Ch. 5 - Volcanoes and Volcanic Hazards

Class: GEOL-101

Notes:

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.

St. Helens vs. Kilauea

Presentation content:
Mount St. Helens:

• Eruption blew up the summit, lowering it by 1,350 ft.
• Ejected ash and rock debris over 400 sq. km (160 sq. miles)
• Ash Blankets 2 meters (6 ft) deep.
• Mudflows carried ash, tree, and rock debris 29 km (18 miles)

Kilaua:

• Large amounts of lava and associated fires
• Quiet non-explosive eruptions
• Fountains of lava
• Hawaiian Volcano Observatory has operated on the summit since 1912. Surviving more than 50 eruptive phases.

Notes from the lecture:

• Helens
  • Look at the before and after eruption
  • Over time, trees got removed away
  • Ash is not like snow, it is actually much heavier
  • As this volcanoes become active, it heats and all that ice and water on top of them starts melting and mixing.
• Kilauea
  • You can see the lava flowing, burning things along its flow path
  • Fountain full of lava - still very impressive
  • You don't have that explosiveness - not as violent but different types of hazards

5.2 The Nature of Volcanic Eruptions

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

Eruptions

Presentation content:

• All eruptions involve magma
  • Lava is erupted magma
  • The behavior of magma is determined by temperature, composition, dissolved gases.
• Viscosity is a measure of a material’s resistance to flow.
  • More viscous = greater its resistance to flow.
  • Viscosity controls the nature of an eruption.
• Factors Affecting Viscosity
  • Temperature– hotter magmas are less viscous
  • Composition– greater the silica (SiO2) content the more viscous the magma
  • Dissolved Gases– more viscous magmas trap more gases

Notes from the lecture:

• Behavior is dictated by temperature, composition (silica), and other gases
• Salt, peanut butter are very viscous
• The more viscous, the more trapped gases there are

Dissolved Gases

Presentation content:

• Dissolved water vapor in magma reduces viscosity by inhibiting formation of silica tetrahedra chains.
• Gases expand within a magma as it nears Earth’s surface due to decreasing pressure.
• The violence of an eruption is related to how easily gases escape from magma.

Notes from the lecture:

• Mafic, Intermediate, and Felsic compositions
• Gas is trapped in the rocks
  • Decompression: High -> Low pressure
  • As it goes from a depth of high pressure to a low pressure, those gases expand, that can affect the melting and intensity of the eruption.

Quiescent Versus Explosive Eruptions

Presentation content:

• Quiescent Hawaiian-Type Eruptions
  • Involves fluid basaltic lavas
  • Eruptions are characterized by outpourings of lava that can last weeks, months, or even years
• Explosive Eruptions
  • Associated with highly viscous magmas
  • Eruptions expel particles of fragmented lava and gases at supersonic speeds that evolve into eruption columns

Notes from the lecture:

• Quiescent: Lava being generated
• Explosive: Mt. St. Helens, debris, ash, lava, sliding off the volcano
  • Hydrogen Sulfide associated with it
  • Larger eruption and a lot more of pyroclastic material

5.3 Materials Extruded During an Eruption

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

Lava

Presentation content:

• Lava Flows
  • ~90% of lava is basaltic lava
  • <10% of lava is andesitic lava
  • ~1% of lava is rhyolitic lava
• Aa and Pahoehoe Flows (basaltic lava)
  • Aa flows have surfaces of rough jagged blocks
  • Pahoehoe flows have smooth surfaces and resemble twisted braids of rope
• Block Lavas
  • Composed of andesitic and rhyolitic lava
  • Form short prominent flows
  • Upper surface consists of massive, detached blocks
• Pillow Lavas
  • Formed from outpourings of basaltic lava underwater
  • The flow’s outer skin freezes quickly, but interior lava squeezes out by breaking through
  • Flow is composed of tube-like structures stacked one atop the other
• Lava Tubes
  • Cave-like tunnels
  • Often form in pahoeoe flows
  • Previous conduits for lava
  • Form in the interior of a flow where the temperatures remain high long after the exposed surface cools and hardens.

