Subaqueous Volcanism

Transcription

1 Find the Face

2 Subaqueous Volcanism

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4 Submarine Lavas Most abundant surficial igneous rocks on earth Form in: 1) Mid-ocean ridges- 1-2 km thick 2) Back arc basins 3) Island Arcs 4) Hot Spots 5) Hypabasal Complexes- water saturated seds, magma- pepperites found adjacent to continents or island arcs

5 Controls on Subaqueous Eruptions Composition- < 55% SiO2 -hot, relatively fluid, erupts as fire fountains, flows, cinder lumps Composition > 55%- lower temperature, > viscosity, > volatiles, much more explosive Remember- Eruptions in deep waternonexplosive

6 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 Ancient: 1. Stratigraphic exposure- sections 2. Easier to work on 3. More and better exposures 4. Relationship to resources

7 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

8 Pillow Lavas

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10 Pillow Lava Most distinctive and commonly recognized flow morphology for subaqueous basalts and andesites Also most common volcanic rock is basaltic pillow lava These flows represent sustained subaqueous eruptions with low effusion rates Mostly fissure fed

11 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

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14 Formation Oceans- most common Lakes Rivers Sub- glacial

15 Fissure-fed eruption Hot, sheet flow, coherent Water cooling of flow front Lava eruption continues at same rate Plastic, cooled front cracks in many places Finger-like lava lobes spaghetti

16 Lobe generates individual Pillows. Lava Lobe Top- equidimensional Sides and front- elongate Lobe- large irregular pillow

17 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

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21 Lobe

22 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

23 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

24 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

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28 Gas Cavities- trapped pockets of gas Flat floors, round tops Upper third of pillow

29 Stacked gas cavities

30 Radiating Vesicles (amygdules) All around pillow Top of pillow only

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32 Pillow Shelves: drop in lava Level in lobe- water-quenches Stacked- lava pulses Curved Top Flat Floor

33 Drain Outs- Pillow or Lobe cracks- lava drains away leaving Hollow pillow

34 Multiple Pillow Rinds- rapid growth, Shallow water, quenching and breaking Glassy edge

35 Imploded Pillows: Water pressure On brittle crust- pressure Difference due to shrinkage of Exsolved gas phase

36 Onion Skin Fractures- Cooling Textures and shrinkage

37 Pillow Facies- General

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40 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

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42 Sheet Flows Hyaloclastite-bx base (thin) Massive (thick) center Ropey flow tops (thicker) with or without Bx/hyaloclastite top (thin) 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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44 Ponded Lavas and Lobate Flows Fast spreading ridges- lavas pond or lakes, rift-fill which represents dozens of stacked sheet flows High eruption rates, decreased viscosity In the ponds lava can drain away leaving piles of collapsed rubble. Individual lobate flows represent spill overs from ponds Forms semi-rigid crust which is carried along and atop advancing flow-hummocky tops Tends to have lots of drain back shelfs, hollow flows

45 Feeding Tubes (dikes) and hollow Lobate flows

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47 Other features of Pillows, Sheet and Lobate Flows 1) Varioles- Devitrification Spots

48 2) Varioles: Immiscible Droplets

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50 Amygdules- Size and % - relative water depth

51 Glomerophyric

52 Hyaloclastites Occurrence- Pillow Selvages, Tops of Lobes, Tops and Bottoms of Sheet Flows, Collapse Pits, Pillow Breccias and Self-Peperites, Direct Venting. 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

53 Hyaloclastite Recent submersible dives have found sheet hyaloclastites Occur on flat topped lava flows-no reworking. Submarine fire fountaining- sand-size hyaloclastite granules

54 Most common on slow spreading ridges, seamounts, arc volcanoes, hot spot volcanoes Hyaloclastite >> as water depth <<

55 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.

56 5 mm

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59 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

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62 Pillow Breccias Flow Foot- Auto Breccia Debris Flows Self Peperites (dykes with peperitic margins common in pillow breccias)

63 Flow-Foot Pillow Bx

64 Debris Flows and Avalanches

65 Pillow pieces: Rim and core Core only Angular-blocky Matrix- hyaloclastite?

66 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

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71 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

72 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

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74 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.

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76 Gross characteristics of peperite domains

77 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

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79 Fluid shapes to pillows Bomb-like fragments Quenched and shattered fragments

80 Quenched and shattered fragments

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84 Basalt lava intruded into debris flow

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