Chapter 5 Volcanoes and Other Igneous Activity

1. Chapter 5Volcanoes and Other Igneous Activity

2. Koko Head Crater, Hawaii

3. 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

4. The nature of volcanic eruptions • Viscosity is a measure of a material’s resistance to flow (how sticky) • 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) Felsic = granitic composition = rock is composed almost entirely of light colored silicates • Lower silica content = lower viscosity (e.g., mafic lava such as basalt) Mafic = Basaltic = mostly dark silicates and feldspar

5. Mafic vs. Felsic • Denotes compostition of silicate minerals, magmas and rocks with heavy elements. • Ma from magnesium and FIC from latin word for iron. • Dark • Produced at spreading centers- rifts • Examples include, olivine, pyroxene, amphibole, biotite mica, and plagioclase feldspars. • Denotes composition of silicate minerals, magma, and rocks with low amount of heavy elements. • More silica, oxygen, aluminum, and potassium • Fel for feldspar, Sic –from silica • Light in color • Examples: Quartz, muscovite mica, orthoclase feldspars, granite

6. Rhyolite-light colored igneous rock. Are these mafic or felsic?

7. Basalt – Dark colored igneous rock Remember the oceanic crust is made mostly of basalt

8. What type of igneous rock is this? Would you say Rhyolite or Basalt?

9. 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 (less silica) • Rhyolitic or andesitic lavas = explosive eruptions (more silica)

10. Comparison of eruptions Violent Less Violent • Composition • More silica • Rhyolite – Felsic • Gas Content • Cannot escape easily • Temperature • Composition Less silica Basalt - Mafic • Gas Content • Cannot escape easily • Temperature

11. 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

12. A pahoehoe lava flow Figure 5.5 A

13. Aa lava flow Figure 5.5 B

14. 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

15. A volcanic bomb Bomb is approximately 10 cm long Figure 5.7

16. 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

17. 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

18. Anatomy of a shield volcano Figure 5.10

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

20. Arenal Volcano, Costa Rica - 1997 Notice the vent at the top

21. Arenal Volcano Costa Rica Discussion question; why would people live so close to Arenal?

22. Features of Arenal Stratovolcano Vent Basalt Flow Steep Slopes Gases easily escape

23. Concepcion Volcano, Nicaragua Another stratovolcano

24. Cinder cone volcano Figure 5.14

25. Stratovolcanoor composite volcano

26. 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)

27. Poas Volcano, Costa Rica

28. Poas Volcano, Costa Rica Is this a crater or a caldera?

29. It is a Crater! • Crater - summit depression < 1 km diameter • Caldera - summit depression > 1 km diameter produced by collapse following a massive eruption

30. Stratovolcanos • Common in subduction zones • The magma that forms stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basaltrock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab. • The release of water from hydrated minerals is termed "dewatering," and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. • The water freed from the rock lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. • The magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition (see Classification of igneous rock). • When the magma nears the surface, it pools in a magma chamber under the volcano. There, the relatively low pressure allows water and other volatiles (CO2, S2−, Cl−) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened. Once a critical volume of magma and gas accumulates, the obstacle provided by the volcanic cone is overcome, leading to a sudden explosive eruption.[citation needed] • Central America • Cascades of North America • South America – Andes

31. Anatomy of a composite volcano Figure 5.9

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

33. Mt. St. Helens after the 1980 eruption

34. Profiles of volcanic landforms Figure 5.12

35. 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

36. Nueeardente

37. A nueé ardente on Mt. St. Helens Figure 5.20

38. Lahar flow

39. 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

40. Calderas

41. Formation of Crater Lake, Oregon Figure 5.22

42. 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

43. A lava dome Figure 5.26

44. Volcano in St. Vincent

45. St. Vincent volcano shoeing the lava dome 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 Figure 5.27

48. Shiprock, New Mexico

49. 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

50. Devil’s Tower, Wyoming