# Chapter 5 Study Guide, Version 2: Volcanoes This version follows the chapter section by section. For each section, review the summary, key terms, major ideas, examples, and questions before moving to the next section. ## Chapter Learning Objectives - **LO:1:** Explain the relationship of volcanoes to plate tectonics. - **LO:2:** Identify the different types of volcanoes and their associated features. - **LO:3:** Locate on a map the geographic regions at risk from volcanoes. - **LO:4:** Describe the effects of volcanoes and how they are linked to other natural hazards. - **LO:5:** List the potential benefits of volcanic eruptions. - **LO:6:** Discuss how humans can minimize the volcanic hazard. - **LO:7:** Recommend adjustments we can make to avoid death and damage from volcanoes. ## 5.1 Introduction to Volcanism (LO:1) ### Section summary Volcanism is strongly related to plate tectonics. Most volcanoes occur near active plate boundaries, and about two-thirds of active volcanoes above sea level are associated with the Pacific Ring of Fire. Volcanoes form around vents, which may be roughly circular or elongated fissures. Magma is molten rock beneath Earth's surface; lava is magma that has erupted. Most magma forms in the asthenosphere, the weak upper-mantle layer beneath the lithosphere. Magma is less dense than surrounding rock, so it can rise and collect in magma chambers. ### How magma forms The three principal processes that generate magma are: 1. **Decompression melting:** Pressure decreases when hot mantle rises or when the lithosphere stretches and thins. Melting can occur without an increase in temperature. This process dominates at divergent boundaries, mid-ocean ridges, rifts, and hot spots. 2. **Addition of volatiles:** Water and other volatiles released from a subducting oceanic plate lower the melting temperature of the asthenosphere. This process produces much of the magma above subduction zones. 3. **Addition of heat:** Rising magma transfers heat to surrounding mantle or crust. The heated rock may melt and mix with the original magma, especially at continental hot spots. ### Magma composition and evolution Magma is made of melted silicate rock and dissolved gases. The three major magma types are: | Magma type | Silica content | General character | Volcanic rock | |---------------|---------------:|----------------------------------|---------------| | **Basaltic** | 45-55% | Mafic, hot, low viscosity | Basalt | | **Andesitic** | 55-65% | Intermediate | Andesite | | **Rhyolitic** | More than 65% | Felsic, generally high viscosity | Rhyolite | As magma rises, it may cool, mix with other magma, and assimilate surrounding crust. These processes commonly increase its silica content. The change from primitive basaltic magma toward more silica-rich magma is called **magma evolution**. ### Viscosity and volatile content - **Viscosity** is resistance to flow. More silica and lower temperature generally produce higher viscosity. - Low-viscosity basaltic magma can flow far from its vent and commonly produces effusive eruptions. - High-viscosity andesitic or rhyolitic magma tends to pile up near the vent and can form steep-sided domes. - Dissolved volatiles form expanding bubbles when pressure decreases. Low-viscosity, volatile-poor magma usually erupts passively, while high-viscosity, volatile-rich magma is more likely to erupt explosively. ### Key terms - **Volcanism:** Volcanic activity, including the movement and eruption of magma and lava. - **Magma:** Molten rock beneath Earth's surface. - **Lava:** Magma that reaches Earth's surface. - **Volcanic vent:** An opening through which lava and other volcanic materials are extruded. - **Fissure:** An elongated crack that can act as a volcanic vent. - **Magma chamber:** A near-surface reservoir where magma may accumulate. - **Asthenosphere:** The weak mantle layer from which most magma forms. - **Volatiles:** Compounds such as water, carbon dioxide, and sulfur dioxide that can exist as gases at Earth's surface. - **Viscosity:** Resistance to flow. - **Magma evolution:** The change from primitive basaltic magma toward more silica-rich magma. - **Effusive eruption:** A relatively passive eruption in which low-viscosity lava flows onto the surface. - **Explosive eruption:** An eruption in which volatile-rich, high-viscosity magma fragments and is violently ejected. ### What to be able to explain - Why volcanoes cluster at plate boundaries and hot spots. - How divergent boundaries, subduction zones, and continental hot spots generate magma differently. - Why adding water lowers the melting temperature of mantle rock. - How silica content, temperature, viscosity, and volatile content affect eruption style. - Why basaltic magma commonly forms broad lava flows while rhyolitic magma may form a steep dome. ### Section 5.1 review questions 1. What is magma, and where does most magma come from? 2. Describe decompression melting, addition of volatiles, and addition of heat. 3. How does plate tectonics control the location of volcanoes? 