Geol 11 Module 6B

• Volcano


  Geologic landform where molten rocks, gases and pyroclastic debris erupt through the earth's crust or has done so within the past several million years

  
• Volcanoes may be on land (subaerial) or underwater (subaqueous) such as in the seafloor or underneath lakes

  
• Volcanoes


  • Vary in morphology (shape, size) and eruption styles

  
• Factors that may affect why a volcanic eruption is triggered


  • Influx of new magma supply from a deeper source (possibly a bigger magma chamber)
  • Melting of surrounding country rocks
  • Exsolution of gasses (vesiculation and degassing) during the ascent of the magma to the surface
  • Sudden contact with water

  
• The distribution of active volcanoes is not random. Where magma forms, an active volcano is likely to be present

  
• Magma formation settings


  • Divergent boundaries or spreading centers (decompression melting)
  • Convergent boundaries (flux melting)
  • Mantle plumes/ hotspots (decompression melting and usually hot temperature)

  
• Magma chamber


  • Stores much of the magma as a mush of rocks and molten materials
  • Differs in size, shape, depth and many other characteristics
  • May be connected to more than one magma chamber or a small magma chamber at shallower depth is further connected to a larger magma chamber at depth
• Increased period of volcanic activity


  1. Magma migrates upward from the magma chamber into the vent or a conduit

  2. Magma may flow out into a small circular depression usually at the summit called crater following the central vent

  3. Magma may emerge in secondary sites away from the main crater and in other conduits such as those found at the side of the volcano resulting in a flank eruption

  4. Erupted material accumulates around the vent until it forms a hilly to mountainous appearance

  
• Shield Volcano


  • Largest among all volcanoes
  • Has a roughly circular or oval shape in map view
  • Made up of thin (up to tens of meters thick) but successive basaltic lava flows
  • Owing to its fluidity, the lavas can have high velocities allowing them to spread tens of kilometers away from its source
  • General slope is generally gentle (15o or less) especially close to the summit
  • Very little pyroclastic materials may be present, and most are concentrated in the vent where lava fountaining occurs

  
• Shield volcanoes


  • Mauna Kea
  • Mauna Loa

  
• Cinder Cones


  • Relatively small (usually <300 m high)
  • Made of ejected pyroclastic deposits (usually ash and scoria) from mildly explosive eruptions
  • Have very steep slopes (30 – 40o) due to the stacking of pyroclastic products as determined by the angle of repose
  • Internal layered structure forms due to varying intensities of the explosions that deposit different sizes of pyroclastics
  • Lava flows may also be emitted by these volcanoes in the later stages of eruption and usually in a side vent at the flank

  
• Cinder cones


  • Paricutin volcano
  • Taal volcano

  
• Composite/Stratovolcanoes


  • Have the most majestic shape among the different volcano types - the iconic near conical shape
  • Have relatively steeper slopes compared to shield volcanoes - around 6 to 10° low on the flanks to 30° near the top
  • Made up of alternating tephra (pyroclastic unconsolidated materials which can make up over 50% by volume) and lava flows ranging from andesitic to rhyolitic composition
  • The steep slope close to the summit is produced by the thick, short viscous lava flows that can only travel a short distance away from the vent while the gentler slopes are produced by the accumulation of pyroclastic materials and other volcanic products eroded from the higher elevations of the volcano

  
• Stratovolcanoes


  • Mt. Fuji
  • Mt. Mayon
• Composite volcanoes form at subduction zones, either on ocean-ocean convergent boundaries or ocean-continent convergent boundaries

  
• Shield volcanoes and cinder cones form in areas of continental rifting

  
• Shield volcanoes form above mantle plumes but can also form at other tectonic settings

  
• Sea-floor volcanism can take place at divergent boundaries, mantle plumes and ocean-ocean-convergent boundaries

  
• Lava domes


  • Roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano

  
• Lava domes


  • Composite dome which formed from 1980 to 1986 at the crater of Mt. St. Helens

  
• Calderas


  • Circular to elliptical large depressions with diameters ranging from 1 km to 50 km
  • Form as a result of collapse of a volcanic structure during large volcanic eruptions in stratovolcanoes when voluminous amount of magmas rapidly migrates upward to the surface

