1. Volcanism Volcanic Features Location and Types of Volcanic Activity
Effusive Eruptions  
Explosive Eruptions  
Volcano Forecasting and Planning

2. How and Where Volcanoes Form
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volcano
Volcanic activity takes place primarily at subduction boundaries,
hot spot
4. Some of the magma reaches Earth’s surface, and volcanoes form on the overriding continental plate.
1. Water in the subducted rock is released into the asthenosphere.
Oceanic lithosphere
Continental lithosphere
Asthenosphere
3. The magma is less dense than its surroundings, so it rises.
2. The water lowers the melting temperatures of materials in the asthenosphere, leading to magma formation.

3. How and Where Volcanoes Form
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volcano
Magma and volcanoes also form at hot spots.
hot spot
Kauai
Oahu
Direction of Plate Movement
The Hawaiian Islands formed over a hot spot.
Molokai
Lanai
Maui
Hawaii

4. How Magma Rises
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5. Volcanic Settings

6. Plate Tectonics and Magma Generation
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7. Major Volcanoes Around the World

8. Magma and Erupted Materials
Gases escape easily from basaltic magma, generating relatively quiet eruptions. Hardened basaltic lava flows on land are characterized as pahoehoe or aa; if the lava cools underwater, it is characterized as pillow lava.
Parameters of volcanism:
Viscosity
Chemistry of Magma
Gases within the magma
Water content of magma

9. Magma and Erupted Materials
Viscosity: Viscosity is a measure of how thick (viscous) and sticky a liquid is.
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viscosity
lava
pahoehoe aa
pillow lava
pyroclastic material
pyroclastic flow

10. Lava Flows Lava Flow - A 'stream' of molten rock
'aa' lava flow - jagged, rubbly, broken surface  
'pahoehoe' lava flow - smooth, ropy surface

11. Magma and Erupted Materials
Silica Content
Melting Temp.
Gas Content
Viscosity
Type of Eruption
Location
Least (~50%)
Basaltic Magma
Least
Least viscous
Rarely explosive
Highest
Rifts, oceanic hot spots
Rhyolitic Magma
Most (~70%)
Most
Most viscous
Usually explosive
Lowest
Continental hot spots
AndesiticMagma
Intermediate (~60%)
Intermediate
Sometimes explosive
Subduction boundaries

12. Volcanic Landforms
A volcano’s shape and structure depend on how it erupts and what materials are released. Shield volcanoes are formed by basaltic lava that flows long distances before hardening.

13. Tectonic Settings and Volcanic Activity
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14. Igneous __________- classified by shape, size, and orientation
Intrusions
Igneous __________- classified by shape, size, and orientation

15. There are three types of volcanic cones:
Cinder
______ – explosive eruptions, small but steep slopes, pyroclastics.
_______ – non-explosive eruptions, fluid basaltic lava, gentle broad slopes.
_________ – alternating between lava and pyroclastics, explosive and non-explosive eruptions, steep and tall.
Shield
Composite

16. Volcano Types
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17. Volcanic Landforms Volcanoes
VOCABULARY
A volcano’s shape and structure depend on how it erupts and what materials are released.
shield volcano
cinder cone
composite volcano
Shield volcanoes are formed by basaltic lava that flows long distances before hardening.
lahar
caldera
Shield Volcano
lava plateau

18. Shield and Composite Volcanoes

19. Volcanic Landforms Volcanoes
Cinder cones are formed when molten lava is thrown into the air from a vent and breaks into drops. These drops harden into cinders that form a steep cone around the vent.
Cinder Cone

20. Volcanic Landforms Volcanoes
Composite volcanoes are formed by layers of pyroclastic materials and lava that have erupted in the past.
Composite Volcano

21. Composite volcanoes

22. Debris Avalanche and Eruption of Mount St. Helens, Washington
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23. Nuee ardente: pyroclastic flow, of searing superheated gas and incandescent volcanic ash and dust
Mount Pelee, on the Carribean island of Martinique, 1902 eruption. All but 2 of the more than
20,000 people in the town of St. Pierre were killed.

24. Volcanic Landforms Volcanoes
A caldera is a large crater-shaped basin that forms when the top of a volcano collapses.

25. Formation of Crater Lake
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26. EFFUSIVE ERUPTIONS Generally at hots spots, spreading centers
Mantly comes directly to surface
Hot lava; low viscosity, very mafic, flows easily, gases escape easily
Forms shields, flood basalts

27. EXPLOSIVE ERUPTIONS Found at subduction zones
Magma low temp (800 degrees C), high viscosity, does not flow easilty, more felsic mineralogy, gases trapped, hard to predict explosions
Forms composite volcanoes, cinder cones, calderas, aerial bombs, nuee ardente gas flows, very destructive