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 • •
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 • • • •
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 • • • • •
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