Unit 4 Tectionic activity- physical impacts

• formed in two ways;
  • collapse of surface rock into empty magma chamber 
  • explosive volcanic eruption 
• Explosive caldera formation
  • very large magma chambers filled with silica-rich melt and abundant gas move upwards from depth.
  • very high viscosity that enables them to hold gas bubbles under very high pressures.
  • As it rises to the surface the reduction of pressure causes the gases to expand.
  • break-through occurs the result can be an enormous explosion which blasts away large volumes of rock to form the caldera.

Example:

• Crater Lake Oregon
• Yellowstone supervolcano

• fluid basaltic lava
• released through vents (fissures)
• low magnitude due to constant release of pressure 

• Distinct bursts of fluid lava (basaltic-andesite)
• Explosions usually occur every few minutes at regular or irregular intervals
• Explosions are due to trapped gas bubbles 
• Producces tephra
• Least violent of explosive eruiptions

• volcanic eruption from fracture in earths surface
• underlain by reservoirs of basaltic magma
• Mid-oceanic ridges- divergent plate boundaries

Example

• Iceland, an extention of the Mid-Atlantic ridge
• Hawaiian shield volcanoes

• Steep-sided and cone-shaped
• Made up of layers of ash and lava 
• Andesitic// Ryolitic lava

Example:

• Mount Etna Italy

• Typically hot spot locations where the crust is particually thin or weak
• Balsitic lava
• Only explosive if water gets into vents 
• Gasses escape very easily 
• Gently sloped & made from layers of lava

Example

• Mauna Loa Hawaiian islands 

• Formation; within stratocolcanos 
  • large eruptions empty out the magma chamber in the summit of the composite volcano.
  • It collapses forming a depression.
  • Inside the depression magma oozes out to begin forming a small lava dome.
• Formation; steep sided volcanosAndesitic/ Rhyolitic lava
  • Lava domes form on the steep sides of volcanoes
  • they can become dislodged during an eruption
  • most dangerous type of lava domes due to exposure of molten rock that become pyroclastic flows as the gases are released

Example

• Mt St Helens 
• Mt Pelee Caribbean 

• Smallest and most common volcano
• Occurs in basaltic lava fields or parasitic cones on stratovolcanoes or shield volcanoes.
• Balsitic lava released 
• Tephra released if gas is trapped
• Cinder cones generated by strombolian eruptions
  

Example;

• Sunset crater Arizona US

• Short, violent and relatively small explosion of viscous magma
• Andesitic lava
• Produce tephra, ash clouds, and pyroclastic flows

• Largest and most violent of all eruption types 
• Caused by the fragmentation of gassy very viscousmagma
• Rhyolitiic magma
• Releases enormous amounts of energy
• Produce falls of ash, scoria and lava bombs and pyroclastic

Examples:

• Mt St Helens
• Mt Vesuvius

Intrusive igneous activity- materials injected into the crust are intrusive landforms, which are exposed through erosion or overlaying rock.

Sills: Whin N- England

• parallel bedding planes 
• cracks run horozantally and provide lines of weakness where magma flows 
• As it cools, the magma contracts providing cracks in the rock

Dykes: Channel Islands 

• Cut accross the bedding planes of rock 
• Magma forms cracks and weaknesses.
• It cools and solidifies before reaching the surface forming liniar ridgs where its exposed.

Batholith: Dartmoor

• Large masses of intrsive rock 
• Forms doming on surface
• Heat transfered from the magma created metamorphic rock.