Geology AS terms
Silicic, intermediate, mafic and ultramafic % silica
What % Si02??
66
7

52
6

5
45

1
3
2

4
10

9

8
g

a

b

h

c
d

i

e

f
11. What type of plate margin is shown? (a)

Divergent or constructive

12. What is the compositi...
18
19
20
Bowen’s reaction series

21

23

22
24
What do we mean by the
continuous series (25)
and the discontinuous
series...
27

27

27

27

(mineral name)

This rock formed layer of olivine crystals (or whatever) is called a cumulate.
28

The mag...
These are zoned plagioclase crystals.
33
The core will be more calcium rich.
34

35
The rim will be more sodium rich.
(exp...
In these pictures the paler
(leucocratic) melt has been
“squeezed out” by mafic
minerals settling in the
magma – this is c...
See http://geologicalintroduction.baffl.co.uk/?p=3 for some good igneous pics etc
This is a granite. There are some flakes of biotite mica – but what
is the other mafic mineral in here?
Hornblende…. which...
This is gabbro. So what is the main mafic mineral in here?
(shown close up on the right)

39
40
Augite…. which is a… pyrox...
The upward movement of magma through the crust which
involves magma intruding along joints and bedding planes and
other cr...
42

43
(or is assimilated)
Assimilation will change the composition of a magma - but another
way that magma composition can change is when magmas of
...
Where on this slide might magma mixing take place? (45)
46
vesicular

47
amygdaloidal

What igneous textures are shown here?
48
amygdales

49
phenocrysts

It is easy to confuse amygdales and phenocrysts.
Which is which in this thin section, and ho...
Very vesicular rocks (called pumice) can be very light!
50
51
porphyritic

What igneous texture is shown here?
(larger phenocrysts in smaller “groundmass” of crystals)
porphyritic

It forms due to….
52
53
different stages / rates of cooling
54
equigranular

And the opposite of porphyritic?
(crystals of more-or-less the same size)
equigranular

This tells you that the crystals all…
56
…cooled at a similar rate / speed
55
You know what this
rock is…
57
obsidian
…but what does it tell
you about the cooling
rate?
Super-quick
58

(no time for cr...
59

Small crystal size… but what size specifically?
60
61
…smaller than… 1mm Forms in extrusive rocks
Hypabyssal rocks formed in sills and dykes will cool
62
a bit more slowly – so will have medium-grained
63
texture – like ...
Above 5mm and the texture is called…
65
coarse-grained … and forms in intrusive rocks
64
66
…specifically those formed dee...
However, when there are lots of ‘volatiles’ around in the late
stages of cooling of granite magmas, veins can form with ve...
Special texture often found in viscous volcanic rocks:
68
flow banding
70
69

71

72

73
74
76

75
77
78
Tectonic setting for different volcanic rocks – and WHY??

79 - basalt

80 - andesite
83

82

Rock types?? Tectonic setting?

81
85

84
Deposited from pyroclastic flow …so what is it?

elongated clasts show sense of
‘flow’ (not always present)

Angular clast...
…so what is it?

86
tuff

A few angular clasts
(but not many – mostly ash)
Lots of volcanic ‘bombs’, often rounded

Ash matrix

…so what is it?

87
agglomerate
Geology OCR A-level igneous rocks terms revision interactive - need to download for interactivity / animations / answers t...
Geology OCR A-level igneous rocks terms revision interactive - need to download for interactivity / animations / answers t...
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Geology OCR A-level igneous rocks terms revision interactive - need to download for interactivity / animations / answers to work

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OCR Geology AS-level igneous terms - all terms from Specification covered - interactive so students can self-test

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Geology OCR A-level igneous rocks terms revision interactive - need to download for interactivity / animations / answers to work

