›An artesian well is a distinct type of well due to its formation. (GC Leong, ch04-weathering-mass-movement.md)
›It forms where rock layers are down-folded into a basin shape. (GC Leong, ch04-weathering-mass-movement.md)
›A permeable stratum (aquifer), like chalk or limestone, is sandwiched between two impermeable layers, such as clay. (GC Leong, ch04-weathering-mass-movement.md)
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An artesian well is a distinct type of well characterized by its unique formation in specific geological structures. These wells are created in areas where rock layers have been down-folded into a basin shape. Within this basin, a permeable stratum, known as an aquifer, such as chalk or limestone, is enclosed between two impermeable layers, like clay. Water entering the permeable rocks at the higher edges of this basin gradually seeps downwards due to gravity, accumulating in the basin's lowest part. The surrounding impermeable layers trap this water, preventing it from moving further downwards or escaping upwards, leading to the aquifer becoming saturated under significant hydrostatic pressure. When a well is bored into this pressurized aquifer, the inherent pressure is sufficient to force the water up the bore-hole, causing it to gush onto the surface like a fountain, initially without the need for pumping. However, this pressure can decrease over time, eventually requiring mechanical pumping to extract the water. The depth of artesian wells can vary significantly, ranging from a few metres to thousands. These wells are particularly valuable in arid regions, such as parts of the Sahara and Australia, where the aquifers receive water from areas of higher rainfall, supplying water to otherwise dry regions. Artesian water can serve entire villages or support activities like sheep farming, as seen in the Great Plains of the U.S.A. or Queensland, Australia. Nonetheless, the water may occasionally be unsuitable for agriculture or irrigation if it is hot or contains an excessive amount of mineral salts. Human activities, particularly over-exploitation, can lead to the depletion of groundwater resources from artesian wells, as water is extracted faster than it can be naturally replenished by rainfall.
All key facts
›An artesian well is a distinct type of well due to its formation. (GC Leong, ch04-weathering-mass-movement.md)
›It forms where rock layers are down-folded into a basin shape. (GC Leong, ch04-weathering-mass-movement.md)
›A permeable stratum (aquifer), like chalk or limestone, is sandwiched between two impermeable layers, such as clay. (GC Leong, ch04-weathering-mass-movement.md)
›Water seeps into the permeable rocks from the surface at the basin's edges and collects in the lowest part. (GC Leong, ch04-weathering-mass-movement.md)
›The impermeable layers trap this water, saturating the aquifer under great pressure. (GC Leong, ch04-weathering-mass-movement.md)
›When a well is bored, the pressure forces water to gush to the surface like a fountain. (GC Leong, ch04-weathering-mass-movement.md)
›After some time, the pressure may decrease, necessitating pumping. (GC Leong, ch04-weathering-mass-movement.md)
›The depth of artesian wells varies from a few metres to thousands of metres. (GC Leong, ch04-weathering-mass-movement.md)
›They are most valuable in desert areas (e.g., Sahara, Australia) where aquifers are replenished in higher rainfall regions but supply arid areas. (GC Leong, ch04-weathering-mass-movement.md)
›Water from artesian wells can supply villages (e.g., Great Plains of U.S.A.) or be used for sheep farming (e.g., Queensland, Australia). (GC Leong, ch04-weathering-mass-movement.md)
›The water may sometimes be unsuitable for agriculture or irrigation if it is hot or contains excessive mineral salts. (GC Leong, ch04-weathering-mass-movement.md)
›Over-exploitation can deplete groundwater resources in artesian wells, as extraction may be faster than natural replenishment. (GC Leong, ch04-weathering-mass-movement.md)
Hydrological Cycle
›The hydrological cycle describes the movement of water on, in, and above the Earth.
›The water cycle has been working for billions of years and all life on Earth depends on it.
›Water is the most important element, next to air, required for the existence of life on Earth.
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The Hydrological Cycle, also known as the water cycle, describes the continuous movement of water on, in, and above the Earth. It has been active for billions of years and is fundamental for all life on Earth, making water the most important element next to air. This cycle involves the circulation of water within the Earth’s hydrosphere in its three different forms: liquid, solid, and gaseous phases. It signifies the continuous exchange of water between the oceans, atmosphere, land surface, subsurface, and organisms. Water is considered a cyclic resource, meaning it can be used and re-used as it undergoes this constant circulation from the ocean to land and back. The distribution of water on Earth is uneven, with some areas having abundant supply while others have limited quantities. While the total renewable water on Earth is constant, increasing demand and pollution contribute to water crises.