Notes from the lecture:

• Aa - walk across legos "Aa!"
• Lava tubes takes advantage of existing paths, so lava flows throw them
  • Also can flow below a layer of basaltic crust
• Pillow lavas forms on the ocean floor
  • Cools almost instantaneously, continuous to grow

Gases

Presentation content:

• Volatiles (dissolved gases) make up 1‒6% of the total weight of a magma
• As the magma reaches the surface and the pressure is reduced, the gases expand and escape.
• Composition is about 70% H₂O, 15% CO₂, 5% N, 5% SO₂, and 5% others

Notes from the lecture:

• Change in pressure enables the gas to expand out of the magma

Pyroclastic Materials

Presentation content:

• Volcanoes eject pulverized rock and lava fragments called pyroclastic materials (also referred to as Tephra)
• Particles range in size from fine dust, to sand-sized ash, to very large rocks
• Volcanic ash – fine glassy fragments
  • Welded tuff – fused ash
• Lapilli – walnut-sized material
• Cinders – pea-sized material
• Blocks – hardened or cooled lava >2.5 inches diameter
• Bombs – ejected as hot lava >2.5 inches diameter
• Pumice – light gray or pink porous rock from frothy andesitic and rhyolitic lava
• Scoria – reddish-brown porous rock from frothy basaltic lava

Notes from the lecture:

• Material ejected during an eruption
• Lapilli
  • People buy this for landscaping
• Volcanic bombs
  • They explode
  • The air above cools it, and as it comes down it breaks into lava or more pieces
• Volcanic Blocks
  • larger chunks of rocks ejected from the top part of the volcano

5.4 Anatomy of a Volcano

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

General Features

Presentation content:

• Fissure – a crack develops in Earth’s crust as magma moves toward the surface
• Conduit – a somewhat circular pathway from a fissure to the surface
• Vent – the surface opening of a conduit
• Volcanic cone – the cone-shaped structure created by successive eruptions of lava and pyroclastic material
• Crater – a funnel-shaped depression at the summit of most volcanic cones, generally less than 1 km in diameter
• Caldera – a volcanic crater that has a diameter of >1 kilometer and is produced by a collapse following a massive eruption
• Parasitic cones – a flank vent that emits lava and pyroclastic material
• Fumaroles – a flank vent that emits gases

• Shield Volcano: Mauna Loa, Hawaii
• It is not just one single volcano, it is a series of eruptions
• Any of the volcanoes that are on the Pacific Ring of fire are going to be ...

5.5 Shields Volcanoes

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

General Features of Shield Volcanoes

Presentation content:

• Broad, slightly dome-shaped, and covers large areas
• Produced by mild eruptions of large volumes of basaltic lava
• Most begin on the seafloor as seamounts; only a few grow large enough to form a volcanic island
• Examples include the Hawaiian Islands, the Canary Islands, the Galapagos, and Easter Island. Mauna Loa is the largest volcano on Earth

Notes from the lecture:

• Continuous eruption as the pacific plate continues to move.
• Very large volcanoes

5.6 Cinder Cones

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

General Features of Cinder Cones (Also referred to as Scoria Cones)

Presentation content:

• Built from ejected lava fragments to form steep slope angle
• Rather small size (30–300 m tall) and frequently occur in groups
• Sometimes associated with extensive lava fields—but these generally form in the final stages of the volcano’s life span.
• Paricutin (located 320 km west of Mexico City) is an example of a cinder cone.
  

Notes from the lecture:

• You may actually have a series of several composite cones`

5.7 Composite Volcanoes

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

General Features of Composite Volcanoes (Also referred to as Stratovolcanoes)

Presentation content:

• Large, classic-shaped volcano (symmetrical cone, 1000s feet high, several miles wide at the base)
• Composed of interbedded (generally andesitic) lava flows and layers of pyroclastic debris
• Many are located adjacent to the Pacific Ocean in the Ring of Fire
• Mount St. Helens and Mount Etna are examples

Notes from the lecture:

• Consist of layers

5.8 Volcanic Hazards

Describe the major geologic hazards associated with volcanoes.

• We are showing a strato volcano, probably the most dangerous
  • They see very violent eruptions
• Sulfur dioxide is another hazard, you cannot breathe this
• Collapse is dangerous, because it is sudden.
  • Landslides, flows, can be associated with that as well

Pyroclastic Flows (Also called a nuée ardente)

Presentation content:

• A mixture of hot gases infused with incandescent ash and lava fragments that flows down a volcanic slope.
• Propelled by gravity and move similarly to snow avalanches
• Propelled from the vent at high speeds (60 mph)
  • Pyroclastic flows form from the collapse of tall eruption columns.

