A volcano is an opening, or rupture, in the surface or crust of the Earth or a planetary mass object, which allows hot lava, volcanic ash and gases to escape from the magma chamber below the surface. On Earth, volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are not usually created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust in the interiors of plates

1. Chapter 9
The Wrath of Vulcan: Volcanic Eruptions
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

2. The Wrath of Vulcan: Volcanic Eruptions
Prepared by
Ronald Parker
Earlham College Department of Geosciences
Richmond, Indiana

3. Volcanic Eruptions

What is a volcano?
An erupting vent through which molten rock surfaces.
 A mountain built from magmatic eruptions.

Volcanoes are a clear result of tectonic activity.
 Volcanoes pose a number of hazards to humans.

Mexico City.
 Seattle, U.S.
 Naples, Italy.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

4. Volcanic Eruptions
In 79 C.E. Mt. Vesuvius erupted violently.
 Pyroclastic flows destroyed Pompeii, killing 20,000.
 A record of Roman life was preserved under ash.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

5. Volcanic Eruptions

Unpredictable, dangerous.
Build large mountains.
 Blow mountains to bits.


Eruptions can…
Provide highly productive
soils to feed a civilization.
 Can extinguish a civilization
in a matter of minutes.


Eruptions affect climate.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

6. Volcanic Materials

The products of volcanic eruption take three forms:
Lava flows – Molten rock that move over the ground.
 Pyroclastic debris – Fragments blown out of a volcano.
 Volcanic gases – Vapor and aerosols that exit a volcano.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

7. Lava Flows
Lava can be thin and runny or thick and sticky.
 Flow type depends on viscosity via composition.
 Composition depends on silica (SiO 2), Fe, and Mg.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

8. Lava Compositions

Lavas with high silica / low Fe and Mg are called...


Lavas with low silica / high Fe and Mg are called…


Silicic, felsic, or rhyolitic.
Mafic or basaltic.
Lavas with moderate silica, Fe and Mg are called…

Intermediate or andesitic.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

9. Basaltic Lava Flows
Mafic lava – Very hot, low silica, and low viscosity.
 Basalt flows are often thin and fluid.

They can flow rapidly (up to 100 km/hr).
 They can flow for long distances (up to several 100 km).

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

10. Pahoehoe

Pahoehoe (pa-hoy-hoy) - a Hawaiian word
describing basalt with a glassy, ropy texture.
Pahoehoe forms when extremely hot basalt forms a skin.
 With flow, the skin is rolled into ropy ridges and furrows.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

11. A’a’

A’a’ (ah-ah) is a Hawaiian word describing basalt
that solidifies with a jagged, sharp, angular texture.
A’a’ forms when hot flowing basalt cools and thickens.
 With flow, lava crumbles into shards and fragments.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

12. Lava Tubes
A cooled crust forms on top of a basalt flow.
 A conduit – a lava tube – develops in the flow.
 Tubes prevent cooling, facilitating flow for miles.
 Lava tubes become caves that can transmit water.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

13. Columnar Jointing
Solidified flows may contract with vertical fractures
that are hexagonal in cross-section.
 Columnar jointing indicates lava.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

14. Pillow Basalts
Underwater, basalt cools instantly; it cannot flow.
 It cools to form a rounded blob called a pillow.

The pillow surface is cracked, quenched glass.
 Lava pressure ruptures a pillow to form the next blob.
 The process repeats to form a mound of pillow basalts.


Common on the MOR.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

15. Andesitic Lava Flows

Higher SiO2 content makes andesitic lavas viscous.
Unlike basalt, they do not flow rapidly.
 Instead, they mound around the vent and flow slowly.

The outer crust fractures, creating rubble.
 Andesitic lava flows remain close to the vent.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

16. Rhyolitic Lava Flows

Rhyolite; the highest SiO2 – the most viscous lava.
Rhyolitic lava rarely flows.
 Rather, lava plugs the vent as a lava dome.
 Sometimes, lava domes are blown to smithereens.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

17. Pyroclastic Debris
Fragmental material ejected from a volcano.
 Glass shards, fragmented lava in a range of sizes.

Ash – Powdery glass shards.
 Lapilli – Pea- to plum-sized material.
 Blocks and bombs – Apple- to refrigerator-sized.

Blocks – Pre-existing rock torn from the volcano.
Bombs – Streamlined fragments of ejected lava.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

18. Pyroclastic Debris

Tephra – Deposits of pyroclastic debris.
Volcanic agglomerate – Lapilli and bombs near the vent.
 Tuff – Lithified ash with or without lapilli.

Air-fall tuff – Accumulations of ash that fall like snow.
Welded tuff (ignimbrite) – Tuff that is deposited while hot.
Hot pyroclastic flow material that fuses before cooling.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

19. Pyroclastic Flows

Pyroclastic flows (or, nuée ardentes - French):
200oC - 450oC avalanches of hot ash that race downslope.
 Moving up to 300 kph, they incinerate all in their path.
 Immediately deadly; they kill everything quickly.
 Many famous examples: Vesuvius, Mt. Pelée, Augustine.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

20. Lahars
Tephra is readily moved by water as a debris flow.
 Known as lahars, these flows are often deadly.

Lahars move rapidly (up to 50 km/hr).
 They have the consistency of wet cement,.
 A distinct hazard to people living in volcanic valleys.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

21. Volcanic Gas

1 to 9% of magma may be gas.


Carbon dioxide (CO2) – Second in abundance.


Water (H2O) – Most abundant dissolved gas.
Sulfur dioxide (SO2) – Rotten egg smell.
Magma composition controls gas content.

Felsic magmas are gas-rich; Mafic magmas are less so.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

22. Volcanic Gas
Gases are expelled as magma rises (P drops).
 SO2 reacts with water to form aerosol sulfuric acid.


Style of gas escape controls eruption violence.
Low viscosity (basalt) – Easy escape; mellow eruption.
 High viscosity (rhyolite) – Difficult escape; violent.


Gas bubbles in rock are
called vesicles.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

23. Volcanic Architecture

Volcanoes have characteristic features.
A magma chamber.
 Fissures and vents.
 Craters.
 Calderas.
 Distinctive profile.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

24. Magma Chamber

Located in the upper crust.
Usually an open cavity or area of highly fractured rock.
 Contains a large quantity of magma.


Some magma cools here to form intrusive rock.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

25. Magma Chamber
Some magma rises via a conduit to the surface.
 Magma may also erupt along a linear tear, a fissure.

Fissure eruptions may display a “curtain of fire.”
 Fissures evolve into discrete vents.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

26. Vents
A vent is a lava outlet on a volcano.
 Vents can form anywhere on the volcano.

Summit vent – Located at the top of the volcano.
 Flank vent – Located on the side of a volcano.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

27. Craters
Crater – A bowl-shaped depression atop a volcano.
 Craters are up to 500 m across; 200 m deep.
 Form as erupted lava piles up around the vent.
 accentuate by summit collapse into conduit.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

28. Caldera

A gigantic volcanic depression.
Much larger than a crater.
 1-10s of kilometers across.

Magma chamber empties.
 Volcano collapses into it.

Crater Lake, Oregon.
 Yellowstone National Park.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

29. Crater Lake Caldera, Oregon

30. Shapes and Sizes
Magma type governs volcano shape and size.
 Categories:

Shield volcanoes – Largest.
 Cinder cones – Smallest.
 Stratovolcanoes – Intermediate in size.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

31. Volcano Types

Shield volcanoes.
Broad, slightly domed-shaped (like an inverted shield).
 Lateral flow of low-viscosity basaltic lava.
 Have a low slope and cover large geographic areas.
 Mauna Loa on Hawaii is a good example.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

32. Volcano Types

Cinder cone – Conical piles of tephra.
The smallest type of volcano.
 Built of ejected lapilli-sized fragments piled up at a vent.
 Slopes are at the angle of repose.
 Often symmetrical with a deep summit crater.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

