Subaqueous Volcanism
Submarine Lavas
• Most abundant surficial igneous rock on earth
• Form in:
1) Mid-ocean ridges- 1-2 km thick
2) Back arc basins
3) Island Arcs
4) Hot Spots
5) Hypabassal Complexes- water saturated
seds, magma- pepperites – found adjacent
to continents or island arcs
Controls on Subaqueous Eruptions
• Composition- < 55% SiO2 is hot, relatively
fluid, erupts as fire fontains, flows, cinder
• Composition > 55%- lower temperature, >
viscosity, > volatiles, much more explosive
• Remember- Eruptions in deep waternonexplosive
Ancient Vs Modern
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Each is useful in improving our understanding
of subaqueous volcanism.
Modern:
1. Fresh and unaltered
2. See and map surface features
3. Water depth known
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Ancient:
1.
2.
3.
4.
Stratigraphic exposure- sections
Easier to work on
More and better exposures
Relationship to resources
Submarine Lavas
• Pillow Lavas- lowest eruption rates, gentle
slopes
• Sheet Flows- > eruption rate, > slope
• Lobate Flows- Ponded lava and lava lakes
with lobate flows representing spill overs
Pillow Lavas
Pillow Lava
• Most distinctive and commonly recognized
flow morphology for subaqueous basatls
and andesites
• Also most common volcanic rock is
basaltic pillow lava
• These flows represent sustained
subaqueous eruptions with low effusion
rates
• Mostly fissure fed
• Slow spreading ridges tend to have more pillow
lavas, pillows common on sea mounts
• Seafloor that is covered by pillows is topo
irregular with a relief of 1-5m, pilows tend to be
piled irregularly over one another
• Pillows also form conical piles
• Abundant on flanks of active shield and
composite Island Arc and hot spot volcanoes
Formation
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Oceans- most common
Lakes
Rivers
Sub- glacial
Fissure-fed eruption
Hot, pahoehoe, coherant
Water cooling of flow front
Lava eruption continues at same rate
Plastic, cooled front cracks in many
places
Finger-like lava lobes
sphagetti
Lava Lobe
Lobe generates individual
Pillows.
Top- equidimensional
Sides and front- elongate
Lobe- large irregular pillow
Shape and Size of Pillows
• In most outcrops this is a function afforded by a 2
dimensional exposure.
• Pillow shaped have been referred to as:
a) Bun and balloon
b) Elongate
c) Spherical
d) Amoeboid
e) Mattress
Size- few inches to several feet, highly variable
Chilled Rims- glassy
Individual pillows separated by
1) Thin glassy selvages and
quenched, granulated
spalled rim material
2) Chert, sulfides, sediments
Carbonate, epidote, chlorite
Interpillow material < 5% of a
Pillowed unit
The various shapes and sizes of
observed pillows are consistent
with pillows representing a
branching, intertwined mass of
Interconnected tubes that appear
as separate, isolated pillows in
two dimensions
Broken and Re-entrant selvages:
Identical to chilled rims but
re-entrant selvage occurs within the pillow
as projections roughly perpendicular
to pillow marginflow direction and budding.
Broken selvages- budding and flow
direction
Gas Cavities- trapped pockets
of gas
Flat floors, round tops
Upper third of pillow
Stacked gas cavities
Radiating Vesicles (amygdules)
All around pillow
Top of pillow only
Curved Top
Flat Floor
Pillow Shelves: drop in lava
Level in lobe- water-quenches
Stacked- lava pulses
Multiple Pillow Rinds- rapid growth,
Shallow water, quenching and breaking
Glassy edge
Imploded Pillows: Water pressure
On brittle crust- pressure
Difference due to shrinkage of
Exsolved gas phase
Onion Skin Fractures- Cooling Textures and shrinkage
Pillow Facies- General
Sheet Flows
• Most common at fast spreading centers
• Resemble pahoehoe in surface textures
and are tube feed
• No difference in composition between
these and pillows
• Difference due to eruption rates, topo,
temperature of eruption and thus viscosity
• Sheet can change into pillows away from
vent
Sheet Flows
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Hyaloclastite-Bx base (thin)
Massive (thick) center
Ropey flow tops with or without
Bx/hyaloclastite top (thin) (3cm to 2m)
Centers massive, fine-med. Gr., if no
vesicles hard to tell from sills.
