Unit 9 Exam Questions Flashcards
Suggest two reasons why some places within volcanic regions have a very high hazard level. (6 marks)
Proximity to the Vent
Areas located close to the volcanic vent experience the most intense hazards due to exposure to primary volcanic phenomena such as pyroclastic flows, lava flows, and tephra (volcanic ash and rock fragments). Pyroclastic flows, consisting of hot gas and volcanic materials, can travel at speeds of over 100 km/h and reach temperatures of 1000°C, making them one of the deadliest hazards. For example, in the 1902 eruption of Mount Pelée in Martinique, a pyroclastic flow destroyed the city of Saint-Pierre within minutes, killing nearly all of its 30,000 inhabitants.
Topography and Wind Influence
The shape of the land (relief) can amplify the hazard level. Valleys and steep slopes can direct and accelerate pyroclastic flows and lahars (volcanic mudflows), increasing the risk in certain areas. Additionally, prevailing winds can determine the spread of volcanic ash, which can travel thousands of kilometers. For instance, during the 2010 eruption of Eyjafjallajökull in Iceland, ash clouds were carried across Europe by prevailing winds, disrupting air travel for weeks.
Suggest two reasons for variations in ash fallout from a volcanic eruption (6 marks)
Wind Direction and Strength
The prevailing westerly winds carried ash from the eruption in a predominantly eastward direction.
Stronger winds increase the dispersion distance, while weaker winds lead to more localized ash deposits.
Variability in wind direction on different days influenced the spread of ash in different directions, as seen in the map.
Height of the Eruption Column and Ash Load
The higher the volcanic ash column, the further ash can travel due to the influence of atmospheric currents.
Ash particles settle at different rates:
Larger particles (e.g., volcanic bombs) fall close to the vent.
Fine ash can remain suspended in the atmosphere for long distances before settling.
The eruption’s intensity influences how far ash spreads—stronger eruptions inject ash higher into the stratosphere.
Proximity to the Volcano
Areas closer to the vent experience thicker ash deposits due to the immediate fallout of heavier material.
As distance increases, ash deposition becomes thinner and more dispersed.
Topography and Ash Accumulation
Valleys and mountain ridges channel ash movement, influencing local patterns.
For example, ash may accumulate more in valleys due to wind funnelling, while high ridges might receive less deposition.
Explain the variations in the warning times and hazard durations of different natural disasters.
Earthquakes – Shortest Warning Time and Short Duration
Warning Time: Earthquakes occur suddenly as tectonic stress is released along faults. While some precursors exist (e.g., foreshocks, gas emissions, and ground deformation), they are unreliable for precise prediction, making warning times very short (seconds to minutes).
Hazard Duration: The main earthquake event typically lasts for seconds to minutes, but aftershocks and secondary hazards (e.g., landslides, tsunamis, and fires) can extend the hazard’s impact for days or weeks.
Hurricanes – Moderate Warning Time and Moderate Duration
Warning Time: Hurricanes form over several days as tropical storms develop over warm ocean waters. Meteorological satellites and forecasting models allow for several days of warning before landfall, giving time for evacuation and preparedness.
Hazard Duration: The most intense impacts of a hurricane last a few hours to a couple of days. However, flooding, storm surges, and infrastructure damage can prolong the disaster’s effects for weeks in affected regions.
Volcanic Eruptions – Longest Warning Time and Longest Duration
Warning Time: Volcanic eruptions often provide weeks to months of warning due to precursors such as:
Increased seismic activity
Gas emissions (e.g., sulfur dioxide)
Ground deformation due to rising magma
Hazard Duration: Eruptions can last days, weeks, or even years, as seen with Kīlauea in Hawaii (erupted continuously from 1983 to 2018). Some volcanoes remain active for extended periods, with ongoing ash clouds, lava flows, and pyroclastic activity.
Explain two reasons why the number of deaths from mass movement events varies. (6 marks)
- Physical Factors: Slope Angle, Geology, and Rainfall Intensity
Steeper slopes are more prone to fast-moving, high-impact landslides, leading to higher casualties, as seen in the Himalayas and Andes.
