Geophysics

From Wikipedia, the free encyclopedia

Age of the sea floor. Much of the dating information comes from magnetic anomalies.

 

Computer simulation of the Earth's magnetic field in a period of normal polarity between reversals.

 

Geophysics /dʒiːoʊfɪzɪks/ is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial relations; and analogous problems associated with the Moon and other planets.

Although geophysics was only recognized as a separate discipline in the 19th century, its origins date back to ancient times. The first magnetic compasses were made from lodestones, while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD. Isaac Newton applied his theory of mechanics to the tides and the precession of the equinox; and instruments were developed to measure the Earth's shape, density and gravity field, as well as the components of the water cycle. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, and geophysics played an essential role in the development of the theory of plate tectonics. 

Geophysics is applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental protection. In Exploration Geophysics, Geophysical survey data are used to analyze potential petroleum reservoirs and mineral deposits, locate groundwater, find archaeological relics, determine the thickness of glaciers and soils, and assess sites for environmental remediation.

Physical phenomena

Geophysics is a highly interdisciplinary subject, and geophysicists contribute to every area of the Earth sciences. To provide a clearer idea of what constitutes geophysics, this section describes phenomena that are studied in physics and how they relate to the Earth and its surroundings.

Gravity

A map of deviations in gravity from a perfectly smooth, idealized Earth.

The gravitational pull of the Moon and Sun give rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide.

Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases. Measurements of gravitational acceleration and gravitational potential at the Earth's surface and above it can be used to look for mineral deposits. The surface gravitational field provides information on the dynamics of tectonic plates. The geopotential surface called the geoid is one definition of the shape of the Earth. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents (such as with very narrow canals).

Heat flow

A model of thermal convection in the Earth's mantle. The thin red columns are mantle plumes.

 

The Earth is cooling, and the resulting heat flow generates the Earth's magnetic field through the geodynamo and plate tectonics through mantle convection. The main sources of heat are the primordial heat and radioactivity, although there are also contributions from phase transitions. Heat is mostly carried to the surface by thermal convection, although there are two thermal boundary layers – the core-mantle boundary and the lithosphere – in which heat is transported by conduction. Some heat is carried up from the bottom of the mantle by mantle plumes. The heat flow at the Earth's surface is about 4.2 × 10¹³ W, and it is a potential source of geothermal energy.

Vibrations

Illustration of the deformations of a block by body waves and surface waves (see seismic wave).

 

Seismic waves are vibrations that travel through the Earth's interior or along its surface. The entire Earth can also oscillate in forms that are called normal modes or free oscillations of the Earth. Ground motions from waves or normal modes are measured using seismographs. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source. The locations of earthquakes provide information on plate tectonics and mantle convection.

Recording of seismic waves from controlled sources provide information on the region that the waves travel through. If the density or composition of the rock changes, waves are reflected. Reflections recorded using Reflection Seismology can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas. Changes in the travel direction, called refraction, can be used to infer the deep structure of the Earth.

Earthquakes pose a risk to humans. Understanding their mechanisms, which depend on the type of earthquake (e.g., intraplate or deep focus), can lead to better estimates of earthquake risk and improvements in earthquake engineering.

Electricity

Although we mainly notice electricity during thunderstorms, there is always a downward electric field near the surface that averages 120 volts per meter. Relative to the solid Earth, the atmosphere has a net positive charge due to bombardment by cosmic rays. A current of about 1800 amperes flows in the global circuit. It flows downward from the ionosphere over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the clouds and sprites above. 

A variety of electric methods are used in geophysical survey. Some measure spontaneous potential, a potential that arises in the ground because of man-made or natural disturbances. Telluric currents flow in Earth and the oceans. They have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field. The distribution of telluric current density can be used to detect variations in electrical resistivity of underground structures. Geophysicists can also provide the electric current themselves.

Electromagnetic waves

Electromagnetic waves occur in the ionosphere and magnetosphere as well as in Earth's outer core. Dawn chorus is believed to be caused by high-energy electrons that get caught in the Van Allen radiation belt. Whistlers are produced by lightning strikes. Hiss may be generated by both. Electromagnetic waves may also be generated by earthquakes. 

In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction. Alfvén waves are magnetohydrodynamic waves in the magnetosphere or the Earth's core. In the core, they probably have little observable effect on the Earth's magnetic field, but slower waves such as magnetic Rossby waves may be one source of geomagnetic secular variation.

Electromagnetic methods that are used for geophysical survey include transient electromagnetics, magnetotellurics, surface nuclear magnetic resonance and electromagnetic seabed logging.

Magnetism

The Earth's magnetic field protects the Earth from the deadly solar wind and has long been used for navigation. It originates in the fluid motions of the outer core. The magnetic field in the upper atmosphere gives rise to the auroras.

Earth's dipole axis (pink line) is tilted away from the rotational axis (blue line).