• Can get fingers of massive lava extending
into bx/hyaloclastite-Pepperite
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Ponded Lavas and Lobate Flows
Fast spreading ridges- lavas pond or lakes which
represent dozens of stacked lobate flows
High eruption rates, decreased viscosity
In the ponds lava can drain away leaving piles of
collapsed rubble of the lobate flows that formed the
ponds
Individual lobate flows represent spill overs from
ponds
Forms semi-rigid crustwhich is carried along and
atop advancing flow-hummocky tops
Tends to have lots of drain back shelfs, hollow
flows
Limu O Pele(Kind of fragment)
• Found in subaerial pahoehoe and
subaqueous sheet flows.
• Not found in shallow water pahoehoe• Thus depth indicator-800-3000m?
• Represents entrapment of water in a
flowing lava with steam generation and
expansion.
• Steam bubble expands rapidly enough to
burst into fragments
Limu O Pele
• In submarine envirnment limited bubble
expansion
• Get bubble wall shards
Limu O Pele
• Get massive flow with fragmental zones in
it, may extend through lava if thin enough
• Or massive flow with pockets of fragments
at base
• Key is fragment shape!
Other features of Pillows, Sheet and
Lobate Flows
1) Varioles- Devitrification Spots
Hyaloclastites
• Occurrence- Pillow Selvages, Tops of Lobes,
Tops and Bottoms of Sheet Flows, Collapse
Pits, Pillow Breccias and Self-Peperites, Direct
Venting. Recently hyloclastite
• Definition: Felsic or mafic, formed by quenching
and shattering of hot lava coming into contact
with external water.
• Old Term- Palagonite Tuff- not pyroclastic
• Hyaloclastite >’s as water depth decreases
Hyaloclastite
• Recent submersible dives have found
“sheet” hyaloclastites
• Occur on flat topped lava flows-no
reworking.
• Submarine fire fountaining- sand-size
hyaloclastite granules
• Most common on slow spreading ridges,
seamounts, arc volcanoes, hot spot
volcanoes
• Rare on fast spreading ridges- sediment
lacking
• Hyaloclastite >> as water depth <<
Characteristics: Sharp right angle corners, planar surface, lack
of abundant convex and concave shapes (pyroclasts), breaks
Across gas bubbles. Size- < 1 to about 10mm. Smaller in
Shallower water-more vigorous interaction because of gas
Exsolving.
Pillow-Hyaloclastite Deltas
• Lava from land into sea or lake, above
glacial ice and flow into meltwater
• Massive into pillow fingers surrounded by
hyaloclastite
• Mark old shore lines
Pillow Breccias
• Flow Foot- Auto Breccia
• Debris Flows
• Self Peperites (dykes with peperitic
margins common in pillow breccias)
Debris Flows and Avalanches
Pillow pieces: Rim and core
Core only
Angular-blocky
Matrix- hyaloclastite ?
Self-Peperites: Lava Lobes that intrude into there own
Unconsolidated hyaloclastite. Hyaloclastite comes form direct
Quenching at fissure, pillow rims, top of lobe and pillow flows
Peperites
• Intrusion of hot lava into wet sediment or ash
(pyroclastic material), or hyaloclastite, or
subaqueous, unconsolidated debris flows.
• Leads to quenching and fragmentation of the
lava, disagregation, quenching, and vesiculation
of the sediment/ash and water
• Can also lead to powerful fuel-coolant explosive
eruptions
• Deeper water tend to get large peperitehypabyssal complexes
Hypabyssal Complexes
• Hypabyssal complexes occur where you
have thick, unconsolidated sediments.
• Occur here because density of rising
magma is greater than seds (water
saturated and unconsolidated)
• Magma thus spreads laterally within seds
to form sed-sill complexes
• Also can initiated mass flow of seds on
flanks of marine volcanoes
PEPERITES
• Definition: A genetic term applied to a rock formed essentially
in situ by disintegration of magma intruding and mingling with
unconsolidated or poorly consolidated, typically wet
sediments, hyaloclastite or ash.
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Importance:
– Provides field evidence for mechanisms of magma-water/sediment
interaction including FCI
– Information on vent processes relevant to Surtseyan eruptions
– Important in paleoenvironmental reconstruction and sequence mapping
for it demonstrates contemporaneity of magmatism and sedimentation
or subaqueous deposition of ashy material.
– Hydrothermal alteration and mineralization, fluid flow around
synvolcanic intrusions.