Molecular clouds are among the largest structures of interstellar space. They consist of cold and relatively dense (1012 particles/cm3) collections of matter in molecular form. Molecules in these clouds can become excited by collision with other particles or by interacting with radiation. When either happens, the molecules reach a higher energy state when they are excited, and when they relax to a lower energy state, they emit a photon that can then be detected by astronomers. Molecules are more complex than atoms, so they can produce a greater variety of energy released during changes in rotation, electron transitions, and vibrations, each releasing a characteristic photon emission. Most molecular clouds are located in very dusty and dense areas in interstellar space, so energy released in these processes in the ultraviolet, optical, and most infrared wavelengths is absorbed by the local dust clouds, but photons and energy released at radio-wave frequencies moves through this medium and can be detected from Earth.
Spectra from molecular clouds reveal that they consist mostly of molecular hydrogen (H2), but molecular hydrogen does not emit or absorb radio wave radiation, so it is not useful as a probe. But the spectra emitted from other molecules have proven useful for studying molecular clouds. some of the most useful include carbon monoxide, hydrogen cyanide, ammonia, water, methyl alcohol, and formaldehyde, and many dozens of other complex molecules. These molecules are used as tracers of the physical and chemical makeup of the molecular clouds and are interpreted to have formed in the clouds. The spectral lines from molecular clouds can also be used to determine the composition of the clouds, their temperature, density, and distribution. One of the major discoveries about molecular clouds made in the past few decades is that the clouds are not isolated bodies, but form giant molecular cloud complexes, as large as 50 parsecs across, each containing millions of stars. The Milky Way Galaxy alone has more than 1,000 known molecular clouds.
See also astrophysics; constellation; cosmology; galaxies.
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Comins, Neil F. Discovering the Universe. 8th ed. New
York: W. H. Freeman, 2008. National Aeronautics and Space Administration. Goddard Space Flight Center Astronomical Data Center Quick Reference Page, Interstellar Medium (ISM) Web page. Available online. URL: http://adc.gsfc.nasa.gov/adc/ quick_ref/ref_ism.html. Accessed April 30, 2002. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West, 1991.
island arcs, historical eruptions Island arcs are belts of high seismic activity and high heat flow with chains of active volcanoes, bordered by a submarine trench formed at a subduction zone. They form where plates of oceanic lithosphere are subducted beneath another oceanic plate, and the down-going oceanic lithosphere may be subducted to 500 miles (700 km) or more. A related type of volcanic arc, an Andean or continental-margin volcanic arc, forms on the edge of a continental plate where an oceanic slab is subducted beneath the edge of the continent. In both types of arcs fluids forced out of the subducting slab at 60-100 miles' depth (100-160
km) cause the mantle above the subducting slab to melt partially, and these magmas migrate upward to form the island or continental margin arc. In most cases these arcs are located 90-120 miles (150-200 km) from the trench, with the distance determined by the dip of the down-going slab.
Volcanic arcs developed above subduction zones have several different geomorphic zones defined largely on their topographic and structural expressions. The active arc is the topographic high with volcanoes, and the back arc region stretches from the active arc away from the trench; it may end in an older rifted arc or continent. The arc is succeeded seaward by the fore-arc basin, a generally flat topographic basin with shallow- to deep-water sediments, typically deposited over older accreted sediments and ophiolitic or continental basement. The accretion-ary prism includes uplifted, strongly deformed rocks scraped off the downgoing oceanic plate on a series of faults that branch off from the subduction zone thrust fault. Some accretionary prisms are 50-100 miles (80-160 km) wide, and can be thousands of miles long. The world's largest accretionary prisms currently extend around the Pacific Ocean rim, including the southern Alaska accretionary prism, the Franciscan complex in California, and prisms in Japan and the southwest Pacific. The trench may be several to five or more miles (10 km) deep below the average level of the seafloor in the region and marks the boundary between the overriding and under-thrusting plates. The outer trench slope is the region from the trench to the top of the flexed oceanic crust that forms a few hundred-meter-high topographic rise known as the forebulge on the down-going plate.
Trench depressions are triangular shaped in profile and typically partly to completely filled with graywacke-shale sediments derived from erosion of the accretionary wedge, and deposited by sedimentladen, fast-moving, down-slope flows known as turbidity currents. The resulting sedimentary rock types have a characteristic style of layering and an upward decrease in grain size, and are known as turbidites. Turbidites may also be transported by currents along the trench axis for large distances, up to hundreds or even thousands of miles from their ultimate source in uplifted mountains in the convergent plate boundary orogen. Flysch is a term that applies to rapidly deposited deep marine syn-orogenic clastic rocks that are generally turbidites. Chaotic deposits known as olistostromes that typically have clasts or blocks of one rock type, such as limestone or sandstone, mixed with a muddy or shaly matrix also characterize trenches. These are interpreted as slump or giant submarine landslide deposits. They are common in trenches because of the oversteepening of slopes in the wedge. sediments that get accreted may also include pelagic (deep-water) sediments initially deposited on the subducting plate, such as red clay,
Cross section of island arc showing the physiography and geologic processes involved during subduction and formation of the arc
siliceous ooze, chert, manganiferous chert, calcareous ooze, and windblown dust.
The sediments are deposited as flat-lying turbidite packages, then are gradually incorporated into the accretionary wedge complex through folding and the propagation of faults through the trench sediments. subduction accretion accretes sediments deposited on the underriding plate onto the base of the overriding plate. It causes the rotation and uplift of the accretionary prism, a broadly steady process that continues as long as sediment-laden trench deposits are thrust deeper into the trench. New faults will typically form and propagate beneath older ones, rotating the old faults and structures to steeper attitudes as new material is added to the toe and base of the accretionary wedge. This process increases the size of the overriding accretionary wedge and causes a seaward-younging in the age of deformation.
Parts of the oceanic basement to the subducting slab are sometimes scraped off and incorporated into the accretionary prisms. These tectonic slivers typi cally consist of fault-bounded slices of basalt, gabbro, and ultramafic rocks; rarely, partial or even complete ophiolite sequences can be recognized. These ophiol-itic slivers are often parts of highly deformed belts of rock known as mélanges. Mélanges are mixtures of many different rock types typically including blocks of oceanic basement or limestone in muddy, shaly, serpentinitic, or even a cherty matrix. Mélanges are formed by tectonic mixing of the many different types of rocks found in the fore arc, and are one of the hallmarks of convergent boundaries.
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