﻿MAY 
  4, 
  1922 
  MOREY 
  : 
  CRYSTALLIZATION 
  PRESSURE 
  IN 
  MAGMAS 
  227 
  

  

  On 
  the 
  other 
  hand, 
  conditions 
  may 
  be 
  such 
  that 
  a 
  much 
  greater 
  

   pressure 
  must 
  be 
  developed 
  before 
  the 
  gases 
  are 
  able 
  to 
  force 
  their 
  way 
  

   to 
  the 
  surface. 
  It 
  may 
  be 
  assumed 
  that 
  eruptions 
  will 
  then 
  take 
  

   place 
  at 
  less 
  frequent 
  intervals, 
  since 
  more 
  time 
  must 
  elapse 
  for 
  the 
  

   cooling 
  process 
  which 
  occasions 
  the 
  crystallization, 
  and 
  that, 
  on 
  ac- 
  

   count 
  of 
  the 
  greater 
  pressure, 
  the 
  resulting 
  eruptions 
  will 
  tend 
  to 
  be 
  

   catastrophic. 
  In 
  a 
  previous 
  paragraph 
  it 
  was 
  stated 
  that 
  in 
  the 
  case 
  

   of 
  an 
  incompetent 
  crust 
  the 
  building 
  up 
  of 
  pressure 
  as 
  the 
  result 
  of 
  

   cooHng 
  and 
  crystallization 
  would 
  continue 
  until 
  the 
  pressure 
  was 
  

   relieved 
  by 
  the 
  escape 
  of 
  the 
  volatile 
  material. 
  It 
  might 
  be 
  that, 
  if 
  

   the 
  crust 
  were 
  of 
  sufhcient 
  strength, 
  a 
  fairly 
  large 
  proportion 
  of 
  the 
  

   liquid 
  magma 
  would 
  crystallize 
  before 
  a 
  pressure 
  had 
  been 
  built 
  up 
  

   of 
  sufficient 
  magnitude 
  to 
  cause 
  an 
  eruption. 
  These 
  conditions 
  may 
  

   determine 
  the 
  formation 
  of 
  a 
  new 
  volcano, 
  such 
  as 
  Monte 
  Nuovo, 
  

   Jorullo, 
  or 
  Chinyero, 
  and 
  may 
  also 
  explain 
  the 
  renewed 
  activity 
  of 
  a 
  

   volcano 
  whose 
  vent 
  has 
  been 
  plugged 
  by 
  solidified 
  lava. 
  In 
  such 
  a 
  

   case, 
  in 
  which 
  a 
  considerable 
  amount 
  of 
  crystallization 
  has 
  taken 
  place, 
  

   the 
  non-crystallized 
  material 
  ejected 
  will 
  represent 
  the 
  "mother 
  

   liquor' 
  ' 
  remaining 
  after 
  the 
  segregation 
  of 
  those 
  minerals 
  which 
  are 
  the 
  

   first 
  to 
  crystallize 
  under 
  the 
  conditions 
  prevailing. 
  These 
  may 
  be 
  

   the 
  femic 
  minerals; 
  in 
  which 
  case 
  the 
  mother 
  liquor 
  will 
  be 
  enriched 
  

   in 
  the 
  more 
  salic 
  minerals, 
  quartz 
  and 
  the 
  feldspars, 
  and 
  the 
  water 
  

   content 
  will 
  be 
  correspondingly 
  increased.^ 
  The 
  tendency 
  for 
  the 
  

   heavier 
  femic 
  minerals 
  to 
  differentiate 
  by 
  settling 
  will 
  be 
  great, 
  es- 
  

   pecially 
  since 
  the 
  density 
  difference 
  between 
  the 
  femic 
  and 
  salic 
  

   minerals 
  will 
  be 
  increased 
  by 
  the 
  presence 
  in 
  the 
  salic 
  melt 
  of 
  the 
  ac- 
  

   cumulated 
  water. 
  We 
  should 
  therefore 
  expect, 
  irrespective 
  of 
  the 
  

   original 
  composition 
  of 
  the 
  magma, 
  that 
  paroxysmal 
  eruptions 
  would 
  

   be 
  characterized 
  by 
  the 
  ejection 
  of 
  salic 
  lava. 
  The 
  presence 
  or 
  absence 
  

   of 
  traces 
  of 
  the 
  differentiated 
  materials 
  will 
  be 
  erratic, 
  depending 
  on 
  the 
  

   completeness 
  of 
  the 
  differentiation 
  in 
  relation 
  to 
  the 
  original 
  situation 
  

   of 
  the 
  material 
  examined. 
  Moreover, 
  since 
  the 
  salic 
  lavas 
  are 
  in 
  

   themselves, 
  when 
  freed 
  from 
  water, 
  viscous 
  even 
  at 
  their 
  melting 
  

   points, 
  and 
  since 
  the 
  temperature 
  at 
  this 
  stage 
  will 
  have 
  been 
  greatly 
  

   lowered, 
  on 
  the 
  sudden 
  expansion 
  following 
  the 
  disruption 
  of 
  the 
  re- 
  

   straining 
  crust, 
  the 
  ejected 
  material 
  will 
  be 
  shattered 
  into 
  small 
  

   fragments. 
  It 
  will 
  be 
  seen 
  that 
  the 
  above 
  conclusions 
  are 
  in 
  accord 
  

   with 
  the 
  well-known 
  characteristics 
  of 
  catastrophic 
  eruptions; 
  salic 
  

  

  ^ 
  N. 
  L. 
  BowEN. 
  The 
  later 
  stages 
  of 
  the 
  evolution 
  of 
  the 
  igneous 
  rocks. 
  Journ. 
  Geol., 
  

   Suppl. 
  to 
  Vol. 
  23, 
  No. 
  8. 
  1915. 
  

  

  