Hazards Ch.5 Volcanoes

Vent

opening where lava is ejected

fissures

circular or elongated vent

craters

decompressions at top of volcano caused by explosion or collapse

Calderas

large craters formed from violent collapse of a cone

crater size

~1 mile

caldera size

~ 12+ miles

magma

molten rock BELOW surface of volcano

Lava

molton rock ABOVE surface of volcano

Cause of volcanos

Active plate boundries, hot spots

where does magma form

asthenosphere

where does magma melt

lithosphere

magma generating processes (3)

decompression melting, addition of volatiles, addition of heat

Decompression melting

melting of mantle material that occurs when material rises into a region of lower pressure.

where does most lava extrude from

2/3 mid ocean ridges

addition of volatiles

lower melting temperature of rock

what chemicals are in volatiles (3)

water, carbon dioxide, sulfur dioxide

Volatiles example

salt melts ice below freezing point (32F-17F)

What tectonic setting where volatiles occur

continental-ocean plate boundries

addition of heat

melting occurs if temperature exceeds melt temperature of rock at that depth, making magma rise through mantle and crust releasing heat allong the way

assimilation

cooler, shallow rock melts and mixes with rising magma

Where does addition of heat occur

anywhere where magma is moving upwards

composition of magma/lava

melted silicate rocks and dissolved gasses

types of silica content (3)

basaltic, andesitic, rhyolitic

where is basaltic found

ocean islands (40-55% silica)

where does andesitic found

island and volcanic arcs (55-65% Silica)

where is rhyolitic found

continental volcanoes (>65% silica)

which magma is most common

basaltic

how does basaltic form

by melting of asthenosphere

basaltic composition

mafic

what does cooled basaltic lava form

basalt

rhyolitic composition

felsic

what does cooled rhyolitic form

rhyolite

andesitic composition

intermediate

what does cooled andesitic form

andesite

hottest to coolest magma type

basaltic—> andesitic—>rhyolitic

how are rhyolitic and andesitic formed

magma rises from asthenosphere, chemical changes due to cooling and crustal assimilation, becomes for felsic over time (magma evolution)

what is the reate of magma evolution dependant on

amount of volatiles(more volitiles-more evolution), how fast magma rises through lithosphere (slow rise=more evolution)

viscosity affects (3)

flow of lava, eruption style, shape of volcano

volatile content determines

how explosive an eruption is

viscosity

resistance to flow in a fluid

high viscosity

high resistance (molasses)

low viscosity

low resistance (water)

what determines viscosity

silica content (high silica=high viscosity), temperature (high temp=low viscosity)

types of magma from most to least viscosity

rhyolitic—>andesitic—> basaltic

volatility

decompression at top of volcano causing explosion

high volatile

explosive

rapid decompression result

explosive release of volitiles

low volatile

nonexplosive erruption

most explosive to least lava types

rhyolite—>andesite—>basaltic

effusive eruption

outpouring of low viscosity lava

pyroclastic debris

fragments, gas, and more explode violently high into atmosphere

Volcanic Explosivity Index (VEI)

relative scale used to compare explosiceity between different volcanoes, based on quantitative/qualitative observation

VEI Factors

height of erruption plume, volume of ejected material, overall violence of eruption

Volcanic Structue Types (5)

Composite, lava domes, shield,cinder cones, continental calderas

Other name for composite volcanoes

stratovolcanoes

composite volcanoes magma evolution

basalt to rhyolite

composite volcanoes viscosity

high viscosity, doesnt flow far and causes steep sides

composite volcanoes volatility

high volatile content, high water content

composite volcanoes examples (3)

mount st. helens, mount rainer, mount fuji

composite volcanoes eruption content

lava and pyroclastic deposits

how far do composite volcanoes send pyroclastics

>20 miles high

what is the most deadly/destructive volcano type

composite volcanoes

lava dome size

small

lava dome magma evolution

andesitic to rhyolitic

lava dome viscosity

low to moderate viscosity

are lava dome effusive or explosive

effusive

how can lava dome create a explosive eruption

lava dome collapse

how do lava domes form

in the crater after a large eruption of a stratovolcano

lava dome example

mount unzen, Japan

shield volcanoes type of magma

basaltic

shield volcanoes viscosity

low viscosity

shield volcanoes volatility

low volatility

how do shield volcanoes become explosivewhen they come in contact with water

how do shield volcanoes form

decompression melting

types of shield volcanoes

hawaiian and icelandic

Hawaiian shield volcanoes size

largest volcanoes in the world

Hawaiian shield volcanoes slopes

<10 degrees (very flat everythings under ground

what can Hawaiian shield volcanoes form

lava tubes

what do Hawaiian shield volcanoes resemble

warriors shield

Hawaiian shield volcanoes example

mauna loa, hawaii

lava tubes

magma travelling for miles below surface

how do lava tubes stay hot

rock walls insulate tubes

what happens when erruption ends

lava tubes drain and leaves behind cavern

Icelandic shield volcanoes explosive or effusive

effusive

Icelandic shield volcanoes vent

no central vent, eruption comes from long fissure

Icelandic shield volcanoes nick name

curtain of fire

Icelandic shield volcanoes produce

flood basalts

flood basalts

massive outpouring of lave from fissures

flood basalts examples

snake river idaho, columbia plateau flood basalts (both cover ~50000mi

Cinder cones magma

basalt and small peices of cinder

cinder

small peices of vesicular black or red lava

Cinder cones volatility

intermediate volatility formed when lava meets ground water

where do Cinder cones usually appear

on flanks of larger volcanoes along normal faults and along cracks and fissures

tephra

volcanic rock around vent

cinder cone vent

accumulation of tephra around vent

1943 paricutin mexico

lava flow overran nearby village killing many crops and livestock, no human deaths

continental calderas form when

magma chamber partially collapses leading to collapse of land/volcano above

most violent erruptions

continental calderas

continental calderas volatility

very volatile

rarity of continental calderas

10 occured in the past 1 million years