Volcanoes - shapes & eruptive patterns
Volcanoes are grouped according to their eruptive patterns and shape:
Composite or Stratovolcano
(Majority of volcanoes on earth full into this category)
- Gentle lower slopes, but steep upper slopes.
- Concave upward.
- Small summit crater.
- Highly variable. Alternating basaltic to rhyolitic (stiff silica) lavas and tephra with an overall andesite composition.
- Can explosively emits gases, ash, pumice.
- Eruptions can be accompanied by lahars - mudflows.
- Examples are Krakatoa in Indonesia, Mt. Pinatubo in the Philippines, and Mt. St. Helens in Washington State, USA. Read for more information and volcano images
Scoria Cone or Cinder Cone volcano
- Cone shaped.
- Straight sides with steep slopes - formed by loose, fragmented cinders that fall close to the vent.
- Large bowl shaped summit crater.
- Smallest type, with heights generally less than 300 meters.
- Basalt tephra,occasionally andesitic.
- Very gentle convex slopes. Shield volcanoes can be very big.
- Basalt lava flows.
- Very fluid lava that flows in long-lasting, relatively gentle eruptions.
- Explosions are minimal.
- An example is Mt. Kilauea (in Hawaii, USA).
Additional volcano types:
- A bulbous (rounded) volcano that forms when very viscous lava barely flows.
- Example: Mont Pelée in Martinique.
Rhyolite Caldera Complex:
- The most explosive volcanoes.
- After eruption, the result is a caldera (crater) caused when the area around the vent collapses.
- Examples: Yellowstone in Wyoming, USA and Lake Taupo in New Zealand (which erupted around A.D. 80).
Types of volcanic rock
- Basalt: Black shiny rocks with only a few crystals are usually basalt. The Hawaiian islands are mostly made up of basalts and are famous for their beautiful black-sand beaches
- Rhyolite: White shiny rock with many crystals and often many bubble holes.
- Andesites: Light gray and usually have large box-shaped crystals called plagioclase. Typically found in the Andes Mountains, South America.
What gases do volcanoes emit?
Water, carbon dioxide, sulfur dioxide, hydrogen chloride and hydrogen fluoride. The latter three are strong poisons and cause pollution.