Volcanic Eruptions and Hydrovolcanism

Find the Face

Volcanic Eruptions and Hydrovolcanism

Ocean Ridges Continental Rifts

Subduction Zones: Continental Oceanic Back Arc Basins

Hot Spots Plumes

Cinder Cones Composite Volcanoes Shield VolcanoesCinder Calderas and Domes

Lava has a great resistance to flow- viscosity- it has a high viscosity water- pours- low viscosity molasses- strings- moderate viscosity Lava high- 100,000 to 200,000 times that of water

High viscosity-flows Because Is erupted at a very high temperature Heavy- 2.5 to 3.5 heavier (denser) than waterthus flows under its own weight

Viscosity decreases with Increasing Temperature But increases with increased Crystal content

Triggering Volcanic Eruptions

What triggers an Eruption? Most magma reservoirs underlying worlds active volcanoes are in a delicate state of gravitational balance with enclosing rocksneutral buoyancy Thus in this delicate state many factors can lead to an eruption

Triggering an Eruption Can divide factors into 3 processes Below chamber In Chamber Above Chamber

Below-Rise of new magma into chambercan cause over pressurization and failure of wall rocks Within-Differentiation-cooling-volatile increase; stoping-magma rises-volatile increase Above-Earth Tides-elastic rise and fall of surface up to 40cm a day Pinatubo and pressure changre

What Triggers and Eruption Pressure Change (> in magma, < in lithosphere) Earthquake Volatiles Magma Mixing-New Injections of magma Differentiation Force of M + V is > then strength of rocks in conduit, fractures, or volcanic edifice.

Tectonic Events Nearly simultaneous eruption of 2 or more volcanoes Soufriere and Pelee- 1902-May 7 and 8 Unzen and Pinatubo- 1991-June Regional stress- earth tides which fluctuate with time Full Moon-Tidal stresses

Volatiles Pressure of load of overlying rocks at the level of shallow magma chambers range from 500 bars to 2 kbars Add to this the strength of the rocks- 70-160 bars Magma pressure has to exceed this Water rich magmas can do this-volatiles coming out of solution exert a tremdous force

Volatiles 1 m 3 of rhyolite with dissolved volatiles stored in a shallow magma chamber can expand to 670 m 3 of fragmental material and gas upon reaching atmospheric pressure Can expand 1600 fold as it comes out of solution Faults-synvolcanic

Intensity and Type of Eruption Mass eruption rate-rate of flow out of vent/fissure- measure of eruption intensityheight and length; column collapse Temperature Viscosity-Composition Volatile Content: Fragmentation and vesiculation, accelerate magma out of vent Vent Geometry

VEI-Volcanic Explosivity Index Measures magnitude of volcanic eruptions Based on volume of explosive products plus height of the eruption column, duration of eruption Each successive category equals a 10 fold increase in explosive power 0 to 8

Hydrovolcanism-Trigger for Explosive Eruptions Hydrovolcanic processes involve the interaction between magma and external water This interaction may be passive or explosive Magmatic processeslittle or no interaction with external water

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

Hydrovolcanism-Water and Magma

Weak to extremely powerful

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

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)

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

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

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?

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

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

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

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

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 100m.

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 cannot escape Can generate enormous excess pressure with high explosive potential

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

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

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

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

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

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

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

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

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

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

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