Igneous Rocks

Question 1

Silicic minerals

Answer 1

Quartz
Orthoclase
Plagioclase
Muscovite

Question 2

Mafic minerals

Answer 2

Biotite
Hornblende
Augite
Olivine

Question 3

What is Acid/silicic magma?

Answer 3

> 66% SiO
C-O
most explosive

Question 4

What is Intermediate magma?

Answer 4

55-65% SiO

O-O

Question 5

What is Basic/Mafic magma?

Answer 5

45-55% SO

Mid-plate got spits and oceanic ridges

Question 6

What is Ultrabasic/Ultramafic magma?

Answer 6

<44% SiO
Asthenosphere
Least explosive

Question 7

Grain size

Answer 7

Coarse- >5.0mm
Medium- 0.1-5.0mm
Fine- <0.1mm

Question 8

Example of a silicic rock?

Answer 8

Granite

Question 9

Examples of mafic/basic rocks?

Answer 9

Gabbro
Dolerite
Basalt

Question 10

Example of an Ultra mafic/ Ultrabasic rock?

Answer 10

Peridotite

Question 11

What is holocrystalline?

Answer 11

Rocks that are wholly crystalline I.e entirely composed of silicate crystals

Question 12

What is glassy?

Answer 12

Rocks that are composed of natural glass such as obsidian. This texture develops when lava cools very quickly and is unable to form crystals

Question 13

What is porphyritic?

Answer 13

The rock contains large crystals called phenocrysts set in a matrix of finer crystals.

Phenocrysts form first at higher temperatures where they were unrestricted.

The matrix formed lager when magma had cooled at a lower temperature

Question 14

What is euhedral?

Answer 14

Crystals that show well developed crystal faces. They crystallise first at higher temperatures and were unrestricted. Usually phenocrysts.

Question 15

What is anhedral?

Answer 15

Crystals that show poorly developed crystal faces. They formed at the late stage at lower temperatures so their development was restricted

Question 16

What is groundmass?

Answer 16

Mosaic of small crystals that form in the spaces left behind by earlier formed crystals

Question 17

What is granular?

Answer 17

Rocks composed of similar sized silicate crystals. This texture is formed when all the crystals begin to form at the same temperature and grew at the same rate

Question 18

What is vesicular?

Answer 18

Holes seen in lava rocks that are created by escaping gas bubbles

Question 19

What is fragmental tuff?

Answer 19

Volcanic rocks composed of angular rock, mineral and ash that has been blasted out of a volcano following an eruption

Question 20

Granite

Answer 20

Crystalline
Coarse
Plagioclase Feldspar showing cleavage 
Anhedral Biotite 
Euhedral crystals of Orthoclase Feldspar 
Quartz

Question 21

Gabbro

Answer 21

Coarse grained
>5mm
Very slow cooling 
Basic magma 
Magma chamber

Question 22

Dolerite

Answer 22

Medium grained
0.1- 5mm 
Moderately slow grained 
Basic magma 
Small and shallow intrusions 
Dyke or sill

Question 23

Basalt

Answer 23

Fine grained
<0.1mm
Rapid cooling
Surface lava

Question 24

Pillow lava

Answer 24

Ball shaped masses
Eruption of lava in water 
1m in size 
Kidney bean shaped 
Upper convex surface 
Lower concave surface

Question 25

Columnar jointing

Answer 25

Six sided columns of igneous rocks created by contracting magma or lava during cooling

Question 26

Amygdales

Answer 26

Secondary minerals that infill vesicles

Question 27

Pahoehoe

Answer 27

Ropey structure seen on the surface of lava flow caused by compression of surface lava in ridges as it is slowing more slowly that the lava below

Question 28

Aa

Answer 28

A block structure seen on the surface of lava. Formed by the rapid loss of gas from the lava as it erupts

Question 29

Dyke

Answer 29

Sheet of igneous rock that is formed when magma is injected into a crack ``` It is discordant Few cms to a few kms Creates baked margins Medium grained Fine grained chilled margins ```

Question 30

Will

Answer 30

Magma is injected along a bedding plane Concordant