1. Volcanism and Extrusive Rocks
Chapter Four
Volcanism and Extrusive Rocks

2. VOLCANOES AND VOLCANISMS
VOLCANO: LANDFORM THAT RESULTS FROM THE ACCUMULATION OF LAVA AND ROCK PARTICLES AROUND AN OPENING (OR VENT) IN EARTH’S SURFACE
VOLCANISM: SET OF GEOLOGICAL PROCESSES THAT MARK THE ASCENT OF MAGMA TO THE EARTH’S SURFACE
~ 2000 YRS VOLCANOES ERUPTED
50 VOLCANOES/YEAR
VOLCANOES ARE WINDOWS INTO THE EARTH
WATER AND AIR CAME DUE TO VOLCANIC ERUPTIONS
HYDROSPHERE AND ATMOSPHERE

3. Volcanism and Earth’s Systems
Atmosphere originally created from gases released by magmas
Hydrosphere produced by condensation of volcanic water vapor
Biosphere both positively and negatively influenced by volcanism
Lava flows and ash weather to produce fertile soils
Violent eruptions can destroy nearly all life in their paths
Large amounts of ash and volcanic gases in atmosphere can trigger rapid climate changes and contribute to mass extinctions

4. Volcanic Eruptions Lava is produced when magma reaches Earth’s surface
Explosive eruptions can produce rapidly cooled rock fragments called pyroclasts
Size range from dust (ash) to boulders (blocks and volcanic bombs)
Calm oozing of magma out of the ground produces lava flows
Lava flows and pyroclasts pile up to form volcanoes

5. Classification of volcanoes
ACTIVE EXTINCT
ACTIVE CURRENTLY NOT ERUPTED FOR
OR ,000 YRS OR SO
ERUPTED RECENTLY
e.g.,
DORMANT VOLCANO:
NOT ERUPTED RECENTLY (~1000 YRS OR SO)
LIKELY WILL ERUPT IN THE FUTURE
VOLCANOES PRESENT IN ALL CONTINENTS EXCEPT AUSTRALIA
ALL MAJOR OCEAN BASINS
INDONESIA HAS 76 ACTIVE VOLCANOES
JAPAN HAS 60 & USA HAS 53 ACTIVE VOLCANOES

6. Lava tube formation

7. Causes for Volcanism GAS IN VOLCANIC MAGMA (1-9% IN MOST CASES)
COMPONENT GASES (WATER VAPOR, CO2)
RISING MAGMA RELEASES GASES
EXPLOSIVE PRESSURE BY GAS
MAGMA VELOCITY:
-RESISTANCE TO FLOW
-HEAT REDUCES VISCOSITY
-VISCOSITY INCREASES WITH SILICA CONTENT
-EXPLOSIVENESS INCREASES WITH VISCOSITY

8. Types of Lava . Basaltic Lava- low viscosity and non-explosive
Pahoehoe (pay-HOY-hoy) and a a (AH-AH)
Andesitic Lava – more viscous than basaltic and erupts
Rhyolitic lava- highly viscous and erupts explosively
Pyroclastic materials
Tephra- volcanic dust, ash, cinders (lapilli) and volcanic bombs
Welded tuff (fussed tephra particles)
Pyroclastic flows or Nuees ardents (noo-Ay AR-dant)
Lahars (hot volcanic mudflows)
Water/air pollution, global climate change

9. Basaltic lava flow

10. Basaltic pillow lavas

11. Basaltic pillow lavas-contd.

12. Basaltic pillow lavas-contd.

13. Tephra Layers

14. Pyroclastic Flows

15. Lahars

16. Gas emissions

18. Extrusive Textures Vesicular/Frothy - trapped gas bubbles
Vesicular basalt
Pumice
Fragmental - particles blasted apart by explosive eruptions
Dust and ash (<2 mm)
Cinders (2-64 mm)
Blocks and bombs (>64 mm)

19. Volcanic Landforms Vent - opening through which lava erupts
Crater - basin-like depression over the vent at the summit of the volcano
Caldera - volcanic depression much larger than the original crater, having a diameter of at least 1 km

20. Volcanic Landforms Determined by the composition of Lava
Shield volcanoes…gentle slope, effusive eruption-
non-explosive
Crater and Caldera
Volcanic Domes
Produce great amount of solid volcanic fragments
Composite (Stratovolcanoe)
Alternating layers of pyroclast and solid lava
Pyroclastic Cones (Cinder Cones)
Loose pyroclastic materials around a vent

21. Types of Volcanoes Shield volcanoes Cinder cones Broad Gently sloping
Composed of solidified lava flows
Cinder cones
Small
Steep-sided
Composed of a pile of loose cinders

22. Lava-plateau formation

23. The Gorda ridge

24. Submarine volcano, infiltration

25. Submarine volcano, eruption

27. Volcanic domes, gas escape

28. Volcanic domes, explosion

29. Mount Mazama

30. Mount Mazama-contd

31. Mount Mazama-contd.

32. Mount Mazama-contd.

33. Mount Mazama-contd.

34. PYROCLASTIC MATERIAL TEPHRA: CLASSIFIED BY SIZE RECORDERS IN LAKES
SMALL: VOLCANIC DUST & ASH- 1 MICRON
MEDIUM: CINDERS OR LAPILLI (>1-62 MICRON)
LARGE: VOLCANIC BOMBS (>64 MICRON)
RECORDERS IN LAKES
VOLCANIC MUDFLOWS-LAHAR
SECONDARY VOLCANIC EFFECTS:
ACIDIC PRECIPITATION
GLOBAL CLIMATIC CHANGES-
REDUCED SUNLIGHT
REDUCED TEMPERATURE

35. PRODUCTS OF VOLCANISM OCCURRENCE: MAFIC LAVA (BASALT)-MOST COMMON
ANDESITIC LAVA – INTERMEDIATE
FELSIC LAVA – LEAST COMMON

36. ERUPTIVE STYLES ERUPTIVE STYLES:
EFFUSIVE ERUPTION: NONEXPLOSIVE, BASALTIC
CENTRAL VENT ERUPTIONS
PRODUCE SHIELD VOLCANOES
PRODUCE CALDERAS AND FLANK ERUPTIONS
FISSURE ERUPTIONS ON LAND PRODUCE LAVA PLATEAUS OR FLOOD BASALTS
SUBAQUEOUS ERUPTIONS (MAINLY OCEAN)
SUBGLACIAL ERUPTIONS (MAINLY UNDER GLACIERS)

37. Volcanic Hazards
Life threatening situation- explosive ejection of pyroclastic materials
Water and air pollution
Forest fire
Global climate change
Mitigation
Avoid area… zoning
Structural/strategic defense- lava wall, warning system, evacuation plan, measuring and predicting

38. Ash flow eruption

39. Extrusive Rocks and Gases.
Extrusive Rocks and Gases
Scientific Investigation of Volcanism
Rocks, gases and events from observed eruptions compared to similar lavas elsewhere to infer past activity
Rock Composition
Rhyolite - high silica; light color
Basalt - low silica; dark color
Andesite - intermediate silica and color

40. Chapter Summary - 4
Benefits of Volcanic Eruption/Environmental Effects
Active, Dormant, and Extinct Volcano
Pyroclastic Volcanoes
Definition of Viscosity
Relationship between Viscosity, Temperature, Si content in magma
Basaltic, Andesitic, Rhyolitic Lava – Most volcanic eruptions occur in what type of lava
Classification of Tephra
Definition of Lahar
Contents of Volcanoes
Volcanoes, Pyroclastic occurrence
Effective means of dealing with volcanic hazards