Volcanic Hazards Program

Smithsonian Institution National Museum of National History Global Volcanism Project

Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

5.1 Compare and contrast the 1980 eruption of Mount St. Helens with the most recent eruption of Kilauea, which began in 1983.

5.2 Explain why some volcanic eruptions are explosive and others are quiescent.

5.3 List and describe the three categories of materials extruded during volcanic eruptions.

5.4 Draw and label a diagram that illustrates the basic features of a typical volcanic cone.

5.5 Summarize the characteristics of shield volcanoes and provide one example of this type of volcano.

5.6 Describe the formation, size, and composition of cinder cones.

5.7 List the characteristics of composite volcanoes and describe how they form

5.8 Describe the major geologic hazards associated with volcanoes.

5.9 List volcanic landforms other than shield, cinder, and composite volcanoes and describe their formation.

5.10 Compare and contrast these intrusive igneous structures: dikes, sills, batholiths, stocks, and laccoliths.

5.11 Summarize the major processes that generate magma from solid rock.

5.12 Explain how the global distribution of volcanic activity relates to plate tectonics.

Concept Checks 

5.1

1. Briefly compare the 1980 eruption of Mount St. Helens to a typical eruption of Hawaii’s Kilauea Volcano.

5.2

1. List these magmas in order, from the highest to lowest silica content: mafic (basaltic) magma, felsic (granitic/rhyolitic) magma, intermediate (andesitic) magma.

2. List the two primary factors that determine the manner in which magma erupts.

3. Define viscosity.

4. Are volcanoes fed by highly viscous magma more or less likely to be a greater threat to life and property than volcanoes supplied with very fluid magma?

5.3

1. Contrast pahoehoe and aa lava flows.

2. How do lava tubes form?

3. List the main gases released during a volcanic eruption.

4. What is scoria? How is it different from pumice?

5.4

1. Distinguish among a conduit, a vent, and a crater.

2. How is a crater different from a caldera?

3. What is a parasitic cone, and where does it form?

5.5

1. Describe the composition and viscosity of the lava associated with shield volcanoes.

2. Are pyroclastic materials a significant component of shield volcanoes?

3. Where do most shield volcanoes form—on the ocean floor or on the continents?

5.6

1. Describe the composition of a cinder cone.

2. How do cinder cones compare in size and steepness of their flanks with shield volcanoes?

5.7

1. What name is given to the region having the greatest concentration of composite volcanoes?

2. Describe the materials that compose composite volcanoes.

3. How does the composition and viscosity of lava flows differ between composite volcanoes and shield volcanoes?

5.8

1. Describe pyroclastic flows and explain why they are capable of traveling great distances.

2. What is a lahar?

3. List at least three volcanic hazards besides pyroclastic flows and lahars.

5.9

1. Describe the formation of Crater Lake.

2. How do the eruptions that created the Columbia Plateau differ from the eruptions that create large composite volcanoes?

3.What type of volcanic structure is Shiprock, New Mexico, and how did it form?

5.10

1. What is meant by the term country rock?

2. Describe dikes and sills, using the appropriate terms from the following list: massive, discordant, tabular, and concordant.

3. Distinguish among batholiths, stocks, and laccoliths in terms of size and shape.

5.11

1. Explain the process of decompression melting.

2. What role does water play in the formation of magma?

3. Briefly explain one way that basaltic magma can generate felsic magma.

5.12

1. Are volcanoes in the Ring of Fire generally described as effusive or explosive? Provide an example that supports your answer.

2. How is magma generated along convergent plate boundaries?

3. Volcanism at divergent plate boundaries is most often associated with which magma type?

4. What is thought to be the source of magma for most intraplate volcanism?

Concepts In Review Volcanoes and Other Igneous Activity 

 5.1  Mount St. Helens Versus Kilauea

Compare and contrast the 1980 eruption of Mount St. Helens with the most recent eruption of Kilauea, which began in 1983.Volcanic eruptions cover a broad spectrum from explosive eruptions, like that of Mount St. Helens in 1980, to the comparatively quiet eruptions of Kilauea.

5.2 The Nature of Volcanic Eruptions

Explain why some volcanic eruptions are explosive and others are quiescent.

Key Terms: magma, lava, effusive eruption, viscosity, eruption column

*The two primary factors determining the nature of a volcanic eruption are a magma’s viscosity (a fluid’s resistance to flow) and its gas content. In general, magmas containing more silica are more viscous. Temperature also influences viscosity; hot lavas are more fluid than relatively cool lavas.

