Igneous Review 2 Mostly Extrusives

1. Igneous Review 2Mostly Extrusives Igneous Rock Summary.doc will be helpful http://www.soest.hawaii.edu/GG/HCV/kilauea.html

2. The Nature of Volcanic Eruptions • Factors determining the “violence” or explosiveness of a volcanic eruption • Composition of the magma –Silica Content • Temperature of the magma • Amount of dissolved gases in the magma • Composition and Temperature control the viscosity (resistance to flow) of magma. Viscous magmas cannot release gasses coming out of solution, and explode lava as it freezes. http://www.soest.hawaii.edu/GG/HCV/kilauea.html

3. The Nature of Volcanic Eruptions • Water has very low viscosity, cold molasses high viscosity • Factors affecting Viscosity • Temperature - hotter magma is less viscous (more fluid). Basaltic (mafic) magmas (Olivine, Pyroxene, Ca-Feldspars) are hotter than Granitic (felsics) (Quartz, K- feldspars) • Composition (Silica content) - Felsic lava (e.g. rhyolite) is most viscous due to high silica content - intermediate lavas (e.g. andesite) viscous. - mafic lava (basalt) has lower viscosity - more fluid-like due to lower silica content

4. The Nature of Volcanic Eruptions • Factors affecting explosiveness • Dissolved Gases • Gases come out of solution and expand in 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 – trapped gasses expand and shatter solidifying lavas, causing explosions http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html

5. The Nature of Volcanic Eruptions • Summary • Fluid basaltic lavas generally produce quiet eruptions (Hawaiian lava flows) • Viscous lavas (rhyolite or andesite) produce more explosive eruptions (Yellowstone & Mt. St. Helens hot ash explosions)

6. Viscous Andesitic Lava over crater floor Source:Eugene Iwatsubo/Cascade Volcano Observatory, USGS

7. Very Viscous Rhyolite Flow Source:Martin Miller Viscous, short path

8. Materials extruded from a Basaltic Volcano • Lava Flows • Basaltic lavas are much more fluid • Types of basaltic flows • Pahoehoe lava (- twisted or ropey texture) • Aa lava (rough, jagged blocky texture) • Dissolved Gases • 1-6% of a magma by weight • Mainly H2O vapor and CO2 and SO2

9. Typical a’a’ flow

10. Fluid basalt forms lava tubes Checking Bowens Reaction Series

11. Products of Explosions • Pyroclastic materials – “Tephra” Propelled through the Air Types of pyroclastic debris • Dust 0.001 mm and Ash < rice sized • Cinders or Lapilli - pea to walnut-sized material Particles larger than lapilli • Bombs - > 64 mm ejected as hot lava -Surtsey Is. Bombs the size of busses

12. A volcanic bomb Bomb is approximately 10 cm long

13. Tephra forms Tuff St. Lucia Anecdote Source:Gerald & Buff Corsi/Visuals Unlimited, Inc.

14. Tephra layers fine away from source

15. Pumice • Felsic magmas with high water content may bubble out of a vent as a froth of lava. • Quickly solidifies into the glassy volcanic rock known as Pumice. http://volcanoes.usgs.gov/Products/Pglossary/pumice.html

16. Types of Volcanoes 1 • Shield volcano - Largest • Broad, slightly domed-shaped • Composed primarily of basaltic lava • Generally cover large areas • Produced by mild eruptions of large volumes of lava • Mauna Loa on Hawaii is a good example

17. Shield Volcano (Hawaii's K’ilaueau Volcano) Shield Volcanoes are often in a chain of islands. They have basaltic lava, which is NOT very viscous, so it easily releases it’s gasses. Hence explosive pyroclastic eruptions are rare. Source:Jeff Greenberg/Visuals Unlimited, Inc.

18. A size comparison of the three types of volcanoes

19. A Shield Volcano A Shield Volcano

20. Mars Olympus Mons Caldera 23 km high

21. Types of Volcanoes - 2 • Cinder cone - Smallest • Built from ejected lava fragments (mainly cinder-sized) • Steep slope angle • Rather small size • Frequently occur in groups

22. Cinder Cone

23. A Cinder Cone Fountain Typical of divergent margins

24. Sunset Crater – a cinder cone near Flagstaff, Arizona

25. Types of Volcanoes - 3 • 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 & several miles wide at base) • Composed of interbedded lava flows and layers of pyroclastic debris • Above subduction zones

26. A composite volcano

27. Mt. St. Helens – a typical composite volcano (prior to eruption) Composite volcanoes typically have intermediate silica, andesitic magma. Gasses are trapped in the magma. When it erupts out onto the surface, low pressure causes dissolved gasses to come out of solution just as the lava is freezing. The lava explodes, Resulting in a nuee ardente.

28. Mt. St. Helens after 1980 eruption

29. Pyroclastic Flows AKA nuée ardente • explosive mix of rock, gas and heat • only with felsic & intermediate magma • consists of ash, pumice, other fragments • material propelled from vent at high speed

30. St Helens Eruption Sequence How would Scientists Monitor this Process? Seismometers Tilt Meters

31. Composite Volcanoes –continued • Most violent type (e.g., Mt. Vesuvius, Mt. St. Helens, Mt. Pinatubo) • Often produce a nuée ardente • Fiery pyroclastic flow made of hot gases infused with ash and other debris • Move down the slopes of a volcano at speeds up to 200 km per hour • Forms Welded Tuff http://volcanology.geol.ucsb.edu/pfs.htm

32. A nueé ardente on Mt. St. Helens

33. Pyroclastics on upper slopes may produce a lahar, which is a volcanic mudflow. Heat of volcanics melts ice. Lahars

34. Volcano Features • General Features • Opening at the summit of a volcano • Crater - steep-walled depression at the summit, generally less than 1 km diameter • Caldera - a summit depression typically greater than 1 km diameter, produced by collapse following a massive eruption. • Vent – opening connected to the magma chamber via a pipe

35. Calderas • Calderas form by collapse of evacuated magma chamber • Steep-walled depressions at the summit • Size generally exceeds 1 km in diameter

36. 4700 BC S Oregon Mt Mazama Eruption and Caldera Collapse Ngorongoro Crater in Tanzania similar 2 mya

37. Caldera of Mt. Mazama now filled by Crater Lake

38. Plumes

39. Flood Basalts Hot Spot currently forming Hawaii Hey, the plate changed direction !

41. Flood Basalts • Fluid basaltic lava extruded from crustal fractures called fissures • e.g., Columbia River Plateau, • Deccan Traps in India • Cover huge areas • Plumes from Mantle

42. Flood Basalt erupted from fissures - Snake River Plain, southern Idaho Plume Activity

43. Volcanic landforms • Lava Domes • Bulbous mass of congealed lava • Most are associated with explosive eruptions of silica-rich magma http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html

44. Viscous magmas St Helens Lava Dome

45. Volcanic landforms • Volcanic Pipes and Necks • Pipes are short conduits that connect a magma chamber to the surface • Volcanic necks (e.g., Devils Tower in Wyoming and Ship Rock in New Mexico) are resistant vents left standing after erosion has removed volcanic cone

46. Formation of a volcanic neck

47. Spanish Peaks and Radiating Dikes (southern CO)

48. Plutonic igneous activity • Types of intrusive igneous features • Dike– a sheetlike injection into a fracture Discordant - cuts across pre-existing • Sill– a sheetlike injection into a bedding plane Concordant - lies parallel to bedding

49. Some intrusive igneous structures

50. A sill in the Salt River Canyon, AZ Sill: Sediments above and below sill are baked. Lava Flow, just baked below.