volcanic behavior is affected by: 1 composition, 2 temperature 3 dissolved gases volatiles all of which various

1. Volcanic �Behavior� is affected by: 1) Composition, 2) Temperature 3) Dissolved gases (Volatiles) All of which variously affect viscosity, i.e., the �thickness� or resistance to flow of the lava. Gabbro/Basalt (Mafic) � 50% silica Diorite/Andesite (Intermediate) � 60% silica Granite/Rhyolite (Felsic) � 70% silica Longer �chains� of silica tetrahedra = greater viscosity.

2. Volatiles (Dissolved gases) � tend to increase the fluidity of the magma. Are present in the lighter fractions of the magma, after the iron-rich dark minerals have �dropped out�. The gases, in sufficient quantities have the ability to drive the eruption. Gases in basaltic lavas cause the �lava fountains� shown in Figure 5.5, pg. 131. Because of the low silica content and high temperature, the fountains �erupt freely�, without major explosions.

3. In contrast, in high silica intermediate and rhyolitic lavas, the lower temperature and higher silica content causes plugs & lava domes to develop in the volcanic neck (or vent), especially after the explosion. The result is: The �irresistible force� (the gases) meets the �immovable object� (the solidified plug), pressure builds and eventually, the gases generally win. Basaltic magmas produce shield volcanoes and cinder cones. Intermediate magmas generally produce composite volcanoes. Felsic magmas produce caldera-type volcanoes.

4. See Figure 5.1 (p. 127) Before and After pictures of Mt. St. Helens.

5. The hot, fluid nature of basalt flows produce their own characteristic features. Basaltic flows � ropy �pahoehoe� (Figure 5.7b) and rough, jagged �aa� (Figure 5.7b) flows. Pahoehoe forms when the �plastic� flow surface meets an obstruction.

6. Pahoehoe flows on Kilauea

8. Volcanic gases can include (as cited on page 98) water vapor, carbon dioxide, nitrogen, sulfur dioxide, chlorine, hydrogen chloride, and argon. Ejecta � volcanic bombs. From larger vents, bombs can be blown high enough to become streamlined before impact. From spatter vents, ejecta can include ribbons, and clots resembling �cow dung�.

9. Spatter mound erupted within Aden Crater

12. Aden Crater � Shield Volcano

13. Other types of basaltic eruptions and materials include continental flood basalts, ex., Aden & Afton Basalts (NM), Columbia River Basalts (p. 147)

14. Basalt ejecta from shield volcanoes & spatter vents

17. Composite volcanoes are generally associated with Island arc systems or Continental arc systems, i.e., inland from subduction zones. Volcanic materials are generally mafic to intermediate (more common). Aside from the lahars, the pyroclastic flows (Fig. 5.22) present a great danger near these volcanoes. Often called �Nue� Ardente� (glowing cloud) ash flows can travel 100+ mph and have been traced 60 miles from eruptive center.

19. Crater sizes Kilauea caldera (pg. 145) � 2 - 3 mi. across. Crater Lake caldera (pg. 145) � 6 mi. across. Yellowstone caldera (pg. 145) � 43 mi. across. Yellowstone-Type Calderas - supervolcanoes Craters � tens of miles across. Area �swelled� by rising pluton can cover 100+ sq. mi.. Histories can last millions of years. Last major Yellowstone eruption � 630,000 years ago (estimated). 3 major episodes over last 2.1 m.y.. Long Valley caldera, CA and Valles caldera, NM are also youthful. Ash flows from calderas have been traced 100 miles from their sources.

20. Kilauea caldera, �atop� Kilauea volcano.

21. Smaller caldera within Kilauea caldera.

23. Older Caldera, modified by erosion and faulting. Crater � 10�s of miles in diameter

24. Because Intrusive Igneous Rocks represent solidified magma (below the surface), they are only exposed by: Erosion, Faulting bringing the rocks to the surface, Both Erosion and Faulting Because of the slow cooling rate, intrusives have larger grains than extrusives. Intrusives may have served as conduits for extrusives to reach surface.

25. Intrusive igneous activity � Internal processes, many related to overlying volcanic activity. Relationships to local geologic structures � Discordant intrusions cut across local structures. Concordant intrusions are parallel to local (especially sedimentary) structures. Shapes � tabular (dikes or sills), massive (batholiths, stocks, pegmatites), & combination (laccoliths). Dikes � discordant. Sills � concordant. See p. 151 for examples of both.

27. Dikes � tabular/ discordant

29. Examples of Laccoliths include La Sal Mts. and Henry Mts. in Utah

30. Another example of Laccoliths are the Sierra Blanca Peaks in West Texas. Erosion has revealed the �flat bottom� over limestone.

