Download Nature and Products of Volcanic Eruptions

The Nature and Products
of Volcanic Eruptions
Chapter 4
(Pages 88 – 124)
The Nature of Volcanic Eruptions
(Page 89)
Factors determining the “violence” or
explosiveness of a volcanic eruption
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
The above three factors actually control
the viscosity of a given magma which in
turn controls the nature of an eruption.
The Nature of Volcanic Eruptions
Viscosity is a measure of a material’s resistance to flow
(e.g., Higher viscosity materials flow with great difficulty)
Factors affecting viscosity (Pg. 90)
1) Temperature - Hotter magmas are less viscous
2) Composition - Silica (SiO2) content
- Higher silica content = higher viscosity
(e.g., felsic lavas have a high viscosity)
- Lower silica content = lower viscosity or more fluid-like
behavior (e.g., mafic lavas have a low viscosity)
The Nature of Volcanic Eruptions
In Summary
• Fluid basaltic lavas generally produce
quiet eruptions
• Highly viscous lavas (rhyolite or andesite)
produce more explosive eruptions
Materials extruded during an
Eruption (Pg. 93 – 95)
Lava Flows
• Basaltic lavas are much more fluid
• Types of basaltic flows
– Pahoehoe lava (resembles a twisted or ropey
texture)
– Aa lava (rough, jagged blocky texture)
A Pahoehoe lava flow
A Typical aa flow
Materials extruded during an
Eruption (Pg. 108)
Lava Plateau
• forms from one or more fissures, which are
fractures that extend to the depths of the
mantle.
• These do not form from volcanic craters.
Plate Tectonics and Igneous Activity
(Pg. 116 – 122)
Global distribution of igneous activity is
not random
• Most volcanoes are located within or near
ocean basins
• Basaltic rocks are common in both
oceanic and continental settings, whereas
granitic rocks are rarely found in the
oceans
Distribution of some of the world’s
major volcanoes (Pg. 117)
Plate Tectonics and Igneous Activity
Igneous activity along plate margins
1) Spreading centers
- The greatest volume of volcanic rock is produced
along the oceanic ridge system
- Mechanism of spreading
- Lithosphere pulls apart
- Less pressure on underlying rocks
- Results in partial melting of mantle
- Large quantities of basaltic magma are produced
Plate Tectonics and Igneous Activity
Igneous activity along plate margins
2) Subduction zones
– Occur in conjunction with deep oceanic trenches
– Descending plate partially melts
– Magma slowly moves upward
– Rising magma can form either
– An island arc if in the ocean
– A volcanic arc if on a continental margin
Plate Tectonics and Igneous Activity
Igneous activity along plate margins
2) Subduction zones (Continued)
– Associated with the Pacific Ocean Basin
– Region around the margin is known as the
“Ring of Fire”
– Most of the world’s explosive volcanoes
are found here
Plate Tectonics and Igneous Activity
3) Intraplate volcanism
- Activity within a tectonic plate
- Associated with plumes of heat in the
mantle
- Form localized volcanic regions in the
overriding plate called a hot spot (Pg.122)
– Produces basaltic magma sources in oceanic
crust (e.g., Hawaii and Iceland)
– Produces granitic magma sources in
continental crust (e.g., Yellowstone Park)
Plate Tectonics and Igneous Activity
3) Intraplate volcanism
- Students should know that the
Yellowstone National Park and Hawaiian
island chain examples differ based on; -- volcano type,
- molten composition, and
- eruption style.
Volcanism on a tectonic
plate moving over a hot spot
Volcano Type Characteristics
Global Effects of Volcanic Activity (p. 122-124)
some global effects of volcanic activity will be
short-term, whereas others will be long-term.
Short-term global effects of volcanic activity include:
1.
2.
3.
4.
Volcanic material can block out sunlight causing short-term cooling.
Volcanic material can disrupt air travel.
Erupting volcanic material can quickly create new land.
Volcanoes and associated material can be destructive (e.g., death to
organisms, property damage, road damage).
5. Volcanoes release sulphur dioxides and nitrogen oxides which can mix with
water vapour in the atmosphere leading to increased, short-term, acid
precipitation.
Global Effects of Volcanic Activity (p. 122-124)
some global effects of volcanic activity will be
short-term, whereas others will be long-term.
Long-term global effects of volcanic activity include:
1. Volcanoes release gases like carbon dioxide and water vapour, which in
large amounts, could contribute to global warming and climate change.
2. Volcanoes release sulphur dioxides and nitrogen oxides, which can mix
with water vapour in the atmosphere leading to increased, long-term, acid
precipitation.
3. Volcanoes create fertile soils which enhance agriculture.
4. Volcanoes, depending on number, frequency, and eruption size, could
contribute to global cooling and the origin of ice ages, due to the blocking
out of the sun. Plants failing to photosynthesize could result in total
collapse of food webs and ecosystems.