  
• Calderas


  • Crater lake Oregon
  • Taal caldera
• Fissures


  • Linear, elongated fractures where very fluid, basaltic lavas erupt
  • Lava fountains and lava curtains may be created in these fractures and may last for a few hours or days

  
• Volcanic Explosivity Index (VEI)


  • Measure of the eruption size (explosiveness)
  • Based on a number of things (e.g. plume height, volume, etc.) that can be observed during an eruption
  • Assigned VEI number uses a logarithmic scale - VEI 2 is 10x more massive compared to an eruption of VEI 1

  
• Volcanic eruptions

  • Pinatubo eruption in 1991 reached a VEI of 6
  • Largest eruption only reached up to VEI of 8 (Taupo Volcano, Yellowstone)

  
• Factors that determine the violence of eruption

  • Composition of magma
  • Temperature of magma
  • Dissolved gasses in magma
  • Viscosity
• Low-viscosity magmas like basalt, cause the rapid migration of magma to the surface. With this, formed gas bubbles as magma exsolves can escape quickly to the surface which produces non-explosive eruptions
• In felsic viscous magmas, high polymerization of silica limits flow movement and impedes the ascent of the gas bubbles. As gas continues to exsolve, more bubbles are prevented from rapidly escaping, thus increasing the overall pressure on the magma column until the gas ejects explosively from the volcano
• Effusive type (Non-explosive) eruption


  Magma rises through the surface and flows out of the volcano as a fluid material called lava. Gases can escape easily in this scenario

  
• Explosive type eruption


  Magma is torn apart as it rises and reaches the surface into pieces known as pyroclasts. Dissolved gases in the magma cannot escape as easily which leads to pressure build up and eventually leading to a violent eruption episode

  
• Hawaiian eruption


  • Characterized by the effusive emission of highly fluid basaltic lavas with low gas contents
  • Steady lava fountaining (may last for days) and the production of thin lava flows
  • Generally a non-explosive eruption type

  
• Icelandic eruption


  • Similar to Hawaiian style of eruption but the main difference is that in Hawaiian volcanoes, the greatest outpouring of lava is through the main vent rather than side fissures

  
• Easily which leads to pressure build up and eventually leading to a violent eruption episode

  
• Specific Types of Volcanic Eruption


  • Hawaiian
  • Icelandic
  • Strombolian
  • Pelean
  • Phreatic
  • Phreatomagmatic
  • Vulcanian
  • Plinian

  
• Hawaiian


  • Characterized by the effusive emission of highly fluid basaltic lavas with low gas contents
  • Steady lava fountaining (may last for days) and the production of thin lava flows

  
• Icelandic


  • Similar to Hawaiian style of eruption but the greatest outpouring of lava is through the main vent rather than through side fissures

  
• Strombolian


  • Intermittent, episodic, and short-lived (a few minutes to a few hours unlike Hawaiian), explosive outbursts of pasty (partially molten) lava sometimes accompanied by booming blasts
  • No sustained eruption column (cloud of eruption) is produced

  
• Pelean


  • Large quantity of pyroclastic materials is blown out of a central crater, fall back (instead of rising into a cloud), and form tongue-like, glowing avalanches (can be clearly visible at night) called nuee ardentes

  
• Phreatic


  • Caused by vaporization of surface/ground waters with no magma-water contact
  • Driven by steam produced by heating and expansion of groundwater due to an underlying hot source
  • Ash, and pyroclastic material (rock fragments derived from pre-existing rocks) are ejected by the volcano
  • Lava is not produced

  
• Phreatomagmatic


  • Generated by the interaction of magma with abundant surface water
  • The contact between magma and surface/ground water causes an explosion due to thermal contraction
  • Gases, ash, reworked rocks as well as newly formed rocks from the quick cooling of lava (termed juvenile fragments) are ejected by the volcano