CH. 9 – IGNEOUS ACTIVITY

Importance of Volcanic Activity

1) Destructive – predict eruptions2) Landforms created3) Clues to Earth’s interior

(temp, pressure, composition)

Importance of Volcanic Activity

4) Clues about internal processes (dynamic activity)

Viscosity of Magma

Viscosity = liquid’s resistance to flow

Low viscosity = low resistance to flow (flows __________)

High viscosity = high resistance to flow (flows ____________)

Viscosity depends on:

1) Temperature of magmaHotter magma = _____ viscosityCooler magma = _____ viscosity

Viscosity of Magma

2) CompositionMafic magma = ___________

- _______ viscosity magmaFelsic magma = ___________

- _______viscosity magma- doesn’t occur as lava flows

Viscosity of Magma

3) Dissolved gas contentMore dissolved gas = lower

viscosityAs gases escape, lava becomes

_______ viscous

Materials extruded in eruptions

1) Pyroclastic debris (solid)“pyro” = fire; “clastic” = piecesDefined by particle sizea) Dust-sized = volcanic ashb) Gravel-sized = cinders

Pyroclastic Debris

c) Boulder-sized = two types:i) angular = volcanic blocks- pre-existing rx blown apart by eruption

Pyroclastic Debris

ii) rounded = volcanic bombs- ejected magma cools in flight- streamlined by air

Materials extruded in eruptions

2) Lava – most common is mafica) Pahoehoe lava- smooth, wrinkled surface- low viscosity (very hot lava)

Mafic lava features

b) A’a lava- sharp jagged surface- cooler lava, so higher viscosity- thicker lava flows

Mafic lava features

c) Lava tubes- tunnels form under crust of lava- adds width to volcanoes

Mafic lava features

d) Columnar basaltEx: Devil’s Tower, WY

Giant’s Causeway, Ireland

Mafic lava features

e) Pillow lava- underwater cooling of lava

Materials extruded in eruptions

3) Gases- gases expand as magma

approaches surface - causes eruptions

Gases

Most common gas = _________ - source of water for oceans and

atmosphere (“outgassing”)Also carbon dioxide, sulfur,

chlorine

Types of Volcanoes

1) Shield volcano- largest- gently sloping cone b/c mafic

magma flows easily from vent - cones grow wider than tall

Shield Volcanoes

Calm eruptions (gases escape)Ex: Hawaiian Islands, Galapagos Isl.

Types of Volcanoes

2) Composite Cone - layers of lava and pyroclastic

debris - steep symmetrical cone

Composite Cones

- intermediate lava (high viscosity)

- violent eruptions (gases trapped)

Mt. St. Helens

Composite Cone

Located in:- Circum-Pacific belt (“Ring of

Fire”)- Mediterranean Belt- Caribbean Isl.

Volanic activity map

Composite Cones

Ex: Mt. St. HelensAndes Mtns.Mt. Etna, ItalyCarribbean Isl.

Composite Cones

Associated features:a) Lahar – super-saturated ash (mudflow )Sources of water?

Lahar

- follows river channels - causes extensive floods,

property damage, & deaths

Associated features:

b) Nuee ardente (“glowing avalanche”)

- dense steam cloud of toxic gases and red-hot ash

- travels down the cone at very high wind speeds

Nuee ardente

Ex: St. Pierre, Martinique in 1902

Types of Volcanoes

3) Cinder cones- smallest volcano- built of loose cinders- steep-sided - end-stage of eruption cycle

Cinder cones

Short eruption lifespan- few weeks up to a yearEx: Paricutin, Mexico (1943) Sunset Crater, AZ

Paricutin Sunset Crater

Size comparison of 3 volcanoes

Types of Volcanoes

4) Other volcanic features a) Caldera - summit collapses into emptied

magma chamberEx: Crater Lake, OR

Other volcanic features

b) Volcanic Neck- eroded remnant of volcanoEx: Ship Rock, NM

Other volcanic features

c) Lava Plateau (Flood Basalt)- mafic lava erupts out of fissures

(cracks in crust)- magma covers hundreds of

miles

Flood basalts

Lava Plateau

Ex: Columbia Plateau in Oregon, Washington, Idaho

Intrusive Structures

Most magma cools at depth- exposed only after uplift &

erosion- also called ___________

After crystallization

Pluton Classification

Criteria: 1) Shape

Tabular = linearMassive = “blob-like”

Pluton Classification

2) Relationship to pre-existing rock (“country rock”)

Discordant – cuts acrossConcordant –parallel to/between

After crystallization

Intrusive Structures

1) Dike - tabular discordant pluton- magma travels in fractures- often associated with volcanic

activity (Ex: Ship Rock, NM)

Intrusive Structures

2) Sill- concordant tabular pluton- magma squeezes between

layers- shallow intrusion

Sill

Usually mafic magma b/c low viscosity

- often basalt b/c shallow intrusions cool quickly

Question:

How can you tell if a layer of basalt in sedimentary rocks is a sill or a buried lava flow?

CLUES:

Sill

Intrusive Structures

3) Laccolith (“laccos” = lens)- concordant massive pluton- magma thickens as it cools - arches overlying rock into

dome Ex: Henry Mtns., Utah

Laccolith diagram

Intrusive Structures

4) Batholith (“bathos” = deep)- discordant massive pluton- 100’s of mi.2 (largest pluton) - graniteEx: Sierra Nevada Mtns., CA

Uplift & erosion

Intrusive Structures

Label structures on diagram at end of study guide.