The type of rock and soil affects slope stability:
Loose, unconsolidated sediments (e.g., clay, sand, volcanic ash) absorb water and trigger rapid landslides after heavy rain.
Harder rocks (e.g., granite, limestone) may break into large boulders, causing rockfalls that affect only localized areas.
Intense or prolonged rainfall increases soil saturation, leading to catastrophic slope failure. For example, the 2006 Leyte landslide in the Philippines was triggered by weeks of heavy rainfall. - Human Factors: Population Density and Infrastructure Development
Areas with high population density suffer greater casualties when landslides occur, especially if settlements are built on unstable slopes.
Poorly constructed buildings collapse more easily during landslides, increasing death tolls. In contrast, wealthier nations enforce building codes and land-use regulations to minimize risk.
Lack of warning systems and disaster preparedness leads to higher fatalities. Countries with early warning systems (e.g., Japan, USA) can evacuate residents before a disaster, reducing casualties.
Explain why volcanoes are not found at all types of tectonic plate boundaries. (6 marks)
- Where Volcanoes Do Occur
At Divergent Boundaries (Constructive Margins)
Plates move apart, creating a gap where magma rises from the mantle.
This results in volcanic activity along mid-ocean ridges (e.g., Mid-Atlantic Ridge).
At Convergent Boundaries (Destructive Margins)
When an oceanic plate subducts beneath a continental plate, the subducted plate melts.
This forms magma, which rises to create volcanoes (e.g., Andes Mountains in South America).
- Where Volcanoes Are Not Found
At Collision Boundaries (Continental-Continental Convergence)
When two continental plates collide, neither is subducted, meaning no magma is generated.
Instead, mountain ranges form due to the crust thickening and folding (e.g., Himalayas).
No magma = No volcanoes at these plate boundaries.
At Conservative Boundaries (Transform Margins)
Plates slide past each other rather than moving apart or colliding.
No crust is created or destroyed, and no magma is produced.
Example: San Andreas Fault (California), where earthquakes occur but volcanic activity is absent.
Explain why the depth of focus of earthquakes varies from place to place. (6 marks)
- Shallow-Focus Earthquakes (0–70 km deep)
Occur at divergent (constructive) and transform (conservative) plate boundaries.
At divergent boundaries, plates move apart, creating tensional stress, which causes fractures in the upper crust, leading to shallow earthquakes.
At transform boundaries, plates slide past each other, causing friction and sudden fault slippage, producing high-impact shallow earthquakes (e.g., San Andreas Fault, California). - Intermediate-Focus Earthquakes (70–300 km deep)
Found at subduction zones (convergent boundaries), where an oceanic plate sinks beneath a continental plate.
The subducting plate compresses and fractures, causing earthquakes at increasing depths as it descends into the mantle.
Example: Andes Mountain subduction zone (Nazca Plate beneath South American Plate). - Deep-Focus Earthquakes (300–700 km deep)
Occur only at subduction zones, where the subducting slab sinks deep into the mantle.
The intense pressure and temperature cause minerals to change phase, releasing energy as deep-focus earthquakes.
These earthquakes are less destructive at the surface due to their depth.
Example: Japan Trench (Pacific Plate subducting beneath the Eurasian Plate).
Explain why the location of tornadoes varies. (6 marks)
- Air Mass Interaction and Atmospheric Conditions
Tornadoes form when warm, moist air meets cold, dry air, creating violent thunderstorms with strong wind shear.
The Great Plains of the USA (Tornado Alley) experience frequent tornadoes due to:
Warm, humid Gulf air colliding with cold Arctic air.
Flat terrain allowing uninterrupted storm development.
Other tornado-prone areas include Bangladesh, Argentina, and South Africa, where similar air mass interactions occur. - Seasonal and Climate Variations
Tornado frequency varies by season:
Spring & summer months have warmer, more unstable air, increasing tornado frequency.
Winter tornadoes are rare due to cooler, more stable atmospheric conditions.
Climate change may shift tornado-prone areas due to changing jet stream positions. - Topographic Influence
Flat landscapes (e.g., USA Midwest, Argentina Pampas) allow tornadoes to form without obstruction.
