1. What Controls Volcanic Eruptions?
Ch 12.2

2. Some eruptions are quiet…
Kilauea, Hawaii

3. Some eruptions are EXPLOSIVE!
Mt. Pinatubo
Montserrat
Mt. St. Helens

4. What Controls Volcanic Eruptions?
How much water vapor (a gas) and other gases are trapped in the erupting magma
How much silica is contained in the magma

5. water vapor and carbon dioxide are dissolved in magma
Trapped Gases
water vapor and carbon dioxide are dissolved in magma
When magma rises to surface, gases escape because they are under less pressure
Some magma is thin and fluid- this lets gases escape easily- resulting in quiet eruption
Other magma is thick and stiff- when gases erupt it results in explosive eruption

6. Water Vapor Water may be added to magma when ocean crust rock is melted at convergent plate boundaries

8. Composition of magma
Silica-rich magma is called granitic/rhyolitic magma
Results in explosive eruptions
Ex. Soufriere Hills in Montserrat
Characteristic of convergent plates
Silica-poor magma is called basaltic magma
Results in quiet eruptions
Ex. Iceland
Characteristic of divergent plates

9. Basaltic magma may result in different three lava types:
Pahoehoe Aa
Pillow (cools
the fastest)

10. Pahoehoe lava: Hot, thin, fast flowing
harden with a relatively smooth surface
Often has a ropy or wrinkled appearance

11. Pahoehoe lava

12. Aa lava:
Cooler, thicker, slow moving
Hardens with a rough, jagged, sharp edge surface

13. Pillow Lava:
Lava cools underwater
resembles stuffed pillows

14. Explosive Magma/Lava Granitic/Rhyolitic Magma high in silica
high amount of trapped gases
very viscous
lead to explosive type eruptions
Soufriere Hills volcano-active since 1995
At what type of boundary would granitic/rhyolitic magma form?
convergent boundaries

15. Composition of magma
Andesitic magma – more silica than basaltic magma, but less than granitic magma
Common at convergent boundaries
Named after the Andes Mountains in South America
Also violent eruptions
Ex. Krakatau- one of the most violent eruptions in history

16. Andes Mtns

17. Krakatau 1883 Sound heard 3,000 mi away!
13,000 x more powerful than nuclear blast at Hiroshima in WWII

18. Anak Krakatau (Child of Krakatau)

19. Forms of Volcanoes Depends on: 3 basic types of volcanoes
Type of magma it was formed from
Type of eruption- quiet or explosive
3 basic types of volcanoes
Shield
Cinder cone
Composite

20. 1. Shield volcanoes
Result of quiet eruptions of basaltic lava flows (no ash)
May form volcanic cones or fissures- long cracks that result if flood basalts (Ragged Mtn, Lamentation Mtn)
Ex. Hawaiian Islands and the Columbia Plateau of Northwest US

21. Shield volcanoes
Mauna Kea, Hawaii

22. Shield volcanoes
Mauna Loa, Hawaii

23. Shield volcanoes

24. 5 Shield volcanoes make up the island of Hawaii

25. Fissure eruption

26. Fissure eruptions
Common at divergent plate boundaries, mid-ocean ridges

27. Columbia Plateau

28. 2. Cinder Cones Small base, steep-sided, loosely consolidated
Up to 1000 feet tall
Life span of a few years
Commonly built from solidified rock thrown through the air called tephra
Has violent eruptions, dangerous when close

29. 2. Cinder Cone Volcanoes
Paricutin, Mexico ( , newest volcano in Western Hemisphere, 1000 killed)

30. Cinder Cone Volcanoes
Paricutin

31. Cinder Cone Volcanoes
Pu'u ka Pele on flank of Mauna Kea volcano, Hawaii

32. Cinder cone example: Paricutín, Mexico
1943

33. Cinder Cone Volcanoes

34. Composite Volcano (aka stratovolcano)
most beautiful, but most deadly
characteristic shape
made of alternating layers of lava and tephra- periods of quiet, then explosive eruptions
Mt. Fuji, Japan- one of the most photographed volcanoes in the world because of its near perfect symmetry
Ex. Mt. St. Helens

35. Composite Volcanoes

36. What are the three types of magma in order from least viscous to most viscous?
a. Basaltic
b. Andesitic
c. Granitic

37. Tephra
Names are based on size:
Ash
Lapilli/Cinders
Bombs/Blocks

38. Volcanoes
Volcanoes eject different types of material. This material is known as tephra or pyroclastic material.
There are 3 main types:
Ash-less than 2 mm in diameter
Lapilli/cinders-2mm-64 mm in diameter
Blocks or Bombs-larger than 64mm

39. Ash

40. Lapilli/Cinders
Mt Pinatubo, Philippines 1991

41. Bombs

42. Blocks