Types of Volcanic cone:

Composite / Strato-volcano

Shield Volcano (also called Hawaiian)

Cinder Cone - one made from solid fragments emitted from the vent. The shape of the cone depends on the angle of rest of the fragments: larger fragments produce steeper, but less stable slopes. Finer particles are carried further, and come to rest on more gentle slopes

e.g Cerro Negro in Nicaragua

Can be built on the terrain of older lava flows (see notes on Mt. Fuji)

Types of Eruptions

Gentle

Violent (Plinian) - refers to Pliny the Elder, who witnessed eruption of Vesuvius.

Phreatic: when hot, gas-charged magma encounters groundwater or seawater, vast quantities of super-heated steam are formed. This produces a phreatic or steam eruption. The best known of these was Krakatoa in 1883.

• Small island formed from volcanic cones. Walls of volcano ruptured and seawater poured into magma chamber
• Huge explosion (5000x Hiroshima atomic bomb)
• Eruption heard in Australia over 2000km away
• Ash covered an area of 700,000 square kilometers
• Reduced sunlight reaching earth by 13%
• Generated tsunami 40m high which drowned 36000 people
• Island was destroyed, leaving a 300m deep water covered depression

Other types:

Strombolian: named after volcano in Italy - Infrequent, violent eruptions made up of lava

Vulcanian: infrequent ejects of lava fragments and ash which produce a cinder cone

Pelean: named after Mt. Pelee on Martinique - characterised by pyrodastic flows, also called 'nuees ardentes' (glowing clouds)

There is a scale for eruptions in the same way as there is the Richter scale for earthquakes; it is called the Volcanic Explosive Index. It is based on the amount of material ejected, the height of the cloud it causes and the amount of damage caused. Any explosion over level 5 is classed as large. Logarithmic, like the Richter (10x larger for each step up) Never been a level 8: Tambora in Indonesia was 7, Mt. St. Helens was 5, Krakatoa was 6

Other Features:

Diatreme

Sometimes magma solidifies in the vent and never reaches the surface. The various pipes and vents fill with basalt. The slow cooling can lead to the formation of diamonds, which are formed from carbon under great pressure e.g Kimberly mines in S. Africa - when only a vent is blocked, tends to be known as a plug.

e.g is Shiprock in New Mexico, USA

Fissure Eruptions

Imagine basaltic lava flowing out of a crack in the surface tens of kilometres long and covering large areas. These have occurred throughout history, and as recently as 1783 in Iceland, when a 32km long fissure opened up, and spewed out 12 cubic kilometres of basalt, which killed 20% of the whole population.

The lava spreads out rather than piles up and creates a lava plateau. They bury the pre-existing topography.

e.g Columbia plateau in NW USA

Area along Columbia River: Oregon and Washington states. Area of 200000 square kilometres. Some flows more than 100m thick, and spread as far as 60km from the source. New landscape then begins to form on the lava flows with new river valleys forming Another area is the Deccan traps, or plateau in India.