Indogangetic Plain

The Indo-Gangetic Plain is the active foreland basin of the India-Asia collision, with sediments derived from erosion of the Himalaya Mountains and carried by numerous rivers that feed into the Indus and Ganges Rivers. Alluvial deposits of the Indo-Gangetic Plain stretch from the Indus River in Pakistan to the Punjab Plain in India and Pakistan, to the Haryana Plain and Ganges delta in Bangladesh. Sediments in the foreland basin extend up to 24,500 feet (7,500 m) thick over the basement rocks of the Indian shield, thinning toward the southern boundary of the basin plain. The plain has very little relief, with only occasional bluffs and terraces related to changes in river levels.

The northern boundary of the plain is marked by two narrow belts known as Terai, containing small hills formed by coarse remnant gravel deposits emerging from mountain streams. Many springs emanate from these gravel deposits forming large, swampy areas along the major rivers. In most places the Indo-Gangetic Plain is about 250 miles (400 km) wide. The southern boundary of the plains is marked by the front of the Great Indian Desert in Rajasthan, then continues eastward to the Bay of Bengal along the hills of the Central Highlands.

The Indo-Gangetic Plain can be divided into three geographically and hydrologically distinct sections. The Indus Valley in the west is fed by the Indus River, which flows out of Kashmir, the Hundu Kush, and the Karakoram range. The Punjab and Haryana Plains are fed by runoff from the Siwalik and Himalaya Mountains into the Ganges River, and fed by the Lower Ganga and Brahmaputra drainage systems in the east. The lower Ganga plains and Assam Valley are lush and heavily vegetated, and the waters flow into the deltaic regions of Bangladesh.

Clastic sediments of the foreland basin deposits under the Indo-Gangetic plain Eocene-Oligocene (about 50-30 million-year old) deposits, grading up to the Miocene to Pleistocene siwalik clastic rocks, eroded from the siwalik and Himalaya ranges. The basement of the Indian shield dips about 15° beneath the Great Boundary and other faults marking the deformation front at the toe of the Himalayas.

See also Archean; convergent plate margin processes; craton; Gondwana, Gondwanaland; greenstone belts; large igneous provinces, flood basalt.

FURTHER READING

Molnar, Peter. "The Geologic History and Structure of the

Himalaya." American Scientist 74 (1986): 144-154. Naqvi, S. Mahmood, and John J. W. Rogers. Precambrian Geology of India. Oxford: Oxford University Press, 1987.

Ramakrishnan, M., and R. Vaidyanadhan. Geology of India. 2 vols. Bangalore, India: Geological Society of India, 2008.

interstellar medium The interstellar medium, which consists of the areas or voids between the stars and galaxies, represents a nearly perfect vacuum, with a density a trillion trillion times less than that of typical stars. The density of the interstellar medium is so low that there are only a couple of atoms per cubic inch (about 1 atom per cubic cm), or roughly a thimbleful of atoms in a volume the size of the

Earth. Interstellar matter includes all of the gas, dust, dark matter, and other material in the universe not contained within stars, galaxies, or galaxy clusters. Despite the extremely low density of the interstellar medium, the vast size of the space is such that the amount of matter in the interstellar medium is about the same as that in all of the stars and galaxies.

Matter in the interstellar medium consists of two main components—gas and dust. In many places in the universe dust forms dark clouds of gigantic size that obscure distant light from passing through, making parts of the universe appear dark. This dust consists of clumps of atoms and molecules whose sizes are comparable to the wavelength of light (10-7 m), and much larger than the interstellar gas. The size of the dust particles explains why dust clouds appear dark. Electromagnetic radiation, such as visible light, can be effectively blocked only by particles of similar or greater size than the wavelength of the incident light (or other radiation), and since the size of the dust is similar to the wavelength of light, dust is an excellent blocker of light and appears dark, blocking light from sources behind the dust clouds. However, longer-wavelength radio waves can pass through dust clouds unimpeded. Gas in the interstellar medium is composed mainly of individual atoms of about 1 angstrom (10-10 m) in size, and a smaller amount of atoms combined into molecules. The size of the interstellar gas is less than the wavelengths of electromagnetic radiation in most of the visible and radio wavelengths, so this radiation, including light, can pass through the gas, with absorption occurring within a specific narrow range of wavelengths. Interstellar dust, however, blocks the shorter wavelength optical, ultraviolet, and X-ray radiation.

The gas and dust in the interstellar medium preferentially absorbs the longer wavelengths of light from the higher-frequency blue area of the spectrum, so stars and galaxies appear redder than they actually are. Despite this "reddening" of the appearance of stars, the spectral signature, and absorption lines in the stars' spectrums, are mostly unaffected by interstellar dust. With this relationship it is possible to measure the spectrum of a star, which will reveal its luminosity and color. Then by measuring the color on Earth, the amount of reddening by interaction with dust in the interstellar medium can be measured, and this in turn reveals information about the amount and type of interstellar dust the light encountered on its transit from the star to Earth.

Interstellar space is quite cold, with an average temperature of about 100 kelvin (which is -173°C, or -279°F), but ranging from a few kelvins (near absolute zero, where all motion of atoms stops) to several hundred kelvin near stars and other sources of radiation.

Cone Nebula pillar of gas and dust. Photo taken by Advanced Camera for Surveys aboard Hubble during space shuttle STS-109 mission in March 2002 (NASA, H. Ford (JHU), G. Illingworth (USCS/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA)

Interstellar gas is made mostly (about 90 percent) of atomic and molecular hydrogen, followed by about 9 percent helium, and 1 percent heavier elements such as carbon, oxygen, silicon, and iron, but the heavier elements have a much lower concentration in the interstellar gas than in stars or in the Earth's solar system. This may be because the heavier elements were combined into molecules to form interstellar dust, whose composition is not well known. Dust is known to include these heavier elements such as silicon, iron, and graphite, as well as ices of water, ammonia, and methane, similar to the "dirty ice" found in comet nuclei. Interstellar dust polarizes light that passes through it; in other words it aligns the light so that the electromagnetic radiation vibrates in a single plane instead of randomly as in unpolarized light. The light becomes polarized because the interstellar dust is made of long, skinny needlelike particles aligned in a specific direction. The cause of this alignment of interstellar dust is the subject of much research and debate but is thought to be caused by the dust particles aligning themselves along weak magnetic field lines in interstellar space.

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