Igneous Rock Summary.doc will be helpful for this chapter’s homework.
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 gasses in the magma
•
Composition and Temperature control the viscosity (resistance to flow) of magma.
Viscous magmas cannot release gasses coming out of solution, and the gasses explode the 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
– gasses 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 gases 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)

Low-Viscosity Basaltic Lava
Source: Phil Degginger/Earth Scenes

Viscous Lava over crater floor
Source: Eugene Iwatsubo/Cascade Volcano Observatory, USGS

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
•
Mainly H
2
O vapor and CO
2
, SO
2, and N
2
,CH
3

A Pahoehoe lava flow

ypical
Broken, often further from vent

Fluid basalt forms lava tubes
Checking Bowens Reaction Series

Materials extruded from a volcano
• 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
Some the size of a Bus

Tephra forms Tuff
Source: Gerald & Buff Corsi/Visuals Unlimited, Inc.
St. Lucia Anecdote

Tephra layers fine away from source

• 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

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

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 gases. Hence explosive pyroclastic eruptions are rare.
Source: Jeff Greenberg/Visuals Unlimited, Inc.

Types of Volcanoes - 2
•
–
Built from ejected lava fragments
(mainly cinder-sized)
– Steep slope angle
– Rather small size
–
Frequently occur in groups

Typical of divergent margins

Sunset Crater – a cinder cone near Flagstaff, Arizona

Types of Volcanoes - 3
•
(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. gases are trapped in the magma. When it erupts out onto the surface, low pressure causes dissolved gases 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

A size comparison of the three types of volcanoes

St Helens
Eruption
Sequence
How would
Scientists
Monitor this
Process?
Seismometers
Tilt Meters

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

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 nue é ardente on Mt. St. Helens

Location of Montserrat
- 1997 Nuée Ardente
Plus massive ashfall. Leaves of trees preserved.
North American plate’s ocean crust
Subducted under Caribbean Plate

Montserrat Eruption with
Nu ée Ardente, 1997
Source: Kevin West/Liaison Agency
19 people died when they were overtaken by a pyroclastic flow

Lahars
Pyroclastics on upper slopes may produce a lahar, which is a volcanic mudflow. Heat of volcanics melts ice.

•
Calderas form by collapse of evacuated magma chamber
•
Steep-walled depressions at the summit
•
Size generally exceeds 1 km in diameter

Caldera of Mt. Mazama now filled by Crater Lake

4700 BC S Oregon
Mt Mazama Eruption and Caldera Collapse
Ngorongoro Crater in Tanzania similar 2 mya

Volcanism on a tectonic plate moving over a hot spot

Flood Basalts
Hot Spot currently forming Hawaii
Hey, the plate changed direction !

Flood Basalts
•
• e.g., Columbia River Plateau,
•
Deccan Traps in India
•
•
Plumes from Mantle

Flood Basalt erupted from fissures - Snake
River Plain, southern
Idaho
Plume Activity

Lava Plateau Formation

Volcanic landforms
•
• 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
•
Laccolith – A mushroom-shaped concordant

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 along plate margins
•
Mid-Ocean Ridges – Basaltic Pillow Lavas
•
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

http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html

Plate tectonics and igneous activity
•
Igneous activity along 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)

The Cascades, Washington State

Plate tectonics and magmatism
•
Intraplate volcanism
•
Associated with plumes of heat in mantle
•
Form localized volcanic regions in the overriding plate called a hot spot
–
Produces basaltic magma sources in oceanic crust (Hawaii)
–
Produces granitic magma sources in continental crust (Yellowstone Park)
–
These differences are predicted by a
Crust-Melting model of Granite generation

http://www.mt-fuji.co.jp/Photo/Photo.html