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AGGLOMERATE (from the Lat. agglomerar...
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See also:AGGLOMERATE (from the See also:Lat. agglomerare, to See also:form into a See also:ball, glomus, glomeris) , a See also:term used in See also:botany, meaning crowded in a See also:close cluster or See also:head, and, in See also:geology, applied to the accumulations of coarse volcanic ejectamenta such as frequently occur near See also:extinct or active volcanoes. Agglomerates in the See also:geological sense, with which this See also:article is concerned, consist typically of blocks of various igneous rocks, mixed often with more or less material of rudimentary origin and embedded in a finer-grained See also:matrix, similar in nature to the coarser fragments. As distinguished from See also:ordinary ash beds or tuffs, they are essentially coarser, less frequently well-bedded; they are less persistent and tend to occur locally, but may attain a very See also:great thickness. Showers of See also:fine ash may be distributed over a wide See also:area of See also:country and will form thin layers of great extent. Coarser accumulations gather only near the actual foci of eruption (craters, fissures, &c.). When the activity of a volcanic vent comes to an end, the orifice is often choked by masses of debris, which will in See also:time become compacted into See also:firm agglomerates. Hence rocks of this type very commonly See also:mark the sites of necks, the remains of once-active volcanic craters. In this connexion they are of especial See also:interest to geologists, as it is always important to be able tc locate the exact points at which volcanic products, such as lavas and ash-beds, were emitted. The blocks in agglomerates vary greatly in See also:size. Some are See also:thirty or See also:forty feet in See also:diameter, and weigh many tons; these are usually pieces of the strata through which the See also:volcano has forced an outlet. They are never far from the See also:crater; most of them, in fact, See also:lie within its boundaries, and cases are known in which enormous masses of this See also:kind (See also:half an See also:acre in area) have been found in such situations. They are masses which have been dislodged, by fissures and landslides, from the crater's walls and have tumbled into the cavity. Pieces of See also:sandstone, See also:limestone and shale occur in the agglomerates mixed with volcanic materials, and very often have been baked and partly recrystallized by contact with the hot igneous rocks and the gases discharged by the volcano. At See also:Vesuvius such blocks of altered limestone are See also:rich in new minerals and are well known to collectors. Agglomerates also are usually full of volcanic bombs. These are spongy globular masses of See also:lava which have been shot from the crater at a time when liquid molten lava was exposed in it, and was frequently shattered by the sudden outbursts of See also:steam. These bombs were more or less viscous at the moment of ejection and by rotation in the See also:air acquired their spheroidal form. They are commonly one or two feet in diameter, but specimens as large as nine or twelve feet have been observed. There is less variety in their See also:composition at any volcanic centre than in the See also:case of the See also:foreign blocks above described. They correspond in nature to the lava which at the time fills the crater of the volcano, and as this varies only very slowly the bombs belong mostly to only a few kinds of See also:rock and are similar in composition to the lava flows. Crystalline masses of a different kind occur in some See also:numbers in certain agglomerates. They consist of volcanic minerals very much the same as those formed in the lavas, but exhibiting certain peculiarities which indicate that they have formed slowly under pressure at considerable depths. Hence they See also:bear a resemblance to plutonic igneous rocks, but are more correctly to be regarded as agglomerations of crystals formed within the liquid lava as it slowly See also:rose towards the See also:surface, and at a subsequent See also:period See also:cast out by violent steam explosions. The sanidinites of the See also:Eifel belong to this See also:group. At Vesuvius, See also:Ascension, St See also:Vincent and many other volcanoes, they form a not inconsiderable See also:part of the coarser ash-beds. Their commonest minerals are See also:olivine, See also:anorthite, See also:hornblende, See also:augite, See also:biotite and See also:leucite.
Agglomerates occur wherever volcanoes are known. In many parts of See also:Britain they attain a great development either in beds alternating with lavas or as the material occupying necks. In the latter case they are often penetrated by dikes. They also show a steep, angular, See also:funnel-shaped See also:dip (e.g. See also:Arthur's Seat, See also:Edinburgh), and may contain thin layers of See also:clay or ashy See also:sand-See also: Additional information and CommentsThere are no comments yet for this article.
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