1. Volcanoes!
Chapter 10

2. Origin of Magma
Magma originates when solid rock, located in the crust and upper mantle, melts.
Factors that influence the formation of magma include…
Heat
Pressure
Water Content

3. Origin of Magma Factors that influence the formation of magma Heat
Additional heat is generated by
Friction in subduction zones
Crustal rocks heated during subduction
Rising, hot mantle rocks

4. Origin of Magma Factors that influence the formation of magma Pressure
Increase in confining pressure causes an increase in melting temperature
Drop in confining pressure can cause decompression melting
Lowers the melting temperature
Occurs when rock ascends

5. Origin of Magma Factors that influence the formation of magma
Water Content
Cause rock to melt at a lower temperature
Plays an important role in subducting ocean plates

6. Volcanoes and Plate Boundaries
Global distribution of igneous activity is not random!
Most volcanoes form along divergent and convergent plate boundaries.
Some volcanoes form far from plate boundaries above “hot spots”.

7. Locations of some of Earth’s major volcanoes

8. Volcanoes and Plate Boundaries
Divergent Boundary Volcanism
The greatest volume of volcanic rock is produced along the oceanic ridge system
Lithosphere pulls apart
Mantle rock rises to fill the gap between the plates.
Decompression melting occurs forming magma.

9. Volcanoes and Plate Boundaries
Convergent Boundary Volcanism
Descending plate partially melts
Magma slowly rises upward
Rising magma can form…
Volcanic island arcs in an ocean (Aleutian Islands)
Continental volcanic arcs (Andes Mountains)

10. Volcanoes and Plate Boundaries
Intraplate Volcanism
Activity within a rigid plate NOT near a plate boundary
Plumes of hot mantle material rise
Form localized volcanic regions called hot spots
Examples include…
Hawaiian Islands (middle of the Pacific Plate)
Yellowstone National Park (Middle of the N. American Plate)

11. The Nature of Volcanic Eruptions
Factors Affecting Eruptions
Viscosity of the magma
Viscosity is a measure of a material’s resistance to flow
Maple syrup vs. water

12. The Nature of Volcanic Eruptions
Factors Affecting Eruptions
Viscosity of magma
Temperature (hotter magma is less viscous)
Composition (silica content)
High silica content – high viscosity (e.g., rhyolitic lava) this creates an explosive eruption.
Low silica content – more fluidlike (e.g., basaltic lava) quiet eruption.

13. The Nature of Volcanic Eruptions
Factors affecting Eruptions
2. Dissolved Gases
Gases trapped in the magma
Violence of an eruption is related to how easily gases escape from magma
Easy escape from fluid magma
Viscous magma produces an explosive eruption

14. Volcanic Material Lava flows Gases
Basaltic lavas are more fluid (low silica content)
Types of lava
Pahoehoe lava (resembles braids in ropes)
Aa lava (rough, jagged blocks)
Gases
One to five percent of magma by weight
Mainly water vapor and carbon dioxide

15. A pahoehoe lava flow

16. A typical aa flow

17. Volcanic Material Pyroclastic materials “Fire fragments”
Types of pyroclastic material
Ash and dust – fine, glassy fragments
Pumice – from “frothy” lava
Lapilli – “walnut” size
Cinders – “pea-sized”
Particles larger than lapilli
Blocks – hardened lava
Bombs – ejected as hot lava

18. Volcanic bombs on Kilauea volcano in Hawaii

19. Volcano Formation
Begins when a fissure or crack develops in the crust as magma is forced to the surface.
Repeated eruptions of lava or pyroclastic material eventually build a mountain called a volcano.

20. Three Main Types of Volcanoes
Shield Volcanoes
Cinder Cones
Composite Cones

21. Types of Volcanoes Shield volcano
Broad, slightly domed (like a warriors shield)
Primarily made of basaltic (fluid) lava
Generally large in size
e.g., Mauna Loa and Mauna Kea in Hawaii

22. Shield volcano

23. Types of Volcanoes Cinder cone
Built from ejected lava fragments that harden in the air
Steep slope angle
Small in size

24. A cinder cone near Flagstaff, Arizona

25. Types of Volcanoes Composite cone (or stratovolcano)
Most are adjacent to the Pacific Ocean along the ‘Ring of Fire’ (e.g., Mt. Rainier)
Magma is viscous with a high silica content
Large size
Composed of layers of lavas and pyroclastic material
Most dangerous and explosive volcano

26. Composite volcano

27. Mount St. Helens – a typical composite volcano

28. Mount St. Helens following the 1980 eruption

29. A size comparison of the three types of volcanoes

30. Other Volcanic Landforms
Calderas
Steep-walled depression at the summit
Formed by collapse
Nearly circular
Size exceeds 1 kilometer in diameter
Lava plateaus
Fluid basaltic lava erupts from crustal fractures called fissures
e.g., Columbia Plateau

31. Crater Lake, Oregon, is a good example of a caldera

32. The Columbia Plateau

33. Other volcanic landforms
Volcanic pipes and necks
Volcanic necks (e.g., Ship Rock, New Mexico) Landform made of magma that hardened in a volcano’s pipe and later exposed by erosion.
Pipes are short conduits that connect a magma chamber to the surface

34. Formation of a volcanic neck

35. Volcanic Hazards Types of volcanoes Composite cone (or stratovolcano)
Often produce
Fiery pyroclastic flow made of hot gases infused with ash
Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour
May produce a lahar – volcanic mudflow

36. Pyroclastic flow Mount St. Helens

37. A lahar along the Toutle River near Mount St. Helens

38. Intrusive igneous activity
Most magma crystallizes within Earth’s crust
An underground igneous body is called a pluton
*Pluto is Roman God of the underworld*
Types of Plutons
Sills - Magma flows between sedimentary layers
Laccoliths - ‘lens shaped’ Magma pushed the overlying rock layers upward.
Dikes - Magma moves into fractures and cuts across rock layers.

39. Intrusive igneous structures exposed by erosion

40. A sill in the Salt River Canyon, Arizona

41. Intrusive igneous activity
Batholith
Largest intrusive body
Much larger than a pluton
Surface exposure 100+ square kilometers
Frequently form the cores of mountain ranges

42. A batholith exposed by erosion