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VOLCANOES
volcanism - any activity which includes movement of magma toward or onto the earth’s surface
lava – molten rock at or above the surface
magma – molten rock below the surface
vent - the opening in the crust through which magma flows onto the surfacevolcano - the structure formed around the vent by layers of volcanic materials
Volcanoes occur mostly in areas near convergent and divergent plate boundaries.
oceanic plate subducts beneath a continental plate
seawater combines with the melting rock and mantle
violently explosive volcanoes
along the edges of the continents
examples: Andes, Cascades
Convergent Plate Boundaries oceanic crust and continental crust
Oceanic plates subduct below other oceanic plates
Chain of volcanoes produced behind the trench called island arcs
Example: Aleutian Islands, Alaska
Convergent Plate Boundaries oceanic crust and oceanic crust
Called mid-ocean ridges or rifts
Oceanic crust spreads apart
Creates new seafloor
Lots of magma comes to the surface here Undersea volcanoes can also be
present
Most go unnoticed, as they occur deep on the ocean floor
Divergent Plate Boundaries oceanic crust and oceanic crust
Mid-Ocean Ridge ExampleIceland
Diverging plates pull apart continents
Rift valleys are formed
have characteristic fissures (cracks in the crust that lava flows through
Example: Great Rift Valley, Africa
Divergent Plate Boundariescontinental crust and continental crust
East African Rift Valleycontinental crust and continental crust
Volcano formed from rift valleyMt.Kilamanjaro,Tanzania,Africa
HOT SPOTSStationary (non-moving)
areas of volcanism within the interiors of lithospheric plates
Plate above the hot spot continues to drift slowly
Plate carries the volcano on the surface away from the hot spot A new volcano begins to form over the hot
spot
Activity in the 1st volcano slows, then ceases.
Hot Spot Volcano ExampleHawaiian Islands
TYPES OF LAVA
Two general types
Depends on the chemical composition of the rocks and minerals
TYPE 1. FELSIC/ANDESITIC LAVAless dense
more viscous or thick/sticky (moves slowly)
lighter in color because it contains more silica (quartz is made of silica)
makes up most of the continental crust
TYPE 2. MAFIC/BASALTIC LAVA
denser
more fluidic
dark-colored
rich in Mg and Fe
forms the oceanic crust
TYPES OF MAFIC/BASALTIC LAVA
aa (pronounced ahh-ahh)
sharp, jagged surfaces
TYPES OF MAFIC/BASALTIC LAVA
pahoehoe (pronounced puh-hoy-hoy)
wrinkled, rope-like surface
VIOLENCE OF ERUPTIONSBasaltic/mafic lava
Very hot
Thin and fluidic
Gasses escape easily
“Quiet” eruptions
VIOLENCE OF ERUPTIONSAndesitic/felsic lava
Cooler flows
Thick (viscous) and “sticky”
H2O & CO2 gasses get trapped inside
Explosive and violent eruptions
LAVA TUBE - A LARGE PIPE FORMED FROM SUCH RAPIDLY COOLING BASALTIC LAVA THAT A HARDENED SHELL FORMS AROUND THE STILL-MOVING MOLTEN ROCK INTERIOR
DANGERS OF LAVA TUBESLooks like normal
ground on top, but can collapse when tourists walk over them
Poisonous gases can escape when lava tubes collapse
SHIELD VOLCANOES- HUGE VOLCANOES MADE OF BASALTIC/MAFIC LAVA, WITH BROAD BASES AND GENTLY SLOPING SIDES - THE HAWAIIAN ISLANDS
CINDER CONES - SMALL, STEEP-SIDED VOLCANOES MADE OF CINDERS AND OTHER PYROCLASTIC MATERIALS; THERE IS LITTLE LAVA FLOW, AND THEY OFTEN OCCUR IN GROUPS
STRATOVOLCANO OR COMPOSITE CONE - LARGE, SYMMETRICAL VOLCANO MADE OF ALTERNATING LAYERS OF VISCOUS ANDESITIC OR FELSIC LAVA AND PYROCLASTIC MATERIALS; THESE ARE VERY DANGEROUS VOLCANOES WITH VIOLENT ERUPTIONS
CRATER - FUNNEL-SHAPED PIT AT THE TOP OF A VOLCANIC VENT, FORMED WHEN VOLCANIC MATERIALS ARE BLOWN OUT OF THE TOP OF THE VENT
CALDERA - LARGE, BOWL-SHAPED DEPRESSION WHICH MAY FORM AT THE SITE OF A VOLCANO AFTER THE MAGMA CHAMBER BELOW EMPTIES OF MAGMA
CRATER LAKE IS A CALDERA WHICH FILLED WITH WATER AFTER THE ERUPTION AND SUBSEQUENT COLLAPSE OF MT. MAZAMA.