Notes from the lecture:

• US GS - US Geological Survey
• A huge evacuation of US personnel in the philiphines
• Can travel very very quickly

Lahars

Presentation content:

• A lahar is mudflow on an active or inactive volcano
• Volcanic debris becomes saturated with water and rapidly moves down a volcanic slope
• Some lahars are triggered when magma melts ice and snow on the summit.
• When Mt. St. Helens erupted in 1980, several lahars were generated
• In 1985, lahars formed during the eruption of Nevado del Ruiz, killing 25,000 people

Notes from the lecture:

• Most destruciton from volcanism in the northern latitudes comes from Lahars
• Nevado del Ruiz
  • Volcanic + Valley
  • The bottom of the valley was a village and basically this flow covered the entire village

Other Volcanic Hazards

Presentation content:

• Volcano-related tsunamis
  • Destructive sea waves can form after the sudden collapse of a flank of a volcano
• Volcanic ash – a hazard to airplanes
  • In 2010, the eruption of Iceland’s Eyjafjallaöku created a thick plume of ash over Europe, stranding hundreds of thousands of travelers
• Volcanic gases – a respiratory health hazard
  • Volcanoes can emit poisonous gases, endangering humans, and livestock

Notes from the lecture:

• Alaska: Volcanos basically collapsing
• Because the landslides, all that material is launched into the water
• Hazard: Displacement of oxygen
  • CO2 and sulfur displace oxygen and you cannot breathe this

Effects of Volcanic Ash and Gases on Weather and Climate

Presentation content:

• Ash particle released from volcanoes can reflect solar energy back into space causing cooling.

Examples:

1. The 1783 Laki eruption in Iceland brought the longest period of below 0 temperatures to New England in 1784.
2. The ash from the eruption of Mount Tambora in 1815 led to the “year without summer” (1816).
3. The El Chichon eruption in Mexico (1982) produced an unusually large amount of SO2 that reacted with water vapor to produce clouds of tiny sulfuric acid droplets.

5.9 Other Volcanic Landforms

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

Calderas

Presentation content:

• Calderas are circular, steep-sided depressions with a diameter >1 km
• Three different types:
  • Crater Lake-type calderas
  • Hawaiian-type calderas
  • Yellowstone-type calderas

Crater Lake-type calderas

Crater Lake-type calderas: Form from the collapse of the summit of a large composite volcano following an eruption; these calderas eventually fill with rainwater

Hawaiian-type calderas

Presentation content:
Hawaiian-type calderas: Form gradually from the collapse of the summit of a shield volcano following the subterranean drainage of the central magma chamber

Notes from the lecture:

• Collapse of Kilauea
• Become completely disconnected from its source of magma, so it will cool
• Less violent, very calm

Yellowstone-type calderas

Presentation content:
Yellowstone-type calderas: Form from the collapse of a large area after the discharge of large volumes of silica-rich pumice and ash; these calderas tend to exhibit a complex history

Notes from the lecture:

• Depressurizing of the magma chamber -> several little volcanoes
• A more violent eruption
• Still active today

Fissure Eruptions and Basalt Plateaus

Presentation content:

• Fluid basaltic lava extruded from fissures blanket a large area, called a large igneous province or basalt plateau.
• Flood basalts appropriately describes these eruptions

Lava Domes

Presentation content:

• A lava dome is a small dome-shaped mass composed of rhyolitic lava.
• As thick lava is squeezed out of a vent, it produces a dome-shaped mass.