31. Yosemite Nat�l Park, part of the Sierra Nevada Batholith � exposed by erosion.

32. Stone Mountain, part of the Stone Mt. Granite batholith (or stock) � exposed by erosion

33. Batholiths rise as buoyant igneous bodies through the warmer, lower crust. In the cooler, brittle upper crust, stoping occurs, whereby the batholith fractures the overlying rock and loose chunks are incorporated into the pluton.

36. Plate Tectonics & Igneous Activity Convergent zone vulcanism (inland from subduction zones) - Circum-Pacific �Ring of Fire� volcanoes mainly composite volcanoes with explosive, volatile-rich magmas. Most of these erupt intermediate (andesitic) lavas and pyroclastics. North America � Cascade Volcanoes, Aleutian Island Arc System South America - Andes Mts. Pacific Island Arcs � Japan, Philipines, Indonesia.

38. Oceanic/oceanic boundaries. When one plate sinks, gravity �pulls� it towards the mantle. Initial vulcanism is basaltic, derived from the melted plate . As the eruptions thicken the overlying plate margin, the increased thickness inhibits rise of basaltic magmas, magmatic differentiation (crystal settling) = more silica-rich (intermediate to felsic), as arc matures. Friction from subduction contri-butes to heat and seawater in sediments contributes to lowered melting points, aiding partial melting and differentiation.

39. Continental Arc Volcanic systems � Cascades, Andes, Mexico volcanoes, Sierra Nevada (old). When oceanic plate begins its subduction beneath continental plate, continental margin sediments, seawater, and partial melting of the oceanic crust triggers the rise of plutons. These plutons, may cause partial melting of the felsic continental crust. The mixing (and crustal thickness), result in intermediate to felsic magmas (diorites and granites).

40. Divergent Plate Boundaries � Uprising mantle plumes (upwhellings) stretch and thin lower crust = decompression melt, which is enhanced as magma nears the surface. Continental Rifts � Continental crust thickness and silica content allows presence of both basaltic and intermediate magmas and lavas. Oceanic rift zones � mafic basalts directly erupted from mantle-derived ultramafic plutons.

41. Intraplate Vulcanism (Oceanic plates) � Unusually strong mantle plume �breaks through� plate. Examples: Hawaii, Emperor Seamount Chain, Canary Islands. Intraplate Vulcanism (Continental plates) - mantle plume erupts within continental crust. Examples: Yellowstone, Long Valley, CA, San Francisco Volcanic Field (Flagstaff, AZ and other N. AZ volcanics), Columbia River Basalts, Snake River Basalts, West Africa (Cameroon) and North Africa (Libya) volcanoes. Yellowstone, Long Valley, Valles � rhyolitic (felsic) Others � generally basaltic/intermediate http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/north_america.html

42. Volcanoes and Climate Ash contributions to stratosphere are generally short-lived, ash eventually settles out. Sulfur dioxide (as aerosols) is more of a concern, is believed to reflect sunlight back into space. Mt. Pinatubo eruption in Philipines caused global cooling for about 2 years. Eruptions of Deccan Plateau flood basalts (Cretaceous Period) may have contributed to global cooling that stressed the dinosaurs. Eruptions of flood basalts in Siberia may have contributed to mass-extinctions at the end of the Permian Period (246 m.y.a).

43. �Volcanic carbon dioxide caused increased Greenhouse Effect in the Cretaceous Period.� This plays into the politically-driven modern hypothesis. http://www.john-daly.com/history.htm How the current hypothesis of carbon dioxide-caused global warming became a �political animal�. Many scientists believe that increased carbon dioxide is the �effect� of global warming, rather than the �cause�.

44. Current atmospheric carbon dioxide content is about 380 ppm = 0.038% Approximately 90% of the modern Greenhouse Effect is due to atmospheric water vapor and clouds. Estimates of Cretaceous atmospheric carbon dioxide range from 1100 ppm (.110%) to 3600 ppm (.360%). Evidence suggests that Cretaceous Period was very warm. When two things happen �at the same time� = �correlation�, not causation.

45. When we find correlations in the past, e.g., atmospheric carbon dioxide rises & temperature rises, this doesn�t tell you which one �leads� and which one �follows�. The computer models used by the IPCC & supporters to support the CO2 rise = temperature rise may not include the effects of atmospheric water vapor and clouds. If temperature rises first � oceans warm and lose dissolved CO2 (�Coca Cola effect�), increased biological activity (animals, bacteria, termites, etc.) = increased CO2 .