1. Volcanoes and Volcanic Hazards
Chapter 5

2. The Nature of Volcanic Eruptions
All eruptions involve magma
Magma is molten rock that usually contains some crystals and varying amounts of dissolved gases
Lava is erupted magma
The behavior of magma is determined by:
Temperature of the magma
Composition of the magma
Dissolved gases in the magma
The above three factors control the viscosity of a magma, which in turn controls the nature of an eruption

3. The Nature of Volcanic Eruptions
Viscosity is a measure of a material’s resistance to flow
The more viscous the material, the greater its resistance to flow
Ex: syrup is more viscous than water
Factors affecting viscosity
Temperature – hotter magmas are less viscous
Composition – silica (SiO2) content
Higher silica content magmas are more viscous (e.g., rhyolitic and andesitic magmas)
Lower silica content magmas are less viscous (e.g., basaltic lavas)
Dissolved gases
Dissolved water in magma reduces viscosity
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

4. Magma
Type
Chemical
Composition
Temperature
(degrees C)
Viscosity
Gas Content
Basaltic
45-55% SiO2;
High in Fe, Mg, Ca; Low in K, Na.
Low
Andesitic
55-65% SiO2;
Intermediate Fe, Mg, Ca, Na, K
Intermediate
Rhyolitic
65-75% SiO2;
Low in Fe, Mg, Ca; High in K, Na
High

5. The Nature of Volcanic Eruptions
Quiescent Versus Explosive Eruptions
Quiescent Hawaiian-Type Eruptions (quiet)
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

6. Materials Extruded During an Eruption
Lava
Lava Flows
~90 percent of lava is basaltic lava
<10 percent of lava is andesitic lava
~1 percent of lava is rhyolitic lava
Aa and Pahoehoe Flows
Composed of basaltic lava
Aa flows have surfaces of rough jagged blocks
Pahoehoe flows have smooth surfaces and resemble twisted braids of rope
Pillow Lavas
Composed of basaltic lavas extruded underwater: found in places like Hawaii
Flow is composed of tubelike structures stacked one atop the other

7. A Typical Aa Flow
A Pahoehoe Lava Flow

8. Pahoehoe

9. Aa

10. Lava tube: A tube formed by cooling and solidifying of the lava walls while fluid lava continued to flow inside.

11. Materials Extruded During an Eruption
Lava (continued)
Block Lavas
Composed of andesitic and rhyolitic lava
Upper surface consists of massive, detached blocks
Gases
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

12. Materials Extruded During an Eruption
Pyroclastic Materials
Volcanoes eject pulverized rock and lava fragments called pyroclastic materials
Particles range in size from fine dust, to sand-sized ash, to very large rocks
Tephra: Pyroclastic material
Volcanic ash – fine glassy fragments
Welded tuff – fused ash
Lapilli – walnut-sized material
Cinders – pea-sized material

13. Materials Extruded During an Eruption
Pyroclastic Materials (continued)
Blocks – hardened or cooled lava
Bombs – ejected as hot lava

14. Materials Extruded During an Eruption
Pyroclastic Materials (continued)
Pumice – light gray or pink porous rock from frothy andesitic and rhyolitic lava
Scoria – reddish-brown porous rock from frothy basaltic and andesitic lava

15. Anatomy of a Volcano

16. Anatomy of a Volcano General Features
Conduit – a fissure that magma moves through to reach the surface
Vent – the surface opening of a conduit
Volcanic cone – a cone of material created by successive eruptions of lava and pyroclastic material

17. Anatomy of a Volcano General Features (continued)
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

18. Shield Volcanoes General Features Broad, slightly dome-shaped
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 shield volcano on Earth

19. Shield Volcanoes

20. Cinder Cones General Features Built from ejected lava fragments
Steep slope angle
Rather small size
Frequently occur in groups
Sometimes associated with extensive lava fields
Paricutin (located 320 km west of Mexico City) is an example of a cinder cone

21. Cinder Cones

22. Composite Volcanoes General Features Also called stratovolcanoes
Large, classic-shaped volcano (symmetrical cone, thousands of feet high and several miles wide at the base)
Composed of interbedded lava flows and layers of pyroclastic debris, thus have pyroclastic eruptions
Many are located adjacent to the Pacific Ocean in the Ring of Fire
Mount St. Helens and Mount Etna are examples

23. Composite Volcanoes

25. Volcanic Hazards Pyroclastic Flows Lahars Other hazards
Volcano-related tsunamis
Volcanic ash
Volcanic gases –
Effects of volcanoes on climate

