Plate Tectonics- Vulcanicity

Major forms of extrusive activity- Volcanoes

Fissure Eruptions

• Occur where an elongated crack in the crust allows lava to spill out over a large area.
• Location: Constructive margins
• Made of: Basaltic rocks
• Eruption: Gentle, persistent
• Example: Antrim lava Plateau, Northern Ireland, Giant’s causeway

Shield Volcanoes

• Form gently sloping cones from layers of less viscous lava. 
• Location: Hot spots and where oceanic crust meets oceanic crust
• Made of: Basaltic rock
• Eruptions: Gentle and predictable
• Example: Mauna Loa, Hawaii

Composite Volcanoes

• Classic pyramid-shaped volcanoes.
• Location: Destructive Margins, on land 
• Made of: Andesitic rocks
• Eruption: Explosive eruption and subsequent layers of lava and ash 
• Example: Mount Etna and Vesuvius

Acid or Dome Volcanoes

• Steep sided volcanoes formed from very viscous lava. Lava cannot travel far and builds up convex cone shape 
• Location: Destructive Margins
• Made of: Rhyolite rocks
• Eruption: Explosive as pressure builds up in blocked vent
• Example: Mount. Pelée, Martinique

Calderas

• Location: Destructive Margins and Hot spots
• Made of: Rhyolite rocks
• Eruption: Explosive as pressure builds up, removes top leaving an opening several kilometres wide.
• Example: Krakatoa, Indonesia; Yellowstone National Park, USA

Minor forms of extrusive activity

As well as volcanoes there are other features on the surface due to vulcanicity.

Geysers

• Water heated by volcanic activity is intermittently discharged of water
• Ejected as a turbulent eruption of water and vapour
• Water is above boiling point
• Examples: Old Faithful, Yellowstone National Park and Geysir, Iceland

Hot Springs

• The emergence of geothermally heated groundwater from the earth’s crust
• Can be found all over the world
• Occur where water temperature is below boiling point
• Examples: Bath and Blue Lagoon, Iceland

Boiling Mud

• Heat water mixes with surface deposits

Fumarole/Solfatara

• Small volcanic areas without cones, produced by gases escaping from the surface
• Example: Rotorua, New Zealand 

Pools

• Hot springs that have enough water comes to the surface to keep the fluid form entirely boiling away and carry away the mud and debris 
• Water is near to boiling point and therefore little grows
• The colour of the pools is because the water is so clear and refracts like a prism