 

The Earth's field is roughly like a tilted dipole, but it changes over time (a phenomenon called geomagnetic secular variation). Mostly the geomagnetic pole stays near the geographic pole, but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals, analyzed within a Geomagnetic Polarity Time Scale, contain 184 polarity intervals in the last 83 million years, with change in frequency over time, with the most recent brief complete reversal of the Laschamp event occurring 41,000 years ago during the last glacial period. Geologists observed geomagnetic reversal recorded in volcanic rocks, through magnetostratigraphy correlation and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on seafloor spreading, a part of plate tectonics. They are the basis of magnetostratigraphy, which correlates magnetic reversals with other stratigraphies to construct geologic time scales. In addition, the magnetization in rocks can be used to measure the motion of continents.

Radioactivity

Example of a radioactive decay chain

 

Radioactive decay accounts for about 80% of the Earth's internal heat, powering the geodynamo and plate tectonics. The main heat-producing isotopes are potassium-40, uranium-238, uranium-235, and thorium-232. Radioactive elements are used for radiometric dating, the primary method for establishing an absolute time scale in geochronology. 

Unstable isotopes decay at predictable rates, and the decay rates of different isotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past geologic eras. Radiometric mapping using ground and airborne gamma spectrometry can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.

Fluid dynamics

Fluid motions occur in the magnetosphere, atmosphere, ocean, mantle and core. Even the mantle, though it has an enormous viscosity, flows like a fluid over long time intervals. This flow is reflected in phenomena such as isostasy, post-glacial rebound and mantle plumes. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.

Geophysical fluid dynamics is a primary tool in physical oceanography and meteorology. The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to the Coriolis effect. In the atmosphere it gives rise to large-scale patterns like Rossby waves and determines the basic circulation patterns of storms. In the ocean they drive large-scale circulation patterns as well as Kelvin waves and Ekman spirals at the ocean surface. In the Earth's core, the circulation of the molten iron is structured by Taylor columns.

Waves and other phenomena in the magnetosphere can be modeled using magnetohydrodynamics.

Mineral physics

The physical properties of minerals must be understood to infer the composition of the Earth's interior from seismology, the geothermal gradient and other sources of information. Mineral physicists study the elastic properties of minerals; their high-pressure phase diagrams, melting points and equations of state at high pressure; and the rheological properties of rocks, or their ability to flow. Deformation of rocks by creep make flow possible, although over short times the rocks are brittle. The viscosity of rocks is affected by temperature and pressure, and in turn determines the rates at which tectonic plates move.

Water is a very complex substance and its unique properties are essential for life. Its physical properties shape the hydrosphere and are an essential part of the water cycle and climate. Its thermodynamic properties determine evaporation and the thermal gradient in the atmosphere. The many types of precipitation involve a complex mixture of processes such as coalescence, supercooling and supersaturation. Some precipitated water becomes groundwater, and groundwater flow includes phenomena such as percolation, while the conductivity of water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such as salinity have a large effect on its motion in the oceans.

The many phases of ice form the cryosphere and come in forms like ice sheets, glaciers, sea ice, freshwater ice, snow, and frozen ground (or permafrost).

Regions of the Earth

Size and form of the Earth

The Earth is roughly spherical, but it bulges towards the Equator, so it is roughly in the shape of an ellipsoid (see Earth ellipsoid). This bulge is due to its rotation and is nearly consistent with an Earth in hydrostatic equilibrium. The detailed shape of the Earth, however, is also affected by the distribution of continents and ocean basins, and to some extent by the dynamics of the plates.

Structure of the interior

Seismic velocities and boundaries in the interior of the Earth sampled by seismic waves.

 

Evidence from seismology, heat flow at the surface, and mineral physics is combined with the Earth's mass and moment of inertia to infer models of the Earth's interior – its composition, density, temperature, pressure. For example, the Earth's mean specific gravity (5.515) is far higher than the typical specific gravity of rocks at the surface (2.7–3.3), implying that the deeper material is denser. This is also implied by its low moment of inertia ( 0.33 M R², compared to 0.4 M R² for a sphere of constant density). However, some of the density increase is compression under the enormous pressures inside the Earth. The effect of pressure can be calculated using the Adams–Williamson equation. The conclusion is that pressure alone cannot account for the increase in density. Instead, we know that the Earth's core is composed of an alloy of iron and other minerals.

Reconstructions of seismic waves in the deep interior of the Earth show that there are no S-waves in the outer core. This indicates that the outer core is liquid, because liquids cannot support shear. The outer core is liquid, and the motion of this highly conductive fluid generates the Earth's field. Earth's inner core, however, is solid because of the enormous pressure.

Reconstruction of seismic reflections in the deep interior indicate some major discontinuities in seismic velocities that demarcate the major zones of the Earth: inner core, outer core, mantle, lithosphere and crust. The mantle itself is divided into the upper mantle, transition zone, lower mantle and D′′ layer. Between the crust and the mantle is the Mohorovičić discontinuity.

