1. NATURE OF
VOLCANIC ERUPTIONS

2. VIOLENCE OF VOLCANIC ERUPTIONS
Factors That influence the Violence of Volcanic Eruptions
Composition of the Magma
Temperature of the Magma
Amount of dissolved Gases in the Magma

3. Composition
&
Viscosity

4. MAGMA & LAVA VISCOSITY As silica content Decreases
Viscosity “gooeyness”- a material’s resistance to flow (how thick it is)
related to Silica content of magma
As silica content Increases
viscosity Increases - thicker magma
As silica content Decreases
viscosity Decreases - runny magma

5. VISCOSITY FACTORS

7. VISCOSITY FACTORS (CONT.)
COMPOSITION
Low % Silica content – Low Viscosity (More fluid)
Basaltic lava
Intermediate % Silica content
Andesitic lava
High % Silica content – High Viscosity (Thicker)
Granitic lava

8. VISCOSITY FACTORS TEMPERATURE
Hotter the magma the Less Viscous (more fluid)
Hawaiian lava:
1170° C ° C
very fluid (runny)
Mt. Saint Helens lava:
500° C - 700° C
very viscous (stiff)

9. VISCOSITY FACTORS (CONT.)
DISSOLVED GASES
H2O & CO2
Gases escape, as magma moves toward the surface and the pressure is reduced.
How easily do gases escape from magma?
Low Viscosity (Fluid) magma
quieter the eruption
Gases can escape easily
Viscous (Thick) magmas
produce violent eruptions
Gases become trapped within cooling magma and build up pressure

10. Volcanic
MATERIAL

11. MATERIALS ASSOCIATED WITH VOLCANIC ERUPTIONS

12. LAVA FLOWS Aa Lava Pahoehoe Lava Basaltic lavas are more fluid
Types of Lava
Aa Lava
Rough, jagged blocks
Pahoehoe Lava
Resembles twisted braids in ropes Aa
Pahoehoe

13. GASES Magma contains 1% - 5% gas Water vapor - H2O (70%)
Carbon dioxide - CO2 (15%)
N2 and nitrogen compounds (5%)
Sulfur compounds (5%)
minor amounts of Cl, H2 & Ar

14. PYROCLASTIC MATERIAL Pyroclastic – means “Fire Fragments”
Pyroclastic – means “Fire Fragments”
Types of Pyroclasts

15. ASH - fine, glassy fragments

16. PUMICE - forms “frothy” lava

17. CINDERS -“pea-sized” with voids

18. LAPILLI - “walnut” size

19. BLOCKS
hardened lava > 64mm

20. VOLCANIC BOMB
ejected as semi-molten
hot lava
> 64 mm

21. What materials are ejected from volcanoes?
Bellringer
You have 5 minutes to answer the questions on a sheet of paper and turn into the basket
What materials are ejected from volcanoes?
Depending on the type of eruption, volcanoes may produce lava flows or eject pyroclastic materials, or both. Also all eruptions emit large amounts of gas.

22. “A volcano is a mountain with hiccups”
VOLCANOES
“A volcano is a mountain with hiccups”

23. ANATOMY
OF A
VOLCANO

24. Parasitic cone Lava Pyroclastic material
ANATOMY OF A VOLCANO
Crater
Caldera
Vent, Conduit (pipe)
Parasitic cone Lava Pyroclastic material

25. Sratovolcano (Composite)
VOLCANO
TYPES
Shield Volcano
Cinder Cone Volcano
Sratovolcano (Composite)
Lava Dome

26. SHIELD VOLCANO Broad & slightly domed Covers large area
Broad & slightly domed
Covers large area
Quiet eruptions
produce large volumes of Basaltic lava
Dark colored lava
Low viscosity
(very runny or fluid)

27. STRATOVOLCANO (COMPOSITE)
Large in size
Iayered - lava & pyroclastics
Most adjacent to Pacific Ocean
Most Violent eruptions
Produce “Lahars”
Type of mudflow
Produce “Nuée ardente”
Fiery cloud
Hot gases mixed with ash
Flows down sides at speeds up to 200 kph

28. CINDER CONE Built from ejected lava fragments (pyroclastics)
Built from ejected lava fragments (pyroclastics)
Have Steep Slopes
Small in size
Frequently occur in groups

30. THREE VOLCANO PROFILES

31. DANGERS OF COMPOSITE CONES MOUNT St. HELENS
Before: 5/18/1980
After: 5/18/1980

32. Nuée ardente & Lahars

33. *What are the three types of volcanoes?
Bellringer
You have 5 minutes to answer the question on a sheet of paper and turn into the basket
*What are the three types of volcanoes?
The three main types of volcanoes are shield volcanoes, cinder cones, and composite cones.

34. Other Volcanic Landforms
EXTRUSIVE IGNEOUS FEATURES

35. VOLCANIC LANDFORMS
Crater
Steep walled depression at summit of volcano

36. CALDERA Collapsed crater generally larger than 1 km in diameter
Collapsed crater
generally larger than 1 km in diameter
Formation:
Collapse - drained magma chamber
Proclastic Explosion - blasts out material

37. VOLCANIC VENT & Conduit
Vent (pipe) – conduit that connects crater to magma chamber

38. VOLCANIC NECKS Magma that hardened in the volcanoes pipe (vent)
Magma that hardened in the volcanoes pipe (vent)
Remains standing after volcano has eroded away
Devils Tower (Wyoming)
Shiprock (New Mexico)
Tsé Bit' A'í - Rock With Wings

39. FLOOD BASALTS Lava flows that cover many square miles of area
fissures that open up in the crust
lava flow out onto the surface

40. FISSURES Fissure - crack in the crust
allows lava to flow onto the surface

41. VOLCANIC PLATEAU Fissure Eruptions
Fissure Eruptions
Volcanic material flows from fractures in crust
Flood Basalts
Yellowstone Plateau
Columbia Plateau