Quick revise

Volcanoes are formed along two types of plate boundary: destructive and constructive. Volcanoes occur where molten rock (magma) comes to the surface of the earth.

The magma rises to the surface through cracks in the crust called vents.

Cone Volcanoes

 When lava is thick and acidic it doesn’t flow far before cooling and solidifying, this causes cone volcanoes (aka acid cone volcanoes) to have steep sides.    

cone volcano

Example: Mount Pelee on Martinique (an island in the Caribbean)

Shield Volcanoes

When lava is runny and thin it can flow a long way before cooling and solidifying, this causes shield volcanoes to have gentle slopes and wide bases built almost entirely of low viscosity basaltic lava flows.  

Shield volcanoes are the largest of the three types.  

The eruptions are generally non explosive due to the low silica content and may last for years

 shield volcanoes

Example: Mount Kilauea in Hawaii (USA), Galapagos Islands, Snake River Plain in Idaho, USA.

Composite Volcanoes (Stratovolcano)

Composite volcanoes are the most deadly of volcano types.

They are made of alternate layers of ash and lava and have steep sides built up by eruptions of intermediate viscosity andesitic lava and explosive tephra.  

Often the lava cools creating a plug which blocks the vent resulting in a huge explosion blowing out the plug    

Composite Volcanoes

Example: Mount St Helens in Alaska (USA), Mount Shasta in California, Mount Rainier in Washington state, and Mount Fuji in Japan.

Human effects of Volcanoes
The effects of volcanoes can be divided into primary and secondary effects.  

Primary effects

  • Produced directly by the volcanic activity.
  • Examples include lava flows, ash-flows, lateral blasts, ash-falls, and gases.

Secondary effects

  • The result of primary effects e.g. mud flows
Lava Flows
  • Lava flows are streams of molten rock.
  • Lava flows can erupt relatively non-explosively and move very slowly (a few meters to a few hundred meters per hour) or they can move rapidly (typically down steep slopes).    
  • Most lava flows are slow enough that they are seldom a threat to human life.
  • Such flows generally follow a predictable course.
  • However, lava flows can cause extensive damage or total destruction by burning, crushing, or burying everything in their paths.
  • Whole villages have been known to completely disappear beneath lava flows.  
  • To avoid such destruction, controlling a lava flow has become important and successful deflection has occurred in the past. photos of lava flows.
Pyroclastic Hazards
  • Volcanic explosions produce volumes of tephra.
  • Tephra is the material blown out of the volcanic vent when an explosion occurs.
  • Ash-flows, lateral blasts, and ash-falls are the types of pyroclastic activity that produce tephra, with composite volcanoes and large calderas the vent sources.
Pyroclastic Flows
  • Pyroclastic flows (also called ash-flows) are high speed avalanches of hot ash, rock fragments, and gas which move down the sides of a volcano during explosive eruptions.
  • These flows occur when the vent area or ash column collapses.
  • Because pyroclastic flows can reach 1500 degrees F and travel at high speeds (160-250 kilometres per hour and up), they are extremely destructive and deadly.
  • Pyroclastic flows are typical of composite volcano eruptions, but are also associated with large caldera systems.
Volcanic Gases
  • All magmas contain dissolved gases that are released during and between eruptive episodes.
  • These gases are predominately steam, followed in abundance by carbon dioxide, compounds of sulphur and chlorine, and lesser amounts of other gases.
  • While they rarely reach populated areas in lethal concentrations, gases can be injected to great heights in the atmosphere by volcanic eruptions, in some cases spreading throughout the globe.
Mud Flows

Mud flows (Lahars / Debris flows) are mixtures of water, rock, ash, sand, and mud that originate from the slopes of a volcano. They can travel over 80 kilometres and commonly reach speeds of 35 to 65 kilometres per hour.  

They contain a high percentage of rock debris look like fast-moving rivers of concrete. Close to a volcano, they have the strength to rip huge boulders, trees, and structures from the ground and carry them for great distances.

Farther downstream the coarser debris settles to the bottom of the flow, leaving mud to continue on to cover everything it passes.

Mud Flows are formed when masses of unconsolidated, wet debris become mobilized, and are commonly start by:

  • Large landslides of water-saturated debris
  • Heavy rainfall eroding volcanic deposits
  • Radiant heat emitted from a volcanic vent suddenly melting snow and ice
  • Pyroclastic flows on the flanks of a volcano
  • Breakout of water from glaciers, crater lakes, or from lakes dammed by volcanic eruptions  

Historically, mud flows have been one of the most deadly of the volcanic hazards.

Advantages of Volcanic Eruptions

Although volcanoes can be very destructive, there are benefits of living near a volcano.

Volcanoes provide resources for energy extraction, also called geothermal resources.  Heat from the earth's crust is being converted to energy. The big advantages to this type of energy are that it is very clean and the resources are nearly inexhaustible.

When a volcano erupts it throws out a lot of ash.  In the short term this ash can be very harmful to the environment, but in the long term the ash layer, which contains many useful minerals, will be converted to a very fertile soil. Residents living nearby will use the rich soil for farming. Even after an eruption people still return because of the fertile soil around the volcano.