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PNEUMATOLYSIS (Gr. 7rvevµa, vapour, a...
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See also:PNEUMATOLYSIS (Gr. 7rvevµa, vapour, and Mete, to set See also:free) , in See also:petrology, the See also:discharge of vapours from igneous magmas and the effects produced by them on See also:rock masses In all volcanic eruptions the gases given off by the molten lavas are powerful agencies. The slaggy clots of See also:lava thrown out from the See also:crater are so full of See also:gas that when they cool they resemble spongy pieces of See also:bread. The lava streams as they flow down the slopes of the See also:volcano are covered with See also: Some geologists believe it is of subterranean origin like the lava itself, and is an essential or See also:original component of the magma. They point to the existence of gases in considerable quantity in meteorites, and, comparing the See also:earth to a great aerolite, insist that it should contain gases in solution like the smaller masses of the same See also:kind. Others hold it more probable that the water has percolated in from the surface, or seeing that many volcanoes stand near the See also:sea margin and by their linear disposition may be disposed along fissures or lines of weakening in the crust, they argue that the water of the sea may have filtered down even in spite of the great outward pressure exerted by the steam generated by contact with the intensely heated rock. The abundance of chlorides and hydrochloric acid is appealed to also in favour of a marine origin for the water. Against this we may See also:place the fact that at great depths whence active magmas ascend the rocks are under so great pressures that every fissure is closed up; in fact in some of the deepest mines the quantity of water found in the workings is often exceedingly small. Probably there is some truth in both theories, but the. See also:balance of See also:probability seems to incline in favour of the view that the water is an original and essential See also:part of the magma and not an introduction from above. See also:Long after a lava has cooled down and become rigid the vapours continue to See also:ooze out through its fissures, and around many volcanoes which are believed to be See also:extinct there are orifices discharging gas in great quantities. This See also:state of activity is said to be " solfataric," and a See also:good example of it is the volcano called the See also:Solfatara near See also:Naples. The numerous "Soufrieres" of the See also:West Indies are further instances. The prevalent gas is steam with sulphuretted hydrogen and carbonic acid. At the Grotto del See also:Cane in the Phlegraean See also:Fields (See also:Italy) the carbonic acid rising from fissures in the bottom of a See also:cave covers the See also:floor as a heavy layer, and a See also:dog placed in the interior of the cave becomes stupefied by the narcotic gas; such gas-springs have been called " mofettes." Around them there is often a See also:deposit of sulphur, produced by oxidation of the sulphuretted hydrogen, and the rocks are bleached, softened and decomposed. White crusts of See also:alum, various sulphates, and sulphides such as See also:pyrites, also See also:carbonates of soda and other bases, are formed by the action of the acid vapours on the volcanic rocks. The final manifestation of volcanic activity in such a region may be the discharge of heated waters, which have ascended from the deep-seated magma far below the surface, and make their See also:appearance as groups of hot springs; these springs persist long after the volcanoes which give rise to them have become quite extinct. It is now believed by a large number of geologists and See also:mining See also:engineers that these ascending hot waters are of See also:paramount importance in the See also:genesis of some of the most important types of ore deposits. Analyses have proved that the igneous rocks often contain distinct though very small quantities of the heavy metals; it is also established beyond doubt that See also:veins of See also:gold, See also:silver, See also:lead, See also:tin and See also:mercury most commonly occur in the vicinity of intrusive igneous masses. At Steamboat in See also:Nevada, hot springs, probably of magmatic origin, are forming deposits of See also:cinnabar. At Cripple See also:Creek, See also:Colorado, and in many other places gold-bearing veins occur in and around intrusive plugs of igneous rock. Tin ores in all parts of the See also:world are found in association with See also:tourmaline granites. In all cases the veins See also:bear See also:evidence of having been filled from below by hot waters set free during the cooling of the igneous intrusions. Volcanic rocks are consequently the See also:parent See also:sources of many valuable See also:mineral deposits, and the agency by which they were brought into their See also:present situations is the volatile products discharged as the magma crystallized. The See also:process was no doubt a long oae and it is most probable that both steam and water took part in it. Above 365° C. water is a gas under all pressures and the action is strictly pneumatolytic; below that temperature steam is changed to water by pressure and the action may be described as hydatogenetic. The distinction is unessential, and in our See also:ignorance of the temperatures and pressures prevailing at considerable depths we lack the means of See also:classification. In what See also:condition the metallic ores are dissolved and by what reactions they are precipitated depends on many factors only partly understood. The tin ores are so often associated with minerals containing boron and fluorine that it is quite probable that they were combined with these elements in some way, but they were deposited in nearly all cases as oxides. Other gaseous substances, such as sulphuretted hydrogen, carbonic acid and hydrochloric acid, probably have an important part in dissolving certain metals; and the alkaline carbonates, sulphides and chlorides have been shown by experiment to See also:act also as solvents. In these ore deposits not only the heavy metals are found, but often a much larger quantity of minerals such as calcite, See also:barytes, fluorspar, See also:quartz and tourmaline which serve as a See also:matrix or See also:gangue, and have been deposited by the same agencies, and often at the same See also:time as the valuable minerals. In their passage upwards and outwards through the rocks of the earth's crust, these gases and liquids not only deposit minerals in the fissures along which they ascend, but attack the surrounding rocks and alter them in many ways. The See also:granite or other plutonic See also:mass from which the vapours are derived is especially liable to these transformations, probably because it is at a high temperature, not having yet completely cooled down. Around the tin-bearing veins in granite there is extensive replacement of See also:felspar and See also:biotite by quartz, tourmaline and white micas (the last-named often See also:rich in lithia). In this way certain types of altered granite are produced, such as See also:greisen (q.v.) and See also:schorl rock (see SCHORL). In the slates adjacent to the tin veins tourmalinization also goes on, converting them into schorl-See also:schists. The alteration of felspar into See also:kaolin or See also:china See also:clay is also a pneumatolytic process, and is often found along with tin veins or other types of mineral deposit; probably both fluorine and carbonic acid operated in this instance along with water. Equally See also:common and important is the silicification of rocks near mineral veins which carry gold, copper, lead and other metals. Granites and felsites may be converted into hard cherty masses of See also:silica. Limestones undergo this transformation very readily; at the same time they are regarded as rocks very favourable to the deposition of ores. Probably the great frequency with which they undergo silicification and other types of metasomatic replacement is one of the See also:main causes of the abundance of valuable deposits in them. The process known as " propylitization," which has extensively affected the andesites of the Hungarian goldfields, is believed to be also a consequence of the action of pneumatolytic gases. The andesites See also:change to dull, soft, greenish masses, and their original minerals are to a large extent replaced by quartz, See also:epidote, chlorite, sericite and kaolin. Around granites intrusive into See also:serpentine and other rocks containing much See also:magnesia, there is often extensive " steatisation," or the deposit of See also:talc and steatite in place of the original minerals of the rock. Some of the See also:apatite veins of See also:Canada and See also:Norway accompany basic rocks of the See also:gabbro group; it has been argued that the apatite (which contains See also:phosphorus and See also:chlorine) was laid down by vapours or solutions containing those gases, which may See also:play a similar part in the basic rocks to that taken by fluorine and boron in the pneumatolytic veins around granites. In the See also:country rock around the veins See also:scapolite (q.v.), a See also:lime alumina silicate, containing chlorine, often is substituted for lime-felspar. These extensive changes attending the formation of mineral veins are by no means common phenomena, but in many plutonic masses pneumatolytic action has contributed to the formation of pegmatites (q.v.). (J. Additional information and CommentsThere are no comments yet for this article.
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