  1. 1. Geology AS terms
  2. 2. Silicic, intermediate, mafic and ultramafic % silica What % Si02?? 66 7 52 6 5 45 1 3 2 4
  3. 3. 10 9 8
  4. 4. g a b h c d i e f 11. What type of plate margin is shown? (a) Divergent or constructive 12. What is the composition of the oceanic crust at b? ultramafic, basaltic, andesitic or granitic? 13. Letter d shows the crust and upper mantle – what is the word for this layer? lithosphere 14. What is the composition of the magma at e? mafic or basaltic or gabbro(ic) artial _______ 15. What process in the asthenosphere produces magma e? p_____ melting 16. What is the name for the lava erupted at g? basalt or MORB (ask!) 17. What two minerals would you be most likely to find in the rock at i? olivine and pyroxene
  5. 5. 18 19 20 Bowen’s reaction series 21 23 22 24 What do we mean by the continuous series (25) and the discontinuous series (26) ?
  6. 6. 27 27 27 27 (mineral name) This rock formed layer of olivine crystals (or whatever) is called a cumulate. 28 The magma composition goes from A to B to C to D This change in composition is because the magma 29 becomes relatively depleted in mafic minerals and 30 therefore relatively saturated in silica 31 32
  7. 7. These are zoned plagioclase crystals. 33 The core will be more calcium rich. 34 35 The rim will be more sodium rich. (explain…)
  8. 8. In these pictures the paler (leucocratic) melt has been “squeezed out” by mafic minerals settling in the magma – this is called… 36 FILTER PRESSING Can you explain why the “squeezed out” melt is leucocratic??
  9. 9. See http://geologicalintroduction.baffl.co.uk/?p=3 for some good igneous pics etc
  10. 10. This is a granite. There are some flakes of biotite mica – but what is the other mafic mineral in here? Hornblende…. which is an… amphibole. 38 37
  11. 11. This is gabbro. So what is the main mafic mineral in here? (shown close up on the right) 39 40 Augite…. which is a… pyroxene.
  12. 12. The upward movement of magma through the crust which involves magma intruding along joints and bedding planes and other cracks in the country rocks is called… STOPING 41
  13. 13. 42 43 (or is assimilated)
  14. 14. Assimilation will change the composition of a magma - but another way that magma composition can change is when magmas of different composition mix together… Unsurprisingly, the term for this is magma44 mixing NOTE: this does not happen that often – magmas of different composition don’t mix together easily – they are different density, viscosity, temperature, etc. – think of oil and water Pinatubo’s eruption is thought to have been caused by this process
  15. 15. Where on this slide might magma mixing take place? (45)
  16. 16. 46 vesicular 47 amygdaloidal What igneous textures are shown here?
  17. 17. 48 amygdales 49 phenocrysts It is easy to confuse amygdales and phenocrysts. Which is which in this thin section, and how can you tell?
  18. 18. Very vesicular rocks (called pumice) can be very light! 50
  19. 19. 51 porphyritic What igneous texture is shown here? (larger phenocrysts in smaller “groundmass” of crystals)
  20. 20. porphyritic It forms due to…. 52 53 different stages / rates of cooling
  21. 21. 54 equigranular And the opposite of porphyritic? (crystals of more-or-less the same size)
  22. 22. equigranular This tells you that the crystals all… 56 …cooled at a similar rate / speed 55
  23. 23. You know what this rock is… 57 obsidian …but what does it tell you about the cooling rate? Super-quick 58 (no time for crystals to form)
  24. 24. 59 Small crystal size… but what size specifically? 60 61 …smaller than… 1mm Forms in extrusive rocks
  25. 25. Hypabyssal rocks formed in sills and dykes will cool 62 a bit more slowly – so will have medium-grained 63 texture – like this microgranite and this dolerite. 64 And what specific crystal size? 1-5mm
  26. 26. Above 5mm and the texture is called… 65 coarse-grained … and forms in intrusive rocks 64 66 …specifically those formed deep underground: plutonic
  27. 27. However, when there are lots of ‘volatiles’ around in the late stages of cooling of granite magmas, veins can form with very large crystals (often 5cm+, even up to metres in diameter…) 67 pegmatite
  28. 28. Special texture often found in viscous volcanic rocks: 68 flow banding
  29. 29. 70 69 71 72 73
  30. 30. 74
  31. 31. 76 75
  32. 32. 77
  33. 33. 78
  34. 34. Tectonic setting for different volcanic rocks – and WHY?? 79 - basalt 80 - andesite
  35. 35. 83 82 Rock types?? Tectonic setting? 81
  36. 36. 85 84
  37. 37. Deposited from pyroclastic flow …so what is it? elongated clasts show sense of ‘flow’ (not always present) Angular clasts (‘bits’) Ash matrix 85 ignimbrite
  38. 38. …so what is it? 86 tuff A few angular clasts (but not many – mostly ash)
  39. 39. Lots of volcanic ‘bombs’, often rounded Ash matrix …so what is it? 87 agglomerate

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