5.4 Tephra Fall and Dispersal Patterns
6 min read•july 30, 2024
Explosive eruptions eject fragmented material called tephra into the atmosphere. This includes , , and , which vary in size and composition. Understanding tephra helps volcanologists interpret eruption dynamics and assess potential hazards.
Tephra dispersal patterns are influenced by wind conditions, eruption parameters, and particle properties. Isopach and isopleth maps help visualize deposit thickness and grain size distribution, providing insights into eruption characteristics and environmental impacts.
Tephra and its components
Composition and size classification
Top images from around the web for Composition and size classification
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
1 of 2
Top images from around the web for Composition and size classification
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
Frontiers | Aeolian Remobilisation of the 2011-Cordón Caulle Tephra-Fallout Deposit: Example of ... View original
Is this image relevant?
1 of 2
- Tephra is the fragmented material ejected from a volcano during an explosive eruption
- Includes ash (< 2 mm), lapilli (2-64 mm), and bombs (> 64 mm)
- Ash consists of fine-grained volcanic particles
- Typically composed of pulverized rock, mineral fragments, and glass shards
- Can be transported long distances by wind due to its small size
- Lapilli are medium-sized tephra fragments
- Often contain a mixture of vesicular (pumice) and non-vesicular (scoria) material
- Deposited closer to the vent compared to ash
- Volcanic bombs are large, semi-molten or solid fragments ejected from the vent
- May have aerodynamic shapes (spindle, ribbon, or cow-dung bombs) due to partial solidification during flight
- Indicate proximity to the vent and high-energy eruptions
Physical properties and implications
- Tephra components vary in density, shape, and surface area
- Density influences settling velocity and transport distance
- Shape affects aerodynamic behavior and deposition patterns
- Surface area impacts interactions with atmospheric moisture and chemical reactions
- Grain size distribution of tephra reflects fragmentation processes and eruption dynamics
- Finer particles indicate more efficient fragmentation and/or higher eruptive energy
- Coarser particles suggest less efficient fragmentation and/or lower eruptive energy
- Tephra composition provides insights into magma properties and eruption style
- Basaltic tephra (scoria) associated with mafic magmas and Strombolian eruptions
- Silicic tephra (pumice) associated with felsic magmas and Plinian eruptions
Tephra ejection and deposition
Eruption column dynamics
- Explosive volcanic eruptions generate a high-velocity mixture of gas and tephra called an eruption column or plume
- Can rise several kilometers into the atmosphere (up to 50 km in some cases)
- Height of the eruption column determined by factors such as:
- Rate of magma discharge (higher rates lead to taller columns)
- Gas content (more gas leads to greater buoyancy and height)
- Atmospheric conditions (wind speed, temperature, and humidity)
- Eruption columns exhibit different regimes based on their behavior and stability
- Buoyant columns rise due to the density difference between the plume and surrounding air
- Collapsing columns occur when the plume density exceeds that of the surrounding air, leading to pyroclastic density currents
Transport and deposition processes
- Tephra is transported laterally by prevailing winds at various altitudes
- Larger particles (bombs and lapilli) fall closer to the vent
- Finer particles (ash) can be carried hundreds to thousands of kilometers
- Tephra settles out of the eruption column and atmosphere
- Forms a deposit that typically decreases in thickness and grain size with increasing distance from the vent
- Deposition influenced by particle size, density, shape, atmospheric conditions, and
- Wet deposition occurs when tephra interacts with atmospheric moisture (rain or snow)
- Leads to the formation of accretionary lapilli and ash aggregates
- Enhances the removal of fine ash from the atmosphere and modifies deposit characteristics
- Tephra fall deposits can be primary or reworked
- Primary deposits are the direct result of an eruption and maintain original bedding and sorting
- Reworked deposits have been modified by post-depositional processes (wind, water, or gravity) and may exhibit altered bedding and sorting
Factors influencing tephra dispersal
Wind conditions
- Wind direction and speed at different altitudes play a crucial role in tephra dispersal
- Strong, consistent winds result in elongated or asymmetric tephra fall deposits
- Variable or weak winds may produce more circular or symmetrical deposits
- Wind shear (change in wind speed or direction with height) can affect the trajectory and spread of the eruption column
- Can lead to the separation of coarse and fine particles at different altitudes
- Seasonal and diurnal variations in wind patterns can influence the long-term distribution of tephra
- Prevailing wind directions may vary depending on the time of year or day
- Long-lived eruptions may produce complex dispersal patterns due to changing wind conditions
Eruption parameters
- Height of the eruption column influences the potential distance of tephra dispersal
- Higher columns generally result in more widespread deposits
- Column height is related to the intensity of the eruption and magma discharge rate
- Particle size is a key factor in tephra dispersal
- Smaller particles (ash) have lower settling velocities and can be transported much farther than larger particles (lapilli and bombs)
- Fine ash (< 63 μm) can remain in the atmosphere for days to weeks and circle the globe
- Eruption duration and pulsation can affect the dispersal and accumulation of tephra
- Prolonged eruptions may lead to the development of complex, multi-layered deposits
- Pulsating eruptions can produce distinct beds with varying grain size and thickness
Interaction of factors
- The interaction between particle size, eruption column height, and wind conditions creates a range of dispersal patterns and deposit characteristics
