VOLCANOES
VOLCANIC ACTIVITY
Volcano - a cone-shaped or dome-shaped initial landform created by the emission of lava and gases from a constricted
opening in the Earth’s surface.
Magma
TYPES OF VOLCANOES
Composite Volcanoes and Stratovolcanoes
Stratovolcanoes are steep-sided cones produced by felsic magmas.
Examples: Mt. Hood, Mt. St. Helens, Mt. Lassen (PLUG DOME variety), Mt. Shasta, Mt. Fujiyama in Japan, and
Mount Mayon in the Phillipines.
Nueé Ardente - “”glowing ash” - a cloud of white-hot gases and fine ash and can flow at very high speeds down
the side of a volcano.
Shield Volcanoes – Enormous sized, very tall, but with very low slope angle (usually 6 0 or less).
Mauna Loa and Mauna Kea are two of the five shield volcanoes that form Hawaii
Cinder Cones – smallest type of volcano.
ParacutÍn, Mexico.
Calderas - central depressions in the center of a volcano
Mt. Mazama, exploded approximately 6600 years ago, leaving behind Crater Lake in Oregon with Wizard Island, a
perfect basaltic cindercone in the center.
Hot Spots – localized regions where plumes of hot mantle material rise below the base of the lithosphere.
Yellowstone National Park, NW Wyoming, and Hawaiian Islands, Pacific Ocean.
Flood Basalts
Basaltic magma is so fluid that, as it rises, it seeks any available route to the surface. One route can be through a volcanic
neck or pipeline/conduit to the surface - creating an active volcano
.
Ash-Flow Eruptions
1) The Toba ash-flow eruption
2) The Long Valley Caldera was formed
Other Dangerous Historic eruptions
1. Tambora - Indonesia - 1815 2. Krakatau 3) Santorini - ?1400 BC? - near the isle of Crete. Appears to be the same type of explosion/eruption as Krakatau.
May have been responsible for the disappearance of the Minoan civilization, and it is possible that the disappearance of
the island gave rise to the legend of Atlantis. - Maybe even an explanation to how Moses crossed the Red Sea?
EXPLOSIVITY CONTROLS OF VOLCANOES?
1. Viscosity – a measure of fluidity of magma – Higher viscosity equals more explosivity
a.
Temperature – Felsic magmas are cooler (600-9000C); mafic magmas as hotter (900-12000C)
b.
Composition – Felsic magmas more viscous, less fluid: mafic magmas less viscous, more fluid
2. Dissolved Gases
a.
Felsic magmas contain more dissolved gases; mafic magmas contain less dissolved gases
3. “Escape” potential
a.
Felsic magmas do not allow gases to ‘escape’ easily; build up more internal pressure
b.
Mafic mamas do allow gases to ‘escape’ easily; less build up internally
ROCK MELTING
Three factors: (1) Increase Temperature
(2) Reduce Overlying Pressure
(3) Add a Flux – usually water