2.2 Tectonic Processes Affecting the Magnitude of Volcanic Eruptions

How do tectonic processes affect the magnitude of volcanic eruptions?

How do tectonic processes result in a volcanic eruption?

volcano

• a landform created when lava erupts on the Earth’s surface

• may form a cone-shaped mountain as more lava

  • erupts and accumulates 

    → over time 

volcanic eruptions occur at

• divergent plate boundaries

• convergent plate boundaries 

How do volcanic eruptions occur at divergent plate boundaries?

1) Plates move apart

• the crust stretches and 

• fractures develop

2) The decrease in overlying pressure causes

• parts of the underlying mantle to melt

  → forming magma 

3) Magma contains dissolved gases and is 

• less dense than the

  • surrounding materials 

4) Hence, magma rises through weak areas in the crust to Earth’s surface to 

• erupt as lava

  • causing a volcanic eruption 

5) The lava cools, solidifies and accumulates over time

• forming a volcano 

How do volcanic eruptions occur at convergent plate boundaries?

1) Plates move towards each other

• the denser plate subducts under the other

2) As the subducting plate sinks into the mantle

• the high-pressure forces water out of the oceanic crust

• water lowers the melting point of the overlying mantle 

  → causing it to melt

  • forming magma 

3) Magma contains dissolved gases and is

• less dense than the 

  • surrounding materials

4) Hence, magma rises through weak areas in the crust to Earth’s surface to

• erupt as lava

  • causing a volcanic eruption

5) The lava cools, solidifies, and accumulates over time

• forming a volcano 

How is the explosivity of volcanic eruptions measured?

determined by how easily dissolved gases can

• escape from magma

  • can’t escape = explosive eruptions

  • escapes easily = effusive eruptions

and by

• magma viscosity

  • high viscosity = explosive eruptions

  • low viscosity = effusive eruptions 

What are the characteristics of high silica magma?

Magma is of

• high viscosity

• as magma rises towards Earth’s surface

  • dissolved gases in the magma

    → cannot escape easily

  • more pressure builds up until

    • gases escape explosively 

  ∴ volcanic eruptions are explosive and violent

• stratovolcanoes 

Exception

• however, there are volcanoes with viscous magma but

  • they don’t result in

    → explosive and violent eruptions

  • because the magma rises in a way that

    • enables the dissolved gases to escape

What are the characteristics of low silica magma?

Magma is of

• low viscosity

• as magma rises towards Earth’s surface

  • dissolved gases in the magma

    → can escape easily

  • less pressure builds up

  ∴ volcanic eruptions are gentle and effusive

• shield volcanoes

How do stratovolcanoes form?

• convergent plate boundaries

1) High viscosity magma rises through weak areas in the crust to Earth’s surface

• erupts explosively as

  • lava, ash and rocks

2) The ash and rocks settle on the sides of the volcano and are later

• covered by the lava

3) Over successive eruptions

• a tall volcano develops

  • consisting of alternating layers of ash and lava

4) The volcano has 

• steep sides

• narrow summit 

  → as the highly viscous lava travels a shorter distance 

  • before cooling and solidifying 

How do shield volcanoes form?

• divergent plate boundaries

1) Low viscosity magma rises through weak areas in the crust to the Earth’s surface

• erupts effusively

2) Over successive eruptions

• a volcano develops 

  • consisting of layers of lava

3) The volcano has

• gently sloping sides 

• broad summit

  → as the less viscous lava travels a longer distance

  • before cooling and solidifying 

How are volcanic eruptions measured?

The Volcanic Explosivity Index (VEI)

• measures the magnitude of volcanic eruptions based on

  • explosivity

    via the following criteria:

1) Volume of Ejected Material

• larger vol. of ejected material

  → higher VEI

2) Height of the Eruption Cloud

• higher height of the eruption cloud

  → higher VEI

3) Duration of Eruption

• longer eruption

  → higher VEI

Scale

• 0 - 8

  • usually shield volcano VEI rating ‘0 - 1’

• logarithmic 

  • an increase of VEI ‘1’ indicates eruption is 

    → 10x more powerful than the previous rating