~30,000 years ago), ice covered >31 million sq km of the northern hemisphere. Today major ice caps remain in Greenland (1.87 million sq km) and Antarctica (>13 million sq km). Glaciers form when snow accumulates, compresses into névé/firn (granular ice), and flows downhill under gravity. Masses of ice moving as sheets over land (continental or piedmont glaciers) or as linear flows down mountain slopes in broad trough-like valleys (mountain and valley glaciers) are called glaciers. Glaciers erode by two processes: **Plucking** (freezing and tearing away blocks) and **Abrasion** (scratching/scouring with embedded debris, producing striation marks and rock flour). Erosion by glaciers is tremendous due to friction caused by the sheer weight of the ice, which can reduce high mountains into low hills and plains. **Highland glacial erosion features:** - **Corrie/Cirque/Cwm**: Armchair-shaped depression where firn accumulates; steep horseshoe-shaped basin with rocky lip; fills with water to form a tarn - **Arête**: Knife-edge ridge between two corries cutting back from opposite sides (e.g. Striding Edge, Helvellyn, UK) - **Pyramidal peak/Horn**: Formed where 3+ corries cut back together (e.g. Matterhorn, Switzerland) - **Bergschrund**: Deep vertical crack at head of glacier in summer - **U-shaped glacial trough**: Wide, flat floor, steep sides; interlocking spurs blunted to truncated spurs; long/ribbon lakes form in overdeepened sections (e.g. Loch Ness, Lake Ullswater) - **Hanging valley**: Tributary valley 'hangs' above main valley after ice melts; stream plunges as waterfall (good for HEP) - **Rock basins and rock steps**: Unequal excavation creates basins later filled by lakes **Moraines:** Lateral (sides), Medial (two glaciers join), Ground (beneath glacier), Terminal/End (at snout), Recessional (formed as ice retreats by stages). **Highland glaciation lowland features:** - **Roche moutonnée**: Resistant rock hummock; upstream side smooth (abrasion), downstream side rough and steep (plucking) - **Crag and tail**: Resistant rock crag protects lee side from erosion; tail of softer rock behind (e.g. Edinburgh Castle Rock) - **Drumlins**: Streamlined egg-shaped hills of boulder clay; formed subglacially; long axis parallel to ice movement - **Eskers**: Long, winding ridges of sand and gravel deposited by meltwater streams beneath glacier - **Kames**: Mounds of sand and gravel deposited by meltwater - **Outwash plain**: Sandy/gravelly plain beyond terminal moraine, deposited by meltwater - **Till plain/Boulder clay**: Mixed boulders and clay deposited by glacier; fertile farmland (e.g. US Midwest, East Anglia) - **Fiord**: Drowned U-shaped valley; typical of Norwegian and south Chilean coasts

Lakes formed by earth movements result from the deformation of the Earth's crust, creating depressions that collect water. This category primarily includes tectonic lakes and rift valley lakes. Tectonic lakes originate from extensive depressions formed by processes such as warping, sagging, bending, and fracturing of the earth's crust. These geological activities lead to large and deep basins where water accumulates. Notable examples of tectonic lakes include Lake Titicaca, which is situated in a high intermont plateau, and the Caspian Sea, recognized as the world's largest lake. Rift valley lakes are a specific type of lake formed by faulting. A rift valley is created when a block of land sinks between two parallel faults, resulting in a deep, narrow, and elongated trough. Water then gathers in these troughs, and their floors often lie below sea-level. The East African Rift Valley is a prominent example of such a formation, containing several significant lakes like Lake Tanganyika (one of the world's deepest) and the Dead Sea (the world's lowest lake).