All key facts
›The hydrological cycle describes the movement of water on, in, and above the Earth.
›The water cycle has been working for billions of years and all life on Earth depends on it.
›Water is the most important element, next to air, required for the existence of life on Earth.
›The distribution of water on Earth is quite uneven, with varying quantities across locations.
›The hydrological cycle is the circulation of water within the Earth’s hydrosphere in different forms: liquid, solid, and gaseous phases.
›It refers to the continuous exchange of water between the oceans, atmosphere, land surface, subsurface, and organisms.
›Water is a cyclic resource and can be used and re-used.
›About 91 per cent of the planetary water is found in the oceans.
›The remaining planetary water is held as freshwater in glaciers and icecaps, groundwater sources, lakes, soil moisture, atmosphere, streams, and within life.
›Nearly 59 per cent of the water that falls on land returns to the atmosphere through evaporation from oceans and other places.
›The remainder of the water that falls on land either runs-off on the surface, infiltrates into the ground, or becomes glacier.
›The renewable water on Earth is constant, but demand is increasing, leading to water crises.
›Components of the Water Cycle include Water storage in oceans, Water in the atmosphere, Water storage in ice and snow, Surface runoff, Stream flow freshwater storage, and Groundwater storage (Table 12.1).
›Processes of the Water Cycle include Evaporation, Evapotranspiration, Sublimation, Condensation, Precipitation, Snowmelt runoff to streams, Infiltration, and Groundwater discharge springs (Table 12.1).
›When rain falls on the earth, some is immediately evaporated and returns to the atmosphere as water vapour.
›Some precipitation is absorbed by plants and gradually returned to the atmosphere by transpiration from leaves.
›Much of the precipitation flows directly off slopes as run-off to join streams and rivers, eventually reaching the seas and oceans.
›A considerable proportion of water from rain or snow percolates downwards into the soil and rocks, filling joints and pore-spaces, forming groundwater.
›Groundwater plays an important part in weathering and mass movement, and serves as a means of natural water storage.
›Groundwater re-enters the hydrological cycle through springs.
Relief of the Ocean Floor
›The oceans are confined to the great depressions of the earth’s outer layer. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›A major portion of the ocean floor is found between 3-6 km below the sea level. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›Ocean floor features are formed by tectonic, volcanic, and depositional processes, similar to those of the continents. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
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The relief of the ocean floor refers to the complex and varied topographical features found beneath the ocean waters, which exhibit characteristics similar to those observed over land. These features include the world’s largest mountain ranges, deepest trenches, and extensive plains. A major portion of the ocean floor is typically found between 3-6 km below sea level. The formation of these submarine features is attributed to tectonic, volcanic, and depositional processes, akin to how continental features are formed.
Geographers categorize the ocean floors into four primary divisions: the Continental Shelf, the Continental Slope, the Deep Sea Plain, and the Oceanic Deeps (or Trenches). In addition to these major divisions, there are numerous minor but significant relief features that predominate in different parts of the oceans. These include mid-oceanic ridges, seamounts, submarine canyons, guyots, and atolls, all contributing to the diverse and rugged topography of the ocean basins.
All key facts
›The oceans are confined to the great depressions of the earth’s outer layer. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›A major portion of the ocean floor is found between 3-6 km below the sea level. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›Ocean floor features are formed by tectonic, volcanic, and depositional processes, similar to those of the continents. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›**Major Divisions of the Ocean Floors:**
›**Continental Shelf:** This is the extended margin of each continent, occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean, showing an average gradient of 1° or less. The shelf typically ends at a steep slope called the shelf break. Its average width is about 80 km, but it can be almost absent in some areas (e.g., coasts of Chile) or very wide (e.g., Siberian shelf in Arctic Ocean, 1,500 km). Depths vary from 30 m to 600 m. Massive sedimentary deposits here are sources of fossil fuels. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›It is the seaward extension of the continent from the shoreline to the continental edge, approximately marked by the 100 fathom (180 metres/600 feet) isobath. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Its width varies greatly, from a few kilometres in the North Pacific off North America to over 160 km (100 miles) off north-west Europe. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›In areas with extremely mountainous coasts, such as the Rocky Mountain and Andean coasts, the continental shelf may be entirely absent. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Off broad lowland coasts like Arctic Siberia, a maximum width of 1,200 km (750 miles) has been recorded. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›A width of 32-160 km (20-100 miles) is generally encountered. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›A gradient of 1 in 500 is common to most continental shelves. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Some continental shelves are considered part of the continent submerged due to a rise in sea-level, such as at the close of the Ice Age when melting ice raised sea-level by several hundred metres. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Smaller continental shelves can also be formed by wave erosion or by the deposition of land-derived or river-borne materials on the off-shore terrace. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Their shallowness allows sunlight penetration, encouraging plankton growth, making them the richest fishing grounds in the world (e.g., Grand Banks off Newfoundland, the North Sea, and the Sunda Shelf). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Their limited depth and gentle slope keep out cold under-currents and increase tide height, which can affect shipping, and most of the world's greatest seaports are located on them (e.g., Southampton, London, Hamburg, Rotterdam, Hong Kong, Singapore). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›**Continental Slope:** Connects the continental shelf and the ocean basins, starting where the shelf bottom sharply drops off. The gradient varies between 2-5°, and depths range from 200 to 3,000 m. This boundary indicates the end of the continents, and canyons and trenches are observed here. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›At the edge of the continental shelf, there is an abrupt change of gradient to about 1 in 20. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›**Deep Sea Plain:** These are gently sloping areas of the ocean basins, considered the flattest and smoothest regions globally. Depths vary between 3,000 and 6,000 m, and they are covered with fine-grained sediments like clay and silt. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›This undulating plain, generally termed the abyssal plain, lies two or three kilometres below sea-level and covers two-thirds of the ocean floor. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Modern sounding devices reveal that the abyssal plain is far from featureless, having extensive submarine plateaux, ridges, trenches, basins, and oceanic islands that can rise above sea-level (e.g., the Azores, Ascension Island). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›**Oceanic Deeps or Trenches:** These are the deepest parts of the oceans, relatively steep-sided, narrow basins, typically 3-5 km deeper than the surrounding ocean floor. They occur at the bases of continental slopes and along island arcs, associated with active volcanoes and strong earthquakes, making them significant for studying plate movements. As many as 57 deeps have been explored: 32 in the Pacific, 19 in the Atlantic, and 6 in the Indian Ocean. — NCERT Class 11 — India: Physical Environment, ch12-water-oceans.md
›These long, narrow trenches plunge to depths of 5,000 fathoms or 9,000 metres (30,000 feet). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Most deepest trenches are found close to the continents, particularly in the Pacific Ocean, rather than in the midst of oceans. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›The greatest known ocean deep is the Mariana Tren
Porous and Permeable Rocks
›The amount of groundwater present and its depth are determined by the porosity and permeability of rocks and soils.
›**Porous rocks** (e.g., sandstone) possess many pore-spaces between their grains.
›Water is easily absorbed by porous rocks and can be stored in their pore-spaces.
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Porous and permeable rocks are characteristics of the earth's crust that significantly influence the presence and depth of groundwater. Porosity refers to the presence of numerous pore-spaces within a rock, such as those found between the grains of sandstone, allowing water to be easily absorbed and stored. Permeability, or perviousness, describes a rock's ability to allow water to pass through it easily.
While most porous rocks are also permeable, this is not universally true. For instance, clay is highly porous due to its fine particles and the pore-spaces between them, enabling it to absorb a great deal of water. However, its pore-spaces are so minute that water cannot move through it easily, rendering clay impermeable. Conversely, granite, being a crystalline rock, is generally non-porous as its individual crystals absorb little to no water. Yet, granite can be pervious if it contains numerous joints or cracks that allow water to pass through. The rate at which solution takes place in rocks is affected not only by their mineral composition but also by their structure, including the presence of pore-spaces in sedimentary rocks where air and water can lodge.
All key facts
›The amount of groundwater present and its depth are determined by the porosity and permeability of rocks and soils.
›**Porous rocks** (e.g., sandstone) possess many pore-spaces between their grains.
›Water is easily absorbed by porous rocks and can be stored in their pore-spaces.
›**Permeable (or pervious) rocks** are those that allow water to pass through them easily.
›Most porous rocks are also permeable.
›Some rocks can be porous but impermeable; for example, clay is highly porous but its very small pore-spaces prevent water from moving through easily.
›Some rocks can be non-porous but pervious; for example, granite is a crystalline, non-porous rock that can be pervious if it has numerous joints or cracks allowing water passage.
›Sedimentary rocks often have pore-spaces between grains where air and water can lodge, contributing to weathering.
›All rocks contain cracks, joints, or pore spaces where water or snow can collect.
›Landslides can occur where permeable debris or rock layers overlie impermeable strata like clay, as water is halted by the clay, creating a slippery surface.