26. Volcanic Hazards Pyroclastic Flows
A pyroclastic flow is a mixture of hot gases infused with incandescent ash and lava fragments that flows down a volcanic slope
Also called a nuée ardente
Propelled by gravity and move similarly to snow avalanches
Material is propelled from the vent at high speeds (can exceed 100 km [60 miles] per hour)
Pyroclastic flows are typically generated by the collapse of tall eruption columns
A surge is a small amount of ash that separates from the main body of the pyroclastic flow
Occasionally, these surges have enough force to knock over buildings and move automobiles
In 1902, the town of St. Pierre was destroyed by a pyroclastic flow from Mount Pelée

27. Pyroclastic Flows

28. Volcanic Hazards Lahars
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 nears the surface of a volcano covered in ice and snow and causes it to melt
In 1985, lahars formed during the eruption of Nevado del Ruiz, killing 25,000 poeple

29. Lahars

30. Lahar

31. Volcanic Hazards Volcano-related tsunamis Volcanic ash Volcanic gases
Other hazards
Volcano-related tsunamis
Destructive sea waves can form after the sudden collapse of a flank of a volcano
Volcanic ash
Jet engines can be damaged when flying through a cloud of volcanic ash
In 2010, the eruption of Iceland’s Eyjafjallaöku created a thick plume of ash over Europe, stranding hundreds of thousands of travelers
Volcanic gases
Volcanoes can emit poisonous gases, endangering humans and livestock
Effects on climate
Ash particle released from volcanoes can reflect solar energy back into space

32. Volcanic Hazards

33. Other Volcanic Landforms
Calderas
Calderas are circular, steep-sided depressions with
a diameter >1 km
Three different types
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: Form gradually from the collapse of the summit of a shield volcano following the subterranean drainage of the central magma chamber
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

34. Formation of Crater Lake-Type Calderas

35. Hawaiian-Type Calderas

36. Formation of Yellowstone-Type Calderas

37. Practice Question
Which of the following choices best explains the difference between a crater and a caldera?
A) A crater is a very large circular depression over 1 km across; A caldera is a small funnel-shaped depression.
B) A crater is a small, funnel-shaped depression; A caldera is a large depression that has a diameter of over 1 km.
C) A crater is a fissure in the crust; A caldera is a collapsed summit.
D) A crater is a large circular depression over 1 km across; A caldera is a small, parasitic cone growing on the flank of the volcano.
Answer: B

38. Other Volcanic Landforms
Large Igneous Provinces
Large igneous provinces cover a large area with basaltic lava
Basaltic lava extruded from fissures blanket a large area, called a large igneous provinces or basalt plateaus
The Colombia Plateau and the Deccan Traps are two examples

39. Other Volcanic Landforms
Lava Domes
A lava dome is a small dome-shaped mass composed of rhyolitic lava
Volcanic Necks and Pipes
A volcanic neck is the remains of magma that solidified in a volcanic conduit
Shiprock, New Mexico, is an example

40. Plate Tectonics and Volcanic Activity
Volcanism at convergent plate boundaries
Occurs at subduction zones, where two plates converge and the oceanic lithosphere descends into the mantle
Volcanic arcs develop parallel to the associated subduction zone trench
The Aleutians, the Tongas, and the Marianas are examples of volcanic island arcs
The Cascade Range is an example of a continental volcanic arc
Most active volcanoes are found along the circum-Pacific Ring of Fire
Eruptions tend to be explosive and associated with volatile- rich, andesitic magma

41. Convergent Plate Volcanism

42. Plate Tectonics and Volcanic Activity
Volcanism at divergent plate boundaries
60% of Earth’s yearly output of magma is from spreading centers
Characterized by a vast outpouring of fluid, basaltic lavas

43. Plate Tectonics and Volcanic Activity
Intraplate volcanism: Hot Spot
Volcanoes that occur thousands of kilometers from plate boundaries
Occurs when a mantle plume ascends towards the surface
Examples include the Hawaiian Islands, the Columbia River Basalts, and the Galapagos Islands

44. Until 1996 Loihi was thought to be an inactive seamount.
It began erupting in 1996 and the eruptions were preceded by a cluster of small earthquakes indicating the movement of magma.

45. The modern active island rests close to the hot spot and its shield volcanoes are fed from the magma that the hot spot generates.

