Volcanoes II

From GeoClasses

Contents 1 Volcanoes 1.1 Anatomy of a volcano 1.1.1 Ring of Fire around the Pacific 1.2 types of volcanoes 1.2.1 shield volcano 1.2.2 composite or stratovolcano 1.2.3 volcanic domes 1.2.4 cinder cones 1.3 Origins of volcanoes 1.3.1 Mid-Ocean Ridge 1.3.2 hot spots 1.3.3 subduction zones 1.3.4 calculate the angle of subduction using distance to volcanoes 1.3.5 Other volcanic features: 1.4 hazards 1.4.1 lava flows 1.4.2 pyroclastic (explosive) activity 1.4.3 poisonous gases 1.4.4 debris flows and mud flows 1.5 Predicting eruptions

Volcanoes

Anatomy of a volcano

• magma vs. lava

eruption of Mount Rinjani, Indonesia: example of lava

lava lamp illustrating a mantle plume or magma chamber

Ring of Fire around the Pacific

types of volcanoes

• review of eruptions styles

shield volcano

• largest volcano, shaped like arc or shield
• usually islands in ocean
• nonexplosive = low SiO2 content in magma
• lava flows down sides = basalt

basaltic ropy lava flow (called pahoehoe)

basaltic blocky lava flow (called aa)

• may have tephra (ejected volcanic debris = ash, cinders, blocks, bombs)
  • tephra forms pyroclastic deposits or rocks
• may create lava tubes underground

lava tube at Craters of the Moon

large lava tube: Valentine Cave in Lava Beds National Monument, CA

• creates a summit caldera - collapse of summit with lava lake inside (Evil Dark Lord)

lava lake inside summit caldera of Mount Erebus, Antarctica

• • examples:

Mauna Loa, Hawaii

Olympus Mons on Mars, the largest volcano in the solar system, 3X higher than Mt Everest

composite or stratovolcano

• cone shaped
• intermediate SiO2 = more viscous = andesite
• made from alternating layers of explosive tephra and ash and lava flow
• steep slope forms from angle of repose of pyroclastic deposits

pyroclastic flow on Mount Merapi, Indonesia, 2006

pyroclastic flow down Mayon volcano in the Phillipines

• can make horizontal explosions like Mt St Helens
• examples: Mt St Helens, Mt. Rainier, Mt. Fuji in Japan

Mt Fuji in Japan

Mt St Helens before the 1980 eruption

Mt St Helens after the 1980 eruption (photo taken 1995)

volcanic domes

• high SiO2 content = rhyolite
• mostly explosives
• examples:

Novarupta, volcanic dome in Alaska

Mt Lassen, WA - one of the largest lava domes

cinder cones

• small volcanoes made of tephra (mostly ash and some bombs)
• sometimes found on flanks of larger volcanoes or along fissures
• examples:

Pu'u O'o cinder cone on Kilauea

Paricutin cinder cone in Mexico; formed over 8 years along a fissure; is now dormant and will not erupt again (characteristic of cinder cones)

• exercise on cinder cones on Earth, the moon, and Mars

Origins of volcanoes

Mid-Ocean Ridge

• basalt from asthenosphere
• if MOR occurs on an island then it makes a shield volcano (Iceland)

bathymetric map (topography underwater) showing the mid-ocean ridge as one continuous seam around the world's ocean

map of earthquakes, first published in 1954 by French seismologist J.P. Rothe, outlining the mid-ocean ridges. (Compare with the bathymetric map)

active geyser in Iceland, an island formed by volcanic activity of the mid-Atlantic ridge

hot spots

• areas within a tectonic plate where magma comes to the surface to produce volcanic activity
  • can be oceanic or continental
• examples:

satellite photo showing the Galapagos Islands in the Pacific Ocean

eruption of Sierra Negro on the Galapagos Islands, 2005

the Hawaiian Island chain, formed by the Pacific Plate moving over a hotspot, seen from space

• Exercise: use the Hawaian Island and Emperor Seamount Chain to determine plate motion and speed.

subduction zones

• creates composite volcanoes
  • rising magma mixes with oceanic crust (andesite)

cartoon showing subduction of oceanic crust under oceanic crust

example of oceanic-oceanic subduction: Japanese Archipelago seen from space

• • or continental crust (higher SiO2 content = rhyolite)
  • creates very explosive eruption, calderas, domes, cinder cones, or composite volcanoes

cartoon showing subduction of oceanic crust under continental crust

cartoon showing plate boundaries associated with the formation of the Cascade Range

the Cascade Range seen from space

calculate the angle of subduction using distance to volcanoes

Other volcanic features:

• flood basalts - huge horizontal accumulations of basalt lava flows that cover the land (ex: Columbia Plateau in Washington, created 17-14 mya)

map showing extent of the Columbia River flood basalt in relation to the Cascade Range

Palouse Canyon cutting through layers of Columbia River flood basalt

• hot springs and geysers - groundwater that is heated by hot rock (ex: Yellowstone)

geyser erupting in Iceland

macaques bathing in an open air hot spring in Nagano, Japan

• caldera explosions - huge eruptions 1000 times the size of Mt St Helens, have not happened in last few 100,000 years (ex: 600,000 years ago, Yellowstone; ex: 700,000 years ago, Long Valley, CA)

map showing trail of Yellowstone hotspot with ages marked in my ago

map showing last 3 caldera explosions in Yellowstone

view of the NE rim of the Yellowstone caldera (see in the background), with the Yellowstone River running in the foreground

hazards

lava flows

• 1. pahoehoe - smooth ropy surface, hotter basalt, moves 1 m/hr
• 2. aa - rough blocky surface, cooler more viscous basalt, moves slower - few meters/day
  • people have tried many methods to stop lava flows, but mixed success

pyroclastic (explosive) activity

• ash fall - can cover large expanses with thick blankets

ash plume from Mt Pinatubo eruption 1991

ash fall after Mt Pinatubo eruption 1991

• lateral blasts (horizontal)

Mt St Helens shortly after 1980 eruption, showing devastation from the lateral blast

• pyroclastic flows - avalanches of hot ash, rock, glass, gas - cannot be outrun
  • movie

Soufriere volcano on the island of Montserrat in the Caribbean, seen from space, showing pyroclastic flows going into the sea

poisonous gases

more about volcanic emissions

• CO2, CO (toxic), SO2 (toxic), H2S (toxic)
• short term = toxic to breathe; long term = creates acid rain

debris flows and mud flows

• lahars (Indonesian name): mudflows created when pyroclastics mix with water to make mudslides or lahars

mudline on riverbank after lahars associated with Mt St Helens eruption 1982

lahar from Mt St Helens eruption in 1982

• when volcanic eruptions melt ice and snow, sediment mixed with water
• areas near volcanoes in Cascades (Mt Rainer) are at risk and also Hawaiian Islands

Predicting eruptions

USGS movie about predicting earthquakes]

• seismic activity (earthquakes)
• underground monitoring
  • eruption preceeded by large amount of magma rising to surface - changes temperature (more melting of snow and ice in mountains), hydrologic (groundwater may heat)
• topographic monitoring (land swells as magma moves up)
• volcanic gases (composition and amount of gas escaping changes before eruption)