Notes from the lecture:

• A small shape volcano forming after a previous eruption

Volcanic Necks and Pipes

Presentation content:

• A volcanic neck is the remains of solidified magma in a volcanic conduit.
• A pipe is a rare type of conduit that originated in the mantle at depths exceeding 150 km.
  • Kimberlite pipes, for example

5.10 Plate Tectonics and Volcanism

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

Volcanism at Convergent Plate Boundaries

Presentation content:

• Occur at Subduction Zones
• Volcanic arcs develop parallel to the associated subduction zone trench
• Volcanic island arc: Aleutians, Tongas, and Marianas Islands
• Continental volcanic arc: Cascade Range

• Most active volcanoes are found along the circum-Pacific Ring of Fire
• Eruptions tend to be explosive and associated with volatile-rich, andesitic magma

The Paciic Ring of Fire

• Shows distribution of volcanoes basically surrounding the Pacific Ocean

Notes from the lecture:

• You have assimilation (magma basically rising)
• Most of these will be strato volcanoes (very large)
• Note we have some exceptions, volcanoes right in the middle of a plate (not a plate boundary)

Volcanism at Divergent Plate Boundaries

Presentation content:

• 60% of Earth’s yearly output of magma is from spreading centers
• Characterized by a vast outpouring of fluid, basaltic lavas.

Notes from the lecture:

• As the crust stretch and thins, magma is allowed to rise
• As basalt melts, reincorporates
  • Basalt is more dense, so there is assimilation of these two different magmas
  • As it mixes it passes to a more felsic composition -> more viscous -> traps more gases

Intraplate Volcanism

Presentation content:

• Volcanoes that occur thousands of kilometers from plate boundaries
• Occurs when a mantle plume ascends towards the surface.
• Large mantle plumes, dubbed superplumes, are thought to be responsible for flood basalts.
• Examples include the Hawaiian Islands, the Columbia River Basalts, and the Galapagos Islands.

Notes from the lecture:

• These are the exceptions to the boundary rule
• We think of these as volcanic hotspots
• As the plate moves a plate is created on the ocean floor
• This plate will be continuously moving

Superplumes/Flood Basalts

• Some of the large plates of interplate volcanism
• Some of these are very large
  • A significant amount of basalt has been erupted to the surface
• These events are significant because we can trace them and connect them to other geologic/climate events
  • Greenhouse gases
  • Modeling
  • Meteorite impacts
  • Extinction of dinosaurs?
• Volcanic ash can stay in the atmosphere for at most 2 decades
• We use them to study plate tectonics
  • At some point Africa and South America were joined, and we can see the hotspot there in the middle

5.11 Monitoring Volcanic Activity

List and describe the techniques used to monitor potentially dangerous volcanoes.

Why?

Presentation content:

• To provide scientific data and to assess hazards
• Most notable changes in a volcanic landscape:
  • Changes in patterns of earthquakes
  • Inflation of the volcano related to rising magma
  • Changes in the amount and/or composition of gases released from the volcano

Notes from the lecture:

• Try to be able to prevent catastrophic events

Remote sensing devices

Presentation content:

• Remote sensing devices greatly enhance ability to monitor volcanoes
  • Limited-accessibility volcanoes
  • Eruptions in progress
  • Ground deformation and SO2 emissions
• A volcano must be monitored for a long time to recognize a difference between "resting state" and "active state."

Notes from the lecture:

• See if there is a lot of topography

End of Chapter 5 - Concept Checks

5.1 Mount St. Helens Versus Kilauea

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

5.2 The Nature of Volcanic Eruptions

1. List these magmas in order, from the highest to lowest silica content: mafic (basaltic) magma, felsic (granitic/rhyolitic) magma, intermediate (andesitic) magma.
2. 4. 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 Materials Extruded During an Eruption

1. Contrast pahoehoe and aa lava flows.
2. How do lava tubes form?
3. List the main gases released during a volcanic eruption.
4. How do volcanic bombs differ from blocks of pyroclastic debris?
5. What is scoria? How is it different from pumice?

5.4 Anatomy of a Volcano

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 Shield Volcanoes

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 Cinder Cones

1. Describe the composition of a cinder cone.
2. How do cinder cones compare with shield volcanoes in terms of size and the steepness of their flanks?

5.7 Composite Cones

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 do the composition and viscosity of lava flows differ between composite volcanoes and shield volcanoes?

5.8 Volcanic Hazards

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 Other Volcanic Landforms

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 Plate Tectonics and Volcanism

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?

5.11 Monitoring Volcanic Activity

1. What three factors do volcanologists monitor in order to determine whether magma is migrating toward Earth’s surface?
2. What volcanic hazard does the warning system installed around Mount Rainier aim to identify?