46. Monitoring Volcanic Activity
Efforts aimed at detecting movement of magma from a subterranean reservoir
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
Increase in ground temperature
Remote sensing devices aid in monitoring limited- accessibility volcanoes
A volcano must be monitored for a long time to recognize a difference between
“resting state” and “active state”

47. Monitoring Volcanic Activity
Monitoring gas emissions
Measuring magma movement
Measuring ground temperature

48. A surface bulge appeared on Mt. St. Helens prior to its eruption.
Surface tilting: recognition of changes in the land surface due to building pressure in the conduit.
A surface bulge appeared on Mt. St. Helens prior to its eruption.
April 8, 1980
April 26
May 2

49. Hazard zones have been distinguished around Mt
Hazard zones have been distinguished around Mt. Shasta based on topography and past experience with eruptions.
Zone 1: areas likely to be affected most frequently. Most future flows from summit eruptions probably would stay within this zone.
Zone 1

50. Hazard zones have been distinguished around Mt
Hazard zones have been distinguished around Mt. Shasta based on topography and past experience with eruptions.
Zone 1: areas likely to be affected most frequently. Most future flows from summit eruptions probably would stay within this zone.
Zone 2: areas likely to be affected by lava flows erupted from vents on the flank of the volcano or that move into zone 2 from zone 1.
Zone 2

51. Hazard zones have been distinguished around Mt
Hazard zones have been distinguished around Mt. Shasta based on topography and past experience with eruptions.
Zone 1: areas likely to be affected most frequently. Most future flows from summit eruptions probably would stay within this zone.
Zone 3: areas likely to be affected infrequently and then only by long lava flows that originate at vents in zones 1 and 2
Zone 3
Zone 2: areas likely to be affected by lava flows erupted from vents on the flank of the volcano or that move into zone 2 from zone 1.

52. Earthquakes: generated as the magma moves up the feeder conduit to the vent.
When viscous magma becomes stuck in the conduit strain energy builds as more magma tries to push out.
Movement takes place in a series of “jerks” as the rock material breaks. Each “jerk” produces an earthquake.
Magnitudes generally less than 5 M.
The more earthquakes the further the magma has moved.

53. The 1992 Eruption of Crater Peak Vent
Mount Spurr, Alaska:
The 1992 Eruption of Crater Peak Vent
USGS
Black bars: earthquake frequency.
Red lines: volcanic eruptions.

54. A combination of approaches is likely the key to short-term prediction.

55. The impact of volcanic eruptions

56. A series of eruptions of Tambora (Indonesia) extruded up to 150 km3 of magma (solid equivalent), much of it into the atmosphere.
Tambora (1815 eruption) was followed in 1816 by the “year without a summer”.
Average global temperature is estimated to have been reduced by 3 degrees Celsius.

57. In June of 1816 there was widespread snowfall throughout the eastern United States.
The normal growing season experienced repeated frosts as cold air extended much more southerly than normal.
Food shortages and starvation are attributed to the deaths of 80,000 people.
The global population was about 1 billion people in 1816.
Our current population is over 7 billion.
The 1816 fatality rate would have resulted in a death toll of nearly 500,000 people due to starvation.

58. Approximately 260,000 people have been killed by volcanoes in historic times…most by a handful of individual eruptions.

59. Yellowstone Caldera
Known for its hot springs and geysers, Yellowstone National Park, is likely the most popular super volcano in the world.
The park sits on an active caldera that rises and sinks in response to magma movement and pressure fluctuations within the Earth.
Over recent years the surface has risen by as much as a metre and sunk back by 1/3 of a metre.
Thousands of small earthquakes are produced as earth surface moves.

60. The magma chamber is only 5 to 13 km below the land surface.
The caldera is 80 km long and 50 km wide.

61. The caldera and its magma chamber are due to a hot spot in the mantle that has moved several hundred kilometres over the past 12.5 million years.
The movement is due to the drift of the north American continent over the hot spot.
Ancient, inactive calderas mark the path of the hot spot.

62. The current caldera was formed with an eruption 640,000 years ago (the Lava Creek Eruption).
This eruption ejected 1,000 km3 of pyroclastic debris.
An earlier eruption (the Huckleberry Ridge Eruption, 2 million years ago) ejected 2,500 km3
of pyroclastic debris.
A smaller eruption happened 1.3 million years ago, releasing 280 km3 of debris.

63. Eruptions appear to have a 600,000 year period (that long between eruptions) so we’re overdue for another one.
Previous eruptions spread ash over thousands of km2 across the US.

64. Heightened monitoring of the Yellowstone Caldera in recent years has led to media concern of an impending eruption.
Government officials and geologists indicate that there have been no clear indicators of high risk at this time.
If such an eruption were to take place, North America and the rest of the world could experience another “Dark Ages”.
Geologists now believe that we will have significant warning time, they have found some chambers that they think would fill before an eruption. This is merely a hypothesis however.