This document provides information about volcaniclastic rocks and pyroclastic materials. It defines pyroclastic rocks as being composed of volcanic materials and volcaniclastic rocks as pyroclastic materials that have been transported and reworked. Pyroclastic materials include volcanic bombs, lapilli, ash, and ignimbrites. These materials are classified based on size. Volcaniclastic rocks include agglomerates, tuffs, and other deposits that form from pyroclastic flows and ash falls.

1. Volcanoclastic rocks: on land and marine
NAME: MIHIR CHAUDHARY
M.SC. SEM 4
ROLL NUMBER:02
PAPER 509
GUIDED BY-DR. RAHUL GAYAKWAD SIR
M.G. Science Institute
(Geology Department)
Navrangpura,Ahmedabad,Gujarat 380009
mihirchaudhary1611@gmail.com

2. VOLCANICLASTIC ROCKS
• Pyroclastic rocks or pyroclastics (derived from the Greek:
meaning fire; and, meaning broken) are clastic rocks composed
solely or primarily of volcanic materials.
• Where the volcanic material has been transported and reworked
through mechanical action, such as by wind or water, these rocks
are termed volcaniclastic.

3.  Pyroclastic rocks may be a range of clast sizes, from the largest
agglomerates, to very fine ashes and tuffs.
 Pyroclasts of different sizes are classified as volcanic
bombs, lapilli, and volcanic ash. Ash is considered to be
pyroclastic because it is a fine dust made up of volcanic rock.
 One of the most spectacular forms of pyroclastic deposit are
the ignimbrites, deposits formed by the high-temperature gas-
and-ash mix of a pyroclastic flow event.

4. CLASSIFICATION
Clast size Pyroclast Mainly
unconsolidated:
Tephra
Mainly
consolidated:
Pyroclastic rock
> 64 mm Block, bomb Agglomerate Agglomerate,
pyroclastic
breccias
< 64 mm Lapillus Layer, lapilli tephra Lapilli tuff,
lapillistone
< 2 mm Coarse ash Coarse ash Coarse (ash) tuff
< 0.063 mm Fine ash Fine ash Fine (ash) tuff

5. AGGLOMERATES
 Agglomerates (from the Latin 'agglomerare' meaning 'to form
into a ball') are coarse accumulations of large blocks of volcanic
material that contain at least 75% bombs.
 These bombs were viscous at the moment of ejection and by
rotation in the air acquired their shape. They are commonly 1 to 2
feet (30 to 60 cm) in diameter, but specimens as large as 12 feet
(3.7 m) have been observed.
 Agglomerates are typically found near volcanic vents and within
volcanic conduits, where they may be associated with pyroclastic
or intrusive volcanic breccias.

6. AGGLOMERATES
Vertical sections through beds of tephra which have built up on top of the andesite lavas.

7. TUFFS
 Tuff (from the Italian tufo) is a type of rock consisting of consolidated volcanic
ash ejected from vents during a volcanic eruption.
 Tuff is sometimes called tufa, particularly when used as construction material,
although tufa also refers to a quite different rock. Rock that contains greater than 50%
tuff is considered tuffaceous.
Welded tuff from Bandelier National
Monument, New Mexico
18.5 million-year-old tuff exposed at Hole In The
Wall, Mojave National Preserve, California.

8. VOLCANIC BOMBS
 A volcanic bomb is a mass of molten rock (tephra) larger than 64 mm (2.5 inches)
in diameter, formed when a volcano ejects viscous fragments of lava during an
eruption.
 They cool into solid fragments before they reach the ground. Because volcanic
bombs cool after they leave the volcano, they do not have grains making
them extrusive igneous rocks.

9. TYPES OF BOMBS
Bombs are named according to their shape, which is determined by the fluidity
of the magma from which they are formed.
 Ribbon or cylindrical bombs form from highly to moderately fluid magma,
ejected as irregular strings and blobs.
 Spherical bombs also form from high to moderately fluid magma. In the case
of spherical bombs, surface tension plays a major role in pulling the ejecta
into spheres.

10. TYPES OF BOMBS
 Spindle, fusiform, or almond/rotational bombs are formed by the same
processes as spherical bombs, though the major difference being the partial
nature of the spherical shape.
 Cow pie bombs are formed when highly fluid magma falls from moderate
height; so the bombs do not solidify before impact (they are still liquid when
they strike the ground).
 Bread-crust bombs are formed if the outside of the lava bombs solidifies
during their flights.
 Cored bombs are bombs that have rinds of lava enclosing a core of previously
consolidated lava.

11. Volcanic bomb found in the Cinder Cones
region of the Mojave National Preserve.
Fusiform lava bomb. Capelinhos
Volcano, Faial Island, Azores.

12. LAPILLI
 Lapilli is a size classification term for tephra, which is material that falls out
of the air during a volcanic eruption or during some meteorite impacts. Lapilli
(singular: lapillus) means "little stones" in Latin.
 By definition lapilli range from 2 to 64 mm (0.08 to 2.52 in) in diameter.
A pyroclastic particle greater than 64 mm in diameter is known as a volcanic
bomb when molten, or a volcanic block when solid.
 Lapilli tuffs are a very common form of volcanic rock typical
of rhyolite, andesite and dacite pyroclastic eruptions. Here, thick layers of
lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which
remain in ancient terrains are formed by the accumulation and welding of
semi-molten lapilli into what is known as a welded tuff.

