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Geology AS Level > IGNEOUS ROCKS > Flashcards

IGNEOUS ROCKS Flashcards

1
Q

Igneous RFM

A

olivine - green/black
augite - 2 cleavage 90°
hornblende - 2 cleavage 60°
biotite mica - perfect basal cleavage
muscovite mica - clear, perfect basal cleavage
plagioclase feldspar - grey
k feldspar - pink
quartz - clear

Silicic - >66%
Intermediate - 66% - 52%
Mafic - 52% - 45%
Ultramafic - <45%

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2
Q

Crystal shape and size

A

Shape

  • eugedral - well formed
  • subhedral
  • anhedral - no form

Size

  • fine - <1mm
  • medium - 1-5mm
  • coarse - >5mm

Relative size

  • equicrystaline - all equal size
  • inequicrystaline - different sizes
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3
Q

Igneous rocks

A

Silicic - >66%

Fine - pumice, obsidian, rhyolite
Medium - microgranite
Coarse - granite, pegmatite

Intermediate - 66% - 52%

Fine - andesite
Medium - microdiorite
Coarse - diorite

Mafic - 52% - 45%

Fine - basalt, scoria
Medium - dolerite
Coarse - gabbro

Ultramafic - <45%

Coarse - peridotite

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4
Q

Pegmatites

A
  • cooled quickly but contain large crystals (cm’s)
  • formed from hydrothermal fluids after most of an intrusion has crystallised
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5
Q

Textures - porphyritic

A
  • 2 stages of cooling result in 2 different crystal sizes
  • slow cooling forms large ehedral crystals in magma (phenocrysts)
  • rapid cooling crystallises remaining magma forming a fine groundmass
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6
Q

Textures - amygdaloidal

A
  • when a lava cools, holes are left where gas bubbles where
  • vesicles rise to the top unless lava is too viscous
  • vesicles infilled by another mineral (e.g calcite,quartz)
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7
Q

Textures - glassy

A
  • no crystals present as a result of very rapid cooling
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8
Q

Textures - flow banding

A
  • layers of dark and light minerals form due to separation within a lava flow
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9
Q

Textures - ophitic

A
  • crystals are enclosed by another mineral
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10
Q

Textures - cumulate

A
  • layers of crystals accumulate at the bottom of an intrusion
  • desner than magma so sink
  • only common in mafic/ultramafic as magma isn’t viscous
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11
Q

Major intrusions - stoping and assimilation

A
  • magma moves upwards and separates pieces of country rock (xenolith)
  • xenoliths are assimilated and become magma
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12
Q

Minor intrusions

A
  • Concordant - intruded along the bedding plane
  • Discordant - intrusion cross cut a bedding plane
  • sill - concordant and parallel to beds
  • dyke - discordant and near verticle
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13
Q

Dyke ring complexes

A
  • ring dykes - blocks of country rock above an intrusion sink downwards as magma chamber empties. Circular faults created fill with magma
  • cone sheets - blocks of country rock above an intrusion are pushed upwards by the pressure of the magma. Circular faults created fill with magma
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14
Q

Chilled and baked margins

A
  • large temperature contrast between country rock and magma

Chilled margin

  • edge of intrusion cools quicker than the rest, forms fine crystals

Baked margin

  • country rock is recrystalised by the heat by contact metamorphism
  • rocks have a sugary texture and become hard and light
  • baked margin size depends on size of intrusion
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15
Q

Density and viscosity of magma

A

Density

  • silicic is least dense, ultramfic is most dense

Viscosity

  • chemistry - more silica = more viscous
  • temperature - hotter = less viscous
    -e.g pahoehoe is hot + runny
    -e.g a’a is cool and viscous
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16
Q

Magma recharge

A
  • magma chambers get partially emptied and recharged
  • different compositions don’t mix well - causes volatiles (co2) to be released which triggers eruption
  • if magma is stored for decades, crystallisation of mafic minerals rob the liquid of Fe and Mg so the liquid evolves to become more silicic
17
Q

Geysers

A

Magma heats groundwater to produce hotsprings
Geysers are hotsprings where steam erupts from the ground

  • water is heated >100°C but can’t boil due to confining pressure
  • some water rises, reduces pressure of water below
  • Flash boiling due to pressure drop
  • steam expands and pushes out remaining water
  • water drains into ground
  • repeat
18
Q

Mafic shield volcanoes

A
  • e.g Manua Loa, Hawaii

Plate location

  • oceanic crust
  • divergent margin, MOR, hotspot

Magma

  • low in silica 52% - 45%
  • non viscous - lots of basalt

Shape

  • wide base, circular, gentle slope
  • lava flows easily

Eruptions

  • effusive and frequent
  • quiet, non explosive, non hazardous
19
Q

Silicic strato volcanoes

A
  • e.g Mount Saint Helens, Washington USA

Plate location

  • continental crust
  • destructive margin, island arcs, cordilleras

Magma

  • high in silica >66% - 52%
  • viscous
  • lots of gas and vesicles
  • pyroclastics

Shape

  • steep sides, lava dome in crater, caldera
  • short lava flows - largest pyroclastic are near the top

Eruptions

  • rare (1000s year intervals)
  • violent, explosive, huge volumes, hazardous
20
Q

Distribution of pyroclastics

A
  • grain size - small particles go further
  • wind - wind blows fine particles
  • energy of blast - more energy = more distance

Ash - <2mm
Lapilli - 2 - 64mm
Block - >64mm (hard rock)
Bomb - >64mm (lava)

21
Q

Calderas

A
  1. a series of violent eruptions remove large volumes of magma
  2. Magma chamber begins to empty and the top begins to collapse down
  3. The entire cone collapse due to more violent eruptions

Geology AS Level (10 decks)

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