The Earth consists of concentric layers. The outermost is the **lithosphere (crust)** divided into **SIAL** (upper continental crust — silica + alumina, density 2.7) and **SIMA** (lower basaltic crust — silica + iron + magnesium, density 3.0). Beneath the crust is the **mantle (mesosphere)** ~2,900 km thick, rich in olivine. The innermost is the **core (barysphere)** — radius 3,476 km, composed mainly of iron-nickel (**NIFE**), temperature ~1,927°C. The innermost core is solid/crystalline despite high temperature due to extreme pressure. **Three classes of rocks:** 1. **Igneous rocks**: Formed by cooling of magma. Plutonic (intrusive, e.g. granite) cool slowly and have large crystals. Volcanic (extrusive, e.g. basalt) cool rapidly with small crystals. Acid igneous rocks (high silica, lighter) vs. Basic igneous rocks (more iron/aluminium, darker, denser). 2. **Sedimentary rocks**: Formed from accumulated sediments, characterised by distinct strata. Contain fossils. Types: Mechanically formed (sandstone, conglomerate, shale), Organically formed (limestone, chalk, coal), Chemically formed (rock salt, gypsum). 3. **Metamorphic rocks**: Existing rocks altered by heat and pressure. Clay → slate; limestone → marble; sandstone → quartzite; granite → gneiss; shale → schist; coal → graphite. **Four types of mountains:** Fold mountains (most widespread — Himalayas, Andes, Alps, Rockies), Block mountains/Horsts (e.g. Vosges, Black Forest), Volcanic mountains (e.g. Mt. Fuji, Mt. Etna), Residual mountains (e.g. Mt. Monadnock, Scottish Highlands). **Orogenic events:** Caledonian (~320 mya — Scotland, Scandinavia); Hercynian (~240 mya — Ural, Pennines, Appalachians); Alpine (~30 mya — Alps, Himalayas, Andes, Rockies — still highest because most recent).

Ice caps represent regions of permanently snow-covered ground, characterized by thick layers of ice. In the Northern Hemisphere, they are primarily found north of the Arctic Circle, specifically confined to Greenland and the highlands of high-latitude areas. In contrast to lowlands which may become ice-free for a few months and support tundra vegetation, ice caps remain permanently frozen. The Southern Hemisphere hosts the most extensive single stretch of ice cap on the continent of Antarctica, where the permanent ice layers can reach depths of up to 3,000 metres. These massive ice bodies are associated with large anticyclones, from which winds may blow.

A cyclone is a large-scale circulation of winds around a centre of low atmospheric pressure. An anticyclone is a circulation around a centre of high pressure. The two differ in rotation direction, wind behaviour, and associated weather. **Tropical Cyclones:** Tropical cyclones form over warm tropical oceans (sea surface temperature above 27°C). They derive energy from the latent heat released as moist warm air rises and condenses. Key characteristics: - Winds spiral inward and upward - A calm "eye" at the centre surrounded by the most intense winds and rainfall in the "eye wall" - Rotate anti-clockwise in Northern Hemisphere, clockwise in Southern Hemisphere - Called hurricanes (Atlantic), typhoons (Pacific), cyclones (Indian Ocean/Bay of Bengal) - Dissipate when they move over land or cool water (lose their energy source) **Temperate Cyclones (Extra-tropical):** Also called depressions or mid-latitude cyclones. They form along the polar front where warm and cold air masses meet. Unlike tropical cyclones, they have frontal systems (warm front, cold front) and do not have a calm eye. They move generally west to east in the westerlies belt. **Anticyclones:** Regions of high pressure with outward-spiralling winds. Associated with clear, settled weather. In Northern Hemisphere, winds spiral clockwise; Southern Hemisphere, anti-clockwise. The Bay of Bengal is more cyclone-prone than the Arabian Sea due to: - Higher sea surface temperatures - Enclosed shape funnelling storm energy - Warmer, more humid conditions throughout the year

Atmospheric circulation is driven by the unequal heating of the Earth's surface. This creates zones of high and low pressure that drive the global wind systems. **Pressure Belts (from equator to poles):** 1. **Equatorial Low Pressure Belt (ITCZ — Inter-Tropical Convergence Zone)**: Around 0°, intense solar heating causes air to rise, creating a low-pressure belt. Calm conditions and heavy rainfall prevail. Also called the Doldrums. 2. **Sub-Tropical High Pressure Belts**: Around 30° N and S. Descending air creates high pressure, resulting in hot deserts on western margins of continents (Sahara, Arabian, Australian deserts). Also called the Horse Latitudes. 3. **Sub-Polar Low Pressure Belts**: Around 60° N and S. Convergence of polar and sub-tropical air masses creates low pressure. 4. **Polar High Pressure Belts**: At the poles, cold air sinks, creating high pressure. **Planetary Wind Systems:** - **Trade Winds**: Blow from sub-tropical highs to equatorial lows; northeast trades (N. Hemisphere), southeast trades (S. Hemisphere). Consistent and reliable; used by early sailors. - **Westerlies**: Blow from sub-tropical highs toward sub-polar lows; from southwest in N. Hemisphere, northwest in S. Hemisphere. Bring rainfall to western coasts of continents in temperate zones. - **Polar Easterlies**: Blow from polar highs toward sub-polar lows. The Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, creating the circular patterns observed in pressure systems.