ACTIVE VOLCANOES CONCENTRATED AT PLATE BOUNDARIES St Helens Iceland East African Rift.
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Transcript of ACTIVE VOLCANOES CONCENTRATED AT PLATE BOUNDARIES St Helens Iceland East African Rift.
ACTIVE VOLCANOES CONCENTRATED AT PLATE BOUNDARIES
St Helens
Iceland
East African Rift
Different types of volcanoes at different tectonic settings
Davidson 4.16
Japan,Aleutians
East African Rift
St Helens,Andes
IGNEOUS ROCKS
SILICATE MATERIAL (WITH GASSES) MELTS AT DEPTH
MELTED MAGMA LESS DENSE THAN SURROUNDING SOLID SO RISES
INTRUSIVE ROCK: SOLIDIFIES ("CRYSTALLIZES”/"FREEZES") AT DEPTH
EXTRUSIVE ROCK: LAVA ERUPTS AT SURFACE & FORMS.
COOLING LAVA RELEASES GASSES INTO ATMOSPHERE
IGNEOUS ROCKS CLASSIFIED BY THEIR CHEMICAL COMPOSITION (HOW MUCH SiO2 ), MINERALS, AND INTRUSIVE OR EXTRUSIVE
QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.“every rock and tree and creature has a life, has a spirit, has a name…”
THREE IMPORTANT PAIRS OF IGNEOUS ROCKS
RHYOLITE / GRANITE (>63% SiO2): MOST OF CONTINENTAL CRUST, DENSITY ABOUT 2.8 g/cm3 , FORMS BY MELTING CONTINENTAL CRUST
ANDESITE */ DIORITE (63-52% SiO2) FROM MELTING OCEANIC CRUST, OFTEN AT SUBDUCTION ZONES
BASALT / GABBRO (<52% SiO2) MOST OF OCEANIC CRUST, DENSITY ABOUT 3.3 g/cm3, FROM MELTING MANTLE, OFTEN AT MIDOCEAN RIDGES
BECAUSE GRANITE IS LESS DENSE THAN BASALT:
CONTINENTS "FLOAT" HIGHER THAN THE OCEAN CRUST SO WE LIVE ABOVE SEA LEVEL
CONTINENTS NEVER SUBDUCT BACK INTO THE MANTLE SO FORMED EARLY IN EARTH HISTORY & ARE OLD (LESS THAN 500 MYR TO 4 BYR), COMPARED TO OCEANIC CRUST THAT SUBDUCTS SO IS YOUNG (0-200 MYR)
*LIKE ANDES
Igneous rock textures formed primarily by cooling rate
LARGE GRAINS SMALL GRAINS LARGE & SMALL GRAINS
MICROSCOPE IMAGE
Davidson 4.4
AS MAGMA COOLS MINERALS FORMDEPENDING ON TEMPERATURE ("FRACTIONAL CRYSTALIZATION")
REVERSE PROCESS OCCURS DURING MELTING ("PARTIAL MELTING")
PROCESS ILLUSTRATED WITH HALF-FROZEN APPLE JUICE
FRACTIONAL CRYSTALIZATION /PARTIAL MELTINGAS MAGMA COOLS SOLID MINERALS THAT "FREEZE" OUT DIFFER IN COMPOSITION FROM REMAINING LIQUID
CRYSTALLIZATION OF MAGMA AS TEMPERATURE DROPS
Davidson 4.6
CRYSTALS SINK TO BOTTOM
SiO2-POOR MINERALS FREEZE OUT
MAGMA BECOMES MORE SiO2-RICH
Evanston’s population “ages” over summer break
Higher temperature, lower viscosity (warm syrup flows more easily than cold)
Viscosity increases with increasing silica content due to silica chains
High viscosity lavas flow slowly and typically cover small areas. Low viscosity magmas flow more rapidly and cover thousands of square km.
Low viscosity magmas allow gases to escape easily.
Gas pressures can build up in high viscosity magmas - so violent eruptions (Blowing through a straw, it's easier to get water to bubble than a milk shake)
MAGMA VISCOSITY- VOLCANIC ERUPTION CHARACTERISTICS LARGELY CONTROLLED BY THE VISCOSITY - "GOOEYNESS" (RESISTANCE TO FLOW) - OF THE MAGMA: LOW VISCOSITY FLUIDS FLOW MORE EASILY THAN HIGH VISCOSITY FLUIDS
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High viscosity magma- SiO2 rich
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Low viscosity magma- low SiO2
MAGMA VISCOSITY & VOLCANO TYPE
High viscosity lavas flow slowly & typically cover small areas, forming composite volcanoes (stratovolcanoes) (e.g. Mt. St. Helens) that explode violently due to trapped gas
Low viscosity lavas flow rapidly & form shield volcanoes (e.g. Hawaii) with flows covering thousands of square kilometers
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Mount Saint Helens- stratovolcano (composite volcano) with viscous dacitic (SiO2-rich) magma containing lots of dissolved gas (mostly water vapor), before 1980 explosive eruption, after, & today
BEFORE
AFTER
pyroclastic flows
high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.
A pyroclastic flow will destroy nearly everything in its path. With rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than 80 km per hour, pyroclastic flows knock down, shatter, bury or carry away nearly all objects and structures in their way. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C, can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses.
www.fs.fed.us/gpnf/volcanocams/msh
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Mount St. Helens as part of the new dome collapses. USDA Forest Service photograph by taken moments after a 3.2 magnitude earthquake triggered the event
on July 18, 2005
Lahar
An Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter.
Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second--much too fast for people to outrun.
ICELAND - Part of the Mid-Atlantic Ridge
- Made of Recently Erupted Basalt
- Formed in past 15 million years
Thingvellir
Eurasian plate
North American plate
20 mm/yr
1973 Eruption on the island ofHeimaey, Iceland
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East Africa Rift - New Spreading Center Forms by
Rifting Continental Crust
Africa is splitting up into Nubia (West Africa) and Somalia (East Africa)
2001 Eruption near Goma, Congo
Along the East African Rift
CHAINS OF ISLANDS & SEAMOUNTS
ACTIVE VOLCANISM NOT ASSOCIATED WITH SPREADING RIDGES
WHAT FORMS THESE “HOTSPOTS”?
HOTSPOT / PLUME HYPOTHESISAssume hotspots result from plumes of hot material rising from great depth, perhaps core-mantle boundary
Plumes would be secondary convection mode, ~ 5% of heat transfer
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LINEAR VOLCANIC CHAINS THOUGHT TO BE DUE TO PLATE MOTION
OVER A FIXED OR SLOWLY MOVING
HOTSPOT
ISLANDS GET OLDER ALONG CHAIN
EVENTUALLY SUBSIDE BELOW SEA
Davidson 7.14
BEND ?BEND ?
HAWAIIAN-EMPEROR BEND - WHAT WE BELIEVED
2002 ERUPTION: BIG ISLAND OF HAWAII
Brian White (CAS 2000)Seth Stein
http://hvo.wr.usgs.gov/cam/index.htm
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YELLOWSTONE- NORTH AMERICAN HOTSPOTACTIVE VOLCANIC, SEISMIC, AND GEOTHERMAL REGION
YELLOWSTONE ERUPTIONS FORM TRACE ACROSS SNAKE RIVER PLAININ EXPECTED PLATE MOTION DIRECTION
YELLOWSTONE-LARGE ERUPTION --->HUGE CALDERA 0.6 MILLION YEARS AGO
EARLIER ERUPTIONS FORM TRACE ACROSS SNAKE RIVER PLAIN
BUT, YELLOWSTONE SEISMIC TOMOGRAPHY PROBLEM
LOW VELOCITY ANOMALY (PRESUMABLY ASSOCIATED WITH HOT UPWELLING)
ONLY IN UPPERMOST MANTLE
MANTLE PLUMES?
NEAT IDEA BUT MANY PROBLEMSUNCLEAR WHETHER HOTSPOTS REFLECT
- HOT PLUMES FROM GREAT DEPTH (CORE-MANTLE BOUNDARY)- LOCALIZED UPPER MANTLE INTRAPLATE VOLCANISM
- OR SOME ARE ONE AND SOME THE OTHER?
The two main ways in which melting occurs in the mantle
MELTING AT LOWER PRESSUREWATER LOWERS MELTING
TEMPERATURE
MIDOCEAN RIDGE SUBDUCTION ZONE
GEOTHERM - TEMPERATURE vs DEPTH
SOLIDUS - MELTING CURVE
Davidson 4.3
Stands 27 kilometres (88,600 feet) high over its base (about three times the height of Everest above sea level)
Caldera is 85 km (53 miles) long, 60 km (37 miles) wide, and up to 3 km (1.8 miles) deep with six overlapping pit craters.
Outer edge is defined by an escarpment up to 6 km (4 miles) tall unique among the shield volcanoes of Mars.
Olympus Mons is roughly the size of the state of Missouri
MARS: Olympus Mons
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What type of volcano is Mt. Doom (Orodruin) in the “Lord of the Rings”?
3 MAIN ROCK TYPES: IGNEOUS, SEDIMENTARY, &
METAMORPHIC
IGNEOUS- COOLS FROM MOLTEN ROCK
SEDIMENTARY- RECOMBINED FRAGMENTS OF MINERALS, ROCKS, &/OR ORGANIC ORIGIN
METAMORPHIC- PRE-EXISTING ROCKS CHANGED IN COMPOSITION, MINERALOGY, OR TEXTURE FROM HIGH TEMPERATURES, PRESSURES, &/OR FLUIDS
IGNEOUS ROCKS classified by chemical composition & cooling rate
Felsic- more FELdspar and SIlica
Mafic- more Magnesium and iron (Fe) ----------------------Texture reflects cooling:
Extrusive/Fine Grain
Intrusive/Coarse Grain
LESS DENSE MORE DENSE
LESS SiO2
SiO2
(Mg,Fe)2SiO4
Davidson 4.4
SINGLE CRYSTAL GROWING FOR INTEGRATED CIRCUIT (IC or MICROCHIP) PRODUCTION
ICs are built on single-crystal silicon substrates of high purity and perfection. Single-crystal silicon is used instead of polycrystalline silicon since the former does not have defects associated with grain boundaries.
- Silicon inside the chamber is melted (Si melts at 1421 deg C).
- A slim seed of crystal silicon (5 mm dia. and 100-300 mm long) is introduced into the molten silicon.
- The seed crystal is withdrawn at a very controlled rate, and grows.
- Wafers are sliced off the crystal and circuits built on them
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http://science.howstuffworks.com/oil-refining2.htm
Successive stages of development of Crater Lake, Oregon
Davidson 4.15