Download Section 9.2

Section 9.2
Volcanic Eruptions
“I was walking along a path
with two friends – the sun
was setting - suddenly the
sky turned blood red I paused, feeling exhausted,
and leaned on the fence there was blood and tongues
of fire above the blue-black
fjord and the city my friends walked on, and I
stood there trembling with
anxiety - and I sensed an
infinite scream passing
through nature."
Edvard Munch
An agonized figure wails against a blood red Norwegian
skyline in Edvard Munch's “The Scream” (1893) in the
National Gallery, Oslo, Norway
The 1883 eruption of the volcano Krakatoa in
Indonesia put so much ash into the atmosphere,
it caused unusually intense sunsets throughout
Europe in the winter of 1883-4, which Munch
captured in his painting.
What FACTORS determine if a volcano
erupts violently or quietly?
Quiet eruptions Hawaii & Iceland
Pahoa Lava Flow
https://www.youtube.com/watch?v=fmCJS
S2YAP0
Explosive,
violent
eruption
Krakatoa
eruption
2007
Destructive landslide
associated with 1980
Mt. St. Helen's
eruption
Before the
1980 eruption
After the VIOLENT
eruption
The summit and side
has been blown away.
Bell-work
Pyroclastics consist of ……
Ash
Cinders
Lapilli
Glass
Pumice/Obsidian
Bombs
Blocks
Tuff (welded ash)
Affiliated with Violent Explosions
Lahar: is a type of mudflow or debris
dense, destructive mass flow composed of a slurry of
pyroclastic material, rocky debris, and
of very hot ash, lava
water. The material flows down from a
fragments, and gases
ejected explosively volcano, typically along a river valley.
(Volcano peaks often have ice and
from a volcano and
snow that instantly melt when met
typically flowing
with the heat of an erupting volcano.)
downslope at great
speed.
PYROCLASTIC FLOW: a
Factors that affect volcanic eruptions:
Pg 121
Felsic magma
Feldspar, silica
Mafec* magma
magnesium and iron
FELSIC magma
•
•
•
•
Light colored minerals:
Quartz, orthoclase Feldspar
More than 60% silica
Lower melting point= colder
temperatures (“thick”)
• HIGH viscosity
MAFIC magma
• Dark colored minerals:
• Magnesium and iron Fe
• Olivine, pyroxene,
Hornblende, augite, biotite
• Less than 50% silica
• Higher melting point= hotter
temperatures (“thin”)
• LOW viscosity
So thick it acts
like a “plug” to
the volcano…
“Thin” enough that it
allows the magma to
move more freely…
Volatiles
• refers to the volatile components of magma
[mostly water vapor and carbon dioxide] that
affect the appearance and explosivity of
volcanoes.
• Volatiles in a magma with a high viscosity,
generally felsic with a higher silica (SiO2)
content, tend to produce eruptions that are
explosive.
• Volatiles in a magma with a low viscosity,
generally mafic with a lower silica content, tend
to vent and can give rise to a lava fountain.
Volatiles Continued:
the effect of trapped gases
• How explosive a volcano will be is
dependent on the gases present as well
– How much gas is present
• If a lot of gas is present the volcano is more likely
to be more explosive
– How easily the gas can escape
• If the gases cannot escape easily, the volcano is
more likely to be more explosive
What happens to bring magma
to the surface?
• How does the type of magma affect the
surrounding pressure? (remember gas is a
volatile that affects the reactivity of
magma)
• Volcanic eruptions and what causes them
• Build up of pressure occurs more
frequently with thicker magma (felsic.)…..
EXPLOSIVE ERUPTION
Volcanic Explosivity Index
How to measure a volcanic eruption
VEI Description
Plume Height Volume
Classification
How often
Example
daily
Kilauea
0
non-explosive <100 m
1000s m3
Hawaiian
1
gentle
100-1000 m
10,000s m3
Haw/Strombolian daily
Stromboli
2
explosive
1-5 km
1,000,000s m3
Strom/Vulcanian weekly
Galeras, 1992
3
severe
3-15 km
10,000,000s m3 Vulcanian
yearly
Ruiz, 1985
4
cataclysmic
10-25 km
100,000,000s m3 Vulc/Plinian
10's of years
Galunggung, 1982
5
paroxysmal
>25 km
1 km3
Plinian
100's of years
St. Helens, 1981
6
colossal
>25 km
10s km3
Plin/Ultra-Plinian 100's of years
Krakatau, 1883
7
super-colossal >25 km
100s km3
Ultra-Plinian
1000's of years
Tambora, 1815
8
mega-colossal >25 km
1,000s km3
Ultra-Plinian
10,000's of years Yellowstone, 2 Ma
Paroxysmal
• a sudden outburst of a violent eruption
• an increase of periodic eruptions
• An eruption with a (V.E.I.) Volcanic
Explosivity Index of 5 (on a scale of 0-8)
Lava Vocabulary Quiz 9.2
1.
pyroclastic
Hot, erupted glass, ash, and rock fragments. ________________
2.
igneous
All lava flows are this type of rock. _____________________
3.
pahoehoe
A basaltic lava that results in smooth, ropelike surfaces: ____________
4.
silica
Magma has a high viscosity if it contains over 60% of ________
5.
explosive
If the magma has a high gas content, the eruptions tend to be ________
6.
felsic
Because of their high silica content, this rock is light colored. ________
viscosity
lava
aa
pahoehoe
pillow lava
pyroclastic
ash
silica
cinders
igneous
metamorphic
non-explosive
mafic
explosive
sedimentary
felsic