(4) Extrusive Rocks

Question 1

Extrusive rocks are

Answer 1

Lava

Question 2

What is a volcano?

Answer 2

• A hill or a mountain that consists of rick formed by the extrusion of lava, or erupted as pyroclastic material
• Lava is magma that extrudes at the surface of the Earth
• Pyroclasitc material is fragmental lava, broken up in explosive eruptions

Question 3

Classification

Answer 3

- Composition (Rock Name)
o	Felsic (Rhyolite)
o	Intermediate (Andesite)
o	Mafic (Basalt)
o	Ultramafic (Komattite)
- Komattites are rare NOW
- Gases are usually 5% of magma (ex: water vapour)

Question 4

Why are some eruptions very explosive and others relatively quiet?

Answer 4

• Eruptive violence determined by several factors
• 1. Amount of gases
• 2. Viscosity = resistance to flow (changes with temperature, silica content, and the amount of dissolved gases (more makes lower viscosity))

Question 5

Mafic rock eruptions

Answer 5

• Hot
• Low silica
• Low gas content (can easier escape) = low viscosity = tends to flow as liquid

Question 6

Felsic lava

Answer 6

• Cooler
• High silica
• High gas content (releases gases suddenly) = high viscosity = explosive eruptions of pyroclastic material

Question 7

Parts of a volcano

Answer 7

• Vent (main opening) (dikes can become vents and craters)
• Crater (The top of the vent)
• Plug Dome (solidified crater)
• Flank eruption (come out of the side and fractures)
• Caldera (blow the top of the mountain off, or crater formed from collapse)

Question 8

Shield Volcanoes

Answer 8

• Mainly made of lava flows, non-violent, mafic composition
• Large, broad, gentle slopes (2-10deg)
• Long life spans, Hawaii, Iceland
• 2 types of flows : Pahoehoe – smooth/ropey surface ; Aa – rough, broken/angular pieces

Question 9

Composite Volcanoes

Answer 9

• Built up by alternating flows and pyroclastics
• Steeper slopes, can grow large (Mt Fuji)
• Eruptions intermittent – long periods of dormancy
• Composition usually intermediate
• Found in two belts : Circum-Pacific ‘ring of fire’, and Mediterranean

Question 10

Cinder Cones

Answer 10

• Mainly pyroclastic material
• Slopes ~30deg
• Can be any composition – usually on flanks of larger volcano, related to concentration of gases – craters of the Moon Park, Idaho

Question 11

Forms during erputions

Answer 11

• Pyroclastic deposits can appear similar to sediments
• Fine ‘ash’ can travel long distances
• Deposits; tuff = ash and cinders ; breccia = coarser fragments
• Vesicular/scoria = (an extremely vesicular basaltic lava) 505% gas bubbles
• Pumice (extremely porous igneous rock)
• Amygdaloidal (vesicles that have been filled in with a secondary mineral)

Question 12

Pyroclastic fall deposits

Answer 12

• Pyroclastic material falls back to the ground after being erupted
• Thick deposits of pyroclastic material may form

Question 13

Other volcanic features

Answer 13

• Fumaroles (expelling sulfur around a vent)
• Geysers (from heat and pressure)
• Flood lava flows
• Phreatic eruptions (eruptions on the ocean floor)
• Lava pillows

Question 14

What happens when lava flows onto the ocean floor?

Answer 14

• Pillowed basalt

- Confining pressure of seawater makes the bulbous pillow shapes

Question 15

Thick lava flows can have columnar jointing (natural cobblestone)

Answer 15

• Common in flood basalts

- Cooling phenomenon