33. Volcano Types

Stratovolcanoes (composite volcanoes).
Large, cone-shaped volcano.
 Composed of alternating layers of lava and tephra.
 Often symmetric; can be odd shapes from landslides, etc.
 Examples include Mt. Fuji, Mt. Rainier, Mt. Vesuvius.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

34. Eruptive Style

Will it flow, or will it blow? Two dominant styles.
Effusive eruptions – Produce lava flows.
 Explosive eruptions – Blow up.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

35. Eruptive Style

Effusive eruptions – Produce lava flows.
Lava flows stream away from vents.
 Lava lakes can form around the vent.
 Can produce huge lava fountains.


Commonly basaltic, these eruptions create shields.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

36. Eruptive Style

Explosive eruptions – Produce pyroclastic flows.
Caused by gas pressure in the more viscous magma.
 Pressure is released explosively.
 Create stratovolcanoes.
 May create calderas.
 Blanket the landscape with tephra.


Andesitic and rhyolitic compositions.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

37. Eruptive Style

Phreatomagmatic eruptions – Less common style.
Volcanic eruption where magma interacts with water.
 Some phreatomagmatic eruptions can be cataclysmic.

A magma chamber breaches and admits water.
Water flashes to steam and blows the entire volcano apart.
Examples: Santorini, Krakatau.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

38. Controls on Eruptive Style

Viscosity – Controls the ease of lava flow.
Basalt – Low-viscosity lava flows away from vent.
 Felsic – High-viscosity lava builds-up at the vent.


Gas Pressure – Greater P favors explosive style.
Basalt – Low-viscosity allows gas release.
 Felsic – High-viscosity prevents gas release.


Environment – Where eruption occurs important.
Subaerial lava flowing on land cools slower than…
 Submarine lava, which is quickly quenched.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

39. Tectonic Settings
Plate motion is a dominant control on volcanism.
 Volcanic types are linked to tectonic settings


Hot spots - Where mantle plumes cut the lithosphere.
Oceanic hot spots.
Continental hot spots and flood basalts.
Mid-ocean ridges – Spreading axes.
 Convergent boundaries – Subduction zones.
 Continental rifts – Incipient ocean basins.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

41. Hot Spot Eruptions

Oceanic hot spot – Plume under an oceanic plate.
Basalt erupts at the seafloor and forms a growing mound.
 A volcano builds above sea level to form an island.
 Then basalt will not quench and can flow long distances.
 Lava builds upward and outward and the island grows.
 Submarine slumps remove large masses of the volcano.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

42. Hot Spot Eruptions

Continental hot spot – Cuts a continental plate.

Often erupts both basaltic and rhyolitic material.
Basaltic – Makes up the mantle plume.
Rhyolitic – Basalt melts the granitic crust it passes through.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

43. Hot Spot Eruptions

Continental hot spots.

Flood basalts – Voluminous lava eruption above a plume.
Thinned lithosphere erupts magma from long fissures.
Lava spreads over large areas; great thicknesses stack up.
With time, a shrinking plume creates “normal” volcanoes.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

44. MOR Eruptions
Most lava erupts along the mid-ocean ridge (MOR).
 MOR-produced oceanic crust covers 70% of Earth.
 Basalt erupted from fissures quenches as pillows.
 Pillow mounds are pulled apart with plate motion.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

45. Convergent Boundaries

Most volcanoes form at convergent boundaries.
Volatiles from subducting plate initiate melting.
 Arc volcanoes develop on overriding plate.

May cut through oceanic or continental crust.

The “Ring of Fire” dominates Pacific margins.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

46. Continental Rifts

Yield an array of volcano types reflecting...
Partial melting of the mantle (mafic magmas).
 Partial melting of the crust (felsic magmas).


Examples:
East African Rift.
 Basin and Range.
 Mid-continent Rift.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions

47. This concludes the
Chapter 9
The Wrath of Vulcan: Volcanic Eruptions
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 9: The Wrath of Vulcan: Volcanic Eruptions