Download Igneous Rocks, Minerals, and Volcanos

Igneous Rocks, Minerals, and
Volcanos
Allan Treiman
LPI
Plan of Talk

Tyrrany of Three
– Three types of volcanos
– Three types of lavas
Volcanos in terms of Lava Properties and
Environments
 Lava Properties in terms of Atoms
 Igneous Rocks and Minerals

Three Types of
Volcanos
Shield
 Composite /
Stratovolcano
 Cinder Cone

But So Many
More …
Caldera Complex ‘SuperVolcano’
 Lava Plateau
 Dome
 Single Flow
 Tuff Ring
 And …

What Controls the Shape of a
Volcano?

Properties of lava
– Viscosity of lava (runny or stiff)
– Dissolved Gas - Explosive or Effusive
– Solid grains in lava.
Volume of lava erupted
 Single or Multiple eruptions
 Environment around eruption

Lava Properties: Viscosity
Different sorts of lavas are stiffer or runnier
 What is lava?

– Molten material in the Earth
– Solidifies at surface conditions

Many sorts of ‘lava’
–
–
–
–
Most common is silicate - abundant SiO44Molten sulfur, carbonate, iron oxide
Mud is not lava on Earth (but “mud volcanos”)
Water is not lava on Earth (but is elsewhere)
Silicate Lavas

Large Range of Viscosity
– Basalt - as runny as motor oil
– Andesite - stiffer than taffy
– Rhyolite/Granite - like window glass

Depends on silica - SiO2
– Basalt: < 52% SiO2 in
chemical analysis
– Andesite: 52 - 63% SiO2
– Dacite: 63 - 68 % SiO2
– Rhyolite: > 68 % SiO2
Andesite
South Sister
SiO 2
TiO2
Al 2 O3
Fe2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
P2O5
H2 O
63.0
1.15
16.24
2.87
2.94
0.08
1.52
4.04
4.86
2.00
0.28
0.68
Sum
99.9
Why does silica
matter?




Si - O bonds much stronger than others
Silica tetrahedra, SiO44- polymerize
In lava, single silica tetrahedra
flow easily, like little balls
In lava, large silicate polymers
flow poorly, like noodles
Why does Water Matter?

Force for explosive eruptions
– Water vapor bubbles out as
magma nears surface
– No vapor, no explosion!
Stiff water-rich magma makes
foam (pumice)& shards of
glassy ash
 Pumice + ash and water vapor
can flow together as a ‘slurry’
= an ash flow

Ash flow = pyroclastic
flow = ‘nuee ardent’
Caldera Complex
“Super-Volcanos”
Yellowstone




Valles Grandes, NM
Caldera is 22 km across
Rhyolite ash flows & domes
Slope outside caldera ~2°
An Invisible Caldera
Complex “SuperVolcano”:
Harney Basin


A shallow basin, slightly east
of our field trip path.
Multiple Ash Flows
– Devine Canyon: 9 mybp
– Prater Creek: 8.4 mybp
– Rattlesnake: 6.4 mybp


Nearly invisible under later
basalts, and erosion
Typical of later cenozoic
geology of Basin & Range!
Crystals in Lava
Solid crystals make lava more viscous
 What kinds of crystals?

– Olivine (Mg,Fe)2SiO4 - green, glassy
– Pyroxene (Ca,Mg,Fe)SiO3 - black/green, breaks
on flat surfaces (cleavage)
– Feldspar - plagioclase (Ca, Na)(Al,Si)Si2O8 clear-white-greenish, glassy, breaks on flat
surfaces.
– Quartz - SiO2 - clear, glassy, curved fractures.
Single Eruption




Paricutin Cinder Cone - 1.4
km3 lava
Columbia River, Grande
Ronde - to 750 km long,
2000 km3 lava
Yellowstone - Lava Creek
Tuff (like at Valles Caldera) ~1000 km3 ash
How much is a cubic
kilometer?
Many Eruptions
Mauna Loa Shield ~75,000 km3 lava
 Columbia River Basalts
~170,000 km3 lava
 Olympus Mons (Mars) ~500,000 km3 volume
 Ontong-Java Plateau ?6,000,000 km3 lava

Environment of
Eruption

Into Air
– Typical

Into Water
– Maar Explosion
– Tuff Ring
– Pillow Lava

Into Ice
– Tuya Buttes
What Controls the Shape of a
Volcano?

Properties of lava
– Viscosity of lava (runny or stiff)
– Dissolved Gas - Explosive or Effusive
– Solid grains in lava.
Volume of lava erupted
 Single or Multiple eruptions
 Environment around eruption

Cinder
Cone

Paricutin,
Mexico
– 1943 - 1952