VOLCANOES

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VOLCANOES!!

• CHAPTER 9

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VOLCANO

• WHAT IS A VOLCANO?
• THE OPENING IN THE EARTHS CRUST THROUGH WHICH
  MOLTEN ROCK, GASES, AND ASH ERUPT AND THE
  LANDFORM THAT DEVELOPS AROUND THE OPENING
• VOLCANOES FORM WHERE MAGMA REACHES THE SURFACE OF
  THE EARTH

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HOW AND WHERE VOLCANOES FORM

• A VOLCANIC ERUPTION OCCURS WHEN MAGMA RISES TO
  THE SURFACE
• THE VOLCANO FORMS WHERE THE MELTING TEMPS AND
  PRESSURES ARE FAVORABLE IN THE EARTHS
  ASTHENOSPHERE

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CONDITIONS THAT FAVOR VOLCANOES

• 3 CONDITIONS MUST BE MET TO ALLOW MAGMA TO FORM
• 1. A DECREASE IN PRESSURE LOWERS THE MELTING
  POINT OF MATERIALS IN THE ASTHENOSPHERE
• 2. AN INCREASE IN TEMPS CAUSES THE MATERIALS TO
  MELT
• 3. AN INCREASE IN THE AMOUNT OF WATER IN THE
  ASTHENOSPHERE LOWERS THE MELTING POINT

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WHERE ARE VOLCANOES?

• VOLCANOES ARE FOUND ALONG RIFT VALLEYS
• ALONG SUBDUCTION BOUNDARIES
• OCEAN TO OCEAN
• VOLCANIC ISLAND ARCS FORM
• OCEAN TO CONTINENT
• MOUNTAIN CHAINS OF VOLCANOES FORM ON THE CONTINENT

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DIVERGENT BOUNDARY

• BELOW A RIFT THE PRESSURE IS LOWER
• THIS LOWERS THE MELTING POINT OF THE ROCK
• COMBINATION OF HIGH TEMPS AND LOWER MELTING
  POINTS CAUSES LOTS OF MAGMA TO FORM
• RISES TO THE SURFACE AT THE RIFT VALLEY

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DIVERGENT BOUNDARY

• MOST OF THE MAGMA THAT REACHES THE SURFACE OF THE
  EARTH DOES SO AT DIVERGENT PLATE BOUNDARIES
• ICELAND IS A PLACE WHERE A DIVERGENT BOUNDARY
  RISES ABOVE SEA LEVEL

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HOT SPOTS

• AN AREA ON THE EARTHS SURFACE WHERE VOLCANIC
  ACTIVITY RESULTS FROM PLUMES OF HOT SOLID
  MATERIAL RISING FROM DEEP WITHIN THE EARTHS
  MANTLE
• EXAMPLE HAWAIIAN ISLANDS
• A HOT SPOT REMAINS IN THE SAME PLACE IN THE
  MANTLE AS THE PLATE MOVES OVER IT

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HAWAIIAN ISLANDS

• THE ISLANDS WERE CREATED BY A HOT SPOT LOCATED
  UNDER THE PACIFIC OCEAN
• THE PACIFIC PLATE MOVED IN A NORTHWESTERLY
  DIRECTION OVER THE HOT SPOT CREATING THE CHAIN OF
  ISLANDS
• THE BIG ISLAND OF HAWAII NOW SITS OVER THE HOT
  SPOT

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HAWAIIAN ISLANDS
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TYPES OF MAGMA

• BASALTIC MAGMA
• 50 SILICA
• LITTLE GAS CONTENT
• LEAST VISCOUS
• ANDESITIC MAGMA
• 60 SILICA
• INTERMEDIATE GAS AND VISCOSITY
• RHYOLITIC MAGMA
• 70 SILICA
• MOST VISCOUS AND GAS CONTENT

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MAGMA

• VISCOSITY REFERS TO THE MAGMAS RESISTANCE TO
  FLOW
• MAGMAS HIGH IN SILICA RESIST FLOWING AND MOVE
  SLOWER
• MAGMAS LOW IN SILICA FLOW MORE EASILY
• BASALTIC MAGMA FLOWS VERY EASILY
• ANDESITIC AND RHYOLITIC MAGMAS DONT FLOW VERY
  EASILY

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MAGMA

• BASALTIC MAGMAS ARE RARELY EXPLOSIVE AND FORM AT
  RIFTS AND OCEANIC HOT SPOTS
• ANDESITIC MAGMAS ARE SOMETIMES EXPLOSIVE AND FORM
  AT SUBDUCTION BOUNDARIES
• RHYOLITIC MAGMAS ARE USUALLY EXPLOSIVE AND FORM
  AT CONTINENTAL HOT SPOTS