PYROCLASTIC MATERIALS
When viscous magma reaches the vent it will expand and release the gasses trapped inside. At this time a violent eruption occurs and lava is broken into pieces of all sizes.
INDIVIDUAL ERUPTIVE FRAGMENTS ARE CALLED PYROCLASTS ("FIRE FRAGMENTS").
TEPHRA (GREEK, FOR ASH) IS A GENERIC TERM FOR ANY AIRBORNE PYROCLASTIC ACCUMULATION. .
Volcanic Dust – pyroclastic material smaller than 0.25mm in diameter
VOLCANIC ASH - PYROCLASTIC MATERIAL SMALLER THAN 2 MM IN DIAMETER
LAPILLI (“LITTLE STONES”) - PYROCLASTIC MATERIAL LESS THAN 64 MM IN DIAMETER
VOLCANIC BOMBS - LARGE BLOBS OF LAVA THROWN OUT OF THE VENT WHICH COOL INTO A SPHERICAL OR OVAL SHAPE
VOLCANIC BLOCKS - HUGE PIECES OF SOLID ROCK BLASTED FROM THE VENT
FISSURES - CRACKS IN THE CRUST THROUGH WHICH LAVA FLOWS
PILLOW LAVA - UNDERSEA BASALTIC LAVA WHICH COOLS RAPIDLY INTO LARGE, ROUNDED BLOBS; ALSO KNOWN AS PILLOW BASALTS
LAHARS ARE VOLCANIC MUDFLOWS WHICH OCCUR WHEN ASH AND DEBRIS BECOME SATURATED WITH WATER AND FLOW DOWN THE VOLCANO’S SLOPES, USUALLY FOLLOWING STREAM VALLEYS.
OTHERS ARE TRIGGERED WHEN HEAT FROM THE VOLCANO MELTS SNOW AND ICE.
7:3 EXTRATERRESTRIAL VOLCANISM
INDICATIONS ARE THAT MANY OF THE PLANETS AND MOONS, INCLUDING OUR MOON, HAVE BEEN VOLCANICALLY ACTIVE IN THE PAST.
THE NEAR SIDE OF THE MOON IS COVERED WITH BASALTIC LAVA FLOWS WHICH FILLED IMPACT CRATERS.THESE ARE CALLED MARIA, AND SUGGEST THAT VOLCANISM WAS ONCE PRESENT ON THE MOON.
SCIENTISTS HAVE NO EXPLANATION FOR LUNAR VOLCANISM, AS THERE IS NO EVIDENCE OF PLATE TECTONICS OR CONVECTION CURRENTS TO SUPPLY THE HEAT REQUIRED FOR SUCH ACTIVITY.
PERHAPS THE HEAT MAY HAVE COME FROM A LONG PERIOD OF INTENSE METEORITE BOMBARDMENT.
IMAGES FROM SPACECRAFT HAVE SHOWN NUMEROUS VOLCANOES AND VOLCANIC FEATURES ON MARS.
THE LARGEST OF THESE IS A HUGE SHIELD VOLCANO, OLYMPUS MONS, WITH ITS SUMMIT THREE TIMES HIGHER THAN MT. EVEREST AND ITS BASE THE SIZE OF ARIZONA.
VIKING LANDING CRAFT HAVE DETECTED “MARSQUAKES”, SO THE PLANET MAY ALSO BE ACTIVE SEISMICALLY.
IO, A MOON OF JUPITER, IS THE MOST VOLCANICALLY ACTIVE BODY IN OUR SOLAR SYSTEM.
BECAUSE IO IS SO BRILLIANTLY COLORED, SCIENTISTS THINK THAT THE VOLCANIC MATERIAL BEING EJECTED FROM NINE KNOWN VOLCANOES IN UMBRELLA-SHAPED PLUMES IS PRIMARILY SULFUR AND SULFUR OXIDES.
EVERY MONTH, ABOUT THE SAME AMOUNT OF VOLCANIC MATERIAL RELEASED FROM THE 1980 MT. ST. HELENS ERUPTION, SPEWS OUT AND COVERS IO’S SURFACE.
Olympus Mons:
A Shield Volcano
on Mars
Earth’s Largest Volcano: Mauna Loa9 km high
120 km across
Olympus Mons:24 km high
550 km across
x 3 =
Mars ≈ 1/2 of Earth’s diameterOlympus Mons ≈ 3 times the size of Earth’s largest VolcanoHuge size disparity due to Mars’ stationary crust, henceenormous hot-spots that build-up in stationary positions
IT IS THOUGHT THAT AS IO IS PULLED BACK AND FORTH IN ITS ORBIT AROUND JUPITER BY JUPITER’S OTHER MOONS, ITS SURFACE MOVES UP AND DOWN, BUILDING UP HEAT FROM FRICTION.
THIS SURFACE MOVEMENT MAY RESULT IN THE MELTING OF THE INTERIOR AND VOLCANISM.