- Strong winds and high columns can disperse fine ash over vast areas
- Weak winds and low columns may result in localized, thick deposits of coarse tephra
- Topography can influence tephra dispersal and deposition
- Mountains or valleys can channel or block winds, altering the trajectory of the plume
- Steep slopes can promote the remobilization of tephra through gravity-driven processes (e.g., lahars)
- Atmospheric temperature and humidity can affect the buoyancy and condensation of the eruption column
- High humidity can lead to the formation of ice crystals and enhance the removal of fine ash
- Temperature inversions can trap tephra in low-lying areas, leading to localized thickening of deposits
Interpreting tephra fall deposits
Isopach maps
- Isopach maps depict lines of equal thickness (in cm or mm) of a tephra fall deposit
- Provide a visual representation of the deposit's extent and spatial variability
- Constructed using data from field measurements of tephra thickness at various locations around the volcano
- Isopach maps can be used to estimate the volume of tephra ejected during an eruption
- Volume calculated by integrating the thickness over the area enclosed by each isopach
- Helps quantify the magnitude and scale of the eruption
- Isopach patterns reflect the interplay of eruption parameters and wind conditions
- Elongated or asymmetric isopachs indicate strong, directed winds
- Circular or concentric isopachs suggest weak or variable winds
- Limitations of isopach maps include:
- Uneven distribution of field measurements due to accessibility or exposure
- Erosion or compaction of the deposit over time
- Difficulty in distinguishing between multiple eruption events or phases
Isopleth maps
- Isopleth maps display lines of equal mass per unit area (e.g., kg/m²) of a specific size class of tephra
- Allow for the assessment of particle size distribution within the deposit
- Useful for determining the dispersal of specific grain sizes (e.g., lapilli or coarse ash)
- Isopleth maps are constructed using data from sieving and weighing samples collected at various locations
- Requires a systematic sampling strategy to capture the spatial variability of grain size
- Isopleth patterns can provide insights into eruption dynamics and fragmentation processes
- Concentric isopleths indicate a steady, sustained eruption with consistent fragmentation
- Irregular or patchy isopleths suggest a pulsating or unsteady eruption with variable fragmentation
- Combining isopleth and isopach maps can help constrain eruption parameters
- The maximum clast size at a given distance is related to the eruption column height and wind speed
- Grain size variations with distance can be used to estimate the total grain size distribution of the erupted material
Applications and limitations
- Interpreting isopach and isopleth maps requires an understanding of the factors influencing tephra dispersal and the limitations of field data collection and interpolation methods
- These maps can be used to:
- Reconstruct eruption parameters (e.g., column height, wind direction, and eruption duration)
- Assess the potential impacts on surrounding areas (e.g., agriculture, infrastructure, and aviation)
- Inform hazard assessments and emergency response planning for future eruptions
- Limitations and uncertainties in interpreting tephra fall deposits include:
- Post-depositional modification by wind, water, or biological activity
- Challenges in correlating tephra layers across large areas or between different eruptive events
- Variability in deposit preservation due to landscape and environmental factors
- Assumptions and simplifications in models used to interpret deposit characteristics (e.g., Gaussian plume models)
- Integration of field observations, remote sensing data, and numerical modeling approaches can help overcome some of these limitations and improve the understanding of tephra dispersal processes and deposit formation.
Key Terms to Review (19)
Agricultural disruption: Agricultural disruption refers to the interruption or alteration of farming activities, typically caused by external factors such as natural disasters, climate change, or human activities. This phenomenon can lead to significant reductions in crop yields, loss of livestock, and long-term changes to agricultural practices and land use, severely impacting food security and the economy.
Air Travel Disruption: Air travel disruption refers to the significant interruptions in flight operations that affect airlines, passengers, and airports, often caused by natural events like volcanic eruptions. When volcanic activity produces tephra fall, it can lead to widespread flight cancellations and delays due to safety concerns related to ash clouds, impacting travel logistics and economic activities.
Ash: Ash refers to the fine particulate material produced during volcanic eruptions, composed mainly of tiny fragments of volcanic rock and glass. This material can be ejected into the atmosphere and carried over long distances by wind, impacting both the environment and human activities. Understanding ash is crucial for assessing its effects on air quality, aviation safety, and regional ecosystems.
Bombs: In volcanology, bombs are large volcanic projectiles that are ejected during explosive eruptions. These solid fragments, which can vary in size and shape, typically range from 64 mm to several meters in diameter and can travel significant distances from the eruption site due to the force of the explosion. Bombs are an important component of tephra fall, as they contribute to the distribution and characteristics of volcanic deposits.
Climatological conditions: Climatological conditions refer to the long-term patterns of weather and climate in a specific region, including factors like temperature, humidity, wind speed, and precipitation. Understanding these conditions is essential for predicting how volcanic eruptions can affect and disperse tephra, as they directly influence the behavior of volcanic ash clouds and their impact on surrounding areas.
