Volcanoes and volcanism

1. Volcanoes and volcanism • Volcanoes represent venting of the Earth’s interior • Molten magma rises within the Earth and is erupted either quietly (lavas) or violently (pyroclastics)

2. Terminology Magma – molten rock sometimes containing suspended minerals and dissolved gases. Magma forms when temperatures rise sufficiently high for melting to occur in the Earth’s crust or mantle. Volcano – a vent at the surface in which magma, solid rock, and gases erupt. Lava – magma that reaches the surface and pours out over the landscape.

3. A-Characteristics of Magma Composition – Controlled by the abundant elements in Earth (Si, Al, Fe, Ca, Mg, Na, K, H, and O). Most common types of magma are: basaltic (~50% SiO2), andesitic (60% SiO2) and rhyolitic (70% SiO2) Magma that solidify on the surface are called extrusive rocks and rocks that solidify below the surface are called intrusive. ~70 - 75% of all magma erupted by volcanoes is basaltic the rest is split between andesitic and rhyolitic.

4. Characteristics of Magma Rock classification chart - USGS

5. Characteristics of Magma Dissolved Gases Comprise a small percentage of the magma (0.2 to 3 wt.%). Although not present in abundance these gases strongly influence the eruption style and explosiveness of the magma. Dominantly H2O and CO2 with small amounts of nitrogen, chlorine, sulfur and argon. Temperature Ranges from ~800 C to ~1200 C Viscosity a substances resistance to flow **dependent on temperature and composition

6. The Viscosities of Foods as Analogs for Silicate Melts From: Baker D. et al. (2004) J. Geosc. Education

7. Explosive Eruptions vs. Effusive Eruptions • Three factors effect the explosivity of a volcano • Temperature of magma • High-temperature, less explosive • Composition of magma • Less silica, less explosive • Gas content of magma • Less gas, less explosive fluidity

8. Volcano types

9. Volcano types: cinder cones • Cinder cones are volcanoes which erupt only during one episode • They are explosive, but small in size • The cone is a pile of pyroclastic debris which piles up at the angle of repose

10. Volcano types: cinder cones • The cinders are generally of basaltic composition • The eruptive activity typically lasts a few months or years

11. Volcano types: shield volcanoes • Shield volcanoes are broad, gently sloping volcanoes • They are composed mainly of basaltic lava flows • This is a view of Mauna Loa, Hawaii, from the cinder cones of Mauna Kea Mauna Loa is the tallest volcano on Earth, as measured from the sea floor

12. Shield volcanoes on Mars • Other planets also have shield volcanoes • This is the largest shield volcano in the solar system, Olympus Mons on Mars • Check out the scale !

13. Shield volcanoes: Earth vs. Mars • Red = Hawaiian chain, which is superimposed on Olympus Mons • this says it pretty well, I think ! Mauna Loa is about here

14. Volcano types: stratovolcanoes • Stratovolcanoes consist of alternating layers of lava and pyroclastics • They are dominantly andesitic in composition • These volcanoes are typical of subduction zones Mt. St. Helens (pre-1980)

15. Volcanic landforms Volcano types: calderas A large depression generally caused by the removal of large quantities of magma from beneath a volcano causing the ground to collapse into an empty space. Aniakchak Caldera, Alaska, formed during an enormous explosive eruption that expelled more than 50 km3 of magma about 3,450 years ago. The caldera is 10 km in diameter and 500-1,000 m deep. Subsequent eruptions formed domes, and explosion pits on the caldera floor.

16. Now we need to answer……… A-How and where do magmas and volcanoes form ? What tectonic environment do these volcanoes occur in and why?

17. How and why do magmas and volcanoes form? -Global distribution of volcanoes

18. 1-Magma generation at hot spots • Basaltic magmas at hot spots are derived from deep within the mantle • the magmas are fed by deep mantle plumes which are stationary relative to the drifting tectonic plates

19. Intraplate Volcanism USGS

20. Hawai’i • Best example of intraplate volcanism • More lava is extruded here constantly than anywhere else on Earth!

21. A Bigger Picture • Looking at Hawai’i, and volcanic seamounts nearby (underwater volcanic islands) Current Hawaiian Islands

22. What is a hot spot? • Some mantle anomaly allows the oceanic or continental lithosphere to melt where it would not normally melt • The anomaly (usually) stays stationary • The plate(s) moves over it

23. The Hawai’ian Hot Spot University of North Dakota • Islands and seamounts get older as you move away from the hotspot

24. Hawaiian island trail Does the kink represent a change in plate direction?

25. 2-Magma generation at mid-ocean ridges • In these zones, the mantle rises and melts, producing magma of silicate composition • the magma continues to rise, and erupts mainly as basaltic lava flows **volcanism and earthquakes are separate issues

26. This rifting process is dramatic on Iceland • Iceland is literally being torn apart by rifting of the two plates… • yet its center is continually renewed by new magma from the mantle… • the same thing is going on under the ocean

