Tectonics Topic 2.2 (Tectonics processes affecting the magnitude of volcanic eruptions)

volcano

landform created when lava erupts onto the Earth’s surface

volcanic eruptions occur at …

1. divergent plate boundaries

2. convergent plate boundaries

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. 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

convergent plate boundaries

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 of the oceanic crust. Water lowers the melting point of the overlying mantle, causing it to melt, forming magma

3. Magma containing dissolved gases and is less dense that 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.

High silica magma

• More viscous magma

• As magma rises towards the earth’s surface, the dissolved gases in the magma cannot escape easily 

• More pressure builds up until gases escape explosively

• Resulting in violent, explosive eruptions

• Generally associated with stratovolcanoes

low silica magma

• Less viscous magma

• As magma rises towards the earth’s surface, the dissolved gases in the magma can escape easily

• Less pressure builds up 

• Resulting in gentle, effusive eruptions

• Generally associated with shield volcanoes

high silica and low silica exception

there are volcanoes with viscous magma that do not result in explosive eruption as the 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 dissolved gases to escape easily 

stratovolcanoes characteristics

• gradient of slope : steeper at the top and gentle at the base

• magnitude of eruption : explosive eruption

• example : Mount Mayon in Philippines

formation of stratovolcanoes

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 

shield volcanoes characteristics

• gradient of slope : gently sloping sides and broad summit

• magnitude of eruption : gentle and effusive eruption

• example : Kilauea in Hawaii, USA

formation of shield volcanoes

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 gentler sloping sides with a broad summit 

measurement of volcanic eruptions

Volcanic explosivity index (VEI) → measures magnitude of different volcanic eruptions based on the explosivity using the following criteria : 

1. Volume of ejected material (greater volume, higher VEI)

2. Height of eruption cloud (greater height, higher VEI)

3. Duration of eruption (longer eruption, higher VEI)