1. Volcanoes and Igneous Rocks
Chapter 6
Volcanoes and Igneous Rocks

2. VOLCANOES AND IGNEOUS ROCKS

3. Objectives
Identify several different categories of volcanic eruptions.
Identify the volcanic hazards.
Describe how temperature, pressure, and water content affect a rock’s melting point.
Identify three properties that distinguish one lava from another.
Distinguish between and identify volcanic and plutonic rocks.
Describe the most common plutonic formations.

4. Volcanoes and volcanic hazards
A vent through which lava, solid rock debris, volcanic ash, and gasses erupt from Earth’s crust to its surface
Can be explosive or nonexplosive

5. Volcanoes and volcanic hazards
Lava
Molten rock that reaches Earth’s surface
Magma
Molten rock, which may include fragments of rock, volcanic glass and ash, or gas

6. Eruptions, landforms and materials
Eruption types
Shield volcano
A broad, flat volcano with gently sloping sides, built of successive lava flows
Produce flood basalts or basalt plateaus

7. Eruptions, landforms and materials
Eruption types
Strombolian
More explosive than Hawaiian
Create loose volcanic rock called spatter cones or cinder cones

8. Eruptions, landforms and materials
Eruption types
Vulcanian
More explosive than Strombolian and, as a result, can generate billowing clouds of ash up to 10 km
Produce pyroclastic flows
Hot volcanic fragments (tephra) that, buoyed by heat and volcanic gases, flow very rapidly

9. Eruptions, landforms and materials
Eruption types
Plinian
Named after Pliny the Elder, who died during eruption of Mount Vesuvius
Most violent eruptions, generating ash columns the can exceed 20 kilometers
Produce steep sided volcanoes, called stratovolcanoes
Composed of solidified lava flows interlayered with pyroclastic material.
Steep sides that curve upward

10. Eruptions, landforms and materials

11. Eruptions, landforms and materials

12. Eruptions, landforms and materials
Viscosity
The degree to which a substance resists flow,
Less viscous liquid is runny
More viscous liquid is thick
Volcanic materials
Pyroclasts
Tephra
Ash
Agglomerates
Tuff

13. Eruptions, landforms and materials
Other volcanic features
Craters
Resurgent dome
Thermal spring
Geysers
Fumaroles

14. Volcanic hazards Primary effects Secondary effects Pyroclastic flows
Volcanic gases
Secondary effects
Related to, but not a direct result of, volcanic activity
Fires
Flooding
Mudslides
Debris avalanche

15. Volcanic hazards

16. Volcanic hazards

17. Volcanic hazards Tertiary and beneficial effects Change a landscape
Affect climate on regional and global scale
Renew mineral content and replenish fertility
Geothermal energy
Provide mineral deposits

18. Predicting Eruptions Establish a volcano’s history
Active
Dormant
Monitor changes and anomalies
Earthquakes
Changes in shape or elevation
Volcanic gases
Changes in ground temperature
Composition of water

19. Predicting Eruptions

20. How, Why and Where Rocks Melt
Heat and pressure inside Earth
Continental crust: temperature rises 30°C/km, then about 6.7°C/km
Ocean crust: temperature rises twice as rapid

21. How, Why and Where Rocks Melt

22. How, Why and Where Rocks Melt
Effect of temperature and pressure on melting

23. How, Why and Where Rocks Melt
Heat and pressure inside Earth
Fractional melt
A mixture of molten and solid rock
Fractionation
Separation of melted materials from the remaining solid material during the course of melting

24. How, Why and Where Rocks Melt

25. How, Why and Where Rocks Melt
Magma
Molten rock below surface
Lava
Magma when it reaches the surface
Differ in composition, temperature and viscosity

26. How, Why and Where Rocks Melt
Lava
Composition
45-75% of magma by weight is silica
Water vapor and carbon dioxide
Temperature
Lavas vary in temperature between 750°C and 1200°C
Magmas with high H2O contents melt at lower temperatures
Viscosity
Lavas vary in their ability to flow
Influenced by silica content and temperature

27. How, Why and Where Rocks Melt

28. How, Why and Where Rocks Melt
The tectonic setting
Lava characteristics influenced by location
Oceanic, divergent margins
Lava is thin with a steep geothermal gradient
Subduction zones
Typically have high water content and melt at lower temperatures
Hot- spots
Lava tends to be hot and basaltic
Build giant shield volcanoes
Continental divergent margins are all different
High silica lava

29. How, Why and Where Rocks Melt

30. How, Why and Where Rocks Melt

31. How, Why and Where Rocks Melt

32. Cooling and Crystallization
The process whereby mineral grains form and grow in a cooling magma (or lava)
Classified as:
Volcanic
Plutonic

33. Rate of Cooling Rapid cooling: Volcanic rocks and textures
An igneous rock formed from lava
Glassy
Aphanitic
Porphyritic
Pumice
Vesicular basalt

34. Rate of Cooling

35. Rate of Cooling Slow cooling: Plutonic rocks and textures
An igneous rock formed underground from magma
Phaneritic-a coarse grained texture
Can have exceptionally large grains

36. Chemical composition
Igneous rocks subdivided into three categories based on silica content
Felsic
Intermediate
Mafic

38. Fractional Crystallization
Separation of crystals from liquids during crystallization
Bowens reaction series
Predictable melting and cooling of minerals

39. Fractional Crystallization

40. Plutons and Plutonism Plutons Batholith
Any body of intrusive igneous rock, regardless of size or shape
Batholith
A large, irregularly shaped pluton that cuts across the layering of the rock into which it intrudes

41. Plutons and Plutonism Dikes Sills
Forms when magma squeezes into a cross cutting fracture and solidifies
Sills
Magma that intrudes between two layers and is parallel to them

42. Mount Saint Helens

43. Critical Thinking
What factors might prevent magma from reaching Earth’s surface?
What reasons can you think of for living near a volcano? Do you think the advantages outweigh the disadvantages?
If you were to heat up a glass beaker full of crushed rock, the beaker would melt before you could finish studying the rock melting process. How do you think geologists study rock melting?