Igneous Rocks

Question 1

what are igneous rocks?

Answer 1

they are rocks formed by the solidification of magma (liquid rock) - sometimes referred to as plutonic

Question 2

what are extrusive igneous rocks?

Answer 2

• formed/solidified at the ground surface where they cool quickly
• lava, pyroclasts
• small crystal size
• often glassy

Question 3

what are intrusive igneous rocks?

Answer 3

• formed/solidified underground where they cool slowly

- large crystal size

Question 4

why do extrusive igneous rocks have small crystals and why do intrusive igneous rocks have large crystals?

Answer 4

As the magma turns solid, it does so by the growth of crystals. If it cools slowly, those crystals have time to grow big, whereas if it cools quickly the crystals are small. So intrusive rocks have large crystals and extrusive rocks have small crystals.

Question 5

name a common extrusive and intrusive igneous rock?

Answer 5

basalt is EXTRUSIVE (dark in colour)

granite is INTRUSIVE (light in colour)

Question 6

what are pyroclasts usually made up of? (large pieces of rock from volcanic eruptions)

Answer 6

volcanic ash and pumice (extrusive)

Question 7

what do the terms mafic, felsic, intermediate and ultramafic relate to?

Answer 7

the description of the mineral composition of the rock

Question 8

is Basalt mafic or felsic? intrusive or extrusive?

Answer 8

basalt is MAFIC and EXTRUSIVE

Question 9

is Granite mafic or felsic? intrusive or extrusive?

Answer 9

granite is FELSIC and INTRUSIVE

Question 10

what is Rhyolite? mafic or felsic? intrusive or extrusive?

Answer 10

Rhyolite is magma that cools at the ground surface and is therefore EXTRUSIVE, however it is FELSIC so it is not the same as basalt

Question 11

what is Gabbro?

Answer 11

Gabbro is magma that cools slowly under the ground and is therefore INTRUSIVE, however it is mafic and so it is not the same as granite

Question 12

do extrusive rocks tend to be finer or coarser compared to intrusive rocks?

Answer 12

extrusive rocks —> finer because the crystals are small because the magma had less time to cool and grow big crystals

intrusive —> coarser because the crystals are large because the magma had more time to cool and therefore grow larger crystals

Question 13

what is mafic rock?

Answer 13

-dominated by magnesium and ferric minerals
-high melting point
-low viscosity
EXAMPLES: basalt which is dark grey to black, Gabbro which is basically the coarse grained intrusive equivalent of basalt

Question 14

what is felsic rock?

Answer 14

-dominated by quartz (silicon dioxide) and feldspar
-light in colour
-lower density
-low melting point
-high viscosity
EXAMPLES: granite light colour intrusive rock, rhyolite is extrusive

Question 15

give an example of an ultramafic rock?

Answer 15

peridotite

Question 16

give an example of an intermediate rock?

Answer 16

diorite and andesite

Question 17

what does porphyritic/porphyry mean?

Answer 17

rocks where there are both larger and smaller sized crystals, this can happen if the magma has started to cool before being ejected in a volcanic eruption and this terms is used to describe the composition e.g andesite porphyry

Question 18

what is a sill?

Answer 18

a horizontal form of igneous rock (see diagram)

Question 19

where are igneous rocks principally formed?

Answer 19

Igneous rocks are formed principally on or near plate boundaries, and the type of rock is related to the type of plate boundary

Question 20

what is a dyke?

Answer 20

a vertical form of igneous rock

Question 21

some info on granite?

Answer 21

• felsic
• intrusive
• coarse grained
• light in colour
• occurs in large batholiths
• main components of composition: quartz: 25% and feldspar 50%
• very strong rock
• UCS 200MPa (USC means unconfined compressive strength)
• SBP 10MPa (safe bearing pressure)
• weathers by slow decay of feldspar to clay, leaving quartz to form sandy soils

Question 22

some info on basalt?

Answer 22

• mafic
• extrusive (volcanic)
• fine grained
• dark in colour
• fine interlocking crystal mosaic
• even stronger than granite
• UCS 250MPa
• SBP 10MPa (safe bearing pressure)
• weathers by rusting and decaying to clay soils
• generally make goof aquifers (hold water)

Question 23

what are the bubbles called in lava and hence what is the name of the structure of lava?

Answer 23

lava often contains a large number of gas bubbles called vesicles - giving a structure described as vesicular, the bubbles are formed when gas comes out of solution when the pressure reduces as the lava escapes to the surface

Question 24

what is tuff?

Answer 24

solidified ash (extrusive)

Question 25

what is obsidian?

Answer 25

a dark, very glassy extrusive igneous rock that cools in a matter of seconds, giving it its distinctive appearance, it fractures conchodially like glass

Question 26

what is the term porphyritic/porphyry?

Answer 26

term used to describe a rock with large crystals in a fine grained background - and this happens when if the magma has started to cool before being ejected in a volcanic eruption so the crystals are of different sizes

Question 27

what is a phenocryst?

Answer 27

a large crystal surrounded by a matrix of finer crystals - it denotes 2 stages of cooling

Question 28

which rocks are darker, mafic or felsic?

Answer 28

mafic

Question 29

give an example of an ultramafic rock?

Answer 29

peridotite

Question 30

give examples of intermediate rock?

Answer 30

diorite and andesite