The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model of the Earth, mineral physics is needed to interpret seismic velocities in terms of composition. The mineral properties are temperature-dependent, so the geotherm must also be determined. This requires physical theory for thermal conduction and convection and the heat contribution of radioactive elements. The main model for the radial structure of the interior of the Earth is the preliminary reference Earth model (PREM). Some parts of this model have been updated by recent findings in mineral physics (see post-perovskite) and supplemented by seismic tomography. The mantle is mainly composed of silicates, and the boundaries between layers of the mantle are consistent with phase transitions.

The mantle acts as a solid for seismic waves, but under high pressures and temperatures it deforms so that over millions of years it acts like a liquid. This makes plate tectonics possible.

Magnetosphere

Schematic of Earth's magnetosphere. The solar wind flows from left to right.

 

If a planet's magnetic field is strong enough, its interaction with the solar wind forms a magnetosphere. Early space probes mapped out the gross dimensions of the Earth's magnetic field, which extends about 10 Earth radii towards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind the magnetic tail, hundreds of Earth radii downstream. Inside the magnetosphere, there are relatively dense regions of solar wind particles called the Van Allen radiation belts.

Methods

Geodesy

Geophysical measurements are generally at a particular time and place. Accurate measurements of position, along with earth deformation and gravity, are the province of geodesy. While geodesy and geophysics are separate fields, the two are so closely connected that many scientific organizations such as the American Geophysical Union, the Canadian Geophysical Union and the International Union of Geodesy and Geophysics encompass both.

Absolute positions are most frequently determined using the global positioning system (GPS). A three-dimensional position is calculated using messages from four or more visible satellites and referred to the 1980 Geodetic Reference System. An alternative, optical astronomy, combines astronomical coordinates and the local gravity vector to get geodetic coordinates. This method only provides the position in two coordinates and is more difficult to use than GPS. However, it is useful for measuring motions of the Earth such as nutation and Chandler wobble. Relative positions of two or more points can be determined using very-long-baseline interferometry.

Gravity measurements became part of geodesy because they were needed to related measurements at the surface of the Earth to the reference coordinate system. Gravity measurements on land can be made using gravimeters deployed either on the surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has been measured by analyzing the motion of satellites. Sea level can also be measured by satellites using radar altimetry, contributing to a more accurate geoid. In 2002, NASA launched the Gravity Recovery and Climate Experiment (GRACE), wherein two twin satellites map variations in Earth's gravity field by making measurements of the distance between the two satellites using GPS and a microwave ranging system. Gravity variations detected by GRACE include those caused by changes in ocean currents; runoff and ground water depletion; melting ice sheets and glaciers.

Satellites and space probes

Satellites in space have made it possible to collect data from not only the visible light region, but in other areas of the electromagnetic spectrum. The planets can be characterized by their force fields: gravity and their magnetic fields, which are studied through geophysics and space physics. 

Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above lunar maria were measured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.

History

Geophysics emerged as a separate discipline only in the 19th century, from the intersection of physical geography, geology, astronomy, meteorology, and physics. However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since the ancient era.

Ancient and classical eras

Replica of Zhang Heng's seismoscope, possibly the first contribution to seismology.

 

The magnetic compass existed in China back as far as the fourth century BC. It was used as much for feng shui as for navigation on land. It was not until good steel needles could be forged that compasses were used for navigation at sea; before that, they could not retain their magnetism long enough to be useful. The first mention of a compass in Europe was in 1190 AD.

In circa 240 BC, Eratosthenes of Cyrene deduced that the Earth was round and measured the circumference of the Earth, using trigonometry and the angle of the Sun at more than one latitude in Egypt. He developed a system of latitude and longitude.

Perhaps the earliest contribution to seismology was the invention of a seismoscope by the prolific inventor Zhang Heng in 132 AD. This instrument was designed to drop a bronze ball from the mouth of a dragon into the mouth of a toad. By looking at which of eight toads had the ball, one could determine the direction of the earthquake. It was 1571 years before the first design for a seismoscope was published in Europe, by Jean de la Hautefeuille. It was never built.

Beginnings of modern science

One of the publications that marked the beginning of modern science was William Gilbert's De Magnete (1600), a report of a series of meticulous experiments in magnetism. Gilbert deduced that compasses point north because the Earth itself is magnetic.

In 1687 Isaac Newton published his Principia, which not only laid the foundations for classical mechanics and gravitation but also explained a variety of geophysical phenomena such as the tides and the precession of the equinox.

The first seismometer, an instrument capable of keeping a continuous record of seismic activity, was built by James Forbes in 1844.

