Stratovolcanoes tend to have highly infrequent

Stratovolcanoes tend to have highly infrequent eruptions -- hundreds of years apart --
and typically form in subduction zones
o Intermediate/Andesitic:
 Thick, high viscosity lava
o Large, cone-shaped volcanoes with steeper slopes
o Explosive, tall, conical volcano composed of one layer of hardened lava, tephra, and
volcanic ash
 Made of alternating layers of lava, tephra, and debris
o Ex. Mount Saint Helens, Mount Rainier, Mount Vesuvius (Pompeii, 79 AD)
 Shield volcanoes:
o These wide, relatively short volcanoes occur when low-viscosity lava flows out with
minimal explosiveness, such as in Hawaiian eruptions
o The lava disperses out over a wide surface area -- sometimes hundreds of kilometers --
building up a shield-shaped dome
o Near the summit, the edifice gets a little steeper, giving the volcano a slightly raised
center
o Many shield volcanoes erupt with great frequency (every few years or so)
o Mafic/Basaltic:
 Low viscosity, high fluid lava
o Effusive, eruptive
o Broad, slightly dome-shaped (like an inverted shield)
o Have a low slope and cover large geographic areas, but are not very tall

o Low profile volcanos, like Hawaii Shield volcanoes
 Ex. Mauna Loa
 Scoria cone volcanoes (cinder cones):
o These relatively small cones are the most common volcano type
o They are characterized by steep slopes on both sides of the edifice, which lead up to a
very wide summit crater
o This edifice is composed of ashy tephra, usually spewed out by Strombolian eruptions
o Unlike stratovolcanoes, many Scoria cone volcanoes have only one eruption event
o Felsic/Rhyolitic:
 Thick, high viscosity lava
o Conical piles of tephra; the smallest type of volcano
o Built of ejected lapilli and blocks piled up at a vent
o Often symmetrical, with a deep summit crater
o Typically, from a single eruption event
o Steep conical hill of loose pyroclastic fragments such as volcanic clinkers, cinders,
volcanic ash and scoria that has been built around a volcanic event
o As the gas-charged lava is blown violently into the air, it breaks into small fragments that
solidify and fall as either cinders, clinkers, or scoria around the vent to form a cone that
often is symmetrical; with slopes between 30–40°; and a nearly circular ground plan.
The Hawaiian Island Chain
 Whilst most volcanic activity happens at plate margins, there are cases of volcanoes erupting in
the middle of plates
o The Hawaiian Islands are formed by volcanic activity, despite the nearest plate margin
being 3,200 km away
o Some geologists have suggested that a ‘hot spot’ in the mantle, which remains
stationary as the Pacific Plate moves over it, explains the existence of the island chain
 The hot spot may represent the top of a mantle plume which originated deep down at the outer
core—lower mantle boundary
o The plate moves in a north westerly direction due to sea floor spreading along the East
Pacific Rise
o As oceanic lithosphere moves away from the hot spot, volcanic activity ceases and it
cools, becomes denser, and slowly subsides

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