14. ARMOURED(OR CORED)LAPILLI
 This lapilli is a variety of accretionary lapilli, though it contains lithic or
crystal cores coated by rinds of coarse to fine ash. Armoured lapilli only form
in hydroclastic eruptions, where significant moisture is present. The vapour
column contains cohesive ash which sticks to particles within it.

15. VOLCANIC ASH
 Volcanic ash consists of fragments of pulverized
rock, minerals and volcanic glass, created
during volcanic eruptions, less than 2 mm (0.079 inches) in
diameter.
 The term volcanic ash is also often loosely used to refer to all
explosive eruption products (correctly referred to as tephra),
including particles larger than 2mm. Volcanic ash is formed during
explosive volcanic eruptions when dissolved gases
in magma expand and escape violently into the atmosphere.

16. Ash cloud from the 2008 eruption of Chaitén
volcano stretching across Patagonia from
the Pacific to the Atlantic Ocean.
Ash plume from Mt Cleveland, a strato
volcano.

17. PYROCLASTIC MATERIAL
The pyroclastic material erupted from an explosive volcanic eruption may be
ejected as fragments resulting following:
• Scoria cones or cinder cones
• Lava tubes
• Ash falls
• Ash flows
• Lahars

18. SCORIA/CINDER CONES
 A cinder cone or scoria cone is a steep
conical hill of tephra (volcanic debris)
that accumulates around and downwind
from a volcanic vent.
 The rock fragments, often
called cinders or scoria, are glassy and
contain numerous gas bubbles "frozen"
into place as magma exploded into the
air and then cooled quickly.
Cinders from
a Pleistocene cinder cone, San
Bernardino Valley, south
eastern Arizona.

19. LAVA TUBES
 A lava tube is a natural conduit
formed by flowing lava which
moves beneath the hardened
surface of a lava flow.
 Tubes can be actively draining
lava from a volcano during an
eruption, or can be extinct,
meaning the lava flow has
ceased and the rock has cooled
and left a long, cave-like
channel.
Thurston Lava Tube in Hawaii Volcanoes National Park, Hawaii. The step mark, more visible on
the right wall, indicates the depth at which the lava flowed for a period of time.

20. ASH FALLS
 The finer material of volcanic
eruption are ejected into the air,
eventually falling to the ground to
form a layer of ash.
 The extent of the ash flow depends
upon the height that the ash ejected
along with the speed and direction of
the prevailing wind at the time.
 Most ash cloud are only ejected 1 km
into the air and fall back onto the
volcanic cone.
ROMAN CITY UNDER ASH
FALL

21. LAHARS
 A lahar is a type of mudflow or debris
flow composed of
a slurry of pyroclastic material, rocky
debris, and water. The material flows
down from a volcano, typically along
a river valley. Lahars are extremely
destructive: they can flow tens of
metres per second, be 140 metres
(460 ft.) deep, and destroy any
structures in their path. Notable lahars
include those at Nevado del
Ruiz and Mount Pinatubo, each of
which killed thousands of people.

22. DIAGENETIC DERIVATIVES
 Many consolidated pyroclastic rocks, which are mixture of volcanic and
detrital or calcareous material have acquired secondary cleavage forming
pyroclastic or ashy slates; sometimes develops new mineral.
 The silica release from these changes produces an extremely hard and tough
slate.
 The Pyroclastic deposits which have been subjected not merely lithification or
re-crystallization but have undergone fundamental chemical changes.
 Secondary silicification affects many volcanic ashes producing extremely hard
rocks such as halleflinta.
 Some of the remarkable modifications of pyroclastic deposits result form the
devitrification or hydration of pumice tuffs.

23. BENTONITE
 Bentonite is an argillaceous rock of peculiar character, unweather, it is
light green or pale greenish yellow in color and has a fracture like that
of hard wax.
 Bentonite absorbs water and many other substances such as organic
dyes in quantities up to about 8 times its own original volume.
Another property is its marked capacity for base-exchange.
 Bentonite consists of montmorillonite or closely allied clay-minerals
in minute crystals. Most samples also contain particles of orthoclase,
plagioclase and biotite and some accessory mineral.
 Bentonite if found in the coastal areas of Kachchh mainland in
Gujarat.

25. FULLER’S EARTH
 Fuller’s earth is an argillaceous rock or clay, which like
Bentonite, has strong powers of adsorption of water, coloring
matters, and especially grease and certain oils.
 For this reason it was formerly used for cleaning and whitening
wools, and is now used for decolorizing clarifying oils.
 Fuller’s earth consist essentially montmorillonite and it is
originated as a result of devitrification of volcanic glass.
 Microscopic structure and mineral constituents shows that they
may be formed due to in situ devitrification of water-laid
pumiceous tuffs.

27. References
 https://www.google.com/search?q=volcanic+bombs&sxsrf=ALeKk009Y1EKZdhiRk
gTyQPa4DyZsz259w:1582171041157&source=lnms&tbm=isch&sa=X&ved=2ahU
KEwj0-
Ia3nt_nAhXjV3wKHeHxDHwQ_AUoAXoECBQQAw&biw=1366&bih=667#imgrc=a
Mqq5O3QokSMuM
 https://www.mindat.org/min-49482.html
 https://pubs.geoscienceworld.org/gsa/geology/article-
abstract/34/8/677/129626/Primary-volcaniclastic-rocks?redirectedFrom=PDF
 https://en.wikipedia.org/wiki/Pyroclastic_rock

28. THANK YOU