Himalayas

From Wikipedia, the free encyclopedia

Himalayas
Aerial view of Mount Everest and surrounding landscape
Highest point
Peak	Mount Everest (Nepal and China)
Elevation	8,848 m (29,029 ft)
Coordinates	27°59′N 86°55′ECoordinates: 27°59′N 86°55′E
Dimensions
Length	2,400 km (1,500 mi)
Naming
Native name	Sagarmatha
Geography
The general location of the Himalayas mountain range (this map has the Hindu Kush in the Himalaya, not normally regarded as part of the core Himalayas).
Continent	Asia

The Himalayas, or Himalaya (/ˌhɪməˈleɪə, hɪˈmɑːləjə/), form a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has many of the Earth's highest peaks, including the highest, Mount Everest. The Himalayas include over fifty mountains exceeding 7,200 m (23,600 ft) in elevation, including ten of the fourteen 8,000-metre peaks. By contrast, the highest peak outside Asia (Aconcagua, in the Andes) is 6,961 m (22,838 ft) tall.

Lifted by the subduction of the Indian tectonic plate under the Eurasian Plate, the Himalayan mountain range runs west-northwest to east-southeast in an arc 2,400 km (1,500 mi) long. Its western anchor, Nanga Parbat, lies just south of the northernmost bend of Indus river. Its eastern anchor, Namcha Barwa, is just west of the great bend of the Yarlung Tsangpo River (upper stream of the Brahmaputra River). The Himalayan range is bordered on the northwest by the Karakoram and the Hindu Kush ranges. To the north, the chain is separated from the Tibetan Plateau by a 50–60 km (31–37 mi) wide tectonic valley called the Indus-Tsangpo Suture. Towards the south the arc of the Himalaya is ringed by the very low Indo-Gangetic Plain. The range varies in width from 350 km (220 mi) in the west (Pakistan) to 150 km (93 mi) in the east (Arunachal Pradesh). The Himalayas are distinct from the other great ranges of central Asia, although sometimes the term 'Himalaya' (or 'Greater Himalaya') is loosely used to include the Karakoram and some of the other ranges.

The Himalayas are inhabited by 52.7 million people, and are spread across five countries: Nepal, India, Bhutan, China and Pakistan. Some of the world's major rivers – the Indus, the Ganges and the Tsangpo-Brahmaputra – rise in the Himalayas, and their combined drainage basin is home to roughly 600 million people. The Himalayas have a profound effect on the climate of the region, helping to keep the monsoon rains on the Indian plain and limiting rainfall on the Tibetan plateau. The Himalayas have profoundly shaped the cultures of the Indian subcontinent, with many Himalayan peaks considered sacred in Hinduism and Buddhism.

Name

The name of the range derives from the Sanskrit Himā-laya (हिमालय, "Abode of Snow"), from himá (हिम, "snow") and ā-laya (आलय, "receptacle, dwelling").^[6] They are now known as the "Himalaya Mountains", usually shortened to the "Himalayas". Formerly, they were described in the singular as the Himalaya. This was also previously transcribed Himmaleh, as in Emily Dickinson's poetry and Henry David Thoreau's essays.

The mountains are known as the Himālaya in Nepali and Hindi (both written हिमालय), the Himalaya (ཧི་མ་ལ་ཡ་) or 'The Land of Snow' (གངས་ཅན་ལྗོངས་) in Tibetan, the Hamaleh Mountain Range (Urdu: سلسلہ کوہ ہمالیہ‎) in Urdu and the Ximalaya Mountain Range (Chinese: 喜馬拉雅山脈; pinyin: Xǐmǎlāyǎ Shānmài) in Chinese.

Geography and key features

A satellite image showing the arc of the Himalayas

 

Marsyangdi valley with Annapurna II

 

In the middle of the great curve of the Himalayan mountains lie the 8,000 m (26,000 ft) peaks of Dhaulagiri and Annapurna in Nepal, separated by the Kali Gandaki Gorge. The gorge splits the Himalayas into Western and Eastern sections both ecologically and orographically – the pass at the head of the Kali Gandaki, the Kora La is the lowest point on the ridgeline between Everest and K2. To the east of Annapurna are the 8,000 m (5.0 mi) peaks of Manaslu and across the border in Tibet, Shishapangma. To the south of these lies Kathmandu, the capital of Nepal and the largest city in the Himalayas. East of the Kathmandu Valley lies valley of the Bhote/Sun Kosi river which rises in Tibet and provides the main overland route between Nepal and China – the Araniko Highway/China National Highway 318. Further east is the Mahalangur Himal with four of the world's six highest mountains, including the highest: Cho Oyu, Everest, Lhotse and Makalu. The Khumbu region, popular for trekking, is found here on the south-western approaches to Everest. The Arun river drains the northern slopes of these mountains, before turning south and flowing through the range to the east of Makalu. 

