Volcanism

1. Volcanism Gulp!

2. Tectonic Settings of Igneous Activity Figure 5.11

3. The World’s Active Volcanoes Fig. 5.28

4. Volcanism Associated with Plate Tectonics Fig. 6.19

5. Material ejectedfrom volcanoes • Lava: Magma that has flowed on the surface of the Earth. • Tephra: Fragments that solidified in the air during eruption.

6. Looking at lava …. Figure 5.4

7. Viscosity: Measure of a fluid’s ability to flow. Higher viscosity = slower flow. (Ketchup has higher viscosity than water.) Looking at lava ….

8. Looking at lava …. • What controls lava viscosity? • Temperature: Higher temperature = less viscous. • SiO2 content: Higher SiO2 = more viscous.

9. Two main types of lava • Mafic or basaltic: • Lower viscosity • Faster lava flows • Calmer eruptions • Felsic or rhyolitic: • Higher viscosity • Slower lava flows • More violent eruptions

10. Basaltic lava • Erupts at 1000° to 1200°C • Can flow as fast as 100 km/hr (but usually a few km/hour) • Can travel as much as 50 km from volcano • Flood basalts: Very fluid basaltic flows that spread out in sheets over the landscape. Layered. Columbia River flood basalts in Washington and Oregon. Figure 6.2

11. Basaltic lava • Basaltic lava flowing downhill forms pahoehoe or aa. • Pahoehoe: (Hawaiian “ropy”). Thin sheet of lava cools on the surface. Skin is twisted and dragged downhill to form “ropes.” • Aa: (Hawaiian “ouch”). Lost gas, is more viscous than pahoehoe. Cools to form thick skin. Skin breaks into jagged blocks. Figure 6.3

12. Pillow lava: Pillow-like blocks of basalt. ~1 m wide. Formed underwater. Blob of basalt extruded underwater (like toothpaste), skin cools quickly (“quenches”) to form glassy rind. Basaltic lava

13. Fissure eruptions Central eruptions Shield volcanoes Domes Cones Stratovolcanoes (composite) Eruptive stylesand landforms

14. Not all lava flows have volcanoes. When low-viscosity lava erupts from cracks in the Earth tens of kilometers long. Make flood basalt provinces. Fissure eruptions Laki fissure (Iceland) erupted in 1783 extruding the largest lava flow in human history (Fig. 6.13).

15. 1971 Fissure Eruption, Kilauea, Hawaii

16. Fissure Eruptions Form Lava Plateaus Figure 6.13

17. Central Eruptions

18. Low-viscosity lava flows (low silica, mafic). Successive lava flows. Gently sloping flanks (between 2 and 10 degrees) Tend to be very large (many 10s of km in circumference) Shield volcanoes Fig. 6.9

19. Mauna Loa -- world’s largest structure -- 10 km above ocean base -- base diameter of 120 km -- took 1 million years to grow from successive lava flows

20. ? Is Mauna Loa about to erupt again?

21. Forms above a volcanic vent Viscous lava — usually silica-rich (or cooler magma) Associated with violent eruptions Volcanic domes Fig. 6.9

22. Mt. St. Helens Lava Dome Lyn Topinka/USGS

23. Cinder Cone • Formed of pyroclastics only • Steep sides — ~30 degrees • Relatively small • Short duration of activity Fig. 6.9

24. Cerro Negro Cinder Cone, near Managua, Nicaragua in 1968 (erupted again in 1995 and 1999) Mark Hurd Aerial Surveys

25. Pyroclast? A volcanic rock fragment ejected into the air during an eruption. Loss of gases due to pressure drop results in explosive eruption. Classified according to size. Volcanic ash <2 mm in diameter. Volcanic bombs: Blobs of lava that cool as they fly trough the air. Can be as big as houses.

26. Pyroclasic Eruption at Arenal Volcano, Costa Rica Gregory G. Dimijian/Photo Researchers Fig. 6.5

27. Volcanic Bomb Fig. 6.5 Science Source/Photo Researchers

28. Lithification of pyroclasts Volcanic tuffs: Rocks created from smaller fragments. Volcanic breccias: Rocks formed from larger fragments.

29. Volcanic Breccia Fig. 6.7 Fig. 5.8 Doug Sokell/Visuals Unlimited

30. Welded Tuff: California 1 foot Gerals and Buff Corsi/Visuals Unlimited

31. Ash-flow Sheets Draping Topography, Japan S. Aramaki

32. Alternating pyroclastic layers and lava flows Slopes intermediate in steepness Intermittent eruptions over long time span Mostly andesite Circum-Pacific Belt (“Ring of Fire”), Mediterranean Belt Composite volcano Fig. 6.9

33. Mt Fujiyama, Japan Fig. 5.15 Raga/The Stock Market

34. Caldera

35. Crater Lake, Oregon

36. Phreatic Eruption An extremely explosive eruption that occurs when hot lava encounters cool seawater. Huge quantities of steam are released. Phreatic eruption on a Pacific island south of Tokyo. Fig. 6.11

37. Hot Spots Fig. 6.19

38. Hot Spots What is a hot spot? A hot spot is the surface expression of a mantle plume. What is a mantle plume? A narrow, cylindrical jet of hot material, rising from deep within the Earth (perhaps the core-mantle boundary) that gives rise to surface volcanism.

39. Hot Spots Fig. 6.22

40. Hot Spots Fig. 6.20

41. Hot Spots Fig. 6.20

42. Hot Spots Fig. 6.20

43. Large Igneous Provinces Fig. 6.21

44. Lava Flows: e.g. Hawaii, 1998 Gas: e.g. Lake Nyos (Cameroon), 1984 1700 people killed Ash fall: e.g. Mt. Pinatubo, 1991 Pyroclastic flows: e.g. Mt. Pelee, 1902 28,000 killed Lahars (mudflows): e.g. Nevado del Ruiz, 1985 23,000 killed Tsunami: e.g. Krakatoa, 1883 36,417 killed Types of Volcanic Hazards

45. May 1990 Eruption of Kilauea, Hawaii

46. San Juan, Mexico. Buried by Paricutin Lava Flows.

47. U.S. Active Volcanoes

48. Before May, 1980 Emil Muench/Photo Researchers

49. After May, 1980 David Weintraub/Photo Researchers