1. 10.2 The Nature of Volcanic Eruptions
Earth Science - Schwartz

2. Review: melted mineral crystals dissolved gasses water
What is the difference between Magma and lava?
Magma that has reached the surface is called lava
Lava cools and hardens to form rock
What are the 3 things Magma is made up of? (see 10.1 notes for answer!)
melted mineral crystals
dissolved gasses
water

3. Factors affecting Eruptions
Primary factors that determine whether a volcano erupts explosively or quietly include
characteristics of the magma- Viscosity specifically
amount of dissolved gases in the magma

4. Viscosity Viscosity= Substances resistance to flow
Viscosity is determined by
Temperature
Chemical composition
More viscous = explosive eruption

5. Factors affecting Eruptions
How does temperature change viscosity?
When you heat maple syrup, it becomes easier to pour. Same thing with Magma, the hotter it is, the less viscous it is!

6. Factors affecting Eruptions
How does chemical composition change viscosity?
More silica= greater viscosity (thicker)
Less silica= less viscosity (thinner)
Silica = SiO2

8. Dissolved Gases Mainly water vapor and carbon dioxide
More trapped gas = Greater Eruption
A vent is an opening in the surface of Earth through which molten rock and gases are released.
Trapped gases provide the force to propel lava

9. Which composition below would have the most explosive eruption?
Which composition would have the quietest?

10. Volcanic Material
Volcanic Material= Explosive eruptions eject lava as well as broken rock, lava bombs, fine ash and dust
Eruptions also emit large amounts of gases
3 Types of Volcanic Material:
1. Lava Flows
2. Gases
3. Pyroclastic Materials

11. Volcanic Material Lava Flows
Silica content and temperature affect lava flow
Types of Lava
Pahoehoe – hotter faster moving basaltic lava
Aa – Cooler slower moving basaltic lava

12. Lava Flows
The chunky brown lava is 'aa' while the ropy black lava is 'pahoehoe'

13. Gases Magma contains varied amount of gases
As pressure is reduced the gases begin to escape (Think opening a soda can!)

14. Pyroclastic Materials
Pyroclastic materials-fragments ejected during eruptions
Includes dust, ash, and rock
Range in size from very fine dust and ash to pieces that weigh several tons.
2 -64 mm called lapilli or cinders
LARGER than 64 mm called blocks when hardened lava
or bombs when ejected as glowing lava

15. Volcano vocab Reminder!
Volcano – a mountain built by repeated eruptions of lava or pyroclastic material often separated by long inactive periods
Crater – steep-walled depression located at the top of many volcanoes

16. Types of Volcanoes
Volcanic activity often starts with a fissure, or crack, develops and magma is forced through it
The form of the volcano is largely determined by the composition of the magma
3 Types :
Shield
Cinder
Composite

17. Shield Volcano
Shield Volcano – produced by the accumulation of fluid basaltic lavas
Shape – broad, slightly domed structure that resembles a warrior’s shield
Most have grown from the ocean-floor to form islands (Hawaii and Iceland)

18. Shield Volcano

19. Cinder Cone Volcano
Cinder Cone – built by ejected lava fragments the size of cinders, which harden in the air
Product of relatively gas-rich basaltic magma
Shape – determined by the steep-sided slope that loose pyroclastic material maintains as it comes to rest
Usually the product of a single eruption that lasts only a few weeks, rarely a few years
The magma in the pipe solidifies after the eruption, and the volcano doesn’t erupt again
Cinder cones are small, ~ meters and less than 700 meters in height
There are thousands of cinder cones around the world

20. Cinder cones

21. Composite Cone Volcano
Most beautiful and potentially dangerous
Also called stratovolcanoes
Large, nearly symmetrical volcanic mountain composed of layers of both lava and pyroclastic deposits
Gas – rich magma
Most explosive eruptions
Most located in the Ring of Fire, Mt. St Helens for example
About 50 of these have erupted in the U.S. in the last 200 years

22. Composite cone

25. Other Volcanic Landforms
Volcanic mountains are not the only landforms that result form volcanic activity
3 other landforms to know
Calderas
Volcanic Necks
Lava Plateaus

26. Other Volcanic Landforms
Calderas
Depression in a volcanic mountain after end of activity
Ex – Crater Lake

27. Formation of Crater Lake

28. Other Volcanic Landforms
Volcanic Neck
Landform made of magma that hardened in a volcano’s pipe and later was exposed by erosion

29. A volcanic neck in the Shiprock Volcanic Field, New Mexico.

30. Other Volcanic Landforms
Lava Plateau
Volcanic landform produced by repeated eruptions of very fluid, basaltic lava
Erupts through long cracks called fissures
Lava spreads out over a wide area

31. Volcanic Hazards Volcanic soil is good for farming
Hazards include lava flows, volcanic ash, pyroclastic flows, and mudflows
Mudflows called lahars occurs when water soaked volcanic rock and ash slide rapidly down hill.