Read Chapter 4, then 3.

ash, lapilli, bombs, tuff

pyroclastic flow, nuée ardente, welded tuff

shield, composite volcanoes

cinder cones (or spatter or tephra cones)

silicic dome, caldera, pillow basalt

thermal springs and geysers

Phaneritic, Aphanitic, Glassy textures

Granite < vs > Rhyolite

Diorite < vs > Andesite

Gabbro < vs > Basalt

Vesicular, Pegmatitic, Porphyritic,

Phenocrysts, Pyroclastic

Plutons, Batholiths, Stocks, Necks, Laccoliths, Sills, Dikes, Xenoliths

Magmatic Differentiation & Bowen's Reaction Series

- Fractional Crystallization

- Partial Melting
 

Volcanoes and their brethren.

Magma - underground molten rock + dissolved gas.

Solidifies into intrusive igneous rock.

Lava - magma that erupts above ground.

Solidifies into extrusive igneous rock.

Composition

rhyolitic magma = 65-75% SiO₂

andesitic magma = 50-65% SiO₂

basaltic magma = 45-50% SiO₂

Temperature of magma is 1000-1400°C.

Rhyolitic = Low Temp, Basaltic = High Temp.

Viscosity (resistance to flow) depends on:

- Temperature: High temp = low viscosity

- Silica content: High silica = high viscosity (polymerization of silica tetrahedrons)

- Pressure: High pressure = low viscosity

Basaltic magma deep in crust has a viscosity similar to that of water => moves thru fractures.

- Gas Content: gases dissolved in magma lowers viscosity. Gases include H₂O, CO₂, SO₂, others.

Basaltic lavas can flow as fast as 10-15 km/hr. Andesitic lavas are more viscous than thick concrete. Rhyolitic? Barely can move.

Eruptions (emanations of lava from the earth):

3 main types: Flowing, fountaining, explosive

Depends on Magma Type

a) Flowing eruptions - mostly basaltic, but may be andesitic, and very rarely rhyolitic. Lava simply flows out of the ground.

* crater (funnel-shaped opening at the top of a volcano) fills up and overflows, leading to lakes and rivers and tubes of lava. Common in Hawaii.

* fissure (narrow fracture in the earth). Fissure eruptions create extensive plains of basalt called flood basalts.

* Pahoehoe (ropy, low visc) and Aa (blocky, high visc) basalts - result from difference in gas content of lava.

b) Fountaining eruptions - Gas bubbling out of solution shoots lava to 1000 meters up.
 

Lava:

c) Explosive eruptions - Gas builds up, can't escape, volcano blows top.
 

Lava:

Lateral eruptions - explosive eruption on the volcano's flank (not top).
 
 

Ex. Mt. St. Helens (WA) - Earthquakes caused a landslide on volcano flank, sudden pressure release on magma allows explosive eruption.

Both Fountaining and Explosive eruptions form Pyroclastic debris, or tephra. - rock fragments and magma blobs ejected during an eruption.

- 3 sizes of pyroclasts: Bombs, Lapilli, Ash.

Ash is smaller than 4 mm diameter

Lapilli is 4 to 64 mm (gravelish size), and

Bombs are big enough to do you in.

Tuff is a rock formed of ash and lapilli.

- Pyroclastic flow - hot, dense cloud of ash moving downhill like an avalanche.
 
 
 
 

1902, Mont Pelée (Martinique) - ash cloud erupted straight up, but was heavier than air and rushed down the mountainside as a nuée ardente (flaming cloud) at speed of 100 km/hr. Burned town of St. Pierre (pop. 28,000) in less than one minute. Air temperature = 700-800°C.

Volcanoes: (4 different: by shape and size)

- Shield volcanoes = Broad, gentle slopes

Mauna Kea, Kilauea.

Lava type:

Form by: .

- Composite or Stratovolcanoes - Tall, steep.

Mt. Fuji, Mt. Rainier,

Ring of Fire.

Lava type:

Form by: .

- Cinder (Spatter, Tephra) Cones - Small, conical.

Sunset Cone, AZ.

Lava type:

Form by: .

- Silicic Domes - Small, dome-shaped

Inside Mt. St. Helens crater.

Lava type:

Form by: .

Other Features associated with volcanoes:

- Caldera - hole where a volcano used to be. Magma chamber collapse yields a big hole.

Crater Lake, Oregon. Also Krakatoa.
 
 
 
 

- Pillow basalt forms wherever basaltic lava erupts underwater, due to rapid chilling.

- thermal springs and geysers = Hot Water.

Thermal springs - water boiling up from underground.

Geysers - eruptions are intermittantly eruptive.

Old Faithful = superheating + spurt.

Igneous Rocks: A) Texture (fabric of rock)

1.

2.

3.

4.

5.

6.

Cooling Rate: Slow cooling = Big crystals.

also Pegmatitic (> 1cm crystals)

and Pyroclastic (sedimentary igneous rock).

B) Composition

Eight important minerals for igneous rocks:
 

Non-ferromag (low Fe + Mg) minerals		Ferromag minerals
Quartz		Biotite
K-feldspar	Na-plagioclase	Amphibole
Muscovite	Ca-plagioclase	Pyroxene
		Olivine

Felsic rock: 0-15% ferromag minerals

Intermediate: 15-40% ferromags

Mafic: 40-95% ferromags

Ultramafic: 95-100% ferromags
 
 

Color  Texture Index:  & Origin:	Felsic	Inter-mediate	Mafic	Ultramafic
Aphanitic/   Extrusive	Rhyolite	Andesite	Basalt	Peridotite
Phaneritic/  Intrusive	Granite	Diorite	Gabbro	Peridotite
Glassy/  Extrusive	Obsidian	Obsidian	Glassy Basalt	(none)
Vesicular/  Extrusive	Pumice	Pumice	Vesicular Basalt	(none)

Granite

Rhyolite

Diorite

Andesite

Gabbro

Basalt

Peridotite

Intrusive Rock Bodies: Plutonic Rx, Plutons
 
 
 
 
 
 
 
 
 
 
 

Batholiths (BIG >100 km² area)

Stocks (Smaller)

Dikes (vertical)

Sills (horizontal)

Necks (vertical pipes)

Laccoliths (dome-like)

- Magma chambers grow upward by Stoping (chunks falling in) and assimilation of Xenoliths