1. Volcanoes: eruptive style and associated landforms

2. Viscosity Resistance to flow
Which test tube contains the fluid with high viscosity? Left? Right?

3. Viscosity
Which eruption was produced by high viscosity lava? What are the clues?
Eruption A
Eruption B

4. Why does one type of lava have a higher viscosity than the other?

5. Why does one type of lava have a higher viscosity than the other?
Tectonic setting
Source of lava
Composition
Basalt: asthenosphere and oceanic crust
Andesite: sediments, water, oceanic crust and continental crust
Lower percentages of silicon and oxygen
Intermediate composition

8. The Silicon Tetrahedron
Acts as a thickening agent
Building block to all rock forming minerals
Higher percentage = higher viscosity
Rhyolite > 65%
Andesite = %
Basalt < 55%

9. Rhyolite is the lava type with the highest percentages of silicon and oxygen
Most violent eruptions

10. Hot spot under continental crust
Notice the direction of plate movement

11. Andesite
Intermediate composition lava

12. Landforms associated with viscous lava
Andesitic lava produces stratovolcanoes
Rhyolitic or dacitic lava produces plugs.

13. Mt. Rainier

14. Mt. St. Helens: before the 1980 eruption
Bulge: plug that is pushed out by magma within the conduit.

15. Mt. St. Helens: after the eruption
Plug dome

16. Mt. St. Helens: dome plug
The plug is nearly the height of the Washington Monument and the width of four football fields.

17. Plug dome: andesitic to rhyolitic in composition

18. Lassen Peak Lassen Peak is a plug dome volcanic landform
Built from felsic lava
One of the largest on Earth
Carved by glaciers during the Ice Age

19. Crater Lake: volcanic caldera

20. Caldera formation and subsequent plug
Volcanic eruption
Large volume of material extruded
Magma chamber empties
Volcano collapses into the empty magma chamber

21. Yellowstone: hot spot under continental crust
Three large eruptions in the last 2 million, 1.3 million and 600,000 years ago
Calderas formed when felsic lava produced enormous eruptions.

22. Yellowstone caldera formation

23. Long Valley Caldera
An enormous eruption 760,000 years ago, forming a caldera

24. Landforms associated with low viscosity lavas
Basaltic lava flows produce shield volcanoes and lava plains or flood basalts.

25. Shield volcano
Mauna Loa is 9 miles high
Built over a long period of time
Associated with basaltic lava

26. Modoc Plateau, northeastern California (extension)
Medicine Lake volcanic field
Mt. Shasta is in the background
Tectonic setting?

27. Columbia River Basalts
Basaltic lava flows from fissures
Layer upon layer of lava flows
Covers continental crust
Columbia River Basalts
14-16 million years old

28. What happened in Iceland?
Eyjafjallajokull's eruption creates an ash cloud that closed Europe’s airports for weeks
Shield volcano eruption under a layer of ice

29. Size comparison

30. Cinder cones: found in most setting
Hawaii
Short lived events
made of cinders
generally about 1000 feet high
Mojave Desert

31. Composition,Viscosity and Eruptive Style
Basalt
Andesite
Rhyolite
Fluid
Viscosity
Pasty
Eruptive Style
Quiet
Violent
Temperature
Cool
Hot

32. The three Vs
Viscosity
Strombolian
Icelandic
Volatiles
Volume
Plinian

33. Volcanic material Pyroclastic debris Lava flow
Pieces of older rock and magma
Ash size to bombs
Smooth or chuncky

34. Volcanic Explosivity Index
Volume of material
How high the eruption column reached
How long the main eruption occurred