Dog on ice? On water? On?

Pyroclastic Fragments, Airfall

Deposits and Cinder Cones

Explosive Eruptions

How a Volcanic Blows Its Top

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-

End up with an

Explosive Eruption

Eruption Column

Gas Thrust

Convection

Column Collapse

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

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

Fusiform Bombs

Spherical

Breadcrust

Fusiform

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

Pyroclastic Breccia- consolidated

aggregate of more than 75% blocks

• 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

Coarse Ash

Fine Tuff

• 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.

Can occur singly, but

Mostly in clusters

Death of hot spot shield volcano- on flanks,

Last eruptive material

Lava cooling, more viscous

Etna-magma and eruptive style

changing??

• 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

• 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

• Shape- central vent-

circular

• Fissure- elongate

•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

Facies of Scoria Cones

• 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