Pyroclastic
Rocks
I.G.Kenyon
Pyroclastic Rocks
Consist of fragmental volcanic
material blown into the atmosphere
by explosive activity
Mainly associated with
felsic volcanoes
Pyroclastic Rocks – 2 Main Groups
Material ejected from the volcano as
liquid globules which solidifies in the
air and is deposited as solid particles
Material ejected from the volcano
as solid fragments, this solid
material has been fractured
by the explosive activity
Materials Ejected in a Liquid State
Pelées Hair
Volcanic Bombs
Pumice
Scoriae
Pelées Hair
A fine mass of
hair-like glass
Formed by lava being
exuded through a small
orifice and blown
about by the wind
Resembles candy
floss in appearance
1cm
Volcanic Bombs
Larger masses of liquid
lava thrown into the air
They rotate and take on
characteristic shapes
Spindle-bombs and breadcrust bombs
are most common-usually vesicular
Vary in size from small droplets
to several cubic metres
Volcanic Bombs
Some bombs have a
characteristic breadcrust
surface, others resemble
cauliflowers or cowpats
depending on the way
they land and solidify.
Bombs develop a rounded or almond
shape as they are twirled through the air.
Volcanic bombs are large
fragments of molten lava up
to 1m in diameter expelled
during an eruption.
Section through a Volcanic Bomb
Highly vesicular interior
5cm
Breadcrust exterior, finer
grained and less vesicular
due to more rapid cooling
Pumice
Highly vesicular material
derived from acid lavas
Very high porosity
and low density
So light that it may
float on water
Pumice
Specimen from
Mt. Teide, Tenerife
Mineralogy: quartz,
feldspar and mica
Volcanic,
felsic igneous
rock
Low density, high
porosity, floats on water
Vesicles up to
3cm in diameter
Microscopic grain size, very rapid
cooling at the earth’s surface
2cm
Scoriae
Associated with mafic lavas
Vesicular but denser than pumice
Globules of lava are ejected and
the exterior chills and solidifies
Interior is still hot and molten
Upon landing they are still soft and
are flattened into pancake shapes
Section through Strombolian Scoriae Cone
Bedding dips 32°SE
The cone has been half excavated for
use in the construction industry
Some layers rich in
volcanic bombs
2m
Strombolian refers to the style of
pyroclastic eruption in which
fragments of incandescent,
vesiculating basaltic magma are
ejected to a moderate height, landing
as solid scoria to form a cone
Strombolian Scoriae Cone
Volcanic bombs occur
up to 50cm in diameter
The structure is very friable
and has an unstable surface
Scoriae clasts range
in size from 3 to 15cm
Material Ejected in a Solid State
Agglomerate-fragments >64mm in diameter
Lapilli – fragments 64mm - 2mm in diameter
Ash, Tuff & Dust – fragments <2mm in diameter
Agglomerate – Volcanic Breccia
Derived from agglomero
meaning ‘gather into a heap’
Formed of volcanic or country rock
in the vent or as part of the cone
Produced by explosive activity which
often shatters the top of the cone
Coarse material is ejected a relatively short
distance before settling back to earth
Comprises angular fragments >64mm
surrounded by finer tuff and lapilli
Agglomerate – Volcanic Breccia
5cm
Large angular fragments
up to 10cm in diameter
Vent
Agglomerate
Large fragments surrounded
by material of ash and lapilli size
Agglomerate–Volcanic Breccia, Arico, Tenerife
Chaotic mixture of boulders over
2m to ash <2mm in diameter
Lapilli – Particles 2 – 64mm
Derived from lapillus
meaning ‘a little stone’
Most commonly small
pea to walnut sized
Tuff – Particles <2mm
The lithified equivalent of volcanic ash
Classified according to the nature
of the pyroclastic fragments
Crystal Tuffs – composed of mainly crystals
Lithic Tuffs – composed of fragments of rock
Vitric Tuffs – composed of glassy fragments
Welded Tuffs (Ignimbrites) – hot fragments
welded together in Nuées Ardentes eruptions
The Formation of Ignimbrites
Associated with Pyroclastic Flows
Nueés Ardentes style eruptions
Glowing fireclouds 300-1000°C
Particles weld together on settling
Activity on Augustine, Alaska, photograph by M.Krafft
The Chimiche Ignimbrite, Arico, Tenerife
It is unwelded as the
particles were cool by the
time they had fallen 10-15km
through the atmosphere
back to earth
15m
This deposit covers more than 150 km2 of the ChimicheArico part of the island. It is thought to represent the
collapse of a 10-15 km high plinian eruptive column
Volcanic Ash – Unconsolidated
material <2mm in diameter
Road cutting in the Guimar Valley, Tenerife
The End