Chapter Four

1
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 EARTHS SURFACE   VOLCANISM
SET OF GEOLOGICAL PROCESSES THAT MARK THE ASCENT
OF MAGMA TO THE EARTHS SURFACE   2000 YRS
------- 600 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 Earths 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 Earths
  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
VOLCANOES ACTIVE EXTINCT ACTIVE CURRENTLY
NOT ERUPTED FOR OR 10,000 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)
• CAUSES FOR VOLCANISM
• 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
17
(No Transcript)
18
Extrusive Textures

• Vesicular/Frothy - trapped gas bubbles
• Vesicular basalt
• Pumice
• Fragmental - particles blasted apart by explosive
  eruptions
• Dust and ash (lt2 mm)
• Cinders (2-64 mm)
• Blocks and bombs (gt64 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 volcanoesgentle 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
• 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
26
(No Transcript)
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
• SMALL VOLCANIC DUST ASH- 1 MICRON
• MEDIUM CINDERS OR LAPILLI (gt1-62 MICRON)
• LARGE VOLCANIC BOMBS (gt64 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

• 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

41
(No Transcript)