*Mafic (basaltic) magmas, which are fluid and have low gas content, tend to generate effusive (nonexplosive) eruptions. In contrast, silica-rich intermediate (andesitic) and felsic (rhyolitic) magmas, which are the most viscous and contain the greatest quantity of gases, are the most explosive.

QUESTION: Although Kilauea mostly erupts in a gentle manner, what risks might you encounter if you chose to live nearby?

5.3 Materials Extruded During an Eruption

List and describe the three categories of materials extruded during volcanic eruptions.

Key Terms: aa flow, pahoehoe flow, lava tube, pillow lava, volatile, pyroclastic material, tephra, scoria, pumice

*Volcanoes erupt lava, gasses, and solid pyroclastic materials.

*Low-viscosity basaltic lava can flow great distances. On the surface, they travel as pahoehoe or aa flows. Fluid lavas congeal and harden at the surface, while the lava below the surface continues to flow in tunnels called lava tubes. When lava erupts underwater, the outer surface is instantly chilled while the inside continues to flow, producing pillow lavas.

*The gasses most commonly emitted by volcanoes are water vapor and carbon dioxide. Upon reaching the surface, gasses rapidly expand, leading to explosive eruptions that can generate a mass of lava fragments called pyroclastic materials.

*Pyroclastic materials come in several sizes. From smallest to largest, they are ash, lapilli, and blocks or bombs. Blocks exit the volcano as solid fragments, whereas bombs are ejected as liquid blobs.

*Bubbles of gas in lava can be preserved as voids in rocks called vesicles. Especially frothy, silica-rich lava can cool to form lightweight pumice, while basaltic lava with lots of bubbles cools to form scoria.

5.4 Anatomy of a Volcano

Draw and label a diagram that illustrates the basic features of a typical volcanic cone.

Key Terms: fissure, conduit, vent, volcanic cone, crater, caldera, parasitic cone, fumarole

*Volcanoes vary in size and form but share a few common features. Most are roughly conical piles of extruded material that collect around a central vent. The vent is usually within a summit crater or caldera. On the flanks of the volcano, there may be smaller vents marked by small parasitic cones, or there may be fumaroles, spots where only gas is expelled.

QUESTION: Label the diagram using the following terms: conduit, vent, lava, parasitic cone, bombs, pyroclastic material. 

5.5 Shield Volcanoes

Summarize the characteristics of shield volcanoes and provide one example of this type of volcano.

Key Terms: shield volcano, seamount

*Shield volcanoes consist of many successive flows of low-viscosity basaltic lava but lack significant amounts of pyroclastic debris. Lava tubes help transport lava far from the main vent, resulting in very gentle, shield-like profiles.

*Most shield volcanoes begin as seamounts that grow from Earth’s seafloor. Mauna Loa, Mauna Kea, and Kilauea in Hawaii are classic examples of the low, wide form characteristic of shield volcanoes.

5.6 Cinder Cones

Describe the formation, size, and composition of cinder cones.

Key Term: cinder cone (scoria cone)

*Cinder cones are steep-sided structures composed mainly of pyroclastic debris, typically having a basaltic composition. Lava flows sometimes emerge from the base of a cinder cone.

*Cinder cones are small relative to the other kinds of volcanoes, reflecting the fact that most form quickly as single eruptive events. Because they are unconsolidated, cinder cones easily succumb to weathering and erosion.

5.7 Composite Volcanoes

List the characteristics of composite volcanoes and describe how they form.

Key Term: composite volcano (stratovolcano)

*Composite volcanoes have a classic symmetrical cone shape and are named because they consist of both pyroclastic material and lava flows. They typically erupt silica-rich magmas of andesitic or rhyolitic composition. They are much larger than cinder cones and form from multiple eruptions over millions of years.

*Because andesitic and rhyolitic lavas are more viscous than basaltic lava, they accumulate at a steeper angle than does the lava from shield volcanoes.

*Mount Rainier and the other volcanoes of the Cascade Range in the northwestern United States are good examples of composite volcanoes.

5.8 Volcanic Hazards

Describe the major geologic hazards associated with volcanoes.

Key Terms: pyroclastic flow (nuée ardente), lahar, tsunami

*The greatest volcanic hazard to human life is the pyroclastic flow. This dense mix of hot gas and pyroclastic materials races downhill at great speed and incinerates everything in its path. A pyroclastic flow can travel many kilometers from its source. Because pyroclastic flows are hot, their deposits frequently “weld” together into a solid rock called welded tuff.