Springs (Groundwater)
›Groundwater forms when rain or snow percolates downwards into the soil and rocks, filling joints and pore-spaces. (ch04-weathering-mass-movement.md)
›Groundwater accumulates above an impermeable rock layer, and the surface of this saturated area is called the water-table. (ch04-weathering-mass-movement.md)
›A spring is an outlet where groundwater, stored in rock, is released onto the surface at points where the water-table reaches the surface. (ch04-weathering-mass-movement.md)
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Springs are natural points where groundwater emerges onto the Earth's surface. Groundwater is formed when precipitation, such as rain or snow, percolates downwards into the soil and rocks, filling joints and pore-spaces. This water then accumulates above an impermeable layer of rock, saturating the permeable rock above it, the surface of which is known as the water-table. A spring occurs precisely where this water-table intersects the land surface, acting as an outlet for the stored groundwater.
Springs are vital components of the hydrological cycle, facilitating the re-entry of groundwater into the surface water system. They also serve as a natural means of water storage. The water emerging from a spring can either seep out gradually or gush forth vigorously, similar to a fountain. The specific characteristics and formation of a spring are largely determined by the geological structure and rock types present, as well as the position of the local water-table. Several distinct types of springs exist, each associated with particular rock arrangements and hydrological conditions.
All key facts
›Groundwater forms when rain or snow percolates downwards into the soil and rocks, filling joints and pore-spaces. (ch04-weathering-mass-movement.md)
›Groundwater accumulates above an impermeable rock layer, and the surface of this saturated area is called the water-table. (ch04-weathering-mass-movement.md)
›A spring is an outlet where groundwater, stored in rock, is released onto the surface at points where the water-table reaches the surface. (ch04-weathering-mass-movement.md)
›Groundwater re-enters the hydrological cycle through springs. (ch04-weathering-mass-movement.md)
›Water from a spring can either seep gradually or gush out like a fountain. (ch04-weathering-mass-movement.md)
›The type of spring is determined by the nature of the rocks and the position of the water-table. (ch04-weathering-mass-movement.md)
›One type of spring emerges in areas of tilted strata where permeable and impermeable rocks alternate, with water appearing at the base of permeable layers. (ch04-weathering-mass-movement.md)
›In well-jointed rocks, springs can form when water percolates to a joint that reaches the surface. (ch04-weathering-mass-movement.md)
›A dyke spring occurs where an impermeable dyke or sill intrudes through permeable rocks, causing the water-table to reach the surface. (ch04-weathering-mass-movement.md)
›In limestone or chalk escarpments, water can issue as a scarp-foot spring at the foot of the scarp or a dip-slope spring near the foot of the dip-slope, where permeable rock lies between impermeable strata. (ch04-weathering-mass-movement.md)
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Ocean Water Temperature Distribution
›Ocean waters are heated by solar energy, similar to land.
›The process of heating and cooling of oceanic water is slower than land.
›Factors affecting ocean water temperature distribution include latitude, unequal distribution of land and water, prevailing winds, and ocean currents.
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Ocean waters are heated by solar energy, similar to land, but their heating and cooling process is slower. The distribution of ocean water temperature varies both spatially (horizontally) and vertically (with depth). Key factors influencing this distribution include latitude, the unequal distribution of land and water, prevailing winds, and ocean currents.
Horizontally, surface water temperature generally decreases from the equator towards the poles as solar insolation diminishes. Oceans in the Northern Hemisphere tend to record higher temperatures than those in the Southern Hemisphere due to greater contact with landmasses. The highest temperatures are typically found slightly north of the equator.
Vertically, ocean temperature generally decreases with increasing depth. A significant feature of this vertical distribution is the **thermocline**, a boundary layer usually beginning between 100-400 meters below the surface, where there is a rapid decrease in temperature. Below the thermocline, temperatures in the deep ocean approach 0°C, accounting for about 90% of the total water volume. In middle and low latitudes, the ocean's temperature structure is described as a three-layer system: a warm surface layer (around 500m thick, 20-25°C), followed by the thermocline layer (500-1,000m thick), and finally a very cold bottom layer extending to the deep ocean floor. In polar regions, due to surface temperatures already near 0°C, only a single layer of cold water extends from the surface to the bottom.
All key facts
›Ocean waters are heated by solar energy, similar to land.
›The process of heating and cooling of oceanic water is slower than land.
›Factors affecting ocean water temperature distribution include latitude, unequal distribution of land and water, prevailing winds, and ocean currents.
›Surface water temperature generally decreases from the equator towards the poles because insolation decreases poleward.
›Oceans in the Northern Hemisphere receive more heat due to their contact with a larger extent of land compared to the Southern Hemisphere.
