Igneous rocks and processes

Question 1

What is a felsic mineral?

Answer 1

Light coloured and silica rich.

Question 2

What is a mafic mineral?

Answer 2

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

Question 3

What is a phenocryst?

Answer 3

A large or conspicuous crystal in an igneous rock.

Question 4

Name five mineral characteristics of quartz.

Answer 4

• Hardness of 7 (can’t be scratched with a steel nail).
• White/grey/transparent.
• No cleavage.
• Vitreous lustre (glassy).
• Composition SiO2.

Question 5

Name five mineral characteristics of feldspar.

Answer 5

• Hardness of 6 (can be scratched with a steel nail).
• K feldspar is pink, plagioclase feldspar is white or grey.
• Good cleavage in two directions.
• Vitreous lustre (glassy).
• Composition K feldspar- KAlSi3O8 plagioclase feldspar - NaAlSi3O8 to CaAl2Si2O8

Question 6

Name five mineral characteristics of hornblende (amphibole).

Answer 6

• Hardness of 5.5.
• Black
• Cleavage at 60°.
• Forms 6-sided crystals.
• Vitreous lustre (glassy).

Question 7

Name five mineral characteristics of augite (pyroxene).

Answer 7

• Hardness of 5.5.
• Black to dark green.
• Two cleavages at 90°.
• Forms 8-sided crystals.
• Vitreous lustre (glassy).

Question 8

Name four mineral characteristics of olivine.

Answer 8

• Hardness of 6.5.
• Light green.
• No cleavage.
• Vitreous lustre (glassy).

Question 9

Name two fine, silicic igneous rocks.

Answer 9

Pumice and Rhyolite

Question 10

Name a medium, silicic igneous rock.

Answer 10

Microgranite

Question 11

Name a coarse, silicic igneous rock.

Answer 11

Granite

Question 12

Name a fine, intermediate igneous rock.

Answer 12

Andesite

Question 13

Name a medium, intermediate igneous rock.

Answer 13

Microdiorite

Question 14

Name a coarse, intermediate igneous rock.

Answer 14

Diorite

Question 15

Name a fine, mafic igneous rock.

Answer 15

Basalt

Question 16

Name a medium, mafic igneous rock.

Answer 16

Dolerite

Question 17

Name a coarse, mafic igneous rock.

Answer 17

Gabbro

Question 18

Name a coarse, ultramafic igneous rock.

Answer 18

Peridotite

Question 19

What texture does obsidian have?

Answer 19

Glassy with conchoidal fracture.

Question 20

What texture does pumice have?

Answer 20

Glassy shards, fine crystals. Vesicular.

Question 21

What texture does rhyolite have?

Answer 21

Fine crystals. Flow banded.

Question 22

What texture does granite have?

Answer 22

Coarse crystals. Porphyritic or equigranular.

Question 23

What texture does andesite have?

Answer 23

Fine, vesicular or amygdaloid or porphyritic or equigranular.

Question 24

What texture does basalt have?

Answer 24

Fine, vesicular or amygdaloidal or porphyritic or equigranular.

Question 25

What texture does dolerite have?

Answer 25

Medium, porphyritic or equigranular.

Question 26

What texture does gabbro have?

Answer 26

Coarse and equigranular.

Question 27

Where do fine crystals form?

Answer 27

They can form in lava extruded onto the surface that cools rapidly over weeks or months. They can also form below the surface as chilled margins at the edges of minor intrusions such as sills and dykes.

Question 28

Where do medium crystals form?

Answer 28

They form below the surface, in minor intrusions (hypabyssal). They cool slowly over a few thousand years.

Question 29

Where do coarse crystals form?

Answer 29

Major intrusions form 10km or more beneath the surface, as batholiths or plutonic rocks. They cool very slowly over millions of years.

Question 30

What process is involved with the nucleation of crystals?

Answer 30

Endothermic process. Requiring energy from the magma.

Question 31

What process is involved with crystal growth?

Answer 31

Exothermic process. Putting energy into the magma, keeping it hot. Insulating effect of rock surrounding deep intrusions act to keep the nucleation rate low which allows crystals to grow to larger ones due to fewer crystals being able to nucleate.

Question 32

What does ‘glassy texture’ mean?

Answer 32

Glassy or Vitreous texture means that there are no crystals, resembles glass. It is a result in very rapid cooling where crystals had no time to form.

Question 33

What does ‘equicrystalline texture’ mean?

Answer 33

All of the crystals are of equal size but may be fine, medium or coarse.

Question 34

How does vesicular/amygdaloidal textured rocks form?

Answer 34

Vesicular texture is where gas bubbles are trapped as the lava is cooled rapidly, leaving holes where the gas was present. The vesicles are usually oval or ellipsoid and elongated parallel to the direction of flow. Amygdaloidal is where vesicles have been later infilled by minerals deposited from percolating groundwater.

Question 35

What does ‘flow banding’ mean?

Answer 35

Flow banding occurs where layers of dark and light minerals form due to the separation of minerals within a lava flow. Aligned parallel to the flow of direction, usually seen in rhyolite.

Question 36

How does ‘porphyritic texture’ form?

Answer 36

When a rock has two stages of cooling which results in two distinct sizes of crystals. Phenocrysts are formed first by cooling slowly.

Question 37

What is ‘Ophitic texture’?

Answer 37

Forms when an elongate crystal is enclosed by another mineral.

Question 38

How does ‘cumulate texture’ form?

Answer 38

When crystals settle out of the magma, typically on the floor of the magma chamber but can be on the walls or roof and accumulate in mutual contact.

Question 39

What is the average geothermal gradient near the surface?

Answer 39

30°

Question 40

What does adiabatic process mean?

Answer 40

Convection within the Earths mantle allows hot, less dense rock to rise and cool, more dense rock to sink without any melting. Adiabatic process is a thermodynamic process in which no heat enters or leaves the system during expansion or compression.

Question 41

What is adiabatic cooling?

Answer 41

When crust or mantle material rises it undergoes expansion and the temperature falls; with no loss or gain of thermal energy (spread over a greater volume).

Question 42

How does water effect the melting process at subduction zones?

Answer 42

Water in the rocks of the oceanic crust is carried down into the hot mantle. As descending plates heat up, the water is released into the mantle rock above the plate. Water lowers the melting points of minerals in the mantle rock and partial melting of the mantle produces magma. Process is known as flux melting.

Question 43

How is magma formed at an oceanic-oceanic plate margin?

Answer 43

Erupting magma will be mafic to intermediate in composition (from overriding plate being oceanic). If magma rises quickly at shallow depths then it will be basalt.

Question 44

How is magma formed at a continental-oceanic plate margin?

Answer 44

Overriding plate is continental, magma rises through thicker, silicic crust and may partially melt due to the increasing temperature. Mafic and silicic magma will be present, the result is mixing the two to give intermediate to silicic. They have different viscosities which makes mixing difficult. Some magma will reach the surface to form intermediate volcanoes, most will be intruded to form granite batholiths.