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Monday, 26 February 2007

Eruption Products

PART ONE - LAVA FLOW TYPES

BASALTIC LAVA


Where do basaltic lava flows erupt from?

  • Shield volcanoes
  • Rift zones
  • Cinder cones

Fluid lava flows can be subdivided into:

  • Pahoehoe Lava -- Surfaces are smooth, billowy, or ropy.
  • A'a Lava -- Surfaces are fragmented, rough, and spiny, with a "cindery" appearance

Pahoehoe Lava



Charateristics of Pahoehoe Lava

  • Relatively thin (1-2m)
  • Very fluid
  • Low Viscosity

Pahoehoe Lava can be further divided into 3 different types:

  • Ropy Pahoehoe - lava surface is bunched up or wrinkled and resembles a coiled rope
  • Sheylly Pahoehoe - contains a billowy flow top with a frothy vesicular surface skin, only a few centimeters thick, overlying large cavities, generally 5-30 centimeters thick
  • Slabby Pahoehoe - contains a series of closely spaced slabs, a few meters across and a few centimeters thick, broken and tilted by mass movement, or drainage, of the underlying lava.

A'a Lava



Characteristics of A'a Lava

  • Cindery fragments broken during the churning action of flow advancement.
  • Thicker than and less viscous than pahoehoe lava.

ANDESITIC LAVA




Characteristics of Andesitic Lava

  • Blocky nature of lava flows
  • High viscosity
  • Relatively high thickness (some thick enough to form lava domes)

DACITIC TO RHYOLITIC LAVA




  • As silica content and polymerisation increases, the viscosity of dacitic and rhyolitic lava increases.
  • Although dacitic-to-rhyolitic lavas typically erupt from stratovolcanoes, they are not as abundant as andesite lava.
  • Viscous dacitic-to-rhyolitic lavas generally ooze out of the volcano's central vent to form symmetrical lava domes

Characterisitics of Dacitic to Rhyolitic lava

  • High viscosity
  • High gas content



posted by the geog bloggers at 11:26:00 pm

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