Understanding Volcanic Eruptions: Factors and Materials

1. Chapter 5 Volcanoes and Other Igneous Activity

2. The nature of volcanic eruptions • Characteristics of a magma determine the “violence” or explosiveness of a volcanic eruption • Composition • Temperature • Dissolved gases • The above three factors actually control the viscosity of a given magma

3. 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 (e.g., felsic lava such as rhyolite) • Lower silica content = lower viscosity (e.g., mafic lava such as basalt)

4. The nature of volcanic eruptions • Dissolved gases • Gas content affects magma mobility • 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 from magma • In summary • Basaltic lavas = mild eruptions • Rhyolitic or andesitic lavas = explosive eruptions

5. 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

6. A pahoehoe lava flow

7. Aa lava flow

8. 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 • Cinders - pea-sized material • Lapilli - walnut-sized material • Particles larger than lapilli • Blocks - hardened or cooled lava • Bombs - ejected as hot lava

9. Eruption of Kilauea Volcano in Hawaii

10. A volcanic bomb Bomb is approximately 10 cm long

11. Volcanoes • 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

12. Volcanoes • 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

13. Anatomy of a shield volcano

14. JDR Life Goal #38. Climb Mauna Kea (13,796 ft) shield volcano in Hawaii. Highest mountain on the planet from base to crest (almost 30,000 ft). Completed with my son Jonathan in May 2000. This shows profile of Mauna Loa shield volcano, as seen from summit of Mauna Kea (benchmark at lower right).

15. Profiles of volcanic landforms

16. Volcanoes • Cinder cones • Built from ejected lava (mainly cinder-sized) fragments • Steep slope angle • Small size • Frequently occur in groups

17. Cinder cone volcano

18. The Paricutin volcano in Mexico erupted in a corn field in 1943, burying the entire town.

19. Volcanoes • Composite cone (stratovolcano) • Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens) • Large, classic-shaped volcano (1000’s 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)

20. Anatomy of a composite volcano

21. Seamounts over hot spots Volcanoes over hot spots have spawned several prominent chains of islands in the Pacific Ocean, as shown here.

22. These volcanoes become increasing younger preceding southerly, then southeasterly. Most of their mass lies unseen, below the current sea level.

23. The Hawaiian Islands As the Pacific Plate moves WNW at a rate of about 6 mm/yr, magma rises through the thin oceanic crust, causing volcanoes. The unrelenting erosion of waves tends to plane off the islands, unless protected by coral reefs

24. The most active area is currently the Kilauea Rift along the southeastern coast of the island of Hawaii. Here large slump blocks produce tensile scarps, which allows molten lava to flow up to the surface more easily, loading the head of the slumps. Loihi is the next island forming.

25. The Pali Escarpment The Pali Escarpment across Molokai and Oahu is a gigantic landslide headscarp, formed when the northern side of those islands detached itself and slid into the ocean, likely on a layer of altered volcanic ash

26. Bathymetry of the detachments Bathymetry surveys off the north coasts of Oahu and Molokai, showing enormous debris fields

27. Massive subaqueous landslide debris fields extending around the Hawaiian Islands, initially identified by J. G. Moore of the U.S. Geological Survey in the n1980s, which working on the 200 miles economic exclusions zone around the islands.

28. Cascade Volcanoes • There are 13 potentially active volcanoes in the Cascade Range of the northwest-ern United States

29. Mt. St. Helens – prior to the 1980 eruption

30. Mt. St. Helens after the 1980 eruption

31. Precursory stages of the Mt St Helens eruption

32. A seismically-induced landslide reduced lateral and vertical confinement, triggering the May 18, 1980 eruption of Mt. St Helens

33. Cross sections thru Mt St Helens showing deep rotational slide blocks which slid off the peak

34. Atmospheric impacts of the May 18, 1980 Mt St Helens eruption

35. The Mt St Helens eruption shot volcanic ash to an altitude of 60,000 feet, into the stratosphere

36. Relative distribution of blown down trees and lahar debris flows in the Toutle River Valley on the north side of Mt St Helens

37. The St Helens blast flattened Douglas fir trees over an area of 400 square kilometers

38. Lahars, debris flows, and debris chocking of rivers caused by the Mt. St. Helens eruption

39. Volcanoes • 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 • Lahar – volcanic mudflow • Mixture of volcanic debris and water • Move down stream valleys and volcanic slopes, often with destructive results

40. A nueé ardente on Mt. St. Helens after the May 1980 eruption

41. Viscous lava flow passing through the village of Goma in the Congo during the eruption of Mt. Nyiragongo in January 2002

42. 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 • Material ejected at high velocities • Example = Yellowstone plateau

43. Formation of Crater Lake, Oregon

44. Other volcanic landforms • Fissure eruptions and lava plateaus • Fluid basaltic lava extruded from crustal fractures called fissures • Example = Columbia River Plateau • Lava domes • Bulbous mass of congealed lava • Associated with explosive eruptions of gas-rich magma

45. A lava dome

46. 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

47. Formation of a volcanic neck

48. The most famous volcanic neck in the United States is Shiprock, New Mexico

49. JDR Life Goal #54: Climb Shiprock, New Mexico, 1700’ climb, to elevation 7,178 ft, in Four Corners area . Completed in June 1973 (after climb, discovered that climbing had been outlawed in 1970).

50. 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