NECK (0. Eng. hnecca; the word appears in many Teutonic languages; cf. Dutch nek, Ger. Nacken; in O. E. the common word was heals; cf. Ger. Hals), that part of the body which connects the head with the trunk (see Anatomy: Superficial and Artistic). The word is transferred to many objects resembling this part of the body in shape or function; it is thus applied to an isthmus, or to the narrowest portion of a promontory, to the narrow part of a musical stringed instrument connecting the head and body, as in the violin, or to a narrow pass between mountains, which in the Dutch form nek, appears in place-names in South Africa. In architecture, the "neck" is that part of the capital just above the "astragal," and the term "necking" is applied to the annulet or round, or series of horizontal mouldings, which separates the capital of a column from the plain part or a shaft. In Romanesque work this is sometimes corded.
In Geology, the term "neck" is given to the denuded stump of an extinct volcano. Beneath every volcano there are passages of conduits up which the volcanic materials were forced, and after the mass has been levelled by denudation there is always a more or less circular pipe which marks the site of the crater. This pipe, which is filled with consolidated ashes or with crystalline lava, is the characteristic of a volcanic neck. Active volcanoes often stand on the sea-bottom and when the eruption comes to an end the volcano is slowly buried under layers of sediment. In tropical seas the coral animals cover over the submarine volcanoes which rise nearly to the surface and form great reefs of limestone around them. Should elevation take place after long ages the removal of the overlying strata will bring the volcanic mass to light, and in the normal course of things this will suffer denudation exactly like a recent volcano. Many instances of this are furnished by the geological history of the British Isles. In Carboniferous times, for example, before the Coal-measures were deposited, a shallow sea occupied the southern part of Scotland and the north of England. Volcanic activity broke out on the seabottom, and many volcanic cones, both small and large, were produced. These have long since been uplifted and the superjacent strata denuded away over a large part of the area which they occupied. In Derbyshire, Fife, the Lothians and the Glasgow district the remains of Carboniferous volcanoes occur in every state of preservation. Some have the conical hills of lavas and ashes well preserved (e.g. Largo Law in Fifeshire); others retain only a small part of the original volcanic pile (e.g. Arthur's Seat, Edinburgh; the Binn of Burntisland) and of the larger number nothing remains but the "neck" which shows where once the crater was situated.
In regions of former volcanic activity necks are the most persistent of all volcanic structures, because the active volcanic magma is located deep within the earth's crust, and the pipe by which it rises to the surface is of great length and traverses a great thickness of strata. Many volcanic necks stand on lines of fault. In other cases there are groups of necks lying in a straight or sinuous line, which may indicate the position of a fracture or at least of a line of least resistance. But in Scotland it is often impossible to adduce any evidence of the connexion between faults or fissures and the position of volcanic necks; and it seems likely that the pressure of the gases in the igneous magma increased till an explosion took place which perforated the rocks above with a clean tubular passage often nearly circular in cross section. This pipe was usually vertical, and nearly uniform in diameter for great depths; the material occupying it, when exposed by denudation, has a circular ground plan, or if shown in vertical section (or elevation) in a cliff is a pillar-shaped mass crossing the bedding planes of the strata nearly at right angles. It terminates upwards in the remains of the volcanic cone and communicates below with the reservoir from which the lavas were emitted, represented in most cases, where it has been exposed, by a large irregular mass (a batholith or boss) of coarsel y crystalline igneous rock. The site of such a neck is generally indicated by a low conical hill consisting of volcanic rock, surrounded by sedimentary or igneous strata of a different kind. The low cone is due to the greater hardness and strength of the volcanic materials and is not connected with the original shape of the volcano. Such hills are common in some parts of Scotland and well-known examples are Arthur's Seat and the Castle Rock (Edinburgh), North Berwick Law, the Bass Rock; they occur also in the Peak district of Derbyshire, and the Wolf Rock off the coast of Cornwall is probably a neck. Two splendid sugar-loaf cones known as the Pitons of St Lucia in the West Indies, rising from the sea with almost vertical sides to a height of nearly 3000 ft., are old volcanic necks. In Texas, New Mexico, Arizona, California and many of the western states of North America geologists have observed conical volcanic hills having all the features which belong to necks.
Where the volcanic rocks are soft and easily disintegrated they may be reduced more rapidly than the strata around them and the position of a neck may be indicated by a cup-shaped hollow; this is the case with some of the diamond-bearing basic pipes of South Africa. Sometimes necks are encountered in underground mining operations; in the coal-field of Fife, for instance, the coals are sometimes replaced by a circular mass of volcanic rock, a quarter of a mile or more in diameter, which rise vertically to the surface. Better examples are the Kimberley diamond mines. The blue-ground (or serpentine breccia) occupies great pipes or funnels, circular in outline with nearly vertical sides, extending downwards to unknown depths; these are undoubtedly the necks of old volcanoes. If any lavas were poured out from these pipes at the surface they have since been carried away by denudation.
