Volcanology Chapter 5: Explosiveness Factors

Study Notes

This document is a table of contents for a larger work on geological natural hazards.
It organizes the topics by sections, such as magma properties, effusive eruptions, explosive eruptions, and volcano monitoring.
It also covers specific examples of hazards, like lava flows on Hawaii, and Mount Etna, as well as mitigation techniques.
Other topics include pyroclastic density currents (PDCs) and lahars, the causes of landslides, and flooding hazards.

Volcanic hazards are the natural processes which can cause harm, such as floods and volcanoes.
Vulnerability includes factors like engineering, economics, and the building and people exposed,
Risk describes the relationship between hazard, vulnerability, and exposure.
Magma is formed from the melting of the Earth's mantle via increases in temperature, pressure, and fluids added.
Subduction causes volcanic arcs
Wide compositional ranges of volatile-rich magmas (andesites to rhyolites) are created from the melt.

Effusive eruptions are characterized by lava flows at the surface
Montserrat has thick lava dome eruptions, rich in silica.
Kilauea displays runny lava flows, silica poor magma.
Lava - magma which reaches the Earth's surface in liquid form.
Tephra - generated by magma fragmentation during explosive eruption. Low-viscosity magma allows bubble escape and gas content.
High-viscosity magma has slow ascent and low gas content.

Explosive eruptions release smaller fragments near the vent
Phreato Plinian style yields larger eruptions which expel fine particles into the atmosphere.
Tephra (fragmental material) consists of lithic clasts, crystals, and pyroclastic (juvenile) materials.
Lapilli (2–64 mm) and ash (<2 mm) are examples of grain sizes in tephra.

Pyroclastic Density Currents (PDCs) are gravity-driven mixtures of gas and solid fragments.
These flows can be high velocity (100 km/hr+) and mobile, traveling down slopes.
PDC deposits typically have a high volume, with high ash content and pumice fragments, hence the term ignimbrites
Lahars are water-sediment slurries which involve liquid and solid interactions
Lahars cause immense damage, requiring high mobility for their destructive power.
These slurries are highly destructive and erosive on slopes.

Monitoring volcano activity requires understanding past behavior and the probability, frequency, and impact of events.
Monitoring approaches include earthquake monitoring and analysis, deformation, gas emissions, ground-based, and satellite-based observations.
Volcano seismicity is a key parameter tracked by monitoring approaches for potential eruption precursors.
These parameters have a major effect on forecast results.

Mitigation plans are often missed or too late, even with monitoring.
This may be due to factors like remote volcano locations and limited understanding.
Subjectivity and simplification of risk quantification are current challenges.
Use of probabilistic methods, like simulations and hazard maps, are widely used for forecasting and mitigation.

Landslides result from downward displacement of slope materials.
Driving forces include gravity (driving force).
Landslide movement types are classified into falls, topples, slides, and flows, based on how the movement takes place, speed, and material composition
Forces that resist landslides (resisting forces) include cohesion, friction, and the presence of geologic materials, electro-static forces form clay, grain size, and angle of internal friction
Factors affecting movements include type of movement, magnitude / size of event, and speed of failure.
Geological factors are crucial in understanding and assessing factors associated with landslides.
Equation for estimating factor of safety: [calculation]

Tsunamis are formed from vertical displacement of the water column, caused by earthquakes, slope failures, volcanic activity, or bolide (e.g meteor/asteroid) impacts

Tsunamis are characterized by extremely long wavelengths with small height.
Wave velocity depends on the depth of the water
Energy of waves doesn't dissipate as much over long distances as the wave moves into shallow water.
The velocity decreases, and the wave height and amplitude increase as the wave moves from deep water to shallow water near the coast
Magnitude and wavelength are also strongly related to one another

Real-time warning systems like DART systems are essential.
Communicating risk through maps, education, and understanding past events are crucial
Planning for potential responses is important. Different choices of method of prediction create different hazards
Mitigation requires a combination of monitoring, warning systems, and infrastructure planning.

Magnitude is controlled by slip and rupture area
Strike-slip faults and Normal faults have differing magnitudes, given different tectonic settings
Shallow, large faults are related to the length of the rupture
Thrust faults happen in subduction zones, have much deeper and wider rupture

Flood events can involve extremely intense rainfall events over short periods, as well as long-duration, larger magnitude events.
Flood intensifying factors - basin factors (stable and variable), weather, and drainage/channel factors.
Increased intensity and frequency of rain events are associated with an increased probability of flooding.
The hydrological cycle plays a significant role in determining flood probabilities and risks.
Mitigation involves flood defences, and floodplain management.

Foreshocks (small quakes preceding large ones), seismic velocity changes, and other geological phenomena may be used to forecast earthquakes
Recurrence time - the average interval between events of a specified magnitude (Mw) - useful for future predictions and hazard assessment.

Building styles vary significantly in their earthquake resilience.
Masonry, wood frames, and reinforced concrete each have differing strengths and vulnerabilities to seismic hazards