Pyroclastic Fragments and Cinder Cones by mercy2beans120

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									Pyroclastic Fragments, Airfall
     and Cinder Cones
Explodes Out Because
     It’s the Gas- You Know
• Like Pepsi Magma has a lot of Gas
  Dissolved in it- Sulfur. Chlorine, water,
  carbon dioxide, fluorine- main ones
• Like Pepsi, dissolved in magma when
  under pressure
• Take the cap off, release pressure, and
  gas starts to come out of solution
  As the magma moves closer to
the surface pressure gets less and
               less

 Gas comes out faster and faster
 So fast tears the magma apart
              and-
               Pyroclasts
• Pyroclastic fragment: This is an “instant”
  fragment produced directly from volcanic
  processes.
• Hydroclastic fragment: Pyroclasts formed
  from steam explosions at magma-water
  interfaces. Also by rapid chilling and and
  mechanical granulation- hyaloclastites
         Volcanic Fragments
• Alloclastic: disruption of pre-existing volcanic
  rocks by igneous processes beneath the earth’s
  surface

• Volcaniclastic: All clastic volcanic materials
  formed by any process of fragmentation,
  dispersed by any transporting agent, deposited
  in any environment.

• Tephra: pyroclastic accumulations without
  regard to size.
   Kinds of Pyroclastic Ejecta
• Juvenile: also called essential, derived
  directly from erupting magma.
• Cognate: also called accessory,
  fragmented co-magmatic volcanic rocks
  from previous eruptions of the same
  volcano
• Accidental: Derived from the subvolcanic
  basement- can be of any composition
Pyroclasts are named according to grain size
• Blocks and bombs- (>64mm in diameter)

• Bombs- clots of magma that are partly or entirely
  plastic when erupted. Dominantly basalt.
• Shape of most bombs are determined by the
  initial fluidity of the magma.
• Other factors include length and duration of flight
  through the air, rate of cooling, rate of expansion
  of vesicles, deformation on impact
• Whole range of terms for bombs
                         Agglutinate (pyroclasts
                          so hot they readily
                         Stick together)- no size
                         conotation




Agglomerate (> 75%
Bombs welded together)
• Blocks- fragments of solid rock- angular or
  blocky.
• Different terms:
  – Cognate or accessory- formed by previous
    eruptions of same volcano
  – Plutonic cognate- coarse grained margins of
    magma chambers- calderas
  – Accidental- subvolcanic crust, rare mantle
• Lapilli- name for gravel-like or cinder-like
  pyroclasts (2-64mm in diameter)
• May be accessory, juvenile, or accidental
Lapilli-stone – Accumulations of lapilli
Sized pyroclasts with > than 75% lapilli

Lapilli-tuff- Mixture of ash and lapilli size
Material where ash forms 25-75%
• Ash-size pyroclasts: (most common size)
  Pyroclasts < 2mm in diameter. Term tuff-
  consolidated equivaent of ash. Divided
  into:
  – Coarse ash (11/16 to 2mm)
  – Fine ash- smaller
• Scoria- Mafic
  vesicular juvenile
  fragments.
  Vesicularity ranges
  from 30 to 90%.
• Cinder- used for
  lapilli-size scoria
• Pumice- Felsic scoria
• Vitric Ash: From
  shattering of scoria
  and/or pumice.
  Typically ash-size
  pieces are curved,
  crescent-shaped,
  triangular pieces of
  glass
              Airfall Deposits
• Pyroclasts that fall out of an eruption column or
  are ballistically ejected
• Mostly ash size plus lapilli bombs
• Mantle topography, bedded
• Wind dependent
• Coarser closest to vent, finer further away
• Ash can travel 1000’s of miles
• Climate changers if ash gets into stratosphere
        Cinder or Scoria Cones
           (Pumice Cones)
• Most common volcano type on land
• High mounds (cone shaped) with small
  craters at the top. Little or no water in the
  eruption- magmatic
• Dominantly mafic and basaltic-low
  viscosity magma
• Mild explosions-sprays to fire fountains
• Usually occur in groups or fields
• Monogenetic Volcanoes
Cone grows from thousands of recurring jets of molten to plastic lumps of
Magma which has been disrupted by mild explosions
The plastic lumps cool to solids with different
 shapes, though most are “clinker” like
(rounded edges). Similar to industrial slag
or “cinder” produced In foundries.
They are vesicular, dominantly lapilli and rough surfaced.
• Conical shape due to fact
  largest fragments and
  largest proportion of
  fragments of all sizes fall
  close to vent or fissure,
  so hill is highest close to
  the vent and decreases
  away from it.
• Angle of slope = angle of
  repose-30 degrees
• Crater filled in by debris
  reworked from top of the
  cone rim
• Young cinder cones consist almost wholly of loose
  material
• With time this becomes cemented by groundwater or
  hydrothermal waters
• Overall the cone is composed of successive showers
  (layers) of fragments which end up being draped, one
  over the other- mantle bedding
• Outer portions- layers regular and continuous
• Inner portions- discontinuous, lenses like because of
  succeeding explosions and slumping
• Same with very outer layers- slumps and slides
• Individual cinder fragments vary from an
  inch to several feet in diameter, most are
  lapilli size
• Over entire cone fragments often relatively
  uniform in size- uniform explosiveness of
  the entire eruption
• Usually find some large bombs embedded
  in cinder-fusiform and cylindrical- craters
  or pits
                        Facies of Scoria Cones

•Inner Crater: Lower part-agglutinate
(welded spatter), feeder dyke, flows

•Upper Crater: Scoria beds, bombs

•Outer Wall: Lower-Scoria and accidental
Fragments

•Upper- scoria beds and bombs

•Talus Slopes
• Cinder cones range in
  height up to 1500
  feet- not much height-
  L of repose
• However can reach
  such a height in a
  week or month-
  Paricutin-1200 feet
  within 2 months, 450
  in the first week

								
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