In the far east of Nepal, the Himalayas rise to the Kanchenjunga massif on the border with India, the third highest mountain in the world, the most easterly 8,000 m (26,000 ft) summit and the highest point of India. The eastern side of Kanchenjunga is in the Indian state of Sikkim. Formerly an independent Kingdom, it lies on the main route from India to Lhasa, Tibet, which passes over the Nathu La pass into Tibet. East of Sikkim lies the ancient Buddhist Kingdom of Bhutan. The highest mountain in Bhutan is Gangkhar Puensum, which is also a strong candidate for the highest unclimbed mountain in the world. The Himalayas here are becoming increasingly rugged with heavily forested steep valleys. The Himalayas continue, turning slightly northeast, through the Indian State of Arunachal Pradesh as well as Tibet, before reaching their easterly conclusion in the peak of Namche Barwa, situated in Tibet inside the great bend of the Yarlang Tsangpo river. On the other side of the Tsangpo, to the east, are the Kangri Garpo mountains. The high mountains to the north of the Tsangpo including Gyala Peri, however, are also sometimes also included in the Himalayas. 

Going west from Dhaulagiri, Western Nepal is somewhat remote and lacks major high mountains, but is home to Rara Lake, the largest lake in Nepal. The Karnali River rises in Tibet but cuts through the center of the region. Further west, the border with India follows the Sarda River and provides a trade route into China, where on the Tibetan plateau lies the high peak of Gurla Mandhata. Just across Lake Manasarovar from this lies the sacred Mount Kailash, which stands close to the source of the four main rivers of Himalayas and is revered in Hinduism, Buddhism, Sufism, Jainism, and Bonpo. In the newly created Indian state of Uttarkhand, the Himalayas rise again as the Garhwal Himalayas with the high peaks of Nanda Devi and Kamet. The state is also an important pilgrimage destination, with the source of the Ganges at Gangotri and the Yamuna at Yamunotri, and the temples at Badrinath and Kedarnath. 

The next Himalayan Indian state, Himachal Pradesh, it is noted for its hill stations, particularly Shimla, the summer capital of the British Raj, and Dharmasala, the centre of the Tibetan community in exile in India. This area marks the start of the Punjab Himalaya and the Sutlej river, the most easterly of the five tributaries of the Indus, cuts through the range here. Further west, the Himalayas form most of the southern portion of the Indian State of Jammu & Kashmir. The twin peaks of Nun Kun are the only mountains over 7,000 m (4.3 mi) in this part of the Himalayas. Beyond lies the renown Kashmir Valley and the town and lakes of Srinagar. Finally, the Himalayas cross the Line of Control into Pakistan and reach their western end in the dramatic 8000 m peak of Nanga Parbat, which rises over 7,000 m (23,000 ft) above the Indus valley and is the most westerly of the 8000 m summits.

Geology

The 6,000-kilometre-plus (3,700 mi) journey of the India landmass (Indian Plate) before its collision with Asia (Eurasian Plate) about 40 to 50 million years ago

 

The Himalayan range is one of the youngest mountain ranges on the planet and consists mostly of uplifted sedimentary and metamorphic rock. According to the modern theory of plate tectonics, its formation is a result of a continental collision or orogeny along the convergent boundary between the Indo-Australian Plate and the Eurasian Plate. The Arakan Yoma highlands in Myanmar and the Andaman and Nicobar Islands in the Bay of Bengal were also formed as a result of this collision.

During the Upper Cretaceous, about 70 million years ago, the north-moving Indo-Australian Plate (which has subsequently broken into the Indian Plate and the Australian Plate) was moving at about 15 cm (5.9 in) per year. About 50 million years ago this fast moving Indo-Australian Plate had completely closed the Tethys Ocean, the existence of which has been determined by sedimentary rocks settled on the ocean floor and the volcanoes that fringed its edges. Since both plates were composed of low density continental crust, they were thrust faulted and folded into mountain ranges rather than subducting into the mantle along an oceanic trench. An often-cited fact used to illustrate this process is that the summit of Mount Everest is made of marine limestone from this ancient ocean.

Today, the Indian plate continues to be driven horizontally at the Tibetan Plateau, which forces the plateau to continue to move upwards. The Indian plate is still moving at 67 mm per year, and over the next 10 million years it will travel about 1,500 km (930 mi) into Asia. About 20 mm per year of the India-Asia convergence is absorbed by thrusting along the Himalaya southern front. This leads to the Himalayas rising by about 5 mm per year, making them geologically active. The movement of the Indian plate into the Asian plate also makes this region seismically active, leading to earthquakes from time to time. 

During the last ice age, there was a connected ice stream of glaciers between Kangchenjunga in the east and Nanga Parbat in the west. In the west, the glaciers joined with the ice stream network in the Karakoram, and in the north, they joined with the former Tibetan inland ice. To the south, outflow glaciers came to an end below an elevation of 1,000–2,000 m (3,300–6,600 ft). While the current valley glaciers of the Himalaya reach at most 20 to 32 km (12 to 20 mi) in length, several of the main valley glaciers were 60 to 112 km (37 to 70 mi) long during the ice age. The glacier snowline (the altitude where accumulation and ablation of a glacier are balanced) was about 1,400–1,660 m (4,590–5,450 ft) lower than it is today. Thus, the climate was at least 7.0 to 8.3 °C (12.6 to 14.9 °F) colder than it is today.

