﻿226 
  JOURNAL 
  OF 
  THE 
  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  VOL. 
  12, 
  NO. 
  9 
  

  

  the 
  supercooled 
  liquid 
  at 
  420° 
  will 
  be 
  59 
  atmospheres. 
  If 
  the 
  mix- 
  

   ture 
  containing 
  9.1 
  per 
  cent 
  water 
  were 
  to 
  cool, 
  without 
  crystallizing, 
  

   from 
  500°, 
  its 
  saturation 
  temperature, 
  to 
  420°, 
  its 
  pressure 
  would 
  fall 
  

   from 
  160 
  atmospheres 
  to 
  about 
  50 
  atmospheres. 
  If 
  at 
  this 
  lower 
  tem- 
  

   perature 
  it 
  should 
  begin 
  to 
  crystallize 
  the 
  pressure 
  would 
  suddenly 
  

   rise 
  to 
  that 
  of 
  the 
  solution 
  in 
  equilibrium 
  with 
  quartz 
  and 
  HKSi205 
  at 
  

   420°, 
  or 
  340 
  atmospheres. 
  

  

  It 
  is 
  evident, 
  then, 
  that 
  as 
  a 
  magma 
  containing 
  water 
  and 
  other 
  

   volatile 
  components 
  cools, 
  with 
  consequent 
  crystallization, 
  the 
  pres- 
  

   sure 
  will 
  rapidly 
  rise 
  from 
  its 
  initial 
  value, 
  and 
  as 
  the 
  cooling 
  con- 
  

   tinues 
  the 
  pressure 
  will 
  increase 
  until 
  the 
  temperature 
  of 
  maximum 
  

   pressure 
  has 
  been 
  reached, 
  or 
  until 
  the 
  pressure 
  is 
  relieved 
  by 
  escape 
  

   of 
  the 
  volatile 
  material. 
  In 
  the 
  first 
  case, 
  which 
  is 
  that 
  in 
  which 
  the 
  

   liquid 
  cools 
  under 
  a 
  crust 
  of 
  sufficient 
  weight 
  and 
  strength 
  to 
  withstand 
  

   the 
  internal 
  pressure, 
  the 
  liquid 
  will 
  solidify 
  as 
  an 
  intrusive 
  mass. 
  In 
  

   the 
  case 
  of 
  an 
  actual 
  magma 
  the 
  fact 
  that 
  water 
  has 
  a 
  critical 
  

   temperature 
  at 
  374° 
  C. 
  has 
  no 
  significance, 
  because 
  of 
  the 
  probability 
  

   that 
  enough 
  material 
  will 
  remain 
  in 
  solution 
  to 
  raise 
  the 
  critical 
  

   temperature 
  of 
  the 
  mixture 
  the 
  requisite 
  amount. 
  The 
  water, 
  

   containing 
  in 
  solution 
  residual 
  material 
  such 
  as 
  dissolved 
  gases, 
  

   boric 
  acid, 
  sulfur, 
  and 
  probably 
  some 
  alkalies, 
  will 
  be 
  available 
  for 
  

   metamorphic 
  processes. 
  

  

  In 
  the 
  second 
  case, 
  that 
  in 
  which 
  the 
  liquid 
  cools 
  under 
  an 
  incom- 
  

   petent 
  crust, 
  when 
  the 
  pressure 
  has 
  reached 
  a 
  certain 
  limiting 
  value 
  it 
  

   will 
  force 
  open 
  a 
  vent 
  for 
  itself, 
  possibly 
  giving 
  rise 
  to 
  a 
  volcanic 
  

   eruption. 
  The 
  phenomenon 
  observed 
  in 
  any 
  particular 
  eruption 
  will 
  

   depend, 
  in 
  large 
  part 
  at 
  least, 
  on 
  the 
  magnitude 
  of 
  the 
  pressure 
  and 
  on 
  

   the 
  composition 
  of 
  the 
  non-volatile 
  portions 
  of 
  the 
  magma, 
  though 
  these 
  

   factors 
  may 
  not 
  be 
  independent. 
  If 
  the 
  vent 
  is 
  a 
  fairly 
  open 
  one, 
  

   enormous 
  pressures 
  probably 
  will 
  not 
  be 
  developed 
  and 
  the 
  escape 
  of 
  

   the 
  water 
  as 
  steam 
  may 
  be 
  comparatively 
  quiet 
  ; 
  this 
  will 
  presumably 
  

   be 
  the 
  more 
  probable 
  in 
  the 
  case 
  of 
  a 
  very 
  fluid 
  lava. 
  The 
  mild 
  

   explosive 
  activity 
  of 
  Stromboli 
  and 
  the 
  yet 
  milder 
  bubbling 
  of 
  Kilauea 
  

   may 
  be 
  examples 
  of 
  this 
  type. 
  It 
  may 
  well 
  be 
  that 
  in 
  both 
  these 
  cases 
  

   the 
  activity 
  is 
  the 
  result 
  of 
  the 
  release 
  of 
  volatile 
  material 
  consequent 
  

   on 
  crystallization, 
  and 
  the 
  rate 
  of 
  release 
  of 
  the 
  volatile 
  material 
  may 
  be 
  

   regarded 
  as 
  a 
  measure 
  of 
  the 
  rate 
  of 
  crystallization 
  in 
  the 
  parent 
  body. 
  

   The 
  difference 
  in 
  violence 
  in 
  the 
  two 
  cases 
  may 
  be 
  determined 
  solely 
  

   by 
  the 
  depth 
  at 
  which 
  crystallization 
  is 
  taking 
  place, 
  and 
  by 
  the 
  size 
  

   or 
  tortuosity 
  of 
  the 
  channel 
  through 
  which 
  the 
  material 
  must 
  pass. 
  

  

  