1. Chapter 7 Fires Within: Igneous Activity

2. Origin of Magma Heating of Rocks Friction during subduction
Radioactive decay
Drop in Pressure

3. Partial Melting Mantle rocks melt first
Then partially melt surrounding continental rocks

4. Volcanic Features
Crater and Vent

5. Volcanic Features
Caldera

6. Formation of Crater Lake
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

7. Plate Tectonics and Volcanoes
The type of magma and volcano depend on
1) Tectonic Settings

8. Plate Tectonics and Volcanoes
The type of magma and volcano depend on
Tectonic Settings
Divergent Plate Boundaries (Mid Ocean Ridges) – Basltic
Intraplate (Hot Spots)
Subduction Zones (Ring of Fire)

9. Plate Tectonics and Volcanoes
The type of Magma depends on
Chemical Composition
Temperature
Dissolved gases
These three factors actually control the viscosity of a magma

10. The Nature of Volcanic Eruptions
Viscosity is a measure of a material’s resistance to flow
Factors affecting viscosity
Temperature—Hotter magmas are less viscous
Composition—Silica (SiO2) content
Higher silica content = higher viscosity
Lower silica content = lower viscosity

11. Magma Viscosity Viscosity = resistance to flow
Viscosity of materials decreases with increasing temperature
Viscosity varies with composition
The Good Earth/Chapter 6: Volcanoes and Other Mountains

12. Viscosity depends upon magma composition (silica content)
Magma Viscosity
Viscosity depends upon magma composition (silica content)
The Good Earth/Chapter 6: Volcanoes and Other Mountains

13. The Nature of Volcanic Eruptions
Dissolved gases
Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure
The violence of an eruption is related to how easily gases escape
In summary
Basaltic lavas = mild eruptions
Rhyolitic or andesitic lavas = explosive eruptions

14. Types of Volcanoes
Figure 7.9

15. Volcanic Structures Types of volcanoes Shield volcano
Broad, slightly domed shaped
Generally cover large areas
Produced by mild eruptions of large volumes of basaltic lava
Example = Mauna Loa on Hawaii

16. Anatomy of a Shield Volcano
Figure 7.8

17. Volcanic Structures Composite cone (stratovolcano)
Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)
Large, classic-shaped volcano (1000s of ft. high and several miles wide at base)
Composed of interbedded lava flows and pyroclastic debris
Most violent type of activity (e.g., Mt. Vesuvius)

18. Anatomy of a Strata Volcano
Figure 7.8

19. Mt. St. Helens—Prior to the 1980 Eruption

20. Mt. St. Helens After the 1980 Eruption

21. Volcanic Structures Cinder cone
Built from ejected lava (mainly cinder-sized) fragments
Steep slope angle
Small size
Frequently occur in groups

22. Cinder Cone Volcano

23. Volcanic Structures Flood Basalt Low viscosity flows
High Volume of Magma
No volcanic structure
Large Volume of Magma

24. Yellowstone

25. Volcano Summary

26. Materials Extruded from a Volcano
Lava flows
Basaltic lavas exhibit fluid behavior
Types of basaltic flows
Pahoehoe lava (resembles a twisted or ropey texture)
Aa lava (rough, jagged blocky texture)
Dissolved gases
1%–6% by weight
Mainly H2O and CO2

27. A Lava Flow
Figure 7.5 B

28. Materials Extruded from a Volcano
Pyroclastic materials—“Fire fragments”
Types of pyroclastic debris
Cinders—Pea-sized material
Lapilli—Walnut-sized material
Particles larger than lapilli
Blocks—Hardened or cooled lava
Bombs—Ejected as hot lava

29. Materials Extruded from a Volcano
Pyroclastic materials—“Fire fragments”
Types of pyroclastic debris
Ash and dust—Fine, glassy fragments
Pumice—Porous rock from “frothy” lava

30. Volcanic Structures General features
Opening at the summit of a volcano
Crater— Summit depression < 1 km diameter
Caldera —Summit depression > 1 km diameter produced by collapse following a massive eruption
Vent —Surface opening connected to the magma chamber
Fumarole—Emit only gases and smoke

31. Volcanic Structures Nuée ardente
Nuée ardente —A deadly pyroclastic flow
Fiery pyroclastic flow made of hot gases infused with ash and other debris
Also known as glowing avalanches
Move down the slopes of a volcano at speeds up to 200 km per hour

32. A Nueé Ardente on Mt. St. Helens

33. Volcanic Structures Lahar—Volcanic mudflow
Mixture of volcanic debris and water
Move down stream valleys and volcanic slopes, often with destructive results

34. Other Volcanic Landforms
Caldera
Steep-walled depressions at the summit
Generally > 1 km in diameter
Produced by collapse
Pyroclastic flow
Felsic and intermediate magmas
Consists of ash, pumice, and other debris
Example = Yellowstone Plateau

35. Formation of Crater Lake, Oregon
Figure 7.17

36. Other Volcanic Landforms
Volcanic pipes and necks
Pipes—Short conduits that connect a magma chamber to the surface
Volcanic necks (e.g., Ship Rock, New Mexico)—Resistant vents left standing after erosion has removed the volcanic cone

37. Other Volcanic Landforms

38. Mud cracks

39. Intrusive Igneous Activity
Columnar jointing occurs as a result of shrinkage fractures that develop when igneous rocks cool

40. Other Volcanic Landforms
Volcanic pipes and necks

41. Intrusive Igneous Activity
Most magma is emplaced at depth in the Earth
Once cooled and solidified, is called a pluton
Nature of plutons
Shape—Tabular (sheetlike) vs. massive
Orientation with respect to the host (surrounding) rock
Concordant vs. discordant

42. Intrusive Igneous Features
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

43. Intrusive Igneous Activity
Types of intrusive igneous features
Dike—A tabular, discordant pluton
Sill—A tabular, concordant pluton (e.g., Palisades Sill in New York)
Laccolith
Similar to a sill
Lens or mushroom-shaped mass
Arches overlying strata upward

44. Igneous Structures
Figure 7.22 B

45. A Sill in the Salt River Canyon, Arizona
Figure 7.23

46. Intrusive Igneous Activity
Intrusive igneous features continued
Batholith
Largest intrusive body
Surface exposure > 100+ km2 (smaller bodies are termed stocks)
Frequently form the cores of mountains

47. End of Chapter 7