Stratovolcanoes tend to have highly infrequent eruptions hundreds of years

Stratovolcanoes tend to have highly infrequent

This preview shows page 16 - 19 out of 24 pages.

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
Image of page 16
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
Image of page 17
o
Image of page 18
Image of page 19

You've reached the end of your free preview.

Want to read all 24 pages?

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture