Volcanoes

1
Volcanoes
2
(No Transcript)
3
volcano

• A volcano is a vent at the Earth's surface
  through which magma (molten rock) and associated
  gases erupt, and also the cone built by effusive
  and explosive eruptions
• The lifespan of a volcano can vary from months to
  several million years

4
volcano
5
volcano
6
volcano

• How they form
• Divergent boundaries (sea floor spreading)

7
volcano

• Convergent boundaries

8
volcano

• Scientists usually consider a volcano active if
  it is currently erupting or showing signs of
  unrest
• Many scientists also consider a volcano active if
  it has erupted in historic time
• Recorded history reaches back more than 3,000
  years but in the Pacific Northwest of the United
  States, it reaches back less than 300 years, and
  in Hawaii, little more than 200 years.

9
volcano

• Dormant volcanoes are those that are not
  currently active (as defined above), but could
  become restless or erupt again
• Extinct volcanoes are those that scientists
  consider unlikely to erupt again

10
Types of volcanic cones or structures

• caldera
• cinder cone
• hornito
• maar
• mud volcano
• shield volcano
• spatter cone
• stratovolcano
• volcanic dome

11
Shield volcano

• Volcanoes with broad, gentle slopes and built by
  the eruption of fluid basalt lava
• The largest volcanoes on Earth are shield
  volcanoes
• Basalt lava tends to build enormous, low-angle
  cones because it flows across the ground easily
  and can form lava tubes that enable lava to flow
  tens of kilometers from an erupting vent with
  very little cooling

12
Shield volcano
13
Stratovolcano

• Sometimes called composite volcanoes
• Steep, conical volcanoes built by the eruption of
  viscous lava flows, tephra, and pyroclastic flows
  
• may erupt a variety of magma types, including
  basalt, andesite, dacite, and rhyolite. All but
  basalt commonly generate highly explosive
  eruptions
• typically consists of many separate vents, some
  of which may have erupted cinder cones and domes
  on the volcano's flanks

14
Stratovolcano
15
Cinder cone

• a steep, conical hill of volcanic fragments that
  accumulate around and downwind from a vent
• range in size from tens to hundreds of meters
  tall
• usually erupt lava flows, either through a breach
  on one side of the crater or from a vent located
  on a flank

16
Cinder cone
17
eruption types

• Effusive eruption
• An eruption dominated by the outpouring of lava
  onto the ground
• aa
• pahoehoe
• obsidian
• pillow basalt

18
eruption types

• Aa - (pronounced "ah-ah") is a Hawaiian term
  for lava flows that have a rough rubbly surface
  composed of broken lava blocks called clinkers

19
eruption types

• Pahoehoe - Hawaiian term for basaltic lava that
  has a smooth, hummocky, or ropy surface

20
eruption types

• Obsidian - dense volcanic glass, usually rhyolite
  in composition and typically black in color
• often formed in rhyolite lava flows where the
  lava cools so fast that crystals do not have time
  to grow

21
eruption types

• Pillow lava - when basalts erupt underwater
  mounds of elongate lava "pillows" formed by
  repeated oozing and quenching of the hot basalt

22
eruption types

• Geyser - hot springs that episodically erupt
  fountains of scalding water and steam.
• Such eruptions occur as a consequence of
  groundwater being heated to its boiling
  temperature in a confined space (for example, a
  fracture or conduit)

23
eruption types

• Phreatic eruptions - steam-driven explosions that
  occur when water beneath the ground or on the
  surface is heated by magma, lava, hot rocks, or
  new volcanic deposits (for example, tephra and
  pyroclastic-flow deposits).

24
eruption types

• Phreatic eruptions

25
eruption types

• Lava fountain - a jet of lava sprayed into the
  air by the rapid formation and expansion of gas
  bubbles in the molten rock
• Lava fountains erupt from isolated vents, along
  fissures, within active lava lakes, and from a
  lava tube when water gains access to the tube in
  a confined space

26
eruption types

• Lava fountain

27
eruption types

• Lava lakes - large volumes of molten lava,
  usually basaltic, contained in a vent, crater, or
  broad depression
• can form (1) from one or more vents in a crater
  that erupts enough lava to partially fill the
  crater (2) when lava pours into a crater or
  broad depression and partially fills the crater
  and (3) atop a new vent that erupts lava
  continuously for a period of several weeks or
  more and slowly builds a crater higher and higher
  above the surrounding ground.

28
eruption types

• Lava lakes

29
eruption types

• pyroclastic flow - a ground-hugging avalanche of
  hot ash, pumice, rock fragments, and volcanic gas
  that rushes down the side of a volcano as fast as
  100 km/hour or more
• temperature within a pyroclastic flow may be
  greater than 500 C, sufficient to burn and
  carbonize wood
• the ash, pumice, and rock fragments may deform
  (flatten) and weld together because of the
  intense heat and the weight of the overlying
  material.

