1. Volcanic eruptions Factors that determine the violence of an eruption
Composition of the magma
Temperature of the magma
Dissolved gases in the magma
Viscosity of magma
Viscosity is a measure of a material's resistance to flow

2. Volcanic eruptions Viscosity of magma Factors affecting viscosity
Temperature (hotter magmas are less viscous)
Composition (silica content)
High silica – high viscosity
Low silica – more fluid
Dissolved gases (volatiles)
mostly H2O and CO2
Gases expand near the surface

3. Volcanic eruptions Viscosity of magma Factors affecting viscosity
Dissolved gases (volatiles)
Provide the force to extrude lava
Violence of an eruption is related to how easily gases escape from magma
Easy escape from fluid magma
Viscous magma produces a more violent eruption

4. A low silica lava flow

5. A high silica flow

6. Pyroclastics
Pyroclastic material: fragments of rock formed during eruptions
Ash and dust: fine, glassy fragments
Lapilli: walnut sized rocks
Bombs: magma ejected into the air cools and forms rounded rocks
Blocks: ejected as large rocks

7. Volcanoes General features
Conduit, or pipe carries gas-rich magma to the surface
Vent, the surface opening (connected to the magma chamber via a conduit)
Crater
Steep-walled depression at the summit
Caldera (a summit depression greater than 1 km diameter)

8. Volcanoes Types of volcanoes Shield volcano
Broad, slightly domed (like a shield)
Primarily made of fluid lava
Generally large size

9. A shield volcano

10. Volcanoes Types of volcanoes Cinder cone
Built from ejected lava fragments
Steep slope angle
Relatively small size

11. Sunset Crater – a cinder cone near Flagstaff, Arizona

12. Volcanoes Types of volcanoes Composite cone (or stratovolcano)
Most are adjacent to the Pacific Ocean (e.g., Mt. Rainier)
Large size
Interbedded lavas and pyroclastics
Most violent type of activity

13. A composite volcano (stratovolcano)

14. Mt. St. Helens – a typical composite volcano

15. Mt. St. Helens after eruption

16. A size comparison volcanoes

17. Other volcanic landforms
Calderas
Steep walled depression at the summit
Formed by collapse
Nearly circular
Size exceeds one kilometer in diameter

18. Crater Lake in Oregon

19. Hot Spots

20. Locations of some of Earth’s major volcanoes

21. Continental and island volcanic arcs

22. Super volcanoes “super eruptions:”
eject magma with a mass greater than 1015 kg, equivalent to a volume greater than 450 km3
explosive eruptions of this magnitude have a volcanic explosive index (VEI) of 8 or above and produce > 1000 km3 of fragmental deposits
A “super volcano” can be defined as a volcano that has produced at least one explosive super eruption.

25. Super eruptions require:
- ENORMOUS volume of highly viscous magma (silica content >60%) accumulating in shallow magma chambers, which are part of even bigger magma reservoirs
- High volatile content (mostly water vapor)
* Thick, relatively low-density crust, common in continents or old island arcs help to create such HUGE magma reservoirs.

26. Explosive eruptions can happen if (when?):
The magma body becomes partially solid and the volatiles are forced into the remaining liquid, making the liquid saturated in gas
Fresh batch of hot magma intrudes in the magma chamber
The gas-rich liquid magma escapes from the semi-solid crystal mush and is stored beneath the roof of the magma chamber, and,
Earthquakes and faulting fracture a magma chamber and/or “shakes up” the magma chamber