The size of necks varies considerably; the smallest may be only 20 or 30 yds. in diameter, the largest are several miles. In this respect they resemble active craters, but no necks have been met with on the earth's surface with dimensions approaching those of the so-called "craters" of the moon. Small necks are usually simple, i.e. they contain only one or two kinds of igneous rock (ashes and dikes) and have been produced, so far as we can judge, by a single eruption. Not infrequently they contain no volcanic rock but are filled with pieces of slate, sandstone or whatever strata the pipe traverses. Such necks must have been produced by a single eruption with an outburst of steam, not followed by lava; the disrupted fragments of the surrounding rocks and the materials tumbling down from the crater's walls ultimately filled up the cavity. Instances occur in Fifeshire and in Shetland, and among the recent volcanoes of the Eiffel there are some which have thrown out more slate and sandstone than lava.
Large necks, on the other hand, are often of complex structure, contain many kinds of rock and seem to have been produced by repeated eruptions, each of which more or less completely cleared out the material obstructing the orifice, and introduced a series of fresh accumulations. The beds of ashes which line the interior of an active crater have in nearly all cases a slope or dip towards a central point where the base of the depression is situated, and in volcanic necks which have been filled with ash (tuffs and agglomerates) this funnel-like inward dip is very constant. If there has been only a single eruption the beds of ashes have a very conformable or uniform arrangement, but if activity has been resumed after a period of quiescence a large part of the old material may have been projected and a new series of beds laid down, transgressing unconformably the edges of the earlier ones. By these structures we can sometimes trace a neck within a neck, or of a lateral crater on the margin of a principal one.
Where the crater has filled up with very coarse ashes, or agglomerate, the bedding is rarely visible. Sometimes large empty craters were occupied temporarijy by lakes, and level sheets of mud and silt have gathered on their floors: hence bedded sediments are not infrequently found in volcanic necks. Mixed with the volcanic ashes and bombs there are often large broken pieces of sedimentary rocks which may have been crystallized and hardened by the heat and vapours emitted by the volcano. Sometimes great fragments of the walls have foundered or collapsed into the crater, and masses of nonvolcanic rock, an acre or more in extent, may occur in a volcanic neck. In Arran, for example, there is a large neck which contains lumps of Cretaceous rocks nowhere else known to occur on the island; they have fallen down from strata once occupying part of the walls of the crater but now removed by denudation.
The lava which rises and flows out from the crater leaves its trace also in the necks. Sometimes it forms thin beds or flows alternating with the tuffs and having the same basin-shaped dip. More commonly it appears as the material filling fissures and pipes, traversing the ashes irregularly or rising as a central plug in the interior of the neck, and sending out branching veins. Occasionally a whole neck is composed of solid crystalline rock representing the last part of the magma which ascended from the underground focus and congealed within the crater. In Mont Pelee, for instance, the last stage of the eruptions of 1902 to 1905 was the protrusion of a great column of solidified lava which rose at one time to a height of 900 ft. above the lip of the crater, but has since crumbled down. The Castle Rock of Edinburgh is a neck occupied by a plug of crystalline basalt. Necks of this kind weather down very slowly and tend to form prominent hills.
After the eruptions terminate gases or hot solutions given out by deep-lying masses of molten rock may find a passage upward through the materials occupying the crater, greatly modifying their mineral nature and laying down fresh deposits. A good example of secondary deposits within a volcanic neck is provided by the Cripple Creek mining district of Colorado. The ore-bearing veins are connected with volcanic rocks and part of these occupy a vertical circular pipe which is a typical volcanic neck. A phonolitic breccia, greatly altered, is the principal rock, and is cut by dikes of phonolite, dolerite, &c. The, country rock is mostly granite and gneiss, and blocks of these are common in the breccia. A large volcano was built up in Tertiary times on the granite plateau, and has since been almost entirely removed by denudation. The gold ores were carried upwards by currents of hot water derived from the volcanic magma and were deposited along cracks and fissures in the materials which occupied the crater, and also in the surrounding rocks (see Volcano).
(J. S. F.)
- Please bookmark this page (add it to your favorites)
- If you wish to link to this page, you can do so by referring to the URL address below.
This page was last modified 29-SEP-18
Copyright © 2018 ITA all rights reserved.