Transcript 5 Review Extrusives Basics l

Igneous Review 2
Mostly Extrusives
Igneous Rock Summary.doc will be helpful
http://www.soest.hawaii.edu/GG/HCV/kilauea.html
The Nature of Volcanic Eruptions
• Factors determining the “violence” or
explosiveness of a volcanic eruption
• Composition of the magma –Silica Content
• Temperature of the magma
• Amount of dissolved gases in the magma
• Composition and Temperature control the
viscosity (resistance to flow) of magma.
Viscous magmas cannot release gasses
coming out of solution, and explode lava as
it freezes.
http://www.soest.hawaii.edu/GG/HCV/kilauea.html
The Nature of Volcanic Eruptions
• Water has very low viscosity, cold molasses high viscosity
• Factors affecting Viscosity
• Temperature - hotter magma is less viscous (more
fluid). Basaltic (mafic) magmas (Olivine, Pyroxene,
Ca-Feldspars) are hotter than Granitic (felsics)
(Quartz, K- feldspars)
• Composition (Silica content)
- Felsic lava (e.g. rhyolite) is most viscous due to
high silica content
- intermediate lavas (e.g. andesite) viscous.
- mafic lava (basalt) has lower viscosity - more
fluid-like due to lower silica content
The Nature of Volcanic
Eruptions
• Factors affecting explosiveness
• Dissolved Gases
– Gases come out of solution and expand in
a magma as it nears the Earth’s surface
due to decreasing pressure
– The violence of an eruption is related to
how easily gases escape from magma –
trapped gasses expand and shatter
solidifying lavas, causing explosions
http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html
The Nature of Volcanic
Eruptions
• Summary
• Fluid basaltic lavas generally produce
quiet eruptions (Hawaiian lava flows)
• Viscous lavas (rhyolite or andesite)
produce more explosive eruptions
(Yellowstone & Mt. St. Helens hot ash
explosions)
Viscous Andesitic Lava over crater floor
Source: Eugene Iwatsubo/Cascade Volcano Observatory, USGS
Very Viscous Rhyolite Flow
Source: Martin Miller
Viscous, short path
Materials extruded from a
Basaltic Volcano
• Lava Flows
• Basaltic lavas are much more fluid
• Types of basaltic flows
– Pahoehoe lava (- twisted or ropey
texture)
– Aa lava (rough, jagged blocky texture)
• Dissolved Gases
• 1-6% of a magma by weight
• Mainly H2O vapor and CO2 and SO2
Typical a’a’ flow
Fluid basalt forms lava tubes
Checking Bowens Reaction Series
Products of Explosions
• Pyroclastic materials – “Tephra”
Propelled through the Air
Types of pyroclastic debris
• Dust 0.001 mm and Ash < rice sized
• Cinders or Lapilli - pea to walnut-sized material
Particles larger than lapilli
• Bombs - > 64 mm ejected as hot lava
-Surtsey Is. Bombs the size of busses
A volcanic bomb
Bomb is approximately 10 cm long
Tephra forms Tuff
St. Lucia Anecdote
Source: Gerald & Buff Corsi/Visuals Unlimited, Inc.
Tephra layers fine away from source
Pumice
• Felsic magmas with high water content
may bubble out of a vent as a froth of lava.
• Quickly solidifies into the glassy volcanic
rock known as Pumice.
http://volcanoes.usgs.gov/Products/Pglossary/pumice.html
Types of Volcanoes 1
• Shield volcano - Largest
–Broad, slightly domed-shaped
–Composed primarily of basaltic lava
–Generally cover large areas
–Produced by mild eruptions of large
volumes of lava
–Mauna Loa on Hawaii is a good
example
Shield Volcano
(Hawaii's K’ilaueau Volcano)
Shield Volcanoes are often in a chain of islands. They have basaltic lava,
which is NOT very viscous, so it easily releases it’s gasses. Hence explosive pyroclastic
eruptions are rare.
Source: Jeff Greenberg/Visuals Unlimited, Inc.
A size comparison of the three
types of volcanoes
A Shield
Volcano
A Shield
Volcano
Mars
23 km high
Olympus Mons
Caldera
Types of Volcanoes - 2
•Cinder cone - Smallest
–Built from ejected lava fragments
(mainly cinder-sized)
–Steep slope angle
–Rather small size
–Frequently occur in groups
Cinder Cone
A Cinder Cone Fountain
Typical of
divergent margins
Sunset Crater – a cinder cone
near Flagstaff, Arizona
Types of Volcanoes - 3
• Composite cone (Stratovolcano)
–Most are located adjacent to the
Pacific Ocean (e.g., Fujiyama, Mt.