Hydrology

Confluence of Indus River and Zanskar River in the Himalayas

 

The Himalayan range at Yumesongdong in Sikkim, in the Yumthang River valley

 

Despite their scale, the Himalayas do not form a major watershed, and a number of rivers cut through the range, particularly in the eastern part of the range. As a result, the main ridge of the Himalayas is not clearly defined, and mountain passes are not as significant for traversing the range as with other mountain ranges. The rivers of the Himalayas drain into two large river systems:

• The western rivers combine into the Indus Basin. The Indus itself forms the northern and western boundaries of the Himalayas. It begins in Tibet at the confluence of Sengge and Gar rivers and flows north-west through India into Pakistan before turning south-west to the Arabian Sea. It is fed by several major tributaries draining the southern slopes of the Himalayas, including the Jhelum, Chenab, Ravi, Beas and Sutlej rivers, the five rivers of the Punjab.
• The other Himalayan rivers drain the Ganges-Brahmaputra Basin. Its main rivers are the Ganges, the Brahmaputra and the Yamuna, as well as other tributaries. The Brahmaputra originates as the Yarlung Tsangpo River in western Tibet, and flows east through Tibet and west through the plains of Assam. The Ganges and the Brahmaputra meet in Bangladesh and drain into the Bay of Bengal through the world's largest river delta, the Sunderbans.

The northern slopes of Gyala Peri and the peaks beyond the Tsangpo, sometimes included in the Himalayas, drain into the Irrawaddy River, which originates in eastern Tibet and flows south through Myanmar to drain into the Andaman Sea. The Salween, Mekong, Yangtze and Yellow River all originate from parts of the Tibetan Plateau that are geologically distinct from the Himalaya mountains and are therefore not considered true Himalayan rivers. Some geologists refer to all the rivers collectively as the circum-Himalayan rivers.

Glaciers

South Annapurna Glacier

 

The great ranges of central Asia, including the Himalayas, contain the third-largest deposit of ice and snow in the world, after Antarctica and the Arctic. The Himalayan range encompasses about 15,000 glaciers, which store about 12,000 km³ (2,900 cu mi) of fresh water. Its glaciers include the Gangotri and Yamunotri (Uttarakhand) and Khumbu glaciers (Mount Everest region), Langtang glacier (Langtang region) and Zemu (Sikkim). 

Owing to the mountains' latitude near the Tropic of Cancer, the permanent snow line is among the highest in the world at typically around 5,500 m (18,000 ft). In contrast, equatorial mountains in New Guinea, the Rwenzoris and Colombia have a snow line some 900 m (2,950 ft) lower. The higher regions of the Himalayas are snowbound throughout the year, in spite of their proximity to the tropics, and they form the sources of several large perennial rivers. 

In recent years, scientists have monitored a notable increase in the rate of glacier retreat across the region as a result of global climate change. For example, glacial lakes have been forming rapidly on the surface of debris-covered glaciers in the Bhutan Himalaya during the last few decades. Although the effect of this will not be known for many years, it potentially could mean disaster for the hundreds of millions of people who rely on the glaciers to feed the rivers during the dry seasons.

Lakes

Gurudongmar Lake in Sikkim

 

The Himalayan region is dotted with hundreds of lakes. Most of the larger lakes are on the northern side of the main range. The most famous is the sacred freshwater Lake Manasarovar, near to Mount Kailas with an area of 410 km² (160 sq mi) and an altitude of 4,590 m (15,060 ft). It drains into the nearby Lake Rakshastal with an area of 250 km² (97 sq mi) and slightly lower at 4,575 m (15,010 ft). Pangong Tso, which is spread across the border between India and China, at far western end of Tibet, and Yamdrok Tso, located in south central Tibet, are among the largest with surface areas of 700 km² (270 sq mi), and 638 km² (246 sq mi), respectively. Lake Puma Yumco is one of the highest of the larger lakes at an elevation of 5,030 m (16,500 ft). 

South of the main range, the lakes are smaller. Tilicho Lake in Nepal in the Annapurna massif is one of the highest lakes in the world. Other notable lakes include Rara Lake in western Nepal, She-Phoksundo Lake in the Shey Phoksundo National Park of Nepal, Gurudongmar Lake, in North Sikkim, Gokyo Lakes in Solukhumbu district of Nepal and Lake Tsongmo, near the Indo-China border in Sikkim. 

Some of the lakes present a danger of a glacial lake outburst flood. The Tsho Rolpa glacier lake in the Rowaling Valley, in the Dolakha District of Nepal, is rated as the most dangerous. The lake, which is located at an altitude of 4,580 m (15,030 ft) has grown considerably over the last 50 years due to glacial melting. The mountain lakes are known to geographers as tarns if they are caused by glacial activity. Tarns are found mostly in the upper reaches of the Himalaya, above 5,500 m (18,000 ft).

