Basic Terminology:

• Ash - Pyroclastic rock less than 2 mm in size

• Felsic - Implies rock high in silica content (viscous)

• Fumaroles - Vents through which volcanoes emit gas fumes and steam after a major eruption

• Lava - The name given to magma after it has erupted from a volcano

• Mafic - Implies rock low in silica content (not viscous)

• Pyroclastic Deposits - The shattering of rock during a volcanic eruption in which gases, rock (varying in size), and lava are ejected from a volcano

• Resurgent Caldera - The process of eruption, collapse, recollection eruption, collapse, and so on that occurs during the life of a volcano

• Volcanic Dome - Domes of lava, squeezed through a volcano; the lava is extremely viscous and barely flows

• Volcanism - The process of magma rising from the interior of the earth through the crust and being deposited on the surface of the earth, where lava (magma on earth's surface) cools into igneous rock

• Volcano - An accumulation of lava that has formed a mountain or hill

• The deeper you go in the Earth, the hotter it gets.  The asthenosphere reaches temperatures of 1100ºC to 1200ºC which is hot enough for rock to melt at these depths of 75 km to 250 km.  The rock that melts in this portion is a main source of magma for volcanoes.

• The friction at a subduction zone can reach great temperatures causing rock to melt and magma to form below the earth's surface.

• Magma chambers form in the lithosphere in large cavities in the rock.  The magma in this rock comes from mantle plumes that originate in the core-mantle boundary.

• SHIELD ERUPTIONS
• Emits from a central vent in a lava cone
• Built from thousands of earlier eruptions of flowing lava
• Spews mafic (basaltic) lava which is not very viscous and flows easily and covers a great area
• Usually no more than 1,500 - 2,000 ft. taller than the surrounding landscape
• Can have diameters of 3 - 4 miles

• CINDER CONES
• Simplest form of volcano
• Cone built from lava ejected from a single vent
• As lava is blown into the air, it solidifies and forms cinders that fall around the vent to form an oval cone
• Bowl shaped crater at the top
• Usually no more than 1,000 ft. taller than the surrounding landscape

• COMPOSITE VOLCANOES (STRATOVOLCANOES)
• Very steep sides
• Built of lava, volcanic ash & cinders that have collected on the slope during volcanic activity
• Usually no more than 8,000 ft. taller than the surrounding landscape
• Has a crater at the peak
• Consists of a single vent or a group of vents all meeting at the top
• Lava escapes through vents in the top or through breaks or fissures in the wall of the volcano
• Vents originate deep in the crust of the Earth
• Lava that solidifies in the cracks and fissures greatly strengthen the volcanic cone

• VOLCANIC DOME
• Formed from small eruptions when the lava is too viscous to flow
• the lava piles up and forms a bulge around the vent
• As the surface layer cools, the hot inner lava creates pressure and causes the outer layer to burst, causing the cooled lava to shatter into little rocks

1. Basaltic Lava
• Has a low silica content (mafic) making it able to flow very rapidly down the side of a volcanic slope
• It has a dark color
• Reaches temperatures of 1000°C - 1200°C
• Types of basaltic lava include:
• Flood basalts
• Erupts on a flat landscape and flows easily
• Forms lava plateaus
• Aa
• Loses all gases and becomes more viscous, so it does not flow as well as flood basalts
• When cools, it forms jagged igneous rock
• Pahoehoe
• Flows like flood basalts
• Forms a glassy skin of quickly cooled lava which is twisted as it flows more
• Pillow Lava
• Forms in the bottom of oceans
• Underwater eruptions eject lava, which rapidly cools due to the cold water, forming bubbles of lava
• The outer shell cools quicker than the inside

1. Rhyolitic Lava
• Very felsic lava which means it has a high concentration of silica which indicates it is very viscous
• Light in color
• Reaches temperatures of 800°C to 1,000°C
• Because it is viscous, it bulges when it cools since it cannot readily flow

1. Andesite Lava
• Lava which has properties between Rhyolitic lava and Basaltic lava

1. Phreatic Explosions
• An explosion of steam caused when magma comes into contact with water and causes superheating of the water, creating steam

1. Volcanic Plugs or Volcanic Necks
• Formed from the depths of the Earth's interior
• Solidified magma that feeds in conduits of inactive, eroded volcanoes
• Diatremes (Tuff-breccia)
• Remnant of explosive eruptions of gas charged magma when the volcano was active

1. Calderas
• The basin shaped depression formed when a volcano collapses
• Due to the fact that the volcano can no longer support itself
• If magma re-enters the vent, it can re-build the walls of the volcano
• This process of breakdown and rebuilding is known as resurgent caldera

1. Lahars
• Wet volcanic debris that causes a mud flow
• Water can come from glaciers or heavy rainfall

1. Hot Springs & Geysers
• Geysers & Hot Springs are formed by groundwater that comes into contact with magma, is heated and returns to earth
• A geyser occurs when water is spewed out of the ground 
• A hot spring occurs when water flows out of the ground

• Somewhat unpredictable
• As with Mount St. Helens, a volcano can explode unpredictably at any angle
• Top or side
• If violent, can blow hot lava miles away
• Volcanic ash deposits can be left great distances from a volcano
• Volcanoes can collapse suddenly due to erosion causing damage to anything on top of it
• If it is a violent eruption, there is less time to get out of the way than if it is a non-violent eruption
• Gases common with volcanoes include carbon dioxide, sulfur dioxide, including nitrogen, hydrogen, carbon monoxide, sulfur & chlorine
• Not good to breathe
• Can get into groundwater
• Kills plants, animals, humans, etc.
• Lava kills and covers anything in its path
• Can cause death
• Pyroclastic Flows
• Move at incredible (200 km/hr) speeds, which gives you no time to get out of its path
• Terrible loss of life
• If it rains, it triggers mudslides which are as treacherous as pyroclastic flows

• Heed warnings given about volcanic eruptions
• Do not build near an active volcano or a possible active volcano if you have concern for the loss of your property
• Even though an eruption is over, make sure everything has had time to settle.  If it has not, there is a chance for mudslides, earthquakes, etc. not to mention the toxic gases that still may be in the area due to the eruption.  Follow the advice given in your area
• If you are near an erupting volcano, stay calm, but get away from it as fast as you can
• Do not build on the top of a dormnet volcano.  If the magma plume no longer exists, all that is below you is hollow rock.  Therefore, it could collapse and cause damage to your property or to yourself

External Links:

• Current Active Volcanoes

• Volcanic Megasite

• Volcanic Activity By State

• United States Geological Survey: Volcanoes

• Volcanoes of the United States

• Volcanoes

•  Press, Frank & Siever, Raymond (2000).  Understanding Earth.  New York:  W. H. Freeman and Company.
• Brantley, Steven R. (1995). Volcanoes of the United States.  Denver, CO:  U. S. Geological Survey.
• Tilling, Robert I. (1998). Volcanoes.  Denver, CO:  U. S. Geological Survey.
• Ebert, Charles H. V. (1988). Disasters.  Bubuque, Iowa:  Kendall/Hunt Publishing Company.