AS Igneous Rocks

Question 1

Igneous rocks

Answer 1

are those that have cooled from magma.

Question 2

Phenocrysts

Answer 2

large or conspicuous crystals in an igneous rock, much larger than the groundmass.

Question 3

Essential minerals

Answer 3

those minerals used to classify igneous rocks. Other minerals that may be present, but are not used for classification, are accessory or secondary minerals.

Question 4

Felsic or minerals

Answer 4

light coloured and silica rich.

Question 5

Magma

Answer 5

molten rock, which is beneath the Earth’s surface.

Question 6

Mafic minerals

Answer 6

dark coloured, silica poor and rich in magnesium and iron.

Question 7

Flow banding

Answer 7

formed by friction as the magma or lava slows down near an interface, aligning the minerals as it moves.

Question 8

Conchoidal

Answer 8

a fracture which results in a curved surface.

Question 9

Vesicular

Answer 9

a textual term for a rock containing vesicles. Vesicles were bubbles of gas which came out of solution as a result of pressure release.

Question 10

Porphyritic

Answer 10

a texture where large crystals, called phenocrysts, are completely surrounded by smaller crystals.

Question 11

Equigranular

Answer 11

when crystals in an igneous rock are of approximately the same size.

Question 12

Amygdaloidal

Answer 12

where there are large vesicles that have been filled with a secondary mineral.

Question 13

Lava

Answer 13

molten rock that cools at the surface.

Question 14

Ophiolites

Answer 14

sections of the Earth’s oceanic crust that have been tectonically moved (obducted) onto continental crust.

Question 15

Extrusion

Answer 15

the emission of magma onto the Earth’s surface where it forms a lava flow.

Question 16

Intrusions

Answer 16

composed of igneous rock formed below the Earth’s surface, where magma is forced into pre-existing rocks.

Question 17

Hypabyssal

Answer 17

when igneous rocks form at relatively shallow depths below the surface.

Question 18

Plutonic

Answer 18

when igneous rocks form deep below the surface.

Question 19

Euhedral

Answer 19

crystals are well formed with good crystal faces.

Question 20

Equant

Answer 20

crystals have all axes the same length. Sub-equant means that almost all axes are the same length.

Question 21

Prismatic

Answer 21

crystals have four or more sides but are elongated in one direction.

Question 22

Anhedral

Answer 22

crystals show poorly formed crystal faces.

Question 23

Subhedral

Answer 23

crystals have some well-formed faces and some poorly formed faces.

Question 24

Obsidian

Answer 24

A glassy, silicic, extrusive igneous rock

Question 25

Pumice

Answer 25

A fine, silicic, vesicular extrusive igneous rock

Question 26

Rhyolite

Answer 26

A fine, silicic, extrusive igneous rock

Question 27

Microgranite

Answer 27

A medium, silicic intrusive igneous rock

Question 28

Granite

Answer 28

A coarse, silicic intrusive igneous rock

Question 29

An igneous rock that forms new continental crust

Answer 29

Granite

Question 30

An igneous rock that forms new oceanic crust

Answer 30

Basalt

Question 31

Andesite

Answer 31

A fine, intermediate extrusive igneous rock

Question 32

Microdiorite

Answer 32

A medium, intermediate intrusive igneous rock

Question 33

Diorite

Answer 33

A coarse, intermediate intrusive igneous rock

Question 34

Scoria

Answer 34

A glassy, mafic, extrusive igneous rock

Question 35

Basalt

Answer 35

A fine, mafic, extrusive igneous rock

Question 36

Dolerite

Answer 36

A medium, mafic, intrusive igneous rock

Question 37

Gabbro

Answer 37

A coarse, mafic, intrusive igneous rock

Question 38

Peridotite

Answer 38

A coarse, ultramafic igneous rock

Question 39

Rock that makes up the mantle

Answer 39

Peridotite

Question 40

Silica content in silicic rocks

Answer 40

> 66%

Question 41

Silica content in intermediate rocks

Answer 41

52 - 66%

Question 42

Silica content in mafic rocks

Answer 42

45-52%

Question 43

Silica content in ultramafic rocks

Answer 43

< 45%