27. One result of these processes • Krafla volcano erupts frequently, producing spectacular fountains of fluid lava

28. 3-Magma generation at subduction zones • During subduction, the subducted oceanic plate is heated as it plunges into the mantle • At a depth of 80-120 km, melting begins, and volcanoes are produced which parallel the subduction zone Andesitic to Dacitic magmas are typical of these volcanoes

29. Volcanic landforms Stratovolcanoes -explosive eruptions -viscous lava -built of interlayered lava and pyroclastic material -usually andesitic in composition Mt. Fuji, Japan

30. Volcanoes and Plate Tectonics

31. Stratovolcano

32. Stratovolcano eruptions Pinchincha, Ecuador Anak Krakatau, Indonesia

34. Indonesia Population: 215 million World’s fourth most populous nation. 60% on island of Java Krakatau Volcanoes: 79 active - 20% of the world total -600 eruptions since 1800

35. Krakatau Eruption Krakatau Height 2575 ft (785 m) Circa 1880 August 27th, 1883 • Blast 10,000 greater than at Hiroshima • >36,000 people killed • 18 Cubic Km material ejected The explosion blew away the northern two-thirds of the island and it was almost instantaneously followed by the collapse of the unsupported volcanic chambers which formed the huge underwater caldera

36. Other Features: calderas

37. NOW, WHAT ABOUT RHYOLITIC VOLCANISM? • SiO2 contents are even greater than is the case in andesitic magmas, therefore....... viscosities are even greater. • So, Incredible resistance to flow!

38. NOW, WHAT ABOUT RHYOLITIC VOLCANISM? • In addition, rhyolitic magmas tend to be richer in H2O, because they form by partial melting of the crust, and melting is only possibly there if H2O is present. • In other words, rhyolitic magmas exsolve more H2O (more bubbles form as the magmas rise), yet the bubbles cannot expand owing to the high viscosity of the magma (they must expand as pressure decreases, i.e., as the magmas rise through the crust) • Recipe for a major disaster!!!

39. Yellowstone National Park, Wyoming,offers an excellent example of rhyolitic volcanism

40. Yellowstone National Park, Wyoming, • The volcanic eruptions, as well as the continuing geothermal activity, are a result of a large chamber of magma located below the caldera's surface. • The magma in this chamber contains gases that are kept dissolved only by the immense pressure that the magma is under. • If the pressure is released to a sufficient degree by some geological shift, then some of the gases bubble out and cause the magma to expand. • This can cause a runaway reaction. If the expansion results in further relief of pressure, for example, by blowing crust material off the top of the chamber, the result is a very large gas explosion.

41. Geysers and fumaroles!

42. The H2O is of near-surface origin, but the heat is due to a batholith (still partially molten) not far below the surface

43. Uplift and ring fracture formation Initial eruptive stage along ring fractures Collapse alongside eruption Remaining lava extruded Smith and Bailey Resurgent dome forms Calderas can host lakes -Active hydrothermal systems Life Cycle of a Caldera

44. SUMMARY • Calderas are primarily rhyolitic • largest explosive eruptions are caldera-related • lava is cool and viscous, rises slowly, allowing pressure to build up • gas percolates slowly through the viscous magma, does not have an easy way to vent • Stratovolcanoes are dacitic-andesitic • have eruptions of intermediate explosivity • can undergo lava flows if lava is mafic enough and hot enough • lava domes if lava is more felsic and cooler • gas does vent, but slowly, through fissures • Shield volcanoes are usually basaltic • lava is very hot and fluid • gases easily pass through magma to be released into the atmosphere • experience gentle, effusive activity • fountaining if pressure builds, usually at the start of an eruption

45. Volcanic activity • In the following slides, I will give you some examples of volcanic activity: • lava flows, including flood basalts • lava domes • pyroclastic falls and pyroclastic flows • lahars and debris avalanches • volcanic gases

46. Volcanic activity: lava flows • This is a basalt lava flow in a channel • Due to its low silica content and high temperature, it is quite fluid (but stickier than maple syrup) • Yet lava usually flows fairly slowly

47. Pahoehoe lava Do you want to walk on pahoehoe ? This is a Hawaiian term for smooth, ropy lava It generally exhibits fluid-like textures

48. Aa lava • This type of lava is quite blocky on the surface, and comparatively cool • Yet below the surface, the lava is fairly massive and much hotter • Do you want to walk on aa ?

49. Fire fountaining • Sometimes, basaltic lava can contain lots of gas • Then, small explosive eruptions form fire fountains • As partially liquid drops fall back to the ground, they may coalesce to form a lava flow

50. Flood basalts • The previous examples represent small-scale activity • But basaltic eruptions can be huge, forming lava plateaus • These huge outpourings may occur quickly (1-3 Ma) and may contribute to mass extinctions