Climate

Upper Mustang

 

The Annapurna range of the Himalayas

 

The vast size, huge altitude range and complex topography of the Himalayas mean they experience a wide range of climates, from humid subtropical in the foothills to cold, dry desert conditions on the Tibetan side of the range. For much of Himalayas – that on the south side of the high mountains, except in the furthest west, the most characteristic feature of the climate is the monsoon. Heavy rain arrives on the south-west monsoon in June and persists until September. The monsoon can seriously impact transport and cause major landslides. It restricts tourism – the trekking and mountaineering season is limited to either before the monsoon in April/May or after the monsoon in October/November (autumn). In Nepal and Sikkim, there are often considered to be five seasons: summer, monsoon, autumn (or post-monsoon), winter and spring.

Using the Köppen climate classification, the lower elevations of the Himalayas, reaching in mid elevations in central Nepal (including the Kathmandu valley), are classified as Cwa, Humid subtropical climate with dry winters. Higher up, most of the Himalayas have a subtropical highland climate (Cwb). 

In the furthest west of the Himalayas, in the west of the Kashmir valley and the Indus valley, the South Asian monsoon is no longer a dominant factor and most precipitation falls in the spring. Srinagar receives around 723 mm (28 in) around half the rainfall of locations such as Shimla and Kathmandu, with the wettest months being March and April.

The northern side of the Himalayas, also known as the Tibetan Himalaya, is dry, cold and generally wind swept particularly in the west where it has a cold desert climate. The vegetation is sparse and stunted and the winters are severely cold. Most of the precipitation in the region is in the form of snow during late winter and spring months. 

Local impacts on climate are significant throughout the Himalayas. Temperatures fall by 6.5 °C (11.7 °F) for every 1,000 m (3,300 ft) rise in altitude. This gives rise to a variety of climates from nearly tropical in the foothills to tundra and permanent snow and ice. Local climate is also affected by the topography: The leeward side of the mountains receive less rain while the well exposed slopes get heavy rainfall and the rain shadow of large mountains can be significant, for example leading to near desert conditions in the Upper Mustang which is sheltered from the monsoon rains by the Annapurna and Dhaulagiri massifs and has annual precipitation of around 300 mm (12 in), while Pokhara on the southern side of the massifs has substantial rainfall (3,900 mm or 150 in a year). Thus although annual precipitation is generally higher in east than the west, local variations are often more important.

The Himalayas have a profound effect on the climate of the Indian subcontinent and the Tibetan Plateau. They prevent frigid, dry winds from blowing south into the subcontinent, which keeps South Asia much warmer than corresponding temperate regions in the other continents. It also forms a barrier for the monsoon winds, keeping them from traveling northwards, and causing heavy rainfall in the Terai region. The Himalayas are also believed to play an important part in the formation of Central Asian deserts, such as the Taklamakan and Gobi.

Ecology

Male Himalayan tahr in Nepal

 

Red panda

 

The flora and fauna of the Himalayas vary with climate, rainfall, altitude, and soils. The climate ranges from tropical at the base of the mountains to permanent ice and snow at the highest elevations. The amount of yearly rainfall increases from west to east along the southern front of the range. This diversity of altitude, rainfall and soil conditions combined with the very high snow line supports a variety of distinct plant and animal communities. The extremes of high altitude (low atmospheric pressure) combined with extreme cold favor extremophile organisms.

At high altitudes, the elusive and previously endangered snow leopard is the main predator. Its prey includes members of the goat family grazing on the alpine pastures and living on the rocky terrain, notably the endemic bharal or Himalayan blue sheep. The Himalayan musk deer is also found at high altitude. Hunted for its musk, it is now rare and endangered. Other endemic or near endemic herbivores include the Himalayan tahr, the takin, the Himalayan serow, and the Himalayan goral. The critically endangered Himalayan subspecies of the brown bear is found sporadically across the range as is the Asian black bear. In the mountainous mixed deciduous and conifer forests of the eastern Himalayas, Red panda feed in the dense understories of bamboo. Lower down the forests of the foothills are inhabited by several different primates, including the endangered Gee's golden langur and the Kashmir gray langur, with highly restricted ranges in the east and west of the Himalayas respectively.

The unique floral and faunal wealth of the Himalayas is undergoing structural and compositional changes due to climate change. Hydrangea hirta is an example of floral species that can be found in this area. The increase in temperature is shifting various species to higher elevations. The oak forest is being invaded by pine forests in the Garhwal Himalayan region. There are reports of early flowering and fruiting in some tree species, especially rhododendron, apple and box myrtle. The highest known tree species in the Himalayas is Juniperus tibetica located at 4,900 m (16,080 ft) in Southeastern Tibet.

Culture

Jain pilgrims paying obeisance to Tirthankar Rishabhdev near Mount Kailash.

 

The Himalayan population belongs to four distinct cultural groups, who throughout history have systematically penetrated the isolated indigenous Himalayan population. Those migrating cultures – Hindu (Indian), Buddhist (Tibetan), Islamic (Afghanistan–Iranian) and Animist (Burmese and southeast Asian) – have created here their own individual and unique place. Their current arrangement, though with a few exceptions, is linked to specific geographical regions, and the relative altitude at which they occur. 

