Tuff Rings and Tuff Cones
Tuff Rings and Cones
• With cinder cones these are the most common
volcanic landforms
• Form in environments where erosion and
reworking are prominent
• However water in these hydrovolcanic deposts
quickly cools leading to precipitation of
numerous secondary minerals.
• These fill pore spaces between ash particles
and thus cements deposits
Precambrian Examples
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Rhinelander-Monico, Wisconsin
Mine Centre
Savant Lake
Wawa
Sturgeon Lake
Confederation Lake
Central B.C.
Mineralization
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Wisconsin-VMS-Lynn, Pelican
Mine Centre-VMS,Diamonds
Savant-VMS, Fe-Au
Wawa-Massive Pyrite
Confederation-VMS
Sturgeon-Footwall-Cinder-None, Hanging
Wall (Mattabi)-Diss. Zn-Cu
• Central B.C. Cu-Au, Fe-Au
Hyalotuffs
• Glassy ash deposits produced by
explosive hydrovolcanic eruptions
• From previous these undergo low
temperature alteration and lithification
immediately after deposition- helps them
survive- Surtsey
Water/magma ratio
• Fluctuates greatly during eruptions which build
tuff rings and cones
• Therefore its not uncommon for:
– Massive pyroclastic flow beds to alternate with thin
surge and fall beds
– Bomb and cinder beds mixed with fine ash, surge and
avalanche beds
– Vesicularity of juvenile fragments to vary widely (due
to quenching and or steam jackets)
– Peperites are not uncommon
Tuff Cones
• Small monogenetic volcano produced by
hydrovolcanic explosions in shallow water
• Tuff cones have a rim to rim width of 300-5000
m with a maximum deposit thickness at the
crater rim of 100-800m, crater floor above
ground
• Characterized by high profiles and steep slopes
(>25 degrees) composed largely of thick-bedded
pyroclastic flow and surge deposits forming from
eruption fed density currents and bomb-scoria
beds from magmatic eruption column fallout.
Tuff Rings
• Small monogenetic volcano produced by
hydrovolcanic explosions in a terrestrial
environment where shallow ground water is
present (Maars), shallow lakes, or sea shores.
• Smaller than tuff cones with deposit thickness <
100m
• Characterized by low topographic slopes ( 25
degrees), composed primarily of thin-bedded
surge and fall deposits formed by magmatic fall
and eruption fed density currents.
• Density currents turbulent, dilute.
Eruption fed density currents
Pulsating eruptions
Density Currents
Base Surges
• Variety of bed forms based on water or
steam to magma ratios.
• high steam or water will have turbulent
flow (low particle concentration) leading to
a variety of wavy-dune-like bed forms to
slurries and debris flows.
• Low water or steam, high particle
concentration will get laminar or nonNewtonian flow- massive beds
Density Currents
• Juvenile material- dominantly fine ash due
to violence of eruptions
• Scoria or pumice lapilli will exhibit a variety
of vesicularity
• Larger juvenile fragments can retain
bomb-like shapes; plastic when erupted
then encased in a steam envelope
Characteristics of Deposits
• Thin beds in tuff cones and rings
– Thickness: few mm to several cm, most <
1cm
– Form by a large number of short eruptive
pulses- Surtsey > 60 per minute
– Characterized by fine ash with the sporadic
occurrence of lapilli and blocks
Thin Beds
• Plane parallel to wavy and x-bedded (dune
forms). In general x-bedded represents
density currents where the particles are
supported by water (high w/m ratios) or
where steam is the main component.
• Plane parallel beds form from density
currents with low water/magma ratios
(steam driven?) or from airfall.
Thin Beds
• May have soft sediment deformation
features in wetter deposits
• Convolute bedding in is the most common
Folded beds between non-folded beds
Deformed layers a few mm to several cm thick, may extend for several m
Gravity sliding of wet ash
Thin Beds
• Within individual beds
sorting is poor even
though grain size is
small.
• Beds may be graded
THIN BEDS
• Beds of lapilli size
scoria/pumice occur
interlayered with fine
ash beds. Reflects
changing w/m ratios
Thin Beds
• Lapilli to bomb-size
juvenile fragments
may exhibit a peculiar
texture referred to as
cauliflower.
• similar to breadcrust
from magmatic
eruptions
Thin Beds
• Forms as gas trapped
in the interior of the
fragment expands
against the quickly
quenched outer rim.
This causes the
chilled rim to break
and gives the
fragment the look of a
cauliflower
Thin Beds
• Accretionary lapilli: rounded, oblate, or
flattened aggregates of volcanic ash which
may have a solid core (crystal,
pumice/scoria, accessory fragment)
• These exhibit a concentric structure of one
or more ash rings around the core
Thin Beds
• The concentric structure forms from the
oscilation of ash in the eruption column
moving from wetter to drier parts
• Wet ash accretes around crystal or pumice
or an ash lump- to drier part and ash sticks
to it-back to wet and so on
Clotted Lapilli- wet ash lumps stick togetherwet eruptions
Thin Beds
• Bedding sags- formed by the impact of
ballistically ejected bombs, blocks, and
lapilli upon beds capable of being
plastically deformed.
Concentric Impact
Water settling
Angled Impact- subaerial
Lack of impact sags- transport as flow- surge
Thin Beds
• Nested Blocks- large
plastic juvenile clasts
break apart on
impact- jig saw fit
Thin Beds
• Vesiculated Tuffs- Ash hot on deposition,
water boils, leaves behind cavity which
can then be filled to become an amygdule.
Massive Beds
• Composed of coarse to fine ash with
variable lithic and pumice content.
• Beds range from about 1 to 50-60 m thick
and as such may be difficult to id.
• Thick beds may be seperated by thin,
fissile, laminae of fine ash which is a few
mm to 1-2cm thick.
• Cross-bedding and soft sediment
deformational features are rare
Massive beds
• Lapilli-size juvenile fragments are
abundant and exhibit moderate to high
vesicularity
• They are poorly sorted
• May exhibit normal to inverse grading.
• Lithic or accessory fragments range from
block to lapilli-size, are poorly sorted and
may be normally graded
Beds
• Wavy-x-bedded: Turbulent, high
water/particle ratio, water supports
fragments
• Plane parallel- laminar, low water/rock
ratio
• Thin vs thick plane parallel- length of
individual eruptive pulses
Very high water/magma ratios
• Products are vent debris flows and sheet-flood flows.
Ash so wet it moves away from vent as a debris flow
• Resulting deposits range from a few cm’s to over 50m in
thickness and from 10’s of meters to km’s from source.
• Deposits are composed of coarse ash with lapilli to blocksize fragments. These compose 5-50% of the deposit,
are angular to sub angular, mtx support, and poorly
sorted.
• Juvenile fragments are poorly vesiculated and are coarse
ash to lapilli in size
• Flows are massive
• Grade laterslly into well bedded material that looks like
turbidite.
Lack of in Precambrian
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Not recognized
Topo highs, erosion
High energy environment
Shallow water, ground water
• Quick burial
• Syndepositional cementation and
compaction