GEOS 251 — Physical Geology

1. GEOS 251 — Physical Geology 11 February 2014 • Quiz today, 15 minutes, 25 pts • Starts promptly at 11:02 • Closed book and notes; pencil or pen only • Covers topics through Igneous Rocks (4 Feb) • Handout • Lecture Summary

2. Last Time: Magmatic Processes • Magmatic processes - origin • Melt of rock caused by ∆T, ∆P, ∆H2O (tectonics) • Basalt originates by melting of mantle peridotite • Granite originates by melting of mafic crust • Magmatic processes - evolution and emplacement • Magmas are modified by • Removal of crystals (“crystal fractionation”) • Mixing with other magmas or assimilation of other rocks • Emplacement of magmas: buoyancy (lower density) vs. impedance (increasing viscosity) • Result: A spectrum of igneous rocks that varies with tectonic setting

3. Melting produces magmas • Rocks melt with (1) increasing temperature (2) decreasing pressure (3) addition of water— Why do these factors change in the Earth? • Source rocks:— mantle (peridotite)— crust (andesite or shale — most abundant compositions)

4. Causes of Melting • Increase in temperature • Crustal thickening / subduction • Heat from other magmas • Decrease in pressure • Upwelling of mantle • Increase in water content • Subduction zones • Hydrous crust (what minerals?) • Next examine experimental data– a test in the lab

5. heating melting paths (how?) heating melting paths (how?) addition of water melting paths (why?) decompression melting paths (where?) Wet Dry

6. Melting in the Mantle:The dry (decompression) process • Process: rising mantle intersects the melting curve as it decompresses • Where: (1) under mid-ocean ridges, (2) in hot spots (“mantle plumes”) (e.g., Hawaii) • Produces: basalt (pyroxene + plagioclase feldspar) from mantle (olivine > pyroxene > an Al mineral such as Ca-rich plagioclase)

8. Q: Why do we get basalt (a mafic composition) from mantle peridotite (an ultramafic composition)? In a chemically complex material, crystals and melt typically have different compositions. Thus when peridotite melts >>>

9. Analogous to Ice + Salt melting to make Brine (salt water) Removal of low-T melting fraction (“partial melting”) generates mafic melt (basalt) from ultramafic mantle (peridotite)

10. Water from slab(mainly metamorphic reactions, not from sediment) Origin of magmatic arcs • Cycling of water back into the mantle via subduction of hydrous minerals; their breakdown triggers melting • Where does the liquid water go? • Where does the heat beneath arcs originate?

11. Melt generation processesby tectonic setting Melting by heating of lower crust Decompression melting Water-induced melting

12. Changes in magmas byfractionalcrystallization • Can you think of analogies?

13. Bowen’s Reaction Series

15. How might fractional crystallization lead to the concentration of rare elements? • Tourmaline (gemstone) • Contains the rare elements • lithium (Li, Z =3) • boron (B, Z=5) • Can anyone suggest why these elements might be rare? • Why might these very light elements not readily substitute in common mineral structures?

17. Sedimentation and volcanism • Both take place at the Earth’s surface • Both involve fluids • Water and air for sedimentary rocks; but cool • Melt (± air or water) for volcanic rocks; but hot

18. Lecture 8: Volcanoes and Volcanism • Volcanic rocks and processes • Ejecta (lavas, pyroclastics, gases), volcaniclastics (lahars, sands, muds) • Other features: earthquakes, geothermal systems • Types of volcanoes • Basic types: central vent, fissure, caldera . . . • Controls on types: magma composition, T, flux • Tectonic distribution and controls • Next time finish with: Volcanoes and people: hazards, resources (geothermal, minerals, water, soil), climate • Then on to: Weathering and erosion (Chap. 16, 5)

19. What happens in volcanoes?Why does this happen?What do volcanoes produce?How are volcanoes constructed?How do they differ and why?(what, where, how come)

20. What comes out –volcanic products

21. Volcanic MaterialsTwo groups – what are they? • Lavas — move (mainly) as massive magma • Massive to brecciated flows (pahoehoe, aa, pillows) Fragmental materials • Pyroclastics — fragmented magma / rock • Ash (and lapilli), cinders, bombs (increasing size) • Pyroclastic flows (“ash flows” or “ignimbrites” can be welded by heat, depending on size) • Volcaniclastics — reworked, mainly by water • Lahars / mudflows — start on or near volcano • Epiclastic rocks (reworked volcanic sands / muds)

