2.2 Tetconic processes affecting magnitude of volcanic eruptions

Question 1

How do tectonic processes result in volcanic eruptions?

Answer 1

• A volcano is a landform created when lava erupts onto the Earth’s surface.
• It may form a cone-shaped mountain as more lava erupts and accumulates over time.
• it occurs at:
  1. Divergent plate boundaries
  2. Convergent plate boundaries

Question 2

Volcanoes at divergent plate boundaries

Answer 2

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. Therefore, magma rises through weak areas in the crust to the Earth’s surface to erupt as lava, causing a volcanic eruption.
5. The lava cools, solidifies and accumulates over time, forming a volcano.

Question 3

Volcanoes at convergent plate boundaries

Answer 3

1. Plates move towards each other and the denser plate subducts under the other.
2. As the subducting plate sinks into the mantle, the high pressure forces water out its 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. Therefore, magma rises through weak areas in the crust to the Earth’s surface to erupt as lava, causing a volcanic eruption.
5. The lava cools, solidifies and accumulates over time, forming a volcano.

Question 4

Why are some volcanic eruptions more explosive than others?

Answer 4

• The explosivity of volcanic eruptions depends on the how easily dissolved gases can escape from the magma.
• how easily dissolves gases escape depends on the viscosity of magma
• high silica magma (more viscous)
• low silica magma (less viscous)

Question 5

High Silica Magma

Answer 5

• more vicious magma (like honey)
• as magma rises towards the earth’s surface, dissolves gases in magma cannot escape easily
• more pressure builds up until the gases escape explosively
• resulting in violent, explosive eruptions
• generally associated with stratovolcanoes
• however, there are volcanoes with viscous magma that do not result in an explosive eruption as magma rises in a way that allows gases to escape

Example:
- while Mount Merapi is a stratovolcano, its eruption in 2006 was not explosive as the viscous magma rose in a way that allowed dissolves gases to escape easily

Question 6

Low silica magma

Answer 6

• less viscous magma (like water)
• as magma rises towards the earth’s surface, the dissolved gases in the magma can escape easily
• less pressure built up
• resulting in gentle, effusive eruptions
• generally associated with shield volcanoes

Question 7

Stratovolcanoes

Answer 7

Stratovolcanoes form when:
1. High viscosity magma rises through weak areas in the crust to the Earth’s surface and 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 consisting of alternating layers of ash and lava develops.
4. As the highly viscous lava travels a shorter distance before cooling and solidifying, the volcano has steep sides and a narrow summit.
Example:
- Mount Mayon in the Philippines.

Question 8

Shield volcanoes

Answer 8

Shield volcanoes form when:
1. Low viscosity magma rises through weak areas in the crust to the Earth’s surface and erupts effusively.
2. Over successive eruptions, a volcano consisting of layers of lava develops.
3. As the less viscous lava travels a longer distance before cooling and solidifying, the volcano has gently sloping sides with a broad summit.
Example:
- Kilauea in Hawaii, USA.

Question 9

How are volcanic eruptions measured?

Answer 9

• The Volcanic Explosivity Index (VEl) measures the folitude of different volcanic eruptions based on explosivity using the following criteria:
  1. Volume of ejected material: The greater the volume of ejected material, the higher the VEl.
  2. Height of the eruption cloud: The greater the height of the eruption cloud, the higher the VEl.
  3. Duration of the eruption: The longer the eruption, the higher the VEl.
• Measured on a scale of 0 to 8.
  —————
• Effusive eruptions are generally given a value of 0 or 1, as they are non-explosive with less than 0.0001 km³ of material ejected.
  Example:
• The ongoing effusive eruption of Kilauea, Hawaii, which is non-explosive.
• A value of 8 represents a mega-colossal explosive eruption that can eject more than 1,000km³ of tephra, with an eruption cloud column height of over 25km.
  —————
  Example:
• Toba Volcano in Northern Sumatra, Indonesia, 74,000 years ago.
• The volcano erupted approximately 2,800km³ of material, which covered India and parts of Southeast Asia in over 15 cm of ash.
  —————
• VEI scale is logarithmic - an increase of ‘1’ on the VEl scale indicates an eruption 10 times more powerful than the number before it.