Pyroclastic eruptions and their deposits Based on power point lectures by Wendy Bohrson

1. Pyroclastic eruptions and their deposits Based on power point lectures by Wendy Bohrson

2. Introduction • Explosive volcanism involves transfer of fragmented volcanic material (+gases and lithics)from depth onto Earth’s surface. • Systems of transport and deposition distinguished for three majors types of pyroclastic deposits: fall, flow, surge. • Transport and deposition function of characteristics: particle trajectory, solids concentration, extent to which concentration fluctuates with time, presence/absence of cohesion.

3. Review of fragmentation • Rising magma can begin to fragment when bubble volume reaches 65-70 volume percent. • Fragmentation can occur because bubbles become over-pressured and burst • Can also occur because melt film between bubbles is so thin that they act as brittle materials. Thin films burst when stress exceeds their strength.

4. Review of fragmentation • When bubbles burst, the material changes from a mixture of bubbles in a continuous stream of melt to droplets of melt in a continuous stream of gas. • Changes drastically the viscosity and density of mixture. • Mixture accelerates up the conduit (can reach supersonic speeds) • Mixture of gas and particles that exits eruption conduit is called an eruption column.

5. Eruption Column Eruption column defined as: • Droplets of melt (molten) and quenched melt (glass particles) • Crystals • Country rock/Wallrock (lithic fragments) • All dispersed in a continuous gas phase

6. Eruption Column: General Overview • Mixture erupted out of conduit/vent/crater vertically or laterally (sub-vertically) at velocities up to several hundred m/s. • Initially, density of mixture is greater than surrounding atmosphere. • As material is thrust upward, incorporates (mixes with) cooler, surrounding air into column. • Atmosphere heats up and the density of mixture becomes lower than surrounding atmosphere. • Eventually, mixture has same density as surrounding atmosphere/air. • Friction at outer boundaries (between air and column causes some gravitational fallout of particles).

7. Parts of the Eruption Column • Gas thrust region • Convective ascent region • Umbrella region

8. Parts of the Eruption Column: More Detail • Mixture of pyroclasts and gas jetted 102-103 of meters into atmosphere by initial acceleration • Nozzle velocity defined as maximum velocity to which pyroclasts+gas can be accelerated by expansion of magmatic gas. • 100 m/s for Strombolian/Hawaiian to >600 m/s for Plinian • Nozzle velocity controlled by mainly by volatile content in magma, which controls explosive pressure ni fragmentation zone. • Jet/Gas thrust phase typically extends up to several km above vent; column width narrow. Gas Thrust/Jet Region

9. Parts of the Eruption Column: More Detail Convective Ascent Region • Because gas thrust region is highly turbulent, cool surrounding air mixed into column. • Air is heated and resulting expansion decreases bulk density of mixture. • Transition occurs when bulk density less than that of surrounding atmosphere. • Forces driving motion dominated by buoyancy and mixture rises (hot air balloon). • Mixture rises as an convective eruption column or plume. • Width of column increases.

10. Parts of the Eruption Column: More Detail • Convective part can rise 10s of km upward. • Vertical velocities of plume vary from 10-100 m/s. • Velocity function of source conditions. • Velocity maxima reached in core of plume. • At edges, particles encounter velocities that are insufficient to keep particles aloft. • Some fall back to surface (more in a minute on this topic). Convective Ascent Region

11. Parts of the Eruption Column: More Detail • Density of atmosphere decrease with height. • Thus convective part of plume will eventually reach a level of neutral buoyancy. • Buoyancy no longer the driving force: plume will start to move laterally at a level Hb. • Excess momentum will carry some particles higher. Top of plume is Ht. • Lateral movement forms the distinctive mushroom or umbrella-shaped region. Umbrella Region

12. Transformation to Tephra Fountain • When jet does not incorporate enough air into the mixture to maintain buoyancy, rising jet will decelerate until height where velocity reaches zero. • Plume density in all (or part of) the column greater than atmosphere, particles will fall back to surface. • Reflects column collapse. • Jet transforms into tephra fountain. • Leads to formation of pyroclastic density currents.

13. Eruption Columns and Plumes

14. Rabaul, 1994 • On the morning of September 19, 1994, two volcanic cones on the opposite sides of the 3.8 mile (6 km) Rabaul caldera begun erupting with little warning. • This photo shows the large white billowing eruption plume is carried in a westerly direction by the weak prevailing winds. • At the base of the eruption column is a layer of yellow-brown ash being distributed by lower level winds.

15. Tongariro, 1975 • A vulcanian explosion from Ngauruhoe (Tongariro) volcano in New Zealand on February 19, 1975, ejects a dark, ash-laden cloud. • Large, meter-scale ejected blocks trailing streamers of ash can be seen in the eruption column. • Blocks up to 20 m across were projected hundreds of meters above the vent.

16. Another type of volcanic plume • Another type of volcanic plume forms in association with pyroclastic flows and surges, which are mixtures of hot particles and hot gases that are denser than surrounding atmosphere. • As flows travel away from source, sedimentation of particles from base of flow and heating of entrained air decreases bulk density. • These secondary or co-ignimbrite plumes generated from tops of flows by buoyant rise. • Allows plumes to have much larger areal distribution.

17. Formation of a co-ignimbrite plume • 1980 Mt. St Helens good example of formation of a co-ignimbrite plume. • Pyroclastic flow moving at 100 m/s covered an area of 600 km2. • When flow decelerated, finer particles became buoyant because of heating of entrained air. • Secondary or co-ignimbrite plume ascended to 25 km above surface of Earth.

18. Formation of Co-Ignimbrite Plume Pyroclastic flow heats up entrained air. In addition, sedimentation occurs. Larger, denser particles deposited at base of flow. Thus, because of both of these processes, concentration of particles and thus density of material decreases. Eventually, density less than that of surrounding atmosphere. Buoyant cloud/plume develops.

19. Formation of Co-Ignimbrite Plume Co-ignimbrite plume lacks gas thrust/jet region. Begins ascent with relatively low velocity. Second, source area and radius tend to be much larger than those of the primary plume. Will also develop an umbrella region.

20. Pyroclastic Flow and Co-Ignimbrite Plume, Pinatubo, 1991

21. Makian, Indonesia, 1988 • A vigorous eruption column rises above Indonesia's Makian volcano in this July 31, 1988, view from neighboring Moti Island. • The six-day eruption began on July 29, producing eruption columns that reached 8-10 km altitude. • Pyroclastic flows on the 30th reached the coast of the island, whose 15,000 residents had been evacuated. • A flat-topped lava dome was extruded in the summit crater at the conclusion of the eruption.

22. Another type of volcanic plume • Another type of volcanic plume forms in association with pyroclastic flows and surges, which are mixtures of hot particles and hot gases that are denser than surrounding atmosphere. • As flows travel away from source, sedimentation of particles from base of flow and heating of entrained air decreases bulk density. • These secondary or co-ignimbrite plumes generated from tops of flows by buoyant rise. • Allows plumes to have much larger areal distribution.

23. Transport vs. Depositional Systems • Transport system: responsible for movement of the assemblage of fragmented material (including gas) • Depositional system: controls on the way in which the material comes to rest to form a deposit.

24. Transport Systems Two major classes identified in explosive eruptions. • Vertical plumes: dominant trajectory of motion is initially upward. These generate fall deposits via deposition from wind-driven clouds at elevations of several to 10s of km above Earth’s surface. • Laterally moving systems: dominant trajectory of motion is initially sideways. Generate surge and flow deposits from gravity-controlled, ground-hugging density currents (i.e., pyroclastic density currents). • Note that there are complications to this simple division-- For example, secondary/co-ignimbrite plumes.

25. Transport Systems Leads to three types of transport systems • Fall: high buoyant plume carries all but densest(largest) particles up to 10s of km high; particles are sedimented from plume. Dispersal controlled by wind direction. • Surge: ground-hugging relatively dilute density current with gradual downward increase in density. Not influenced by wind, but can gnerate a secondary plume. • Flow: ground-hugging concentrated (relatively dense) density current, often with accompanying secondary cloud.

26. Pyroclastic Density Currents For laterally moving systems, two end-member types of transport systems have been identified: • Dilute: referred to as pyroclastic surge. • Concentrated: referred to as pyroclastic flow. • Note that these represent a spectrum, with gradations between.

28. Gravity current In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference. Typically, the density difference is small enough for the Boussinesq approximation to be valid. Gravity currents are typically of very low aspect ratio (that is, height over typical horizontal lengthscale). The pressure distribution is thus approximately hydrostatic, apart from near the leading edge (this may be seen using dimensional analysis). Thus gravity currents may be simulated by the shallow water equations, with special dispensation for the leading edge which behaves as a discontinuity.The leading edge of a gravity current is a region in which relatively large volumes of ambient fluid are displaced. Mixing is intense and head is lost. According to one paradigm, the leading edge of a gravity current 'controls' the flow behind it: it provides a boundary condition for the flow.The leading edge moves at a Froude number of about unity; estimates of the exact value vary between about 0.7 and 1.4. Gravity currents are capable of transporting material across large horizontal distances. For example, turbidity currents on the seafloor may carry material thousands of kilometres. Gravity currents occur at a variety of scales throughout nature. Examples include oceanic fronts, avalanches, seafloor turbidity currents, lahars, pyroclastic flows, and lava flows.

29. Pyroclastic flows are a common and devastating result of some volcanic eruptions. They are fast moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h. The gas is usually at a temperature of 100-800 degrees Celsius. The flows normally hug the ground and travel downhill under gravity, their speed depending upon the gradient of the slope and the size of the flow.

30. Pyroclastic Flow: High-speed avalanches of hot ash, rock fragments, and gas move down the sides of a volcano during explosive eruptions or when the steep edge of a dome breaks apart and collapses. These pyroclastic flows, which can reach 1500 degrees F and move at 100-150 miles per hour, are capable of knocking down and burning everything in their paths.

31. Pyroclastic Density Currents: Concentrated Currents or Flows • Has solids in concentrations of 10s of volume percent. Thus are higher density than surges. • Have a free surface, above which solids concentration decreases sharply. • Transport material by fluidization. Most flows considered laminar. • Velocities vary, buy typically 10s of m/s. Can be much faster. Velocities of up to several hundred m/s inferred based on heights of obstacles overcome by flows.

32. Pyroclastic Flow: High-speed avalanches of hot ash, rock fragments, and gas move down the sides of a volcano during explosive eruptions or when the steep edge of a dome breaks apart and collapses. These pyroclastic flows, which can reach 1500 degrees F and move at 100-150 miles per hour, are capable of knocking down and burning everything in their paths. Pyroclastic Surge: A more energetic and dilute mixture of searing gas and rock fragments is called a pyroclastic surge. Surges move easily up and over ridges; flows tend to follow valleys.

33. Pyroclastic Density Currents: Dilute Currents or Surges • Contain less than 0.1-1.0% by volume of solids, even near ground surface. Thus are low density. • Are density-stratified, with highest particle concentration near ground surface. • Transport material primarily by turbulent suspension. • Transport systems modeled as one that loses particles by sedimentation. Depletes the system of mass. Eventually, system may become buoyant, in which case becomes a plume. • Velocities vary, buy typically 10s of m/s. Can be much faster.

34. Structural Differences between Surge and Flow In basal m to 10s of m, surges show increase in density due to sedimentation. Also show decrease in mean velocity due to increased ground friction (drag). Deposits generated by sedimentation through basal zone. Lower solids concentrations than flows.

35. Structural Differences between Surge and Flow Much higher solids concentration than surge. Particles concentrated in basal deposit m to 10s of meters. Highest velocity in this region. Rapid transition between high velocity, high concentration region and overriding cloud. Deposition occurs both because of ground friction and also because the flow eventually comes to rest.

36. Depositional Systems • Clasts in explosive eruptions have a period of transport, and yet, all particles eventually come to rest. • Deposition system concept that allows investigation of processes operating in final stages of movement; essentially the transition from mobile to immobile particles.

37. Controls on Depositional Characteristics There are four fundamental controls on how deposition occurs. • Clast trajectory: vertical to horizontal--> controls whether deposit mantles surface, or has evidence of lateral depositional characteristics. • Concentration of particles: from low to high--> determines degree of sorting, scale of bedforms. • Presence/absence of cohesion. Cohesion will result in rapid and irreversible deposition. Increases slope angle of deposition as well. • Presence/absence of fluctuation of particle concentration with time: steady vs. unsteady--> single deposit or succession of deposits.