4. What is viscosity, and what factors determine magma viscosity? 5. Explain the relationship between magma composition, viscosity, and gas content. ## 5.2 Volcano Types, Formation, and Eruptive Behavior (LO:2) ### Section summary Volcanoes differ in size, shape, composition, and eruptive behavior. Their characteristics depend on the setting where magma forms, the amount of magma evolution, and the volatile content of the melt. Viscosity and volatile content are the primary controls on whether an eruption is effusive or explosive. ### Major volcano types | Volcano type | Composition and volatiles | Shape and materials | Typical behavior | |-------------------------------------|----------------------------------------------------------|-----------------------------------------------------------------------|-------------------------------------------------| | **Stratovolcano or composite cone** | Andesitic to rhyolitic; high volatile content | Steep cone built from alternating lava flows and pyroclastic deposits | Explosive and effusive eruptions | | **Lava dome** | Andesitic to rhyolitic; low to moderate volatile content | Small, steep-sided mound formed by viscous lava piling up near a vent | Slow growth, but dome collapse can be explosive | | **Shield volcano** | Basaltic; low volatile content | Broad, gently sloping volcano built from many lava flows | Mostly effusive, far-traveling lava flows | | **Cinder cone** | Basaltic; low to moderate volatile content | Small, steep cone of scoria and other tephra around a vent | Tephra ejection with associated lava flows | | **Continental caldera** | Rhyolitic; high volatile content | Broad depression formed by explosive eruption and collapse | Rare, enormous explosive eruptions | ### Volcano features and eruption styles - A **crater** is a relatively small depression around a volcanic vent, commonly associated with explosive extrusion. - A **caldera** is a much larger depression formed when a magma chamber is partly emptied and the overlying volcano or land surface collapses. - **Hawaiian-type eruptions** are effusive basaltic eruptions, commonly associated with shield volcanoes. - **Icelandic-type eruptions** are effusive basaltic eruptions from elongated fissures, producing a curtain of fire. - **Strombolian-type eruptions** are mildly explosive basaltic eruptions commonly associated with cinder cones. - **Pelean-type eruptions** involve viscous magma, lava-dome growth or collapse, and pyroclastic flows. - **Plinian and ultra-Plinian eruptions** produce tall eruption columns and abundant pyroclastic debris, commonly from stratovolcanoes or continental calderas. - Basaltic eruptions can become explosive when magma contacts groundwater, snow, or ice. These are called phreatomagmatic or Vulcanian-type eruptions. ### Volcanic Explosivity Index The **Volcanic Explosivity Index (VEI)** compares eruptions using plume height, volume of ejected material, and the violence of the eruption. Except for the lower end of the scale, each increase in VEI represents about a tenfold increase in ejecta volume. VEI is useful for comparing eruptions, but it does not by itself determine total hazard. Location, duration, linked hazards, population, infrastructure, and evacuation capacity also determine losses. ### Case study: Mount Unzen, Japan Earthquake activity beneath Mount Unzen began in 1989. Earthquake foci became progressively shallower and moved toward the summit, indicating that magma was rising. Explosive eruptions began in 1990, and lava-dome collapse produced pyroclastic flows. In 1991, a pyroclastic flow killed 43 people, including three volcanologists. The case demonstrates that earthquake monitoring can reveal magma movement, lava-dome growth can create continuing danger, and people may remain at risk even after evacuation has begun. ### What to be able to explain - Why stratovolcanoes are steep and commonly explosive. - Why shield volcanoes are broad and commonly effusive. - Why lava domes can be small but dangerous. - How cinder cones form from scoria and tephra. - How large calderas form and why they remain hazardous for long periods. - How the VEI compares eruptions and why it is not a complete risk measure. ### Section 5.2 review questions 1. How is a crater different from a caldera? 2. Describe the major volcano types and their compositions. 3. What characteristic of magma controls a volcano's shape, and how? 4. How are Icelandic-type and Hawaiian-type eruptions alike and different? 5. Why can stratovolcanoes and lava domes have different explosive behavior even when their compositions are similar? 