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

1. Geology AS terms

2. Silicic, intermediate, mafic and ultramafic % silica
What % