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Eruption Columns and Fall Deposits
Pyroclastic Deposits
Pyroclastic Fall Deposits
Mantle Topography:
Mostly parallel to bedding
Well sorted
Often graded
Pyroclastic Flow Deposits
Characteristics:
Topographically constrained
Poorly sorted
Often graded
Pyroclastic Surge Deposits
Characteristics:
Partially topographically constrained
Exhibits cross-bedding
Intermediate sorting
Often graded
Ejecta Classification
Tephra:
Definition: All ejecta blown through the air by explosive eruptions.
Particle Size Scale for Volcanic Tephra (Table 7.1):
| Type | Name | Maximum Diameter (mm) | Φ No. |
| ---------------- | ------------- | ---------------------- | -------------- |
| Volcanic Blocks | Bombs | >256 | -8 |
| | | 128 | -7 |
| | | 64 | -6 |
| | | 32 | -5 |
| | | 16 | -4 |
| | | 6 | -3 |
| | | 2 | -2 |
| | | 1 | -1 |
| | | 0.5 | 0 |
| | | 0.25 | 1 |
| | | 0.125 | 2 |
| | | 0.062 | 3 |
| | | 0.031 | 4 |
| | | 0.016 | 5 |
| | Dust | <0.008 | 6 |
| | | 0.004 | 7 |
Ejecta Emplacement
Ballistic Fall:
Surge:
Ash cloud-derived fall
Ballistic Tephra: Types
Bomb Types:
Spindle bomb
Cow-pie bomb
Breadcrust bomb
Blocks:
Volcanic block
Bomb
Sag
Ash Classification
Primary Volcaniclastic:
Definition: Particles mobilized directly by explosive or effusive volcanism and not stored at any time prior to arrival at the site of deposition.
Particle Size Distribution:
Size
Diameter (mm)
Extremely Fine Ash
<1/16 | | Very Fine Ash | 1/16-1/8 | | Fine Ash | 1/8-1/4 | | Medium Ash | 1/4-1/2 | | Coarse Ash | 1/2-1 | | Very Coarse Ash | 1-2 | | Fine Lapilli | 2-4 | | Medium Lapilli | 4-16 | | Coarse Lapilli | >64
Consolidated Types
Extremely Fine Tuff
Very Fine Tuff
Fine Tuff
Medium Tuff
Coarse Tuff
Very Coarse Tuff
Fine Lapilli-Tuff
Medium Lapilli-Tuff
Coarse Lapilli-Tuff
Breccia
Accretionary Lapilli
Pumice
Pele's Hair
Describing Tephra Fall
Sorting:
Proportions of clast sizes
Fall deposits are very well sorted and clast-supported.
Linking Deposits to Eruption Type
Relation to Magma Viscosity and Temperature:
Increasing viscosity and temperature lead to varying types of eruptions:
Hawaiian:
Clasts such as Pele's hair and tears, basaltic pumice, and ribbon bombs.
Scoria:
Fusiform, finned, and almond-shaped bombs.
Silicic:
Pumice bombs and breadcrust.
Commonly Associated Eruption Types:
Vulcanian and Strombolian: Blocks and bombs.
Table 7.2:
Relationship of common kinds of ejecta to modes of emplacement.
Eruption Column Structure
Regions of Eruption Columns:
Gas Thrust Region:
Ascending magma vesiculates and fragments, speeds of hundreds of meters per second, and gains up to a few km in jet phase. Entrains ambient air.
Convective Region:
Buoyancy causes upward acceleration, able to rise tens of kilometers for several hours, and can carry larger tephra upwards.
Umbrella Region:
Neutral buoyancy height (HB), where buoyancy is no longer driving the upwards motion, leading to continued rise to total plume height (HT).
Height of Eruption Columns
For sustained eruptions, height is empirically related to discharge rate (, where ). If eruption column height can be observed, intensity can be estimated.
Tephra Fallout and Dispersal
Tephra sedimentation from the base of the umbrella cloud occurs when terminal velocity exceeds the upward velocity of the cloud.
Isopleths:
Lines joining points of the same maximum clast size.
Effect of Winds on Plumes and Isopleth Maps
Plumes deflect downwind, influenced by height of plume and different winds at various atmospheric levels.
In umbrella clouds, radial spreading occurs against strong winds. Isopleth maps can infer column height and wind direction/speed based on the deposit distribution.
Tephra Hazards
Potential issues such as:
Jet aircraft risks
Roof collapses
Respiratory problems
Abrasion of vehicles
Power generation impacts
Slippery roads/runways
Clogging of drains
Fragmentation vs. Dispersal
D: The area enclosed by an isopach contour representing 1% of the maximum thickness.
F: The percent of tephra < 1mm measured along an axis of dispersal where the isopach is 10% of maximum thickness.
Downwind Changes in Deposit Characteristics
Observations include:
Decrease in thickness
Decrease in maximum grain size
Decrease in median grain size
Increase in sorting
Decrease in median terminal fall velocity
Eruption Types
Hawaiian Eruptions:
Produce small volumes of tephra with little to no fine ash. Clasts often coalesce and may include cow pat bombs. If emission rates are low, clasts cool leading to spindle bombs
Smaller particles like Pele’s hair and tears are carried by the wind.
Strombolian Eruptions:
Concentrated volumes of scoria and angular clasts, generally <500m in eruption height and no fine ash or Pele's hair.
Commonly result in formations of Strombolian or scoria cones, often used for road material.
Vulcanian Eruptions:
Characterized by large explosions leading to sustained plumes reaching heights of 5-10 km with large ballistic bombs and fine ash present.
Plinian Eruptions:
Eruptions can reach above the tropopause (~30 km), producing significant fine ash and pumice, with well-sorted deposits including:
Cap layer of well-sorted, unstratified ash
Pyroclastic flow deposit with poorly sorted mix
Cross-laminated pyroclastic surge beds
Well sorted basal pumice fall beds
Ultra-Plinian Eruptions:
Column heights exceed 45 km with very wide tephra dispersal. Example: Taupo eruption A.D. 181, indicated by thick pumice fall deposits.
Surtseyan Eruptions:
Limited dispersal, fine-grained with variances in sizes, common bomb sags and lithic fragments.
Recommended Reading
Cas, R. & Wright, J. (2012). Volcanic successions modern and ancient: A geological approach to processes, products and successions. Springer Science & Business Media. Chapters 5.1-5.3, 6