6. Explain how large caldera eruptions occur and why they are so dangerous. ## 5.3 Geographic Regions at Risk from Volcanoes (LO:3) ### Section summary Volcanic activity is not randomly distributed. Most volcanoes occur at plate boundaries, but hot spots and continental rifts also produce volcanic hazards. ### Regions to locate - **Pacific Ring of Fire:** Surrounds much of the Pacific Basin and contains many subduction-related, explosive volcanoes. Important areas include Alaska, the Pacific Northwest, the Andes, Japan, Indonesia, the Philippines, and New Zealand. - **Mid-ocean ridges:** Divergent boundaries where decompression melting produces extensive basaltic volcanism. Iceland is an above-sea-level part of the Mid-Atlantic Ridge. - **East Africa:** Rift-related volcanism occurs where tectonic plates are pulling apart. - **Hawaii:** Oceanic hot-spot volcanism produces large shield volcanoes. - **Yellowstone and Long Valley:** Continental hot-spot regions capable of large, infrequent caldera-forming eruptions. ### North American risk In the contiguous United States and Canada, the greatest local volcanic hazard is in: - the Pacific Coast mountains from northern California through Oregon and Washington into southern British Columbia; - Alaska, British Columbia, and the Yukon; - the Yellowstone area; - Long Valley, California; and - isolated volcanic fields in parts of the western United States. The eastern two-thirds of the United States and Canada have little or no local volcanic activity because they are far from active plate boundaries, major rift zones, and known hot spots. However, a large eruption in the western part of North America could spread ash and ash clouds far from the source, affecting aviation, agriculture, transportation, and communities. ### Population and risk Approximately 500 million people live close to volcanoes. Densely populated countries with many active volcanoes, including Japan, Mexico, the Philippines, and Indonesia, are especially vulnerable. The risk depends not just on volcanic activity but also on population density, land use, warning time, and preparedness. ### Section 5.3 review questions 1. Which part of the United States has the greatest local volcanic hazard, and why? 2. Why are there no major local volcanic hazards along the East Coast of the United States and Canada? 3. Why can a region far from a volcano still be at risk from ash? 4. Locate the Ring of Fire, Iceland, East Africa, Hawaii, Yellowstone, and Long Valley on a map and identify their tectonic settings. ## 5.4 Effects of Volcanoes (LO:4) ### Section summary Volcanic hazards are divided into **primary effects**, which result directly from an eruption, and **secondary effects**, which result from the primary activity or its interaction with the environment. Approximately 500 million people live close to volcanoes, so a volcanic event near a densely populated area can become catastrophic. ### Primary effects - Lava flows. - Ash fall and other pyroclastic deposits. - Pyroclastic flows. - Lateral blasts. - Volcanic gases and acidic aerosols. ### Secondary effects - Lahars, debris flows, and mudflows. - Volcanic landslides and debris avalanches. - Flooding from melted snow and ice or blocked drainage. - Fires started by lava or hot pyroclastic material. - Tsunamis caused by volcanic collapse, landslides, or explosions entering the sea. - Temporary global or regional cooling after sulfur dioxide and ash enter the atmosphere. ### Lava flows and lava tubes Basaltic lava can move quickly near a vent but usually slows as it cools. **Pahoehoe** has a smooth or ropey surface. **Aa** is cooler, thicker, slower, and blockier. Most lava flows move slowly enough for people to evacuate, but they can destroy homes, roads, forests, farmland, and other property. Lava tubes allow molten rock to travel beneath a cooled surface. When the lava drains, the tube may remain as a cavern and can create a hazard if its roof collapses. When lava enters the ocean, it can shatter into sand-sized particles and create new black-sand beaches. Thus, lava flows can destroy existing land while also creating new land. ### Pyroclastic activity and tephra **Pyroclastic debris**, or **tephra**, is explosively ejected volcanic material. It ranges from ash smaller than 2 mm, to lapilli, to larger blocks and bombs. Ash can travel hundreds or thousands of kilometers, depending on eruption size and wind. Ash fall can: - destroy crops and vegetation; - contaminate surface water; - irritate the eyes and respiratory system; - add enough weight to roofs to cause collapse; - damage aircraft engines; and - disrupt transportation and communities far from the volcano. ### Pyroclastic flows and lateral blasts Pyroclastic flows are hot, fast mixtures of gas, ash, pumice, and rock. They can exceed 400 km/hr and reach temperatures near 1,000 degrees C. They are among the most lethal volcanic hazards and can destroy everything in their path. They can form when: 1. an eruption column becomes too dense and collapses; 2. an explosion destroys part of a volcano and sends material sideways as a lateral blast; or 3. a viscous lava dome grows and collapses. ### Volcanic gases Water vapor and carbon dioxide make up most volcanic gas emissions. Other hazardous gases include carbon monoxide, sulfur dioxide, and hydrogen sulfide. - Dense carbon dioxide can collect in low areas and suffocate people and animals. - Sulfur dioxide and acidic particles can produce vog, acid rain, respiratory problems, and contaminated water. - Lava entering seawater can create a corrosive steam cloud containing hydrogen chloride. ### Lahars, debris flows, and mudflows A **lahar** is a volcanic debris flow or mudflow formed when ash and loose volcanic material become saturated with water. Water may come from melted snow or ice, heavy rainfall, or a lake. Lahars are generally cooler than pyroclastic flows and may occur during or after an eruption. They can travel tens of kilometers down valleys and may reach communities long after the eruption begins. Old lahar deposits are therefore important evidence when preparing hazard maps. ### Case study: Nevado del Ruiz, Colombia, 1985 An eruption melted snow and ice on Nevado del Ruiz. Water mixed with ash and pyroclastic debris, forming lahars that traveled down river valleys and nearly destroyed Armero. About 21,000 people died. The case demonstrates why hazard maps, warnings, and evacuation plans must account for flows that begin high on a volcano but travel far downstream. ### Case study: Mount Rainier, Washington The Osceola and Electron mudflows show that very large lahars have traveled far from Mount Rainier in the past. Hundreds of thousands of people now live in areas covered by old flows. Automated acoustic-flow monitors can detect the ground vibrations of moving lahars and provide time for evacuation to higher ground. ### Case study: Mount St. Helens, Washington, 1980 Mount St. Helens awoke after more than a century of dormancy. Earthquakes and a rapidly growing flank bulge preceded the eruption. On May 18, 1980, an earthquake triggered a massive debris avalanche, followed seconds later by a lateral blast. Pyroclastic flows, ash, mudflows, floods, and landslides followed. The eruption: - killed 57 people; - devastated about 600 km2 of land; - destroyed more than 100 homes through associated flooding; - sent ash across much of the United States; and - caused more than $1 billion in damage. The event illustrates how one eruption can produce a sequence of linked hazards rather than one isolated effect. ### Case study: Volcanic landslides and tsunamis Giant volcanic landslides have occurred on Hawaii and the Canary Islands. A future submarine debris avalanche could displace seawater and generate a very large tsunami. These events are rare, but their potential effects could extend far beyond the volcanic island where the landslide begins. ### Section 5.4 review questions 1. Describe the primary and secondary effects of volcanic eruptions. 2. Compare ash fall and pyroclastic flows in terms of temperature, speed, reach, and danger. 3. What gases are emitted during volcanic eruptions, and how can they be hazardous? 4. Explain how volcanoes produce lahars and other debris flows. 5. Why are old lahar deposits important for hazard assessment? 6. Use Mount St. Helens or Nevado del Ruiz to trace a sequence of linked volcanic hazards. ## 5.5 Linkages between Volcanoes and Other Natural Hazards (LO:4) ### Section summary Volcanoes interact with tectonics, the atmosphere, water, ice, slopes, and human systems. The linked hazards may be more damaging than the initial eruption alone. ### Major linkages - **Earthquakes:** Rising magma fractures surrounding rock, producing earthquake swarms and tremors. Earthquakes may also trigger landslides or volcanic flank collapse. - **Fires:** Lava and hot pyroclastic material can ignite vegetation, buildings, and gases released from heated organic matter. - **Landslides:** Volcanic slopes can fail because they are steep, fractured, altered, or overloaded by lava and pyroclastic deposits. - **Floods:** Eruptions can melt snow and glacial ice, create jokulhlaups, block rivers, and redirect drainage. - **Lahars:** Meltwater or rainfall can mix with ash and loose volcanic debris and flow down valleys. - **Tsunamis:** Volcanic landslides, flank collapse, or explosions entering the ocean can displace seawater. - **Climate change:** Large eruptions can inject sulfur dioxide into the stratosphere. The resulting aerosols can temporarily cool the atmosphere. - **Human networks:** Ash clouds can disrupt aircraft and transportation across countries or continents even when the eruption itself is local. ### Example: Eyjafjallajokull, Iceland, 2010 The eruption shows several linkages at once. Magma interacted with glacial ice, producing floods and fine ash. Ash was carried toward Europe, where it grounded aircraft and affected millions of travelers. Hydrologic monitoring helped warn people about flood pulses, while seismic and GPS data revealed volcanic unrest. ### Section 5.5 review questions 1. Why are earthquakes commonly associated with volcanic eruptions? 2. How can volcanic activity produce fires, landslides, floods, lahars, and tsunamis? 3. How can a volcanic eruption temporarily affect global climate? 4. How did Eyjafjallajokull demonstrate that a local hazard can have international effects? ## 5.6 Natural Service Functions of Volcanoes (LO:5) ### Section summary Volcanoes are hazardous, but they also provide important natural services and resources. These benefits can encourage people to live near volcanoes, increasing exposure to future eruptions. ### Benefits of volcanoes - **Fertile soils:** Weathering of volcanic rock releases nutrients that support crops such as coffee, maize, pineapple, sugarcane, and grapes. - **Geothermal power:** Volcanic heat can provide electricity, building heat, and industrial energy. It can be renewable when used no faster than it is naturally replenished. - **Mineral resources:** Volcanic systems provide metals such as gold, silver, copper, nickel, lead, and zinc, as well as pumice, tuff, perlite, scoria, basalt, and volcanic clay. - **Recreation and education:** Hot springs, spas, hiking, snow sports, tourism, and observing eruptions all depend on volcanic landscapes. - **Creation of new land:** Lava builds islands such as Hawaii and Iceland and can extend coastlines and create black-sand beaches. - **Atmosphere and hydrosphere:** Ancient volcanic gases and water vapor contributed to Earth's early atmosphere and hydrologic systems. ### Benefit-risk tradeoff The same fertile soils, geothermal resources, minerals, and scenic landscapes that benefit society can attract settlement near active volcanoes. Natural services therefore can increase exposure and risk. ### Section 5.6 review questions 1. How do volcanic rocks contribute to human and animal food supplies? 2. What is geothermal energy, and what is its primary advantage over fossil fuels? 3. List the mineral, recreational, scientific, and land-building benefits of volcanoes. 4. Why can volcanic benefits increase volcanic risk? ## 5.7 Human Interactions with Volcanoes (LO:6) ### Section summary Unlike some hazards, volcanic eruptions cannot currently be prevented or controlled. Human activity does not appear to control the timing or severity of eruptions. The practical goal is to reduce exposure, vulnerability, deaths, property damage, and disruption. ### Why people live near volcanoes People may live near volcanoes because: - they were born there and have family or cultural ties; - all available land on an island may be volcanic; - volcanic soils are fertile; - volcanic landscapes provide jobs and income; - people underestimate the probability of an eruption; or - economic circumstances prevent them from choosing a safer location. Age, length of residence, personal experience, knowledge, and trust in authorities can affect how people perceive the hazard. ### How humans increase losses Human vulnerability rises when people: - build on volcano flanks or in valleys followed by lava flows and lahars; - construct homes and critical facilities on old lava, pyroclastic, or lahar deposits; - ignore hazard maps or fail to enforce land-use restrictions; - assume that a dormant volcano is permanently safe; - lack evacuation routes, shelters, supplies, and communication systems; or - fail to coordinate scientists, emergency managers, government officials, media, and residents. ### Volcanic crisis A **volcanic crisis** develops when scientists judge that an eruption may occur soon. Scientific information alone is not enough. Clear communication is necessary so that officials and residents understand the uncertainty, recognize the danger, and act before routes are blocked. ### Section 5.7 review questions 1. Why can humans not prevent or control volcanic eruptions? 2. Why do people continue to live near active or potentially active volcanoes? 3. How can land-use decisions and poor communication increase volcanic losses? 4. What groups need to communicate during a volcanic crisis? ## 5.8 Minimizing the Volcanic Hazard (LO:6 and LO:7) ### Section summary The most effective hazard reduction combines forecasting, monitoring, hazard mapping, land-use planning, communication, evacuation, and preparedness. A forecast is a probabilistic statement about the likely time, place, and character of an eruption. It is less precise than a prediction. Most volcanic activity cannot yet be forecast accurately, but monitoring can identify unrest and sometimes provide useful warning. Forecasting is generally more successful at volcanoes with a well-understood history, such as Hawaiian volcanoes and Mount Pinatubo. ### Monitoring volcanic activity #### Seismic activity Shallow earthquakes and tremors can occur as rising magma fractures rock and gas bubbles form and burst. Increasing numbers of earthquakes, larger earthquakes, or earthquake foci that migrate upward can indicate rising magma. Seismic activity is useful but not perfectly reliable; Mount St. Helens did not show an additional increase immediately before its catastrophic eruption. #### Land-surface changes Magma movement can cause tilt, swelling, uplift, subsidence, and cracks. GPS, satellite radar, aerial photography, and ground instruments can detect these changes. Kilauea's summit commonly swells and tilts before an eruption and subsides during the eruption. #### Thermal, magnetic, and hydrologic monitoring Rising magma can heat surrounding rock, melt snow or ice, alter magnetic properties, and change water levels or flow. Satellite remote sensing, infrared photography, magnetic surveys, and hydrologic stations help detect these changes. #### Gas emissions Changes in the amount or chemical composition of carbon dioxide and sulfur dioxide can indicate magma movement. Changes in gas emissions helped scientists study Mount St. Helens and Mount Pinatubo. #### Geologic history Mapping and dating old lava flows, ash layers, pyroclastic deposits, and lahar deposits reveal the type, size, frequency, and geographic reach of past eruptions. This information is used to create hazard maps and guide land-use planning. ### USGS alert systems The USGS uses two related four-level systems: | Ground alert level | Aviation color code | General meaning | |--------------------|---------------------|-----------------------------------------------------------------------------------| | **Normal** | Green | Typical background, noneruptive state | | **Advisory** | Yellow | Elevated unrest above background level | | **Watch** | Orange | Heightened or escalating unrest, or an eruption with limited hazards | | **Warning** | Red | Hazardous eruption imminent, underway, or suspected; significant ash for aviation | Ground and aviation levels usually correspond, but they may differ. For example, an eruption may pose limited danger to people on the ground while producing enough ash to create a serious aviation hazard. ### Community adjustments - Map lava, ash, pyroclastic-flow, lahar, gas, flood, and landslide hazards. - Keep homes, schools, hospitals, emergency facilities, and critical infrastructure out of the most dangerous areas when possible. - Establish evacuation zones, routes, transportation plans, and shelters before an eruption. - Maintain seismic, deformation, gas, thermal, magnetic, and hydrologic monitoring. - Use sirens, alerts, radio, television, social media, and other communication systems. - Educate residents, visitors, officials, responders, and the media. - Install automated lahar detection systems where communities lie in lahar paths. - Conduct drills and maintain supplies and emergency plans. ### Limits of engineering control Lava-flow diversion, barriers, and hydraulic chilling have had mixed success. They may help with slow, small flows under favorable conditions, but they cannot be expected to stop large flows. Evacuation and land-use planning are generally more dependable than trying to control an eruption after it begins. ### Case study: Mount Pinatubo, Philippines, 1991 Scientists monitored earthquakes, studied the volcano's past, and educated local communities about explosive eruptions and debris flows. More than 60,000 people were evacuated before the catastrophic eruption. The event demonstrates the value of combining scientific monitoring with public education and government action. ### Case study: Professional profile Volcanologist Chris Eisinger described the importance of observing gas emissions, eruption cycles, seismic activity, and changing conditions in the field. The profile also emphasizes that even experienced volcanologists can be killed when an active volcano erupts with little warning. ### Section 5.8 review questions 1. How can seismic activity indicate an approaching eruption? 2. How is land-surface monitoring used to detect volcanic activity? 3. What thermal, magnetic, hydrologic, and gas changes can signal volcanic unrest? 4. Why is geologic history important for forecasting and hazard mapping? 5. Explain the USGS ground alert levels and aviation color codes. 