Weak to extremely powerful

Transcription

1 Hydrovolcanism Thus there is a continuum between magmatic and hydrovolcanic processes And thus between subaerial and subaqueous volcanism-hydrovolcanic processes active in both environments Continuum represent by water- Its presence or absence during an eruption Amount of water compared to quantity of magma

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3 Weak to extremely powerful

4 Products of Hydrovolcanism Passive or thermal shocking Pillow Lavas Hyaloclastites Self Peperites Lobe Lavas Explosive- deposits called Hyalotuffs Pyroclastic flows and surges Ash showers Lapilli Falls Debris flows

5 Hydrovolcanic Explosions Governed By: A) Dynamics of the water to magma interaction: 1) water/magma ratio 2) confing pressure 3) vent geometry 4) state of the water (liquid, vapor, mix)

6 B) Composition of the magma 1) chemical composition 2) viscosity 3) volatile content C) Volatile fragmentation depth D) Water and sediment depth

7 Volatile Fragmentation Depth Water depth at which volatiles will exsolve from the magma and expand rapidly enough to cuase rapid bubble burst - This has the effect of traring the magma apart and triggering an explosive eruption The VFD is different for magmas of different composition

8 VFD Tholeiitic basalt: Less than 100 m Calc alkaline basalt: less than 200 m Calc alkaline andesite: less than 400 m Alkalic basalt? Calc alkaline-tholeiitic rhyolite: less than 400 to 1000 m?

9 VFD s less than these instantaneous expansion, not just exsolution Gas bubbles compose > 50-70% of magma prior to explosive eruption To get this need shallow water depths

10 Water-Magma Interactions Heat-thermal energy of the magmais rapidly transported into the fluid encountered Kinetic energy of these kind of explosions very high compared to dry (magmatic volotiles only) eruptions. Important boundary condition:

11 Water-Magma Interactions Depth at which magma encounters water when water heated to 100c at a pressure of 1 kb (3km) volume increases only by a factor of 6. At earth s surface steam has a volume that is 2000 times larger than water

12 Water-Magma Interactions Because the volume ratio of steam to water increases with decreasing pressure and therefore depth explosions are most effective in the upper 300m, especially in the upper 100.

13 Water-Magma Interactions When lava flows into water explosive reactions are rare Its only when water and magma come together in an enclosed space, where magma and water vapor canot escape Increasing volume of water cannot escape and it will mix with water Can generate enormous excess pressure withhigh explosive potential

14 Hence need closed or partially closed system, at least emparily Pressure cooker valve At a temperature of 100c and pressure of 1 atmos the ratio of thermal expansion is 15bars/1c; Maean when water becomes heated and its volume remains constant the pressure rises by15 bars for each 1 degree of heating

15 Water and Sediment Depth Magma approaches the surface in dykes which advance by crack propagation/faults Thick piles of unconsolidated sediment is not rigid enough to crack Rising magma may stall out in the sediments Heat of magma boils pore water- shallow get explosive eruption Deep water boils and expansion pushes grains apart-viscous fluid- allows magma to spread laterally- peperites-hypabyssal complexes

16 Hydrovolcanic Processes Produce Deposits and Volcanic Landforms Governed by Water Depth Water/Magma ratio State of water Volatile Content of Magma Chemical Composition of magma

17 Reactions that occur when water and magma interact are divided into 2 major processes Fuel-Coolant (FCI)-Contact- Surface Steam Explosivity Bulk Interactive explosivity

18 FCI Fuel is the magma and it is at a temperature greater than the boiling point of the coolant Coolant is the external water Interaction vaporizes the coolant and chills or quenches the fuel

19 FCI Vaporization commonly occurs at explosive rates (flashing of water to steam) and rapidly converts thermal to mechanical energy Pulsating Eruptions or episodic Abundant water-milder more continuous eruptions

20 BSE Water is trapped close to the magma Water, along with unconsolidated sediment or pyroclastic material, is engulfed in the magma Heat of the magma converts water to steam and resultant expansion tears magma apart

21 Any cooled magma will be torn apart by the associated shock waves

22 Non-Explosive Fragmentation Cooling contraction granulation- higher water/magma ratios Occurs at any water depth Hyaloclatites Planar surfaces, right angle corners, 0 to 20% vesicles Smooth surface flows generate less hyaloclatite than rough surface flows

23 Volcanic Eruptions and Deposits Can, in a general way, be classified on the bases of Water/magma ratio Rate of interaction Volatile content of the magma

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