22. LavasCoherent bodies of magma emplaced by flow at surface • Wide differences in form and textureWhy might this be?

24. Pyroclasticvolcanic products • Fragmental materials produced during eruption (right) • Compare and contrast lavas (above) and clastic sediments

25. Fragmental Volcanic Products • Pyroclastic types • depend on nature of eruption (and thus nature of volcano) • proximal (near the vent) ejecta and more far traveled pyroclastic fall (“air fall” tuff) and pyroclastic flow (ash-flow tuff) deposits • fragment size diminishes with distance from vent • Pyroclastic processes and products • ballistic • mix with air and be carried away (-> pyroclastic fall / ash-fall tuff) • eruptive column collapse (-> pyroclastic flow / ash-flow tuff) • volcano collapse (-> debris avalanches / pyroclastic flows / lahars (cooler)) • Contrast reworked cool materials around volcanoes • mudflows, lahars, and volcaniclastic rocks Many parallels with sedimentary processes (later lectures)

28. Welded tuff – product of an ash flow (why not an ash fall?)

30. Mediterranean exampleswith pyroclastic flows

31. Types of Eruptionswhat are they and what governs their styles? • Types — as seen above Passive – mainly lava flows Explosive — pyroclastic fall and flow deposits • What are the main factors? • Energy for eruption (from what?) Buoyancy, especially dissolved gases that exsolve and expand • Viscosity – how easy it is to deform magma • So what will be the effects of magma composition (and thus geologic setting)?

32. How it stacks up –types of volcanoes

33. Types (geometries) of volcanoes • Central volcanoes (varied compositions) • Cinder cones, domes, stratovolcanoes • Calderas (most commonly felsic) • Fissure eruptions from ring-fractures over collapsing magma chambers • Fissure eruptions (typically mafic) • Flood basalts, mid-ocean ridges • Type (geometry) of volcano largely reflects • Type and volume of magma erupted, thus • Passive vs. explosive eruptions, and • Tectonic setting

34. Cinder cones and basaltic lava flows

35. Lava Domes–More viscous equivalent of cinder cones/flows

36. Stratovolcanoes and Calderas • Stratovolcanoes built of multiple lava flows and pyroclastic rocks (“strata”) • Steep cones, typically of andesitic arc volcanism • Shield volcanoes built of many lava flows • Gentle cones, typically of basaltic volcanism, especially in intraplate (hot spot) settings • Calderas are collapse features formed over rapidly evacuated magma chambers • Most common over rhyolitic magma chamber, commonly without central volcano precursor, but • May also develop late in the history of a cluster of andesitic stratovolcanoes

37. Andesitic stratovolcanoes— Composite (many eruptions), steep sided— Commonly violent (e.g., Mount St. Helens, WA)

38. Mt. Lassen, CA—1915

39. Mount St. Helens, WA—1980

42. Mt. Pinatubo, Philippines--1991

44. Mt. Pinatubo • Crater collapses during eruption to form caldera • Soon fills with water to form a lake

45. Shield volcanoes— Gentle slopes— Basaltic, passive eruptions (e.g., Hawaii)

47. Calderas instratovolcanoes- Andesitic: Crater Lake, Oregon- Basaltic: Kilauea, Hawaii • Crater Lake caldera formed by collapse during & following of massive eruption of ancient Mt. Mazama

48. Rhyolitic calderas• Enormous, violent eruptions; single eruptions (over a few days) can be 100s to 1000s of km3 (e.g., San Juan Mtns, CO)

49. Flood basalts and large igneous provinces • High-volume fissure eruptions related to plumes from deep mantle (hot spots) • Can cover very large areas and can be erupted quickly • Some eruptions contemporaneous with mass extinctions • Most easily recognized on continents • Columbia Plateau / Snake River Plain, Siberian traps (Russia), Deccan traps (India) • But also form large oceanic plateaus • Ontong Java Plateau (SW Pacific), Kerguelen Plateau (Southern Ocean)