Lecture 17 volcanoes

Today: Chapter 9: The Wrath of Vulcan

Volcanic Eruptions

Term Break Extra Credit due this Friday by end of lecture.


Today’s Lecture: Chapter 9
Volcanic activity
Basic types of eruptions
Anatomy of a volcano
Materials erupted
Classes of volcanoes
Basaltic volcanoes
Intermediate volcanoes
Silicic volcanoes
Volcanic hazards
Case Study: Mt. Saint Helens


Types of Volcanic Eruptions

Fig. 09.09ab
W. W. Norton

Central Eruption

Fissure Eruption

Basic Anatomy of a Volcano

Fig. 09.10
W. W. Norton


HY DROTHERMA L
DEPOSITS A S
REPOSITORIES FOR
GEOLOGIC
INFORMA TION

 Most terrestrial hydrothermal
systems are sustained by
magmatic heat sources.
 Variations in the temperature
(and density) of fluids drives
convective circulation in the
crust, producing large-scale
transfers of energy and
materials.
 As hot fluids move through
the crust, they interact
chemically with their host
rocks, leaving behind
distinctive geochemical, and
mineralogical signatures.

Materials extruded during an eruption
Volcanoes erupt:
◆ Lava
◆ Gases
◆ Pyroclastics

Fragmented
rock, ash &
dust


Ash and tephra

Explosive eruption

Volcanic bombs
Side vent
Eroded cone
Lava cone
Sills

p.248-249a

Mud
flows
(older)

Pyroclastic flow
(nuée ardente)

Sequential
ash and lava
layers

Lava flow
Dikes

Fracturing

original artwork by Gary Hincks
Cinder cones

Lava pavement
(cracked/broken)

Old lava dome
Lavas
Sedimentary
rocks
Laccolith

Metamorphic
rocks

Chimney

Contact
metamorphism

Granite

intrusion
(older/cold)
Magma chamber


Fig. 09.24
W. W. Norton


volcanic
pipe/neck

after erosion

before erosion

Shiprock, NM


Fig. 09.04a
Stephen Marshak


Fig. 09.04b
Stephen Marshak


Volcanologists recognize 3 general volcano types:

a. Shield volcanoes

b. Composite cones

c. Cinder cones


Types of Volcanoes
9 km
150 km
Shield volcano (e.g. Hawaii)
3 km

p.248-249c
15 km

original artwork by Gary Hincks

Composite volcano (e.g. Vesuvius)

0.3 km
1.5 km
Cinder cone (e.g. Sunset crater)


What Controls the Form of a Volcano?

Basaltic flow

Andesitic flow

Rhyolitic dome

Rhyolitic spire


Factor I: Magma Viscosity
Silicon-oxygen tetrahedra tend to link
up in magmas and form chains.
In magmas with a high silica content
(e.g. granitic; ~70% by weight) many silica
chains form, especially at the lower eruption
temperature of silica-rich lavas.
This increases the viscosity of the magma.

Viscosity:
Resistance of a fluid to flow.
High viscosity lavas:

- Don’t flow far from eruption center


Factor II: Gas Content of Magma

With viscosity, gas content
Largely determines if an
eruption will be:

Explosive
or
“Gentle”

Non-explosive basaltic eruption


Contrast: Explosive
silicic eruption


The nature of volcanic eruptions

So, in a nutshell…
Amount of dissolved gas

+
Viscosity of magma (silica content
and temperature)

=
Explosiveness of eruption


The nature of volcanic eruptions
What determines if an eruption is

- Explosive?
- “Gentle”?

3 primary factors:
◆ magma’s composition

more silica

more viscous (linking of silicon-oxygen
tetrahedra)


The nature of volcanic eruptions
What determines if an eruption is

- Explosive?
- “Gentle”?

3 primary factors:
◆ Magma’s composition
more silica

more viscous (linking of silicon-oxygen
tetrahedra)

◆ Magma’s temperature
hotter magma

less viscous


The nature of volcanic eruptions
What determines if an eruption is

- Explosive?
- “Gentle”?

3 primary factors:
◆ magma’s composition
more silica

more viscous (chains)

◆ magma’s temperature
hotter magma

Viscosity

less viscous

◆ Amount of gas in magma
Can increase fluidity

Fluidity


Let’s look in more detail at:

a. Shield volcanoes

b. Cinder cones

c. Composite cones

◆ low profile
◆ broad base

◆ basaltic lavas
◆ flows are thin
◆ travel far from
source