Engineering Geology ECIV 2204

Transcription

1 Engineering Geology ECIV Chapter (4) Volcanoes

2 Chapter 4: Volcanoes and Other Igneous Activity

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18 cataclysmic relating to or denoting a violent natural even Eventually the entire surface founders in a cataclysmic event, leading to volcanic resurfacing on a global scale, followed by tectonic deformation of some areas as the new surface settles down.

19

20

21

22

23

24

25

26 Volcanic activity Basic types of eruptions Anatomy of a volcano Materials erupted Classes of volcanoes Basaltic volcanoes Intermediate volcanoes Silicic volcanoes Volcanic hazards

27 Central Eruption Types of Volcanic Eruptions Fissure Eruption Fig ab W. W. Norton

28

29

30 Basic Anatomy of a Volcano Fig W. W. Norton

31 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

32 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 (SiO 2 ) 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)

33 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

34 Materials extruded from a volcano Lava flows Basaltic lavas exhibit fluid behavior Types of basaltic flows Pahoehoe lava (pahoehoe pronounced pah-hoy-hoy) (Resembles a twisted or ropey texture) Aa lava (aa (pronounced ah-ah) rough, jagged blocky texture) Dissolved gases 1% - 6% by weight Mainly H 2 O and CO 2

35 A pahoehoe lava flow

36 Aa lava flow Figure 5.5 B

37 The primary factors that determine the nature of volcanic eruptions include the magma's temperature, its composition, and the amount of dissolved gases it contains.

38 As lava cools, its viscosity increases, its mobility decreases.

39 The viscosity of magma is directly related to its silica content.

40 Rhyolitic lava, with its high silica content, is very viscous and forms short, thick flows.

41 Basaltic lava, with a lower silica content, is more fluid and may travel a long distance before solidification.

42 Dissolved gases provide the force that drive molten rock from the vent of a volcano.

43 The materials associated with a volcanic eruption include lava flows (pahoehoe and aa flows for basaltic lavas), gases (primarily in the form of water vapor), and

44 pyroclastic material (pulverized rock and lava fragments blown from the volcano's vent, which include ashes, pumice (a very light and porous volcanic rock formed when a gas-rich glassy lava solidifies rapidly)), blocks, and bombs).

45 Successive eruptions of lava from a central vent result in a mountainous accumulation of material known as a volcano.

46 Located at the summit of many volcanoes is a steep-walled depression called a crater.

47 Shield cones are broad, slightly domed volcanoes built primarily of fluid, basaltic lava.

48 Cinder cones have steep slopes composed of pyroclastic material.

49 Composite cones, or stratovolcanoes, are large, nearly symmetrical structures built of interbedded lavas and pyroclastic deposits.

50 Composite cones produce some of the most violent volcanic activity.

51 Large composite cones may also generate a type of mudflow known as a lahar(a destructive mudflow on the slopes of a volcano).

52 Most volcanoes are fed by conduits (a channel) or pipes.

53 As erosion progresses, the rock occupying the pipe is often more resistant and may remain standing above the surrounding terrain as a volcanic neck.

54 The summits of some volcanoes have large, nearly circular depressions called calderas that result from collapse following an explosive eruption.

55 Calderas also form on shield volcanos by subterranean drainage from a central magma chamber, and the largest calderas form by the discharge of extremely large volumes of silicarich pumice along ring fractures.

56 Although volcanic eruptions from a central vent are the most familiar, by far the largest amounts of volcanic material are extruded from cracks in the crust called fissures.

57 The term flood basalts describes the fluid, waterlike, basaltic lava flows that cover an extensive region in the northwestern United States known as the Columbia Plateau.

58 When silica-rich magma is extruded, pyroclastic flows consisting largely of ash and pumice fragments usually result.

59 Igneous intrusive bodies are classified according to their shape and by their orientation with respect to the host rock, generally sedimentary rock.

60 Magma originates from essentially solid rock of the crust and mantle.

61 In addition to a rock's composition, its temperature, depth (confining pressure), and water content determine whether it exists as a solid or liquid.

62 Thus, magma can be generated by raising a rock's temperature, as occurs when a hot mantle plume "ponds" beneath crustal rocks.

63 A decrease in pressure can cause decompression melting.

64 Further, the introduction of volatiles (water) can lower a rock's melting point sufficiently to generate magma.

65 Because melting is generally not complete, a process called partial melting produces a melt made of the lowest-meltingtemperature minerals, which are higher in silica than the original rock.

66 Thus, magmas generated by partial melting are nearer to the granitic (felsic) end of the compositional spectrum than are the rocks from which they formed.

67 Most active volcanoes are associated with plate boundaries.

68 Active areas of volcanism are found along oceanic ridges where seafloor spreading is occurring (divergent plate boundaries),

69 in the vicinity of ocean trenches where one plate is being subducted beneath another (convergent plate boundaries), and

70 in the interiors of plates themselves (intraplate volcanism).

71 Rising plumes of hot mantle rock are the source of most intraplate volcanism.

72 Materials extruded during an eruption Volcanoes erupt: Lava Gases Pyroclastics Fragmented rock, ash & dust

73 Ash and tephra Explosive eruption Volcanic bombs Sills Eroded cone Lava flow Side vent Lava cone p a Mud Dikes Cinder cones original artwork by Gary Hincks Lava pavement (cracked/broken) flows (older) Sequential ash and lava layers Fracturing Pyroclastic flow (nuée ardente) Old lava dome Lavas Sedimentary rocks Laccolith Metamorphic rocks Chimney Granite intrusion (older/cold) Contact metamorphism Magma chamber

74 Volcanologists recognize 3 general volcano types: a. Shield volcanoes b. Composite cones c. Cinder cones

75 Types of Volcanoes 9 km 150 km 3 km p c Shield volcano (e.g. Hawaii) 15 km Composite volcano (e.g. Vesuvius) original artwork by Gary Hincks 0.3 km 1.5 km Cinder cone (e.g. Sunset crater)

76 What Controls the Form of a Volcano? Basaltic flow Andesitic flow Rhyolitic dome Rhyolitic spire

77 High viscosity lavas: Silicon-oxygen tetrahedra tend to link up in magmas and form chains. Factor I: Magma Viscosity In magmas with a high silica content (e.g. granitic; ~70% by weight) many silica chains form, especially at the lower eruption temperature of silica-rich lavas. This increases the viscosity of the magma. Viscosity: Resistance of a fluid to flow.

78 Factor II: Gas Content of Magma With viscosity, gas content Largely determines if an eruption will be: Explosive or Gentle

79 Non-explosive basaltic eruption

80 Contrast: Explosive silicic eruption

81 The nature of volcanic eruptions So, in a nutshell (casing) Amount of dissolved gas + Viscosity of magma (silica content and temperature) = Explosiveness of eruption

82 The nature of volcanic eruptions What determines if an eruption is - Explosive? - Gentle? 3 primary factors: Magma s composition more viscous (linking of silicon- tetrahedra) more silica oxygen Magma s temperature hotter magma less viscous

83 The nature of volcanic eruptions What determines if an eruption is - Explosive? - Gentle? 3 primary factors: magma s composition more silica more viscous (chains) magma s temperature hotter magma less viscous Amount of gas in magma Viscosity Fluidity Can increase fluidity

84 a. Shield volcanoes Let s look in more detail at: low profile broad base basaltic lavas flows are thin travel far from source b. Cinder cones c. Composite cones

85