Chapter 7 Fires Within: Igneous Activity

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

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. 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 fluidbehavior • 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 jointingoccurs 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

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