*Lahars are mudflows that form on volcanoes. These rapidly moving slurries of ash and debris suspended in water tend to follow stream valleys and can result in loss of life and/or significant damage to structures.

*Volcanic ash in the atmosphere can be a risk to air travel when it is sucked into airplane engines. Volcanoes at sea level can generate tsunamis when they erupt or when their flanks collapse into the ocean. Those that spew large amounts of gas such as sulfur dioxide can cause respiratory problems. If volcanic gases reach the stratosphere, they screen out a portion of incoming solar radiation and can trigger short-term cooling at Earth’s surface.

5.9 Other Volcanic Landforms

List volcanic landforms other than shield, cinder, and composite volcanoes and describe their formation.

Key Terms: fissure eruption, basalt plateau, flood basalt, volcanic neck (plug)

*Calderas, which can be among the largest volcanic structures, form when the rigid, cold rock above a magma chamber cannot be supported and collapses, creating a broad, roughly circular depression. On shield volcanoes, calderas form slowly as lava drains from the magma chamber beneath the volcano. On a composite volcano, caldera collapse often follows an explosive eruption that can result in significant loss of life and destruction of property.

*Fissure eruptions occasionally produce massive floods of fluid basaltic lava from large cracks, called fissures, in the crust. Layer upon layer of these flood basalts may accumulate to significant thicknesses and blanket a wide area. The Columbia Plateau in the northwestern United States is an example.

*Lava domes are thick masses of high-viscosity, silica-rich lava that accumulate in the summit crater or caldera of a composite volcano. When they collapse, lava domes can produce extensive pyroclastic flows.

*Shiprock, New Mexico, is an example of a volcanic neck where the lava in the “throat” of an ancient volcano congealed to form a plug of solid rock that weathered more slowly than the surrounding volcanic rocks. The surrounding pyroclastic debris eroded, and the resistant neck remains as a distinctive landform.

5.10 Intrusive Igneous Activity

Compare and contrast these intrusive igneous structures: dikes, sills, batholiths, stocks, and laccoliths.

Key Terms: host (country) rock, intrusion (pluton), tabular, massive, discordant, concordant, dike, sil,l columnar jointing, batholith, stock, laccolith

*When magma intrudes other rocks, it may cool and crystallize before reaching the surface to produce intrusions called plutons. Plutons come in many shapes. They may cut across the host rocks without regard for pre existing structures, or the magma may flow along weak zones in the host rock, such as between the horizontal layers of sedimentary bedding.

*Tabular intrusions may be concordant (sills) or discordant (dikes). Massive plutons may be small (stocks) or very large (batholiths). A blister-like intrusion that lifts the overlying rock layers is a laccolith. As solid igneous rock cools, its volume decreases. Contraction can produce a distinctive fracture pattern called columnar jointing.

QUESTION: Label the intrusive igneous structures in the accompanying diagram using the following terms: volcanic neck, sill, batholith, laccolith. 

5.11 Partial Melting and the Origin of Magma

Summarize the major processes that generate magma from solid rock.

Key Terms: partial melting, geothermal gradient, decompression melting

*Solid rock may melt under three geologic circumstances: when heat is added to the rock, raising its temperature; when already hot rock experiences lower pressures (decompression, as seen at mid-ocean ridges); and when water is added (as occurs at subduction zones).

5.12 Plate Tectonics and Volcanism

Explain how the global distribution of volcanic activity relates to plate tectonics.

Key Terms: Ring of Fire, volcanic island arc (island arc), continental volcanic arc, intraplate volcanism, mantle plume, hot spot, superplume

*Volcanoes form at both convergent and divergent plate boundaries as well as in intraplate settings.

*At divergent plate boundaries, where lithosphere is being rifted apart, decompression melting is the dominant generator of magma. As warm rock rises, it can begin to melt without the addition of heat.

*Convergent plate boundaries that involve the subduction of oceanic crust are the most common site for explosive volcanoes—most prominently in the Pacific Ring of Fire. The release of water from the subducting plate triggers partial melting in the overlying mantle. The ascending magma interacts with the lower crust of the overlying plate and can form a volcanic arc at the surface.

*In intraplate settings, the source of magma is a mantle plume—a column of mantle rock that is warmer and more buoyant than the surrounding mantle.

QUESTION: The accompanying diagram shows one of the tectonic settings where volcanism is a dominant process. Name the setting and briefly explain how magma is generated here.