›Winds blowing from land towards oceans can drive warm surface water away, leading to the upwelling of cold water from below, causing longitudinal temperature variation.
›Onshore winds can pile up warm water near the coast, raising local temperatures.
›Warm ocean currents raise the temperature in cold areas (e.g., Gulf Stream along eastern North America and West Europe).
›Cold ocean currents decrease the temperature in warm ocean areas (e.g., Labrador Current near north-east North America).
›Enclosed seas in low latitudes tend to have relatively higher temperatures than open seas.
›Enclosed seas in high latitudes tend to have lower temperatures than open seas.
›The temperature-depth profile for ocean water shows a decrease in temperature with increasing depth.
Vauclusian springs, also known as resurgences, are found in karst regions where underground rivers, flowing through solution-worn passages, re-emerge. (ch04-weathering-mass-movement.md)
›The water-table rises when abundant rain augments groundwater supplies and is lowered in dry periods due to groundwater loss through seepages and springs. (ch04-weathering-mass-movement.md)
›Springs are natural emergence points of groundwater, distinct from wells which are human-made borings. (ch04-weathering-mass-movement.md)
›Other types of springs, such as hot springs, mineral springs, and geysers, are described in Chapter 3. (ch04-weathering-mass-movement.md)
›The **thermocline** is a boundary region characterized by a rapid decrease in temperature.
›The thermocline typically begins around 100-400 meters below the sea surface and extends several hundred meters downward.
›About 90% of the total volume of water in the deep ocean is found below the thermocline, where temperatures approach 0°C.
›In middle and low latitudes, oceans have a three-layer temperature structure:
›**First layer**: Top layer of warm oceanic water, about 500m thick, with temperatures between 20° and 25°C. It is present year-round in tropical regions but only develops during summer in mid-latitudes.
›**Second layer (thermocline)**: Lies below the first layer, 500-1,000 m thick, and is characterized by a rapid decrease in temperature with increasing depth.
›**Third layer**: Very cold, extending up to the deep ocean floor.
›In the Arctic and Antarctic circles, surface water temperatures are close to 0°C, and temperature change with depth is very slight, with only one layer of cold water existing from surface to deep ocean floor.
›The average temperature of surface water of the oceans is about 27°C.
›The average surface temperature gradually decreases from the equator towards the poles.
›The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude.
›Average surface temperatures are around 22°C at 20° latitudes, 14°C at 40° latitudes, and 0°C near the poles.
›Oceans in the northern hemisphere record relatively higher temperatures (average annual ~19°C) than in the southern hemisphere (average annual ~16°C).
›The highest temperature is not recorded exactly at the equator but slightly towards the north of it.
›The maximum temperature of the oceans is always at their surfaces because they directly receive heat from the sun.
›Heat is transmitted to lower sections of the oceans through convection.
›Temperature falls very rapidly up to a depth of 200m, after which the rate of decrease slows down.
›The annual range of temperature in any part of the open ocean is generally less than 5.5°C (10°F). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›The mean annual temperature of surface ocean water decreases from about 21°C (70°F) in equatorial areas to 13°C (55°F) at latitudes 45° N and S, dropping almost to freezing point at the poles. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›The reduction of temperature with latitude is not constant due to interference from warm and cold currents, winds, and air masses. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Warm ocean currents, such as the North Atlantic Drift, raise the surface temperature of coasts (e.g., the Norwegian coast at 60°-70° N remains ice-free throughout the year). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Cold ocean currents, such as the Labrador Current off north-east Canada, tend to reduce the surface-water temperature, causing ports of eastern Canada even at 45°N to be icebound for almost half the year. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›The highest water temperatures are found in enclosed seas in the tropics, e.g., the Red Sea, which records a temperature of 29°C to 38°C (85°F to 100°F). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Arctic and Antarctic waters are so cold that their surface is permanently frozen as pack-ice down to a depth of several metres. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›In the warmer summer, parts of polar ice break off as ice floes that both dilute the water and lower the surface temperature of surrounding ice-free seas. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Vertically, ocean temperature decreases rapidly for the first 200 fathoms (approx. 365m) at a rate of 1°C for every 18 fathoms (1°F for every 10 fathoms), then more slowly until a depth of 500 fathoms (approx. 914m) is reached. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›Beyond 500 fathoms, the temperature drop is scarcely noticeable, less than 1°C for every 180 fathoms (or 1°F for every 100 fathoms). — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch12-the-oceans.md
›In the ocean deeps below 2,000 fathoms (approximately 3,700 metres/12,000 feet), the water is uniformly cold, just a little above freezing point