Understanding Volcanoes: Terminology, Magma Formation, and Eruption Types

1. Volcanoes: Part One

2. New Terminology • Viscosity • Resistance of a material to flow • Ex: honey vs. tar • Volatiles • Gases dissolved in magma

3. Makin’ Magma… • Recall the asthenosphere… • Mostly solid—very close to melting point—small changes in T and P can create melt • Let’s make some Magma!

4. “Liquid Hot Mag-ma…” • Ways to make magma • Increasing T • A solid melts when it becomes hot enough • Decreasing P • Removal of P allows a solid to expand enough to melt • Addition of water • Wet rock melts at lower temperatures than dry rock

5. Magma Environments • Subduction Zones • Presence of water in subducting crust, increasing T and (generally low P) • Generally felsic extrusives

6. Magma Environments • Spreading centers • Ocean plates pull apart, asthenosphere rises to fill gap—decreasing P causes mantle to melt--mafic intrusvies and extrusives Plate motion Plate motion Upwelling mantle

7. Mantle Plumes • Decreasing P—hot mantle material from core-mantle boundary rises through surrounding mantle and melts as it rises • Mafic extrusives

8. Types of Volcanoes

9. Sizes and Shapes

10. Factors Controlling Size, Shape, and Type of Eruptive Activity • Size • Volume of lava erupted • time • Shape • Viscosity/composition • Explosivity • Viscosity/composition • Gas content

11. Basalt • High in Fe, Mg, Ca • Low in silicon • 1000-1200°C • Most common type of melt • Very fluid; up to ~6 mph • Very rarely explosive, mostly “lava rivers” • Deposits generally thin (several meters)

12. Shield Volcano • Gently sloped shield-shaped volcano several km high and 10s of km in diameter • Built up of many overlapping basalt lava flows • Mauna Loa is taller than Mt. Everest (10 km) but most of it is under water • Common above mantle plumes

14. Basalt Eruptions: Lava • Pahoehoe (“ropy”) • Surface cools but internal parts stay molten • “Skin” folds as material moves below

15. Like so

16. Basalt Eruptions: Lava • “Aa” – commonly believed to be expletive after walking on it • moves faster than Pahoehoe • Skin torn into jagged blocks which cascade down front of flow and are over ridden—like the tread on an earthmover

17. Aa lava flow

18. Lava Tubes • Lavas flowing in channels • Top slowly hardens over • Up to 60 km

19. Basalt Eruptions: Pillow Lavas • Erupted under water • Commonly from mid-ocean ridges • Lava in contact with water cools instantly • Lava inside remains hot and can break out

20. (http://www.youtube.com/watch?v=o3BjOapOSGA)

21. Basalt Eruptions: Fire-fountains • Occur when large amounts of gas rich lava erupted • Bubbles expand as magma moves to surface—causes lava to surge high into the air • Erupted lava is partially molten as it hits the ground—spatter • At very high eruption rates, spatter accumulates to form spatter-fed lava flows • Generally from linear fissures

22. Whee

23. Cinder Cone • Low volume, gas-rich, basaltic eruptions create fragments of material which cool quickly in air • Fragments pile up into cones which exist at the angle of stability for a random mass of stuff • Scoria—”lava rock” used in landscaping • Generally small <300m • Paricutín (from 2/20/43 to 1952, 424 m)

25. Convergent MarginVolcanism

26. Evolved Lavas • Partial melting • Not all minerals have the same melting temperature • When a rock is heated, minerals with lowest melting temperatures melt first—generation of Na, K, Si rich magmas • Evolved lavas—high viscosity, high gas content • Andesite • Dacite • Rhyolite

28. Stratovolcano • Also called composite volcanoes • Composed of alternating layers of lava(generally andesitic or dacitic composition) and pyroclastic (ash and blocks) material • Conical profile • Explosive personalities

30. “Evolved” Eruptions: Explosions • Imagine if sticky mass plugs top of volcano… • gas builds up and BOOM

35. “Evolved” Eruptions: Columns • Eruption Columns • Gases separate from magma during ascent due to decrease in pressure • Expand to a froth • Unlike in basalt, viscous magma prevents complete escape • Gas explodes violently at ~surface, propels particles of all sizes into atmosphere • Column propelled upward because it is buoyant—high temp + propelling force of exploding gasses

37. Pyroclastic Material • Anything shot out of a volcano • Range of particle sizes

38. Particles of All Sizes Ash-fall deposit Block Note layering

39. Pyroclastic Flows: Hot stuff, indeed • Occur when large volumes of material are erupted--atmosphere cannot support it • Hot rock and ash flow down sides of volcano at speeds of up to 160 km/hr (100 mph) • Hugs valleys and low spots • Fast (and hot) enough to travel across water • PF video

40. Caldera: Supervolcanoes • Predominantly rhyolite • Large volume eruptions of gas-rich magma causes surface to collapse • Commonly related to mantle plumes beneath continental crust or thinned continental crust • Long Valley Caldera – 600 km3 • A football field ~133,000 miles tall (61% of distance between Earth + Moon)

41. Caldera Formation

42. “Evolved” Eruptions: Lava • Low eruption temperature (compared to basaltic lava): 600-1000°C • Often occur after large, explosive eruptions (the lava is “flat” like a bottle of coke left open for too long) • Viscous: lava often has difficulty breaching crater—lava dome • If crater breached, very thick lava flows-move ~10 ft/day • Some flow fronts reach 150 m (or more)

44. Dacite Rhyolite

45. Case Studies

46. Mt. St. Helens • May 18th, 1980 • “Bulge” on northern flank—caused by development of a crypto-dome—a magma chamber in the body of the volcano • Magnitude 5 earthquake—unstable N. Flank collapses—debris avalanche • MSH height lowered by 1200’ • Left a crater 2 miles wide and .5 miles deep • 230 square miles of wilderness leveled • 58 people killed

47. Yellowstone • Yellowstone national park—three overlapping calderas—caldera at center of park 34 miles wide/1,500’ deep • Last eruption--~650,000 years ago • Repose period-- ~650,000 years • Will it erupt in our lifetime? • If it did, what kind of effects would it have?