ACTIVE VOLCANOES CONCENTRATED AT PLATE BOUNDARIES
Iceland
St Helens
East
African
Rift
Different types of volcanoes at different tectonic settings
Japan,
Aleutians
St Helens,
Andes
East
African
Rift
Davidson 4.16
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
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“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
MICROSCOPE IMAGE
LARGE GRAINS
SMALL GRAINS
LARGE & SMALL GRAINS
Davidson 4.4
FRACTIONAL CRYSTALIZATION /PARTIAL MELTING
AS MAGMA COOLS SOLID MINERALS THAT "FREEZE" OUT DIFFER
IN COMPOSITION FROM REMAINING LIQUID
AS MAGMA COOLS
MINERALS FORM
DEPENDING ON
TEMPERATURE
("FRACTIONAL
CRYSTALIZATION")
REVERSE PROCESS
OCCURS DURING
MELTING ("PARTIAL
MELTING")
PROCESS
ILLUSTRATED WITH
HALF-FROZEN APPLE
JUICE
CRYSTALLIZATION OF MAGMA AS TEMPERATURE DROPS
CRYSTALS
SINK TO
BOTTOM
SiO2-POOR
MINERALS
FREEZE
OUT
MAGMA
BECOMES
MORE
SiO2-RICH
Evanston’s population “ages” over summer break
Davidson 4.6
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
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)
High viscosity magma- SiO2 rich
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Low viscosity magma- low SiO2
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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
AFTER
BEFORE
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.
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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
www.fs.fed.us/gpnf/volcanocams/msh
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
North
American
plate
20 mm/yr
Thingvellir
Eurasian
plate
1973 Eruption on the island of
Heimaey, 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”?
Assume hotspots
result from plumes of
hot material rising
from great depth,
perhaps core-mantle
boundary
HOTSPOT / PLUME HYPOTHESIS
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
BEND ?
ISLANDS GET OLDER
ALONG CHAIN
EVENTUALLY SUBSIDE
BELOW SEA
Davidson 7.14
HAWAIIAN-EMPEROR BEND - WHAT WE BELIEVED
2002 ERUPTION: BIG
ISLAND OF HAWAII
Brian White (CAS 2000)
Seth Stein
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http://hvo.wr.usgs.gov/cam/index.htm
YELLOWSTONE- NORTH AMERICAN HOTSPOT
ACTIVE VOLCANIC, SEISMIC, AND GEOTHERMAL REGION
YELLOWSTONE ERUPTIONS
FORM TRACE ACROSS SNAKE RIVER PLAIN
IN EXPECTED PLATE MOTION DIRECTION
YELLOWSTONELARGE 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 PROBLEMS
UNCLEAR 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
SOLIDUS - MELTING CURVE
GEOTHERM TEMPERATURE
vs DEPTH
MIDOCEAN RIDGE
MELTING AT LOWER PRESSURE
Davidson 4.3
SUBDUCTION ZONE
WATER LOWERS MELTING
TEMPERATURE
MARS: Olympus Mons
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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
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
LESS DENSE
IGNEOUS
ROCKS classified
by chemical
composition &
cooling rate
Felsic- more
FELdspar and
SIlica
Mafic- more
Magnesium and
iron (Fe)
---------------------Texture reflects
cooling:
MORE DENSE
LESS SiO2
SiO2
(Mg,Fe)2SiO4
Extrusive/Fine
Grain
Intrusive/Coarse
Grain
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