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LAVA FLOWS ON LAND

• BASALTIC LAVA FLOWS
• TWO TYPES
• 1. PAHOEHOE
• FLOWS FROM HIGH TEMP LAVA COOLS IN SMOOTH ROPE
  LIKE SURFACES
• 2. AA
• FLOWS FROM COOLER BASALTIC LAVA AND COOLS IN
  ROUGH JAGGED SURFACES

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ASH AND ROCK FRAGMENTS

• IN EXPLOSIVE ERUPTIONS
• PYROCLASTIC MATERIAL
• SOLID FRAGMENTS THAT ARE EJECTED DURING AN
  ERUPTION
• COMBINES WITH FLUID LAVA
• PYROCLASTIC FLOWS WHEN DENSE SUPERHEATED
  CLOUDS OF GAS AND ROCK FRAGMENTS TRAVEL DOWNHILL
• CAN REACH SPEEDS OF MORE THAN 100KM PER HOUR

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ASH AND ROCK FRAGMENTS

• CLASSIFICATION OF PYROCLASTIC MATERIALS
• 1. ASH
• 2. LAPILLI
• 3. BLOCK OR BOMBS

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VOLCANIC LANDFORMS

• SHIELD VOLCANOES
• CINDER CONES
• COMPOSITE VOLCANOES
• CALDERAS
• LAVA PLATEAUS

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HOW VOLCANOES WORK

• http//www.geology.sdsu.edu/how_volcanoes_work/Hom
  e.html

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SHIELD VOLCANO

• BECAUSE OF ITS LOW VISCOSITY THE LAVA TENDS TO
  FLOW A LONG DISTANCE BEFORE IT COOLS AND HARDENS
• IT BUILDS UP IN LAYERS FORMING A MOUNTAIN WITH A
  BROAD BASE AND GENTLY SLOPING SIDES
• CAN SUPPORT VERY HIGH MOUNTAINS
• EXAMPLE MAUNA LOA TOTAL HEIGHT 9170 METERS,
  5000 METERS BELOW SEA LEVEL

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CINDER CONE VOLCANO

• SIMPLEST TYPE OF VOLCANO
• FORMS WHEN LAVA IS THROWN INTO THE AIR FROM A
  VENT
• FALLS, BREAKS INTO FRAGMENTS BEFORE HITTING THE
  GROUND
• FRAGMENTS ACCUMULATE FORMING A CONE SHAPED
  MOUNTAIN
• THESE TYPES USUALLY FORM IN GROUPS NEXT TO LARGER
  VOLCANOES

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COMPOSITE VOLCANOES

• DEVELOP WHEN LAYERS OF MATERIALS ACCUMULATE AFTER
  EACH ERUPTION
• ALSO CALLED STRATOVOLCANO
• MAGMA, WATER AND GASES EXPLODE VIOLENTLY INTO A
  MASSIVE CLOUD OF SUPERHEATED ASH AND STONES
• A LAHAR FORMS WHEN HOT ASH MIXES WITH SNOW AND
  ICE TO FORM A FAST MOVING MUD FLOW

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CALDERAS

• MAGMA CHAMBERS UNDERGROUND THAT ARE PARTIALLY
  EMPTY COLLAPSE
• THE TOP OF THE VOLCANO COLLAPSES INWARD FORMING A
  BASIN
• THIS SOMETIMES FILLS WITH WATER TO FORM LAKES
  EX CRATER LAKE

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CALDERAS

• YELLOWSTONE NATIONAL PARK IS AN ACTIVE CALDERA
• MAGMA SUPERHEATS THE WATER UNDER THE SURFACE OF
  THE EARTH
• THIS IS THE REASON FOR THE GEYSERS AND HOT
  SPRINGS
• THE ERUPTION THAT PRODUCED YELLOWSTONE WAS 1000
  TIMES MORE POWERFUL THAN THE ST. HELENS 1980
  ERUPTION

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LAVA PLATEAUS

• BASALTIC LAVA POURING FROM FISSURES SPREAD ACROSS
  THE LAND FORMING PLATEAUS
• THIS OCCURS AT CRACKS IN THE PLATES OF THE EARTH
• COLUMBIA PLATEAU IN NORTHWEST U.S. IS 164,000 SQ.
  KM IN AREA

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EXTRA-TERRESTRIAL VOLCANOES

• MOON BASALTIC LAVA FLOWS FORMED THE MARIAS
  (DARK AREAS OF THE MOON)
• MARS OLYMPUS MONS, THE LARGEST VOLCANO IN THE
  SOLAR SYSTEM
• 600KM AT THE BASE AND 27KM TALL
• VENUS 1600 ACTIVE VOLCANOES
• IO MOST VOLCANICALLY ACTIVE PLACES IN THE SOLAR
  SYSTEM