Igneous Rocks, Minerals, and Volcanos Allan Treiman LPI

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 ‘Super-Volcano’

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 SiO4

4-

– Molten 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

SiO2 63.0 TiO2 1.15 Al2O3 16.24 Fe2O3 2.87 FeO 2.94 MnO 0.08 MgO 1.52 CaO 4.04 Na2O 4.86 K2O 2.00 P2O5 0.28 H2O 0.68 Sum 99.9

Why does silica matter?

Si - O bonds much stronger than others Silica tetrahedra, SiO4

4- 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 = pyroclasticflow = ‘nuee ardent’

Caldera Complex “Super-Volcanos”

Valles Grandes, NM Caldera is 22 km across Rhyolite ash flows & domes Slope outside caldera ~2°

Yellowstone

An Invisible Caldera Complex “Super-

Volcano”: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