St. Helens)
–Large, classic-shaped volcano
(1000’s of ft. high & several miles
wide at base)
–Composed of interbedded lava flows
and layers of pyroclastic debris
–Above subduction zones
A composite volcano
Mt. St. Helens – a typical composite
volcano (prior to eruption)
Composite volcanoes typically have intermediate
silica, andesitic magma. Gasses are trapped in the
magma. When it erupts out onto the surface, low
pressure causes dissolved gasses to come out of
solution just as the lava is freezing. The lava
explodes,
Resulting in a nuee ardente.
Mt. St. Helens after 1980 eruption
Pyroclastic Flows
AKA nuée ardente
•explosive mix of rock, gas and heat
•only with felsic & intermediate magma
•consists of ash, pumice, other fragments
•material propelled from vent at high speed
St Helens
Eruption
Sequence
How would
Scientists
Monitor this
Process?
Seismometers
Tilt Meters
Composite Volcanoes –continued
–Most violent type (e.g., Mt.
Vesuvius, Mt. St. Helens, Mt.
Pinatubo)
–Often produce a nuée ardente
• Fiery pyroclastic flow made of
hot gases infused with ash and
other debris
• Move down the slopes of a
volcano at speeds up to 200 km
per hour
• Forms Welded Tuff
http://volcanology.geol.ucsb.edu/pfs.htm
A nueé ardente on Mt. St. Helens
Lahars
Pyroclastics on upper slopes may produce a
lahar, which is a volcanic mudflow. Heat of
volcanics melts ice.
Volcano Features
• General Features
• Opening at the summit of a volcano
– Crater - steep-walled depression at the summit,
generally less than 1 km diameter
– Caldera - a summit depression typically greater
than 1 km diameter, produced by collapse
following a massive eruption.
• Vent – opening connected to the magma
chamber via a pipe
Calderas
• Calderas form by collapse of evacuated
magma chamber
• Steep-walled depressions at the summit
• Size generally exceeds 1 km in diameter
4700 BC S Oregon
Mt Mazama Eruption and Caldera Collapse
Ngorongoro Crater in Tanzania similar 2 mya
Caldera of Mt. Mazama now filled by Crater Lake
Plumes
Flood Basalts
Hot Spot currently
forming Hawaii
Hey, the plate changed direction !
Flood Basalts
• Fluid basaltic lava extruded
from crustal fractures called
fissures
• e.g., Columbia River Plateau,
• Deccan Traps in India
• Cover huge areas
• Plumes from Mantle
Flood Basalt erupted
from fissures - Snake
River Plain, southern
Idaho
Plume Activity
Volcanic landforms
• Lava Domes
• Bulbous mass of
congealed lava
• Most are associated with
explosive eruptions of
silica-rich magma
http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html
Viscous magmas
St Helens Lava Dome
Volcanic landforms
• Volcanic Pipes and Necks
• Pipes are short conduits that
connect a magma chamber to the
surface
• Volcanic necks (e.g., Devils Tower
in Wyoming and Ship Rock in
New Mexico) are resistant vents
left standing after erosion has
removed volcanic cone
Formation of a volcanic neck
Spanish Peaks and Radiating Dikes (southern CO)
Plutonic igneous activity
• Types of intrusive igneous features
• Dike – a sheetlike injection into a fracture
Discordant - cuts across pre-existing
• Sill – a sheetlike injection into a bedding
plane Concordant - lies parallel to bedding
Some intrusive igneous
structures
A sill in the Salt River Canyon, AZ
Sill: Sediments above and below sill are baked.
Lava Flow, just baked below.
Why
No
C-C
collisions
Plate tectonics and igneous activity
• Igneous activity above Subduction zones
– Descending plate partially melts
– Magma slowly moves upward
– Rising magma can form either
• A Volcanic Island Arc if ocean-ocean
plate collision (Aleutians, Japan, etc.)
• A Continental Volcanic Arc if oceancontinent plate collision (Sierra Nevada)
Batholiths
Plate tectonics and igneous
activity
• Igneous activity along plate margins
• Ophiolites started at Mid-Ocean Ridges
• Great volumes of volcanic rock produced along
oceanic ridges – New ocean floor
– Mechanism of spreading or “rifting”
• Lithosphere pulls apart and thins
• Less pressure results in partial melting in mantle
http://www.archipelago.nu/SKARGARD/ENGELSKA/ICELAND/surtsey.htm
Ophiolite Suite
Some Serpentine is formed
due to hot water
(called Hydrothermal)
circulation
http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html
Basaltic Pillow Lavas
Black Smokers
Cyprus
Mostly Sulfide Deposits
Field Trip Schedule
http://collections.ic.gc.ca/geoscience/images/detail/F92S0220.jpg
Circulation of hot water in cracks at mid-ocean ridge dissolves metals (Copper, Iron, Zinc, Lead, Barium)
which are re-precipitated as (mostly) sulfide ores. Hydrothermal waters are capable of metamorphism.