Mountain ranges (e.g., Rockies, Himalayas) disrupt wind flow, making tornadoes less common.
Is there a relationship between the number of tornadoes and the number of tornado-related deaths?
- Tornado Strength and Population Density
Not all tornadoes are equally deadly. The Enhanced Fujita (EF) scale ranks tornadoes from EF0 (weak) to EF5 (violent).
A high number of weak tornadoes (EF0–EF1) may cause minimal damage, whereas one EF4–EF5 tornado in a populated area can be catastrophic.
Example: The 2011 Joplin, Missouri EF5 tornado killed 158 people, despite a lower total number of tornadoes that year. - Advances in Early Warning Systems
Improvements in Doppler radar, tornado sirens, text alerts, and emergency preparedness help reduce fatalities.
More deaths occur when tornadoes strike unexpectedly or at night, when people are less prepared.
Example: The 2013 Moore, Oklahoma EF5 tornado had extensive warning, leading to fewer deaths compared to past tornadoes of similar strength. - Building Structures and Preparedness
Storm shelters and reinforced buildings reduce deaths in regions with frequent tornadoes (e.g., Tornado Alley, USA).
Mobile homes and weak infrastructure increase fatality risks, particularly in Southern US states like Alabama and Mississippi.
Example: In the 2020 Tennessee tornado outbreak, most fatalities occurred in areas with older, less storm-resistant buildings. - Seasonal and Geographic Variation
Tornadoes that occur in winter or at night tend to be more deadly, as people are asleep and less aware.
Tornado-prone regions (USA, Bangladesh, Argentina) have different fatality patterns depending on urbanization and preparedness.
Suggest how the mudflows are formed
- Water Source and Initial Trigger
A large volume of water is required to generate a mudflow. This can come from:
Melting glaciers and snowcaps due to volcanic heat.
Heavy rainfall saturating volcanic ash and loose debris, reducing cohesion and allowing material to flow.
Collapse of a crater lake or landslide, releasing stored water and debris.
Example: The 1985 Nevado del Ruiz eruption (Colombia) triggered a deadly lahar that buried the town of Armero, killing over 23,000 people. - Mixing of Water with Volcanic Material
As water flows downhill, it picks up volcanic ash, rock fragments, and loose soil, forming a dense, fast-moving slurry.
The steep slopes of Mount Rainier provide gravitational force, increasing the speed and erosional capacity of the flow. - Path and Movement of the Mudflow
Mudflows follow river valleys and drainage channels, allowing them to travel long distances (up to 100 km or more).
The Electron and Osceola mudflows from Mount Rainier moved along natural valleys into the Puget Sound Lowland, depositing large amounts of sediment.
As the mudflow reaches lower elevation areas, it spreads out, covering large regions with thick layers of volcanic material.
What effects the shaking intensity of an earthquake?
- Distance from the Epicenter and Energy Dissipation
The strongest shaking occurs near the earthquake’s epicenter because this is where the rupture begins and seismic energy is initially released.
As seismic waves travel outward, they lose energy due to attenuation (energy dissipation), causing the shaking intensity to decrease with distance.
This explains why higher-intensity shaking (7 on the scale) is concentrated near the epicenter, while lower intensities (1–3) are reported farther away. - Influence of Local Geology and Soil Type
The type of surface material affects how much seismic energy is amplified.
Soft sediments (e.g., alluvial deposits, clay, loose sand) amplify seismic waves, causing stronger shaking even at distances far from the epicenter.
Bedrock (e.g., granite, basalt) absorbs and dampens seismic waves, leading to lower intensity shaking.
This explains why some areas far from the epicenter still reported strong shaking, especially if they were built on soft, unconsolidated sediments.
Example: In the 1989 Loma Prieta earthquake (California), buildings in areas built on soft sediments (e.g., San Francisco’s Marina District) experienced more damage than those on solid rock. - Fault Lines and Direction of Seismic Energy
Earthquakes often occur along fault lines, where seismic waves propagate in specific directions due to the orientation of the rupture.
If the earthquake rupture moved asymmetrically, shaking may be stronger in one direction and weaker in another.
This could explain why some areas near the epicenter experienced lower shaking, while others farther away recorded higher intensities. - Building Structures and Human Reporting
The map in Fig. 7.1 is based on reported shaking intensity, meaning that variations might be due to differences in building structures and human perception.
Poorly built structures amplify shaking, while well-designed earthquake-resistant buildings may make shaking feel less intense, influencing how reports are recorded.
Explain why some areas are more prone to landslides than others. (6 marks)
- Steep Slopes and Geological Conditions
Areas with steep terrain (e.g., mountain ranges, cliffs, and valleys) experience strong gravitational pull, making them more susceptible to mass movements.
Weak or loose rock formations (e.g., shale, clay, or weathered granite) absorb water easily, reducing cohesion and increasing landslide risks.
Example: The Himalayas and Andes experience frequent landslides due to tectonic uplift, steep slopes, and unstable rock layers. - Heavy Rainfall and Water Saturation
High rainfall intensity or prolonged wet seasons increase soil saturation, making slopes heavier and more prone to failure.
Flash floods and tropical storms can rapidly erode slopes, triggering debris flows and mudslides.
Example: In 2010, heavy monsoon rains in Pakistan triggered deadly landslides, displacing thousands in mountainous areas. - Human Activities (Deforestation and Urbanization)
Deforestation removes tree roots, which help stabilize slopes. Areas with extensive logging or farming on slopes become more vulnerable to landslides.
Urban expansion on steep slopes increases weight and pressure, leading to slope instability and potential collapses.
Example: In Rio de Janeiro (Brazil), informal settlements built on steep hillsides frequently experience deadly landslides after heavy rains.
Briefly explain two causes of landslides other than earthquakes. (6 marks)
- Heavy Rainfall and Water Saturation
Intense rainfall or prolonged wet seasons increase soil water content, making slopes heavier and more prone to failure.
Water infiltration raises pore water pressure, reducing friction and cohesion between soil particles, leading to slope failure.
Flash floods and tropical storms can rapidly erode slopes, triggering debris flows and mudslides.
Example: The 2010 Pakistan monsoon rains triggered widespread landslides, displacing thousands in mountainous areas. - Deforestation and Human Activities
Deforestation removes tree roots, which help bind soil and prevent erosion. Slopes without vegetation are more vulnerable to mass movements.
Unregulated construction on steep slopes adds weight to unstable terrain, increasing landslide risk.
Mining and road construction can undercut slopes, triggering collapses.
Example: In Rio de Janeiro, Brazil, informal settlements built on steep, deforested slopes frequently experience deadly landslides after heavy rainfall.
Explain the factors which influence the hazard of soil liquefaction. (6 marks)
- Soil Composition and Water Saturation
Loose, unconsolidated sediments (e.g., sand, silt) are more prone to liquefaction than dense soils or bedrock.
High groundwater levels or recent heavy rainfall increase soil saturation, reducing soil stability.
Example: The 2011 Christchurch Earthquake (New Zealand) triggered severe liquefaction due to sandy, waterlogged soils. - Earthquake Strength and Duration
Higher magnitude earthquakes with longer shaking duration increase the likelihood of soil liquefaction.
Shallow-focus earthquakes produce stronger ground shaking, making liquefaction more severe.
Example: The 1964 Niigata Earthquake (Japan) caused extensive liquefaction, tilting buildings and infrastructure. - Proximity to Water Bodies and Low-Lying Areas
Coastal regions, riverbanks, and reclaimed land are highly susceptible to liquefaction due to high groundwater levels.
Artificial land reclamation (e.g., in cities like Tokyo or San Francisco) often has poorly compacted fill material, increasing liquefaction risks.
Example: The 1989 Loma Prieta Earthquake caused severe liquefaction damage in San Francisco’s Marina District, which was built on reclaimed land. - Human Activities and Infrastructure Development
Buildings and roads on weak, liquefiable soil increase damage risks.
Excessive groundwater extraction can weaken soil, making it more prone to liquefaction.
Example: Mexico City, built on a former lakebed, is highly vulnerable to liquefaction during earthquakes.
Suggest two physical causes of mass movements generally
- Steep Slopes and Weak Geological Structure
Gravity pulls material downhill, increasing stress on the slope.
Weak or unconsolidated materials (e.g., clay, loose sediment) are more prone to slumping and landslides.
Layered rock structures, particularly those with alternating permeable and impermeable layers, encourage water accumulation, reducing slope stability.
Example: The Dorset Coast (UK) experiences frequent landslides due to clay-rich cliffs absorbing water and becoming unstable. - Heavy Rainfall and Increased Water Content
Prolonged or intense rainfall saturates the soil, increasing its weight and reducing internal friction.
High pore water pressure reduces cohesion, leading to slumping, mudflows, and debris flows.
Storms and flash floods accelerate erosion, further destabilizing the slope.
Example: The 2010 Pakistan floods triggered widespread landslides, displacing thousands in mountainous areas.
Explain why volcanoes can have greater impacts than earthquakes. (7 marks)
- Variety of Hazards from Volcanoes
Volcanoes produce multiple hazards, including:
Lava flows that destroy infrastructure and ecosystems.
Pyroclastic flows that incinerate everything in their path.
Ash fall that disrupts air travel, agriculture, and water supplies.
Lahars (volcanic mudflows) that bury towns under thick layers of debris.
Earthquakes primarily cause ground shaking, which leads to building collapses but has fewer secondary hazards.
Example: The 1991 Mount Pinatubo eruption (Philippines) caused global cooling, ash fall, and lahars that destroyed villages. - Global Environmental and Economic Effects
Large volcanic eruptions inject ash and sulfur dioxide into the atmosphere, leading to climate cooling and agricultural failures.
Example: The 1815 Tambora eruption (Indonesia) caused the “Year Without a Summer”, resulting in global crop failures and famine.
Earthquakes primarily impact local or regional areas, with fewer long-term global consequences. - Duration and Predictability
Earthquakes occur suddenly and last seconds to minutes, whereas volcanic eruptions can last for days, months, or even years.
Example: The Kīlauea eruption (Hawaii, ongoing for decades) continuously reshapes the landscape and disrupts communities.
This prolonged activity means volcanic impacts persist far longer than earthquakes. - Secondary Effects
Volcanic gases (e.g., sulfur dioxide, carbon dioxide) contribute to acid rain, respiratory illnesses, and long-term climate changes.
Landslides and tsunamis triggered by volcanic eruptions can worsen destruction.
Example: The 1883 Krakatoa eruption (Indonesia) generated a massive tsunami, killing 36,000 people.
Suggest reasons for the differences in duration of precursors for various natural hazards e.g. Earthquakes occur suddenly because tectonic stress is released instantly
- Sudden vs. Gradual Hazard Formation
Earthquakes occur suddenly because tectonic stress is released instantly—this makes prediction difficult and means little to no precursors exist.
Volcanoes, hurricanes, and tsunamis develop over time, allowing for precursor detection.
Example: Hurricanes have days to weeks of precursors, while earthquakes may have only seconds of warning. - Detectability and Monitoring
Volcanoes show long-term warning signs (e.g., gas emissions, seismic activity, and ground deformation) before eruption.
Tsunamis have minimal precursors, as they are caused by sudden undersea earthquakes or landslides.
Example: Kīlauea Volcano (Hawaii) had weeks of precursor activity before erupting, while the 2004 Indian Ocean tsunami struck within minutes of an undersea earthquake. - Geographic and Climatic Influences
Hurricanes take time to form over warm ocean waters, allowing meteorologists to track them for days.
Earthquakes are harder to predict because tectonic stress can build for centuries before sudden release.
Example: The San Andreas Fault (California) has built up seismic stress for over a century, but with no major precursor signals.
Explain how pyroclastic flows are formed. (6 marks)
- Column Collapse from an Eruption
During an explosive eruption, a tall column of ash, gas, and tephra is ejected into the atmosphere.
If this column becomes too dense and loses its upward momentum, it collapses, causing material to surge down the volcano’s slopes.
Example: The Mount Pelée eruption (1902, Martinique) produced a pyroclastic flow that destroyed St. Pierre, killing ~30,000 people. - Dome Collapse and Landslides
Some volcanoes form viscous lava domes that block the vent.
When the dome becomes unstable, it collapses under its own weight, generating a hot pyroclastic avalanche.
Example: The Soufrière Hills eruption (Montserrat, 1997) resulted in dome collapse and pyroclastic flows that devastated the island’s capital. - Lateral Blasts from Explosive Eruptions
Occasionally, a sideways explosion occurs instead of a vertical eruption, sending pyroclastic material outward at high speeds.
Example: The Mount St. Helens eruption (1980, USA) caused a massive lateral blast, producing a deadly pyroclastic surge that flattened forests and killed wildlife.
Explain the formation of a tsunami (6 marks)
- Submarine Earthquake and Water Displacement
A megathrust earthquake occurs at a subduction zone, where one tectonic plate is forced beneath another.
The seafloor is suddenly displaced, causing a massive upward push of water.
This displacement creates a series of waves that radiate outward in all directions from the epicenter. - Wave Propagation Across the Ocean
The displaced water generates waves that spread outward in all directions at speeds of up to 800 km/h in deep water.
In deep water, tsunami waves have long wavelengths (100–200 km) and low wave heights (~1 m), making them difficult to detect.
As the waves travel across the ocean, they maintain their energy over long distances. - Shoaling Effect and Coastal Impact
As the tsunami waves approach shallow water, friction with the seabed slows them down, causing the waves to increase in height.
Water often withdraws from the shore before the tsunami arrives, exposing the seabed—a key warning sign.
The tsunami then surges inland, causing widespread destruction.
Example:
The 2011 Tōhoku Earthquake and Tsunami off the coast of Japan caused massive devastation, with waves reaching 40 meters in some areas.
Explain two factors that influence the height of a tsunami on reaching a coastline. (6 marks)
- Water Depth and Seafloor Gradient
In deep water, tsunami waves have a long wavelength and low height, traveling at speeds of up to 800 km/h.
As the wave enters shallower water, friction with the seafloor slows it down, causing wave height to increase—a process known as shoaling.
Steep offshore slopes lead to smaller tsunami heights, while shallow coastal shelves allow for higher waves.
Example: The 2004 Indian Ocean tsunami had devastating impacts on flat coastal areas in Indonesia, Sri Lanka, and Thailand, where waves exceeded 30 meters in height. - Coastal Shape and Orientation
Bays and inlets can funnel tsunami waves, increasing their height and amplifying destructive power.
Coasts directly facing the tsunami’s direction experience higher waves, while islands or headlands may block some energy, reducing impact.
Example: In the 2011 Tōhoku tsunami (Japan), waves were higher in narrow coastal inlets, where they reached up to 40 meters due to wave focusing.
Explain how volcanic hazards may be related to the type of volcanic eruption. (6 marks)
- Effusive Eruptions (Shield Volcanoes)
These eruptions produce low-viscosity, basaltic lava, which flows easily and over long distances.
Main hazards:
Lava flows, which can destroy infrastructure but move slowly, allowing evacuation.
Gas emissions (e.g., sulfur dioxide), which can cause air pollution and respiratory issues.
Example: The Kīlauea volcano (Hawaii) continuously erupts basaltic lava, threatening roads and homes. - Explosive Eruptions (Stratovolcanoes)
These eruptions produce high-viscosity, silica-rich magma, which traps gas, leading to violent explosions.
Main hazards:
Pyroclastic flows (fast-moving clouds of hot gas and ash) that incinerate everything in their path.
Lahars (volcanic mudflows) caused by ash mixing with rain or melted snow.
Ash fall that disrupts air travel and agriculture.
Example: The Mount St. Helens eruption (1980) produced pyroclastic flows and a massive lateral blast. - Phreatomagmatic Eruptions (Water-Magma Interaction)
When magma contacts water, it causes steam explosions, triggering violent eruptions and tsunamis.
Example: The 1883 Krakatoa eruption (Indonesia) triggered a tsunami that killed 36,000 people