6. Why can ground and aviation alert levels differ? ## 5.9 Perception of and Adjustment to the Volcanic Hazard (LO:7) ### Section summary People's perceptions of volcanic hazards vary. Some people have firsthand experience and understand the danger, while others are optimistic, economically constrained, or unfamiliar with the volcano's history. A successful adjustment depends on public understanding, communication, preparedness, and timely evacuation. ### Perception of volcanic hazards People may underestimate the risk because: - the volcano has been dormant for a long time; - previous eruptions were small; - the person has never experienced an eruption; - hazard maps may threaten property values; - people trust barriers or other engineering measures too much; or - economic and social ties make relocation difficult. Education can change perception. Before the Mount Pinatubo eruption, showing people what violent ash eruptions and debris flows could do helped convince residents and officials that evacuation was necessary. Mount Vesuvius illustrates how exposure can increase even when the underlying volcanic hazard remains. The eruption that destroyed Pompeii and Herculaneum in A.D. 79 occurred when the surrounding population was much smaller; nearly 3 million people now live in the area around the volcano. Italy has offered financial incentives to residents of nearby towns who are willing to relocate. The modern risk is therefore greater because many more people and structures are exposed. ### Primary adjustment: evacuation Evacuation is the main adjustment for people in the path of lava, pyroclastic flows, lahars, landslides, toxic gases, ash, or floods. Effective evacuation requires: - clear and trusted warnings; - known routes and shelters; - transportation for people without vehicles; - plans for children, older adults, people with disabilities, and livestock; - early action before roads are blocked; and - continued monitoring after the first evacuation. People should not remain near an eruption to watch it. They should move away from the volcano and out of valleys and channels that can carry lahars or debris flows. ### Personal safety Before an eruption: - Learn the volcano's hazard zones, evacuation routes, and warning systems. - Know whether your home, workplace, school, or travel route lies in a hazard zone. - Prepare transportation, food, water, medication, emergency supplies, and communication plans. During an eruption or warning: - Follow official evacuation instructions immediately. - Avoid pyroclastic flows, lava flows, lahars, gases, unstable slopes, and river valleys. - Protect your eyes and respiratory system from ash and fumes. - Follow official instructions about sheltering, roads, water, and livestock. After an eruption: - Do not return until officials declare the area safe. - Watch for lahars, gas, ash, unstable slopes, contaminated water, damaged roads, and renewed eruptions. - Continue monitoring official alerts and be ready to evacuate again. ### Attempts to control lava flows Hydraulic chilling and walls have sometimes slowed or redirected lava. The 1973 Eldfell eruption on Heimaey, Iceland, provided favorable conditions: the flow was slow, equipment could be delivered, and abundant seawater was available. Fire hoses and pipes helped keep the harbor open. These methods have limits. They are difficult to use, require time and resources, and cannot reliably stop large lava flows. They should not replace evacuation or appropriate land-use planning. ### Survivor story: A close call with Mount St. Helens Reporter Don Hamilton left Spirit Lake and returned to Portland on May 17, 1980, despite the uncertainty surrounding Mount St. Helens. The next morning, the eruption destroyed the area, including Harry Truman's lodge. The story illustrates how personal decisions, official warnings, individual perception, and the difficulty of imagining an extreme event can determine survival. ### Applying the fundamental concepts: Eyjafjallajokull, 2010 Eyjafjallajokull illustrates all five fundamental concepts: 1. **Science helps predict hazards:** Seismic, GPS, and hydrologic data revealed unrest and supported warnings. 2. **Risk information supports decisions:** Iceland's government and residents used prior experience and hazard knowledge to prepare and evacuate. 3. **Hazards are linked:** The eruption produced ash, floods, ice melt, lava, and risks to farms and livestock. 4. **Humans can turn disasters into catastrophes:** A rural setting and low population density limited local deaths, while ash disrupted international aviation. 5. **Consequences can be minimized:** Monitoring stations, evacuation plans, flood-control structures, and aviation restrictions reduced losses. ### Section 5.9 review questions 1. Why do people live near active volcanoes despite the danger? 2. Why was evacuation before the Mount Pinatubo eruption successful? 3. What is the most important personal adjustment to an immediate volcanic threat? 4. What methods have been used to control lava flows, and why have they had mixed success? 