1. VOLCANOES

2. Volcanoes around the World

3. 10.1 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
10.1 The Nature of Volcanic Eruptions
 Factors that determine an eruption
• Composition
Details on next slide
• Temperature
• Dissolved gases
 Viscosity
• Viscosity is the measure of a material's resistance to flow.

4. Comparison of ash plumes
Mt. St Helens was a relatively small eruption compared to prehistoric eruptions

5. 10.1 The Nature of Volcanic Eruptions
 Factors Affecting Viscosity
- Temperature (hotter magmas are less viscous)
- Composition (silica content)
More silica—high viscosity, thicker
2. Low silica—less viscous, more fluid

6. 10.1 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
10.1 The Nature of Volcanic Eruptions
Viscous (thicker) magma produces a more violent eruption.

7. 10.1 The Nature of Volcanic Eruptions
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.
• Provide the force to extrude lava

8. 10.1 The Nature of Volcanic Eruptions
Volcanic Material
10.1 The Nature of Volcanic Eruptions
Lava Flows
• Basaltic lavas are more fluid.
• Types of lava
-Pahoehoe lava (resembles braids in ropes)
Aa lava (rough, jagged blocks)

9. Mayon Volcano, Philippines

10. 10.1 The Nature of Volcanic Eruptions
Volcanic Material
10.1 The Nature of Volcanic Eruptions
• 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.
Yes – write this down- names only!!!
- Ash and dust — fine, glassy fragments
- Pumice — frothy, air-filled lava
- Lapilli — walnut-sized particles
- Cinders — pea-sized particles
- Blocks — hardened lava (large)
- Bombs — ejected as hot lava (large)

11. 10.1 The Nature of Volcanic Eruptions
Types of Volcanoes
10.1 The Nature of Volcanic Eruptions
Anatomy of a Volcano
• A volcano is a mountain formed of lava and/or pyroclastic material.
• A crater is the depression at the summit/top of a volcano
• A conduit carries gas-rich magma to the surface.

12. 10.1 The Nature of Volcanic Eruptions
Types of Volcanoes – 3 main Types
10.1 The Nature of Volcanic Eruptions
Shield Volcanoes
• Broad, gently sloping volcanoes, usually the largest
built from basaltic lavas, shaped like a shield
Magma - Very fluid & the lava can flow great distances – rich in Fe & Mg
Eruptions are usually mild & can occur several times
Ex. Mauna Loa, HI

13. Cinder Cones Cinder Cones - Ex. Paricutin, Mexico
• Small steep sloped volcanoes built primarily of pyroclastic material ejected from a single vent.
Magma: gas rich in basaltic lava, ejects lava
fragments – mostly cinders
- Eruption: typically a single eruption lasting a few weeks to a few years

14. 10.1 The Nature of Volcanic Eruptions
Composite Cones
(a.k.a. Stratovolcano)
• Steep sides w/ alternating layers of ash, cinder, and lava
- Magma: Thick magma contains trapped gases, very viscous, silica rich
- Eruptions alternate between flows and explosions (cinder & ash) & pyroclastics Most violent
Ex. Mt. Shasta, CA
Mt. St. Helens, WA

15. Mount St. Helens Before and After the May 18, 1980, Eruption
                                               

16. Size Comparisons of Volcanoes

17. Different types of volcanoes

18. 10.1 The Nature of Volcanic Eruptions
Other Volcanic Landforms
10.1 The Nature of Volcanic Eruptions
• Calderas are large depressions in volcanoes.
• Nearly circular
• Formed by collapse of the vent
Lava Plateaus
• Fluid basaltic lava extruded from crustal fractures called fissures.

19. 10.2 Intrusive Igneous Activity
Plutons
10.2 Intrusive Igneous Activity
Plutons are intrusive igneous structures that result from the cooling and hardening of magma beneath the surface of Earth.

20. 10.2 Intrusive Igneous Activity
Plutons
10.2 Intrusive Igneous Activity
• Sills and laccoliths are plutons that form when magma is intruded close to the surface.
- Sills resemble buried lava flows and may exhibit columnar joints. Store magma.
- Laccoliths are lens-shaped masses that arch overlying strata upward. Transport magma.
Sill

21. 10.2 Intrusive Igneous Activity
Plutons
10.2 Intrusive Igneous Activity
• Batholiths are large masses of igneous rock that formed when magma intruded at depth, became crystallized, and subsequently was exposed by erosion. Ex) Mount Rushmore
• An intrusive igneous body must have a surface exposure greater than 100 square kilometers to be considered a batholith.

22. 10.2 Intrusive Igneous Activity
Origin of Magma
10.2 Intrusive Igneous Activity
Magma originates when rock located in the lithosphere partially melts.

23. 10.2 Intrusive Igneous Activity
• The geothermal gradient — Earth’s natural temperature is not sufficient to melt rock in lithosphere
• Additional heat is generated by:
- friction in subduction zones
- rocks heated during subduction
- rising, hot mantle rocks

24. 10.3 Plate Tectonics and Igneous Activity
Lava

25. Where Do Volcanoes Occur?
Mostly occur at plate boundaries
Convergent plate boundaries: 75% of active volcanoes
Ex: Ring of Fire
Divergent plate boundaries: as plates move apart, magma rises to fill the space
- Ex: Iceland, Mid-Atlantic Ridge

26. 10.3 Plate Tectonics and Igneous Activity
Convergent Plate Boundaries
10.3 Plate Tectonics and Igneous Activity
Ocean-Ocean
• Forms volcanic island arcs in an ocean
Ex. Aleutian Islands
Ocean-Continent
• Forms continental volcanic arcs
Ex. Andes Mountains

27. 10.3 Plate Tectonics and Igneous Activity
Divergent Plate Boundaries
10.3 Plate Tectonics and Igneous Activity
The greatest volume of volcanic rock is produced along the oceanic ridge system.
Ex. Mid-Atlantic ridge
• Lithosphere pulls apart.
• Partial melting occurs
• Oceanic Ridges form

28. 10.3 Plate Tectonics and Igneous Activity
Intraplate Igneous Activity
10.3 Plate Tectonics and Igneous Activity
Intraplate volcanism occurs within a tectonic plate away from plate boundaries.
• Occurs when hotter than normal mantle material called a mantle plume rises to the surface.
• Forms localized volcanic regions called hot spots.
• Ex. Hawaiian Islands (Kilauea) & Columbia Plateau.

29. 10 Most Active Volcanoes