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Geology > Igneous rocks and processes > Flashcards

Igneous rocks and processes Flashcards

1
Q

What is a felsic mineral?

A

Light coloured and silica rich.

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2
Q

What is a mafic mineral?

A

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

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3
Q

What is a phenocryst?

A

A large or conspicuous crystal in an igneous rock.

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4
Q

Name five mineral characteristics of quartz.

A
  • Hardness of 7 (can’t be scratched with a steel nail).
  • White/grey/transparent.
  • No cleavage.
  • Vitreous lustre (glassy).
  • Composition SiO2.
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5
Q

Name five mineral characteristics of feldspar.

A
  • 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
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6
Q

Name five mineral characteristics of hornblende (amphibole).

A
  • Hardness of 5.5.
  • Black
  • Cleavage at 60°.
  • Forms 6-sided crystals.
  • Vitreous lustre (glassy).
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7
Q

Name five mineral characteristics of augite (pyroxene).

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

Name four mineral characteristics of olivine.

A
  • Hardness of 6.5.
  • Light green.
  • No cleavage.
  • Vitreous lustre (glassy).
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9
Q

Name two fine, silicic igneous rocks.

A

Pumice and Rhyolite

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10
Q

Name a medium, silicic igneous rock.

A

Microgranite

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11
Q

Name a coarse, silicic igneous rock.

A

Granite

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12
Q

Name a fine, intermediate igneous rock.

A

Andesite

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13
Q

Name a medium, intermediate igneous rock.

A

Microdiorite

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14
Q

Name a coarse, intermediate igneous rock.

A

Diorite

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15
Q

Name a fine, mafic igneous rock.

A

Basalt

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16
Q

Name a medium, mafic igneous rock.

A

Dolerite

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17
Q

Name a coarse, mafic igneous rock.

A

Gabbro

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18
Q

Name a coarse, ultramafic igneous rock.

A

Peridotite

19
Q

What texture does obsidian have?

A

Glassy with conchoidal fracture.

20
Q

What texture does pumice have?

A

Glassy shards, fine crystals. Vesicular.

21
Q

What texture does rhyolite have?

A

Fine crystals. Flow banded.

22
Q

What texture does granite have?

A

Coarse crystals. Porphyritic or equigranular.

23
Q

What texture does andesite have?

A

Fine, vesicular or amygdaloid or porphyritic or equigranular.

24
Q

What texture does basalt have?

A

Fine, vesicular or amygdaloidal or porphyritic or equigranular.

25
Q

What texture does dolerite have?

A

Medium, porphyritic or equigranular.

26
Q

What texture does gabbro have?

A

Coarse and equigranular.

27
Q

Where do fine crystals form?

A

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.

28
Q

Where do medium crystals form?

A

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

29
Q

Where do coarse crystals form?

A

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

30
Q

What process is involved with the nucleation of crystals?

A

Endothermic process. Requiring energy from the magma.

31
Q

What process is involved with crystal growth?

A

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.

32
Q

What does ‘glassy texture’ mean?

A

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.

33
Q

What does ‘equicrystalline texture’ mean?

A

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

34
Q

How does vesicular/amygdaloidal textured rocks form?

A

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.

35
Q

What does ‘flow banding’ mean?

A

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.

36
Q

How does ‘porphyritic texture’ form?

A

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

37
Q

What is ‘Ophitic texture’?

A

Forms when an elongate crystal is enclosed by another mineral.

38
Q

How does ‘cumulate texture’ form?

A

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.

39
Q

What is the average geothermal gradient near the surface?

A

30°

40
Q

What does adiabatic process mean?

A

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.

41
Q

What is adiabatic cooling?

A

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).

42
Q

How does water effect the melting process at subduction zones?

A

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.

43
Q

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

A

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.

44
Q

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

A

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.

Geology (2 decks)

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