30
eruption types

• pyroclastic flow

31
eruption types

• Strombolian - characterized by the intermittent
  explosion or fountaining of basaltic lava from a
  single vent or crater
• caused by the release of volcanic gases, and they
  typically occur every few minutes or so,
  sometimes rhythmically and sometimes irregularly
• lava fragments generally consist of partially
  molten volcanic bombs that become rounded as they
  fly through the air.

32
eruption types

• Strombolian

33
eruption types

• vulcanian - explosive eruption that ejects new
  lava fragments that do not take on a rounded
  shape during their flight through the air
• moderate-sized explosive eruptions commonly eject
  a large proportion of volcanic ash and also
  breadcrust bombs and blocks

34
eruption types

• vulcanian

35
eruption types

• Plinian - large explosive events that form
  enormous dark columns of tephra and gas high into
  the stratosphere
• Such eruptions are named for Pliny the Younger,
  who carefully described the disastrous eruption
  of Vesuvius in 79 A.D - 2,000 were killed

36
eruption types

• Plinian

37
volcano

• fissure - a fracture or crack in rock along which
  there is a distinct separation an elongate
  fracture or crack at the surface from which lava
  erupts

38
volcano

• caldera - a large, usually circular depression at
  the summit of a volcano formed when magma is
  withdrawn or erupted from a shallow underground
  magma reservoir

39
volcano

• Vent - openings in the Earth's crust from which
  molten rock and volcanic gases escape onto the
  ground or into the atmosphere
• consist of a single circular-shaped structure, a
  large elongate fissure and fracture, or a tiny
  ground crack

40
volcano

• vent

41
volcano

• Volcanic ash - consists of rock, mineral, and
  volcanic glass fragments smaller than 2 mm

42
volcano

• fumaroles - vents from which volcanic gas escapes
  into the atmosphere
• may occur along tiny cracks or long fissures
• may persist for decades or centuries if they are
  above a persistent heat source or disappear
  within weeks to months if they occur atop a fresh
  volcanic deposit that quickly cools

43
volcano

• fumaroles

a fumarole on Kilauea Volcano
44
volcano

• Basalt - hard, black volcanic rock with less than
  about 52 weight percent silica (SiO2)
• it has a low viscosity (resistance to flow)
• basaltic lava can flow quickly and easily move

45
volcano

• Magma - molten or partially molten rock beneath
  the Earth's surface
• When magma erupts onto the surface, it is called
  lava
• Magma typically consists of (1) a liquid portion
  (often referred to as the melt) (2) a solid
  portion made of minerals that crystallized
  directly from the melt (3) solid rocks
  incorporated into the magma from along the
  conduit or reservoir, called xenoliths or
  inclusions and (4) dissolved gases.

46
volcano

• Magma types
• Dacite - light gray, but can be dark gray to
  black. Dacite lava consists of about 63 to 68
  percent silica (SiO2)
• erupts at temperatures between 800 and 1000C.
• Rhyolite - light-colored rock with silica (SiO2)
  content greater than about 68 weight percent
• erupted at temperatures of 700 to 850 C
• Andesite - gray to black volcanic rock with
  between about 52 and 63 weight percent silica
  (SiO2)
• erupt at temperatures between 900 and 1100 C.

47
volcano

• Silica A chemical combination of silicon and
  oxygen (SiO2).

48

• Dacite
• Andesite
• Rhyolite

49
volcano

• Magma

50
volcano

• Tephra - a general term for fragments of volcanic
  rock and lava regardless of size that are blasted
  into the air by explosions or carried upward by
  hot gases in eruption columns or lava fountains
• includes large dense blocks and bombs, and small
  light rock debris such as scoria, pumice,
  reticulite, and ash.

51
volcano

• Tephra
• Tephra erupted by Mount St. Helens on 18 May 1980
  ranging in size from ash (left 2 piles) to
  lapilli (right 2 piles).

52
volcano

• bombs - lava fragments that were ejected while
  viscous (partially molten) and larger than 64 mm
  in diameter
• Many acquire rounded aerodynamic shapes during
  their travel through the air.
• Volcanic bombs include breadcrust bombs, ribbon
  bombs, spindle bombs (with twisted ends),
  spheroidal bombs, and "cow-dung" bombs

53
volcano

• block - solid rock fragment greater than 64 mm in
  diameter that was ejected from a volcano during
  an explosive eruption
• commonly consist of solidified pieces of old lava
  flows that were part of a volcano's cone

54
volcano

• block

55
volcano

• Bombs
• These basaltic lava bombs were erupted by Mauna
  Kea Volcano, Hawaii.