1. Dynamic Earth
Class 13
21 February 2006

2. Volcanic Imagination (Chapter 4, continued) Exploring the Earth’s Interior

3. How do we know about the Earth’s Interior?
Meteorites
Direct observation
High-pressure experimental petrology
Earthquake waves (seismology)

4. Layers of the Earth Crust Rigid upper mantle (+crust = lithosphere)
Asthenosphere
Upper mantle
Lower mantle
Outer core
Inner core

5. Earth’s internal heat Original heat Subsequent radioactive decay
Conduction
Convection

6. Crust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic
uplift
Return to isostatic equilibrium

7. The less dense crust “floats” on the less buoyant, denser mantle
Mohorovicic
Discontinuity
(Moho)

8. Convection in Earth’s Mantle
Convection happens when
Temperature gradient exists
Heat is directional
Conduction operates slowly
Surface area to depth ratio is low
Viscosity of material not extremely high

9. Convection in Earth’s Mantle
Assumptions
Solid mantle behaves fluidly over time
Mantle and core do not mix
Heat generated from within the Earth

10. Temperature vs. Depth

11. Convection as a Possible Mechanism for Plate Tectonics

12. Mantle Tomography Uses numerous seismic data
Uses small changes in speed of seismic waves
Faster wave motion may correspond to denser or colder regions
Slower wave motion may correspond to buoyant or warmer regions

13. Basics of Tomography

14. Tomography of the Mantle

15. Tomography at the Base of the Mantle
At 2770 km

16. Tomography Beneath Active Volcanoes

17. Tomography Beneath Active Volcanoes

18. Volcanic Activity on Earth
Spreading centers (ridges)
Island arc
Hotspots

19. Hotspots Areas with volcanic activity NOT explained by plate tectonics
Mantle beneath may be hot, wet, or chemically different
Commonly active for long time

20. Global Hotspots http://www.hvo.usgs.gov
There is a huge debate right now about hotspots, about how they form and what they are. The word hotspot is a kind of generic term for a region of volcanic activity of unknown cause, meaning volcanic activity not typical of mid-ocean ridge type rifting, nor of islands arcs or of volcanism associated with tectonic boundaries. This map shows the global distribution of hotspots. Many hotspots are located near mid-ocean ridges, but some are isolated and in the middle of tectonic plates. Hawaii is one of these hotspots unassociated with a plate tectonic boundary. I won’t go into the debate much in this talk but rather will talk about the most common explanation for hotspots

21. Hotspot tracks
Flood basalts
Oceanic
plateaus

22. Karoo / Etendeka Flood Basalts

23. Columbia River Flood Basalts

24. Linear Volcanic Chains

25. Hawaiian Islands - Emperor Seamounts

26. The trail of the Hawaiian Hot Spot goes all the way to the far northwest Pacific

27. In addition to the Hawaiian Hot Spot track,
there are several others in the Pacific

30. Age Progressions Along Volcano Chains

31. Hawaiian Islands
Oldest
Youngest

32. Hotspot Observations
Volcanic activity, NOT explained by plate tectonics
Active for long time
Age varies (youngest in opposite direction of plate motion)

33. Hot spots are regions in the Asthenosphere that are hotter than their surroundings

34. Molten magma rises to the surface to form volcanoes, similar to what happens at Spreading Centers

35. Hot spot volcanoes are relatively small, isolated features.

36. Hot spots are believed to be fixed relative to the mantle.
BUT – this is controversial!

37. When a plate moves over a fixed Hot Spot, a linear chain of volcanoes is formed.

38. Hotspot Origins - Mantle Plumes

39. Mantle plumes and eruption sizes

40. Convection in the Mantle

41. Convection and Mantle Plumes

42. Why Linear Chains of Volcanoes?

43. Model of Mantle Plumes
A mantle plume rising beneath a slow-moving plate or continent will “puddle” beneath the lithosphere

44. Model of Mantle Plumes
When eruptions begin, they are voluminous, causing oceanic plateaus and flood basalt provinces

45. Model of Mantle Plumes

46. Model of Mantle Plume

47. Mantle Plume Shape Unknown

48. Model of Mantle Plumes

49. Models of Mantle Plumes
Wolfe et al., Nature, [1997]

50. Tomography at the Base of the Mantle
At 2770 km

51. Instability Causes Mantle Plumes

52. How Can Plate Tectonics and Mantle Plumes Work Together?

53. Why Are Hotspots Important
Associated with Large Volcanic Eruptions
May inject gas and particles into air
May re-landscape large areas
May decrease habitable areas
May make life difficult for some plants and animals (and cause mass extinction)

54. Volcanic Eruptions and the Atmosphere

55. Life on Earth is Difficult!
Earthquakes
Floods
Climate changes (draught, ice ages)
Other weather hazards (tornadoes, cyclones)
Volcanic eruptions
Meteor / asteroid impacts

56. Meteor / Asteroid Impacts

57. Meteor / Asteroid Impacts

58. Environmental Catastrophes and Hotspots
Eruption of Deccan Traps
(Reunion hotspot)
This is a diagram from Courtillot and Renne showing the excellent fit between the timing of environmental crises, recognized as period changes in the geological time scale and the timing of LIP emplacement. The correlation is very good and it’s difficult to ignore that there might be some relationship; however, we don’t know if the relationship is causal or coincidental - whether the LIP emplacement is the actual reason for the environmental crises or whether they both occur as a result of some larger force.
End Cretaceous (65 Ma)

59. Extinction Percentages and Hotspots
This diagram shows the correlation between extinction percentages at the genus level, in the filled gray pattern, with eruption of continental flood basalts superimposed in the dark gray bars. The timing of oceanic plateau are also noted in text only. There does seem to be some correlation between extinction events and flood basalt eruptions, at least in the case of the largest extinction events at the end of the Permian and the Siberian Traps, at the end of the Triassic, and the central Atlantic Magmatic province, and at the Cretaceous tertiary boundary, and the Deccan Traps.

60. Extinction is Forever Dinosaurs - ~65 Ma Trilobites ~ 300 Ma
Giant Ground Sloth ~10 Ka
Trilobites
~ 300 Ma

61. Thursday
Video:
Death of the Dinosaurs