5. How did monitoring and preparation reduce the effects of the Eyjafjallajokull eruption? 6. How has the risk from Mount Vesuvius changed over the past 2,000 years? ## Chapter-Wide Review ### Connections among the sections - **5.1 to 5.2:** Plate setting controls magma formation; magma composition and volatile content help control volcano type and eruption style. - **5.2 to 5.3:** Volcano type and tectonic setting help identify where hazards are likely to occur. - **5.3 to 5.4:** Risk depends on both volcanic activity and the people, buildings, and infrastructure located nearby. - **5.4 to 5.5:** An eruption can trigger several secondary hazards, including floods, lahars, landslides, fires, tsunamis, and climate effects. - **5.6 to 5.7:** Volcanic benefits attract people and economic activity, increasing exposure. - **5.7 to 5.8:** Because eruptions cannot be controlled, society must focus on monitoring, communication, land-use planning, and preparedness. - **5.8 to 5.9:** Scientific warnings reduce risk only when people understand them and take appropriate action. ### Likely comparison questions - How do decompression melting, addition of volatiles, and addition of heat differ? - How do basaltic, andesitic, and rhyolitic magmas differ? - How do silica content, viscosity, and volatile content affect eruption style? - How do stratovolcanoes, lava domes, shield volcanoes, cinder cones, and continental calderas differ? - How does a crater differ from a caldera? - How do effusive and explosive eruptions differ? - How do lava flows, ash fall, pyroclastic flows, gases, and lahars differ? - How can volcanic eruptions trigger floods, landslides, fires, tsunamis, and climate effects? - Why are the Ring of Fire, Iceland, East Africa, Hawaii, Yellowstone, and Long Valley volcanic regions? - How do volcanic hazards differ from volcanic natural service functions? - Why do fertile soils and geothermal energy increase human exposure? - What observations can indicate that a volcano is reawakening? - How do ground alert levels differ from aviation color codes? - Why are evacuation and land-use planning generally more reliable than lava-flow control? ### Practice questions 1. Explain the relationship between volcanoes and plate tectonics. 2. Describe the three processes that generate magma. 3. Compare basaltic, andesitic, and rhyolitic magma. 4. Explain how silica and volatile content affect viscosity and eruption style. 5. Compare the five major volcano types. 6. Explain how a continental caldera forms. 7. Explain what the VEI measures and why it does not determine total risk by itself. 8. Locate major volcanic regions on a map and identify their tectonic settings. 9. Describe the primary and secondary effects of volcanic eruptions. 10. Compare lava flows, ash fall, pyroclastic flows, volcanic gases, and lahars. 11. Trace a hazard chain beginning with rising magma. 12. List the natural service functions of volcanoes. 13. Explain why humans continue to live near active volcanoes. 14. Describe the evidence scientists monitor when a volcano reawakens. 15. Explain why forecasts are useful even when exact prediction is impossible. 16. Describe community and personal adjustments before, during, and after an eruption. 17. Use Mount St. Helens, Mount Pinatubo, Nevado del Ruiz, Mount Unzen, or Eyjafjallajokull to explain how science and human decisions affect losses. ## Quick Review Checklist Before the quiz or test, make sure you can: - explain how volcanoes relate to plate boundaries and hot spots; - identify decompression melting, addition of volatiles, and addition of heat; - compare basaltic, andesitic, and rhyolitic magma; - connect silica, viscosity, volatile content, and eruption style; - identify the major volcano types and their features; - distinguish a crater, caldera, lava dome, pyroclastic flow, lahar, and tephra; - use the VEI as a relative measure of explosivity; - locate the Ring of Fire, Iceland, East Africa, Hawaii, Yellowstone, and Long Valley; - identify primary and secondary volcanic effects; - explain how volcanoes produce floods, landslides, fires, tsunamis, and climate effects; - list the fertile-soil, geothermal, mineral, recreational, and land-building benefits of volcanoes; - explain why human activities generally do not control eruptions but do affect losses; - describe seismic, deformation, thermal, magnetic, hydrologic, gas, and geologic monitoring; - distinguish USGS ground alert levels from aviation color codes; - explain why evacuation, education, land-use planning, and hazard maps reduce risk; - describe the limits of lava-flow control; - recommend personal actions before, during, and after an eruption; and - use the Mount St. Helens, Mount Pinatubo, Nevado del Ruiz, Mount Unzen, and Eyjafjallajokull examples to explain volcanic hazards and adjustments.