There are many cultural aspects of the Himalayas. In Jainism, Mount Ashtapad in Himalayas is a sacred place where the first Jain Tirthankara, Rishabhdeva attained moksha. It is believed that after Rishabhdeva attained nirvana, his son, Emperor Bharata Chakravartin, had constructed three stupas and twenty four shrines of the 24 Tirthankaras with their idols studded with precious stones over there and named it Sinhnishdha. For the Hindus, the Himalayas are personified as Himavath, the father of the goddess Parvati. The Himalayas is also considered to be the father of the river Ganges. Two of the most sacred places of pilgrimage for the Hindus is the temple complex in Pashupatinath and Muktinath, also known as Saligrama because of the presence of the sacred black rocks called saligrams.

The Buddhists also lay a great deal of importance on the mountains of the Himalayas. Paro Taktsang is the holy place where Buddhism started in Bhutan. The Muktinath is also a place of pilgrimage for the Tibetan Buddhists. They believe that the trees in the poplar grove came from the walking sticks of eighty-four ancient Indian Buddhist magicians or mahasiddhas. They consider the saligrams to be representatives of the Tibetan serpent deity known as Gawo Jagpa.

In Indian tradition, Rishabhdev's son Emperor Bharata Chakravartin, after whom India was believed to be named Bharatvarsha attained nirvana at Mount Kailash.

 

The Himalayan people's diversity shows in many different ways. It shows through their architecture, their languages and dialects, their beliefs and rituals, as well as their clothing. The shapes and materials of the people's homes reflect their practical needs and the beliefs. Another example of the diversity amongst the Himalayan peoples is that handwoven textiles display colors and patterns unique to their ethnic backgrounds. Finally, some people place a great importance on jewellery. The Rai and Limbu women wear big gold earrings and nose rings to show their wealth through their jewellery.

Religions

The Taktsang Monastery, Bhutan, also known as the "Tiger's Nest"

 

Likir Monastery in Ladakh

 

Several places in the Himalayas are of religious significance in Hinduism, Buddhism, Jainism and Sikhism. A notable example of a religious site is Paro Taktsang, where Padmasambhava is said to have founded Buddhism in Bhutan. Padmasambhava is also worshipped as the patron saint of Sikkim. There are also Muslim and Hindhu Shaivite Kashmiri Pandit in the area of Kashmir. 

In Hinduism, the Himalayas have been personified as the king of all Mountain – "Giriraj Himavat", father of Ganga and Parvati (form of Adi Shakti Durga).

A number of Vajrayana Buddhist sites are situated in the Himalayas, in Tibet, Bhutan and in the Indian regions of Ladakh, Sikkim, Arunachal Pradesh, Spiti and Darjeeling. There were over 6,000 monasteries in Tibet, including the residence of the Dalai Lama. Bhutan, Sikkim and Ladakh are also dotted with numerous monasteries. The Tibetan Muslims have their own mosques in Lhasa and Shigatse.

Resources

The Himalayas are home to a diversity of medicinal resources. Plants from the forests have been used for millennia to treat conditions ranging from simple coughs to snake bites. Different parts of the plants – root, flower, stem, leaves, and bark – are used as remedies for different ailments. For example, a bark extract from an abies pindrow tree is used to treat coughs and bronchitis. Leaf and stem paste from an arachne cordifolia is used for wounds and as an antidote for snake bites. The bark of a callicarpa arborea is used for skin ailments. Nearly a fifth of the gymnosperms, angiosperms and pteridophytes in the Himalayas are found to have medicinal properties, and more are likely to be discovered.

Most of the population in some Asian and African countries depend on medicinal plants rather than prescriptions and such. Since so many people use medicinal plants as their only source of healing in the Himalayas, the plants are an important source of income. This contributes to economic and modern industrial development both inside and outside the region. The only problem is that locals are rapidly clearing the forests on the Himalayas for wood, often illegally. This means that the number of medicinal plants is declining and that some of them might become rarer or, in some cases, go extinct.

Although locals are clearing out portions of the forests in the Himalayas, there is still a large amount of greenery ranging from the tropical forests to the Alpine forests. These forests provide wood for fuel and other raw materials for use by industries. There are also many pastures for animals to graze upon. The many varieties of animals that live in these mountains do so based on the elevation. For example, elephants and rhinoceros live in the lower elevations of the Himalayas, also called the Terai region. Also, found in these mountains are the Kashmiri stag, black bears, musk deer, langur, and snow leopards. The Tibetan yak are also found on these mountains and are often used by the people for transportation. However, the populations of many of these animals and still others are declining and are on the verge of going extinct.

The Himalayas are also a source of many minerals and precious stones. Amongst the tertiary rocks, are vast potentials of mineral oil. There is coal located in Kashmir, and precious stones located in the Himalayas. There is also gold, silver, copper, zinc, and many other such minerals and metals located in at least 100 different places in these mountains.