Bulletin of the United States Geological Survey Volume 10

Bulletin of the United States Geological Survey Volume 10

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1890 edition. Excerpt: ...the basic magma when tbe final solidification of the rock took place. A similar resorption of porphyritical minerals which have crystallized in an early period of a rock's existence is frequently observed in all varieties of volcanic rocks, as, for example, partly resorbed hornblendes and feldspars, and the rounded quartz grains of rhyolites and porphyries. The idea that these phenomena indicate changes in the physical conditions experienced by the magma and not in its chemical composition is a generally accepted one. Changes of physical conditions.--Let us consider what maybe some of the changes of condition experienced by rock magmas within the crnst of the earth. In the case of a molten, viscous magma situated at some depth beneath the earth's surface the two physical factors which at first appear to exert the greatest influence on its existence as a plastic or fluid magma are temperature aud pressure. Other things remaining constant, the magma will be more plastic the higher the temperature, and with a decrease of the temperature below a certain point consolidation will take place. Consolidation under these circumstances, that is, at considerable depths within the earth, would be ordinarily accompanied by crystallization, as a rapid chilling is only possible when small bodies of magma are brought in contact with cold rocks. The nature of the crystallization will undoubtedly vary within certain limits with the rate of cooling. If, on the other hand, the temperature of the magma is supposed to remain constant and the pressure to vary, and if we assume that an increase of pressure diminishes the mobility of the molecules of the magma by condensing them, we should expect that an increase of pressure would teud to solidify the mass. The...show more

Product details

  • Paperback | 252 pages
  • 189 x 246 x 13mm | 458g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236836952
  • 9781236836953