38. Four Major Controls on Depositional Processes Particle trajectory: vertical yields mantling of topography; horizontal may lead to bedding Particle concentration: low concentration can lead to good sorting (fall); high can lead to poor sorting (flow) Particle cohesion: cohesive particles will preclude slumping, also allow deposit to be placed on steeper slopes. Fluctuation in particle concentration: sustained yield uniformly graded deposit; non-sustained yield succession beds

39. Effect of Particle Concentration vs. Angle of Trajectory Fall: vertical trajectory, low concentration Surge: lateral (horizontal) trajectory, low concentration (thus leads to lower density than flow) Flow: lateral (horizontal) trajectory, high concentration (thus leads to higher density than surge)

40. More on Fall vs. Surge vs. Flow Fall: drape landscape, no cross beds or wave bedforms, well sorted, bedded, evidence for high temperatures (welding) absent Surge: pinch and swell, basal scouring, cross bedding, (i.e., features that express lateral transport), good to poor sorting, sustained high temperatures rare. Flow: thicken into or are confined in valleys because flow is gravity driven, show basal scouring but lack internal bedforms, poor sorting. Sustained high temperatures (welding) typical. High T indicative of efficient transport (little mixing with ambient air).

41. Spectra between Deposition Mechanisms Surge to fall: gradation between the two, depending on wind. Fall to flow: distinction between these two function of ability of material to trap gas. Falls accumulate too slowly to keep gas trapped. Gas required to fluidize pyroclastic material. That is, trapped gas (which expands because it is hot) will support the weight of the particles. Behaves like a liquid. Flows require sedimentation rates of > 1 m/s.

42. Spectra between Deposition Mechanisms Surge to flow: controls not fully understood, but primary control is particle concentration.

43. More on Particle Cohesion • Important in the low T environment: preferentially affect fines. Clumping inferred to occur in wet conditions (e.g., accretionary lapilli). Causes premature deposition of fines, which in turn causes deposits to be more poorly sorted. • Presence of water in low concentrations also increases cohesion, allowing fall and surge deposits to be deposited on surfaces with angles greater than dry angle of repose. • Water in high concentrations will promote soft-sediment deformation and slumping.

44. More on Particle Cohesion • At high temperatures, near source, cohesion of hot clasts can results in formation of over-steepened features such as spatter cones and ramparts. • At a distance from source in pyroclastic flows, when material coalesces, deposit can retain momentum from transport. If deposited on slope, can flow back downhill under influence of gravity. Produces fountain fed lava flows and rheomorphic flow deposits.

45. More on Role of Fluctuation in Particle Concentration • Fluctuations in particle concentration, particularly in fall deposits yield different types of fall deposits (topic for the future). • Differences also evident in surge vs. flow. Surges are modeled to be more variable in transport and deposition systems, whereas flows are interpreted to be more steady-state. • Reflects differences in momentum and length scale of deposition: momentum in flows greater and beds are typically thicker.

46. Review of Ignimbrites Standard ignimbrite flow unit comprises 3 layers: • Layer 1: deposit laid down at flow front during strong interaction with ambient air and ground surface • Layer 2: main deposit • Layer 3: deposit from overriding dilute cloud (co-ignimbrite cloud)

47. Layer 1 • Highly variable in character; suggests that this layer strongly influenced by local topography, etc. • Most common type is ground layer or lithic-rich layer, which is a layer enriched in heavy components like lithic fragments. • Interpretation is that lithics sedimented out of head of pyroclastic flow. • Can also sometimes find a basal surge layer. Interpreted to be the result of surge advancing at the head of the flow.

48. Layer 2 • Layer 2a: variably developed ash layer interpreted to form because of interaction with ground surface. • Layer 2b: normal grading of density particles, such as lithics. Larger lithics concentrated at bottom. • Reverse-grading at top because pumice are less dense than medium. • Also common are lapilli pipes, which are vertical pipes depleted in fines. They are gas escape structures. Fine particles escape with gas.

49. Layer 3 • Layer 3 is ash-cloud layer, which is layer deposited from secondary or co-ignimbrite cloud.

50. Flow unit vs. Cooling unit • Flow unit--individual units that represent distinct depositional events; may follow within minutes, hours, days, or longer • Cooling unit--a package of rock that cooled as a unit. • So an ignimbrite may be composed of a number of flow units, and one or more cooling units.