Computational modeling: Computational modeling is the process of using computer algorithms and simulations to represent, analyze, and predict complex physical phenomena. This technique is particularly useful in understanding tephra fall and dispersal patterns during volcanic eruptions, as it allows researchers to simulate various eruption scenarios and assess their potential impact on the environment and surrounding communities.
Emergency preparedness plans: Emergency preparedness plans are strategic frameworks designed to ensure a community's readiness to respond effectively to natural disasters, such as volcanic eruptions. These plans outline the necessary actions, resources, and coordination required to mitigate risks, protect public safety, and facilitate recovery in the event of an emergency, like tephra fall from a volcanic eruption.
Eruptive column: An eruptive column is a vertical column of volcanic material, such as ash, gas, and tephra, that is expelled during a volcanic eruption. These columns can rise high into the atmosphere and are critical in determining the dispersal patterns of tephra and ash, impacting both local environments and air traffic across large distances.
Eyjafjallajökull: Eyjafjallajökull is an ice cap and stratovolcano located in Iceland, which erupted in 2010, causing significant disruption to air travel across Europe. This volcano is particularly notable for its explosive eruptions, which are closely linked to the interaction between magma and ice, leading to unique tephra fall and dispersal patterns that can impact large areas.
Fallout pattern: A fallout pattern refers to the distribution and accumulation of volcanic materials, such as ash and tephra, that settle on the ground after an explosive eruption. This pattern is influenced by various factors including wind direction, eruption height, and the size of the particles, resulting in varying deposition across different areas surrounding the volcano.
Geochemical analysis: Geochemical analysis is the study of the chemical composition of materials, particularly rocks and minerals, to understand their properties and behaviors. This type of analysis provides critical insights into the processes that shape volcanic systems, helping to assess hazards and interpret tephra fall and dispersal patterns. By examining the chemical signatures in volcanic materials, scientists can deduce the history of eruptions, track magma movement, and evaluate potential risks associated with volcanic activity.
Hazard maps: Hazard maps are specialized tools used to visually represent the potential risks and impacts of natural disasters, specifically volcanic hazards, in a given area. They integrate scientific data and assessments to identify zones that are at risk from events such as lava flows, tephra fall, and other volcanic phenomena. These maps play a critical role in risk assessment, planning for evacuations, and formulating mitigation strategies, allowing communities to prepare for and respond effectively to volcanic events.
Lagrangian Particle Tracking: Lagrangian particle tracking is a method used in fluid dynamics and environmental science to simulate the movement of particles as they are carried by fluid flow. This technique allows researchers to analyze how volcanic materials, like tephra, disperse from an eruption site based on variables such as wind speed, direction, and particle size. It is particularly useful for predicting the dispersal patterns of ash and other volcanic debris, aiding in hazard assessment and risk management.
Lapilli: Lapilli are small volcanic fragments that range in size from 2 to 64 millimeters in diameter, commonly produced during explosive volcanic eruptions. These rock fragments are crucial in understanding volcanic activity, as their size and distribution can indicate the eruption's intensity and the type of material being expelled. When studying the fall and dispersal patterns of tephra, lapilli provide valuable insights into the dynamics of eruption clouds and the processes that affect how these materials settle on the ground.
Mount St. Helens: Mount St. Helens is an active stratovolcano located in the state of Washington, known for its catastrophic eruption on May 18, 1980, which significantly altered the surrounding landscape. This volcano is a classic example of stratovolcanic activity and provides insights into volcanic behavior, pyroclastic flows, and tephra dispersal patterns associated with eruptions.
Plinian eruption: A Plinian eruption is a type of volcanic eruption characterized by the explosive ejection of ash, gas, and pumice into the atmosphere, producing a towering vertical column that can reach high altitudes. These eruptions are often associated with highly viscous magma, leading to significant pyroclastic flows and widespread tephra fallout, which can impact vast areas around the volcano.
Tephra layer thickness: Tephra layer thickness refers to the measurement of the vertical extent of volcanic tephra deposits that accumulate on the ground following an explosive volcanic eruption. This thickness can provide insights into the eruption's intensity, duration, and the volume of material ejected. Understanding tephra layer thickness is essential for reconstructing past volcanic events and assessing their potential hazards in relation to population centers and infrastructure.
Topography: Topography refers to the detailed description of the physical features of a landscape, including its relief, landforms, and the arrangement of natural and human-made structures. This aspect is essential in understanding how different terrains influence various geological and hydrological processes, especially in relation to volcanic activity and its associated hazards. The configuration of the land can impact the movement of materials during events such as mudflows and tephra dispersal, shaping their behavior and effects.
Wind dispersion: Wind dispersion refers to the process by which volcanic ash and other tephra particles are carried away from their source during an eruption by prevailing winds. This phenomenon is crucial for understanding tephra fall patterns, as it influences where the ash lands, its distribution, and potential impacts on the environment and human activities. Factors such as wind speed, direction, and atmospheric conditions all play significant roles in determining how far and in what patterns the tephra will spread.