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 ActivityIgneous 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 Activity3) 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 Activity3) 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. Volcanic material can block out sunlight causing short-term cooling.

2. Volcanic material can disrupt air travel.

3. Erupting volcanic material can quickly create new land.

4. 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.