Igneous Rocks

Question 1

What should be ignored when observing igneous grain size?

Answer 1

Phenocrysts

Question 2

What texture has no grains?

Answer 2

Glassy

Question 3

What does felsic/silicic mean?

Answer 3

Quartz and feldspar (silica rich)

Question 4

What does mafic rocks mean?

Answer 4

Contain magnesium and iron (ferric- ferromagnesium)

Question 5

What is the appearance of mafic rocks?

Answer 5

Darker

Question 6

What is the appearance of silicic rocks?

Answer 6

Lighter

Question 7

Obsidian

Answer 7

• Exception to the rule
• Looks black but is felsic
• Clear in thin section

Question 8

What is magma?

Answer 8

Mixture of molten rock, solid materials (crystals) and volatiles (dissolved gases)

Question 9

What does a minor intrusion mean?

Answer 9

Hypabyssal depth including dykes and sill

Question 10

What does a major intrusion mean?

Answer 10

Plutonic depth include batholiths

Question 11

Equicrystalline texture

Answer 11

• All crystals are of equal size
• Size related to depth+speed of cooling

Question 12

Hypabyssal crystals

Answer 12

• Fine crystals by rapid cooling

Question 13

Plutonic crystals

Answer 13

• Coarse crystals formed by slow cooling

Question 14

Vesicular texture

Answer 14

• Gas bubbles trapped in lava as rapidly cools
• Vesicles parallel to direction of flow
• Basalt + Pumice

Question 15

Amygdaloidal texture

Answer 15

• Vesicles infilled by minerals depostied by percolating groundwater

Question 16

Flowbanding texture

Answer 16

• Layers of dark + light minerals from lava flow
• Aligned parallel to flow direction
• Rhyolite

Question 17

Ophitic texture

Answer 17

• Form when elongate crystal is enclosed by another mineral

Question 18

Cumulate texture

Answer 18

• Forms when crystals settle out of the magma chamber
• Continue growing after they settle

Question 19

Mafic lavas

Answer 19

• Low viscosity flows of basalt
  They cool to form
• Aa lava flows (rough surface)
• Pahodhoe lava flows (smooth surface)

Question 20

Intermediate and silicic lavas

Answer 20

-Viscous flows of a andesite (limited to cones surrounding volcano)
- Tend not to flow easily
- Can form lava domes within crater (flow banding)

Question 21

Pyroclastic material

Answer 21

Magma ejected into the atmosphere as particles

Question 22

Ash (pyroclastic material)

Answer 22

<2mm
Forms pyroclastic rock tuff when consolidated

Question 23

Lapilli (pyroclastic material)

Answer 23

2-64mm
Forms lapilli tuff when consolidated

Question 24

Bombs + blocks (pyroclastic material)

Answer 24

> 64mm
Forms aggromete close to the volcano

Question 25

Nuee ardentes

Answer 25

Gaseous, pyroclastic cloud of magma droplets + ash

Question 26

Igmibrite

Answer 26

Cooled + solidified Nuee ardentes deposit

Question 27

Shield volcanoes

Answer 27

- Gently slopes - Fissure eruptions -Very little pyroclastic material

Question 28

Composite volcanoes

Answer 28

Often silicic magma which has silicic gas (results in pyroclastic flows)