Subaqueous Volcanism

Submarine Lavas

• Most abundant surficial igneous rock on earth• Form in:

1) Mid-ocean ridges- 1-2 km thick2) Back arc basins3) Island Arcs4) Hot Spots5) 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 water-nonexplosive

Ancient Vs Modern• Each is useful in improving our understanding

of subaqueous volcanism.• Modern:

1. Fresh and unaltered2. See and map surface features3. Water depth known

• Ancient:1. Stratigraphic exposure- sections2. Easier to work on3. More and better exposures4. 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

• 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 manyplaces

Finger-like lava lobes

sphagetti

Lava LobeLobe generates individualPillows.

Top- equidimensional

Sides and front- elongate

Lobe- large irregular pillow

Lobe generates individualPillows.

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 balloonb) Elongatec) Sphericald) Amoeboide) Mattress

Size- few inches to several feet, highly variable

Chilled Rims- glassy

Individual pillows separated by

1) Thin glassy selvages andquenched, granulatedspalled rim material

2) Chert, sulfides, sedimentsCarbonate, epidote, chlorite

Interpillow material < 5% of aPillowed unit

The various shapes and sizes ofobserved pillows are consistent with pillows representing a branching, intertwined mass ofInterconnected tubes that appearas separate, isolated pillows in two dimensions

Broken and Re-entrant selvages:

Identical to chilled rims butre-entrant selvage occurs within the pillowas projections roughly perpendicularto pillow margin-flow direction and budding.

Broken selvages- budding and flowdirection

Gas Cavities- trapped pocketsof gas

Flat floors, round tops

Upper third of pillow

Stacked gas cavities

Radiating Vesicles (amygdules)

All around pillow

Top of pillow only

Pillow Shelves: drop in lavaLevel in lobe- water-quenchesStacked- lava pulses

Flat Floor

Curved Top

Multiple Pillow Rinds- rapid growth,Shallow water, quenching and breakingGlassy edge

Imploded Pillows: Water pressureOn 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

• 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

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, lackof abundant convex and concave shapes (pyroclasts), breaksAcross gas bubbles. Size- < 1 to about 10mm. Smaller inShallower 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 coreCore only

Angular-blocky

Matrix- hyaloclastite ?

Self-Peperites: Lava Lobes that intrude into there ownUnconsolidated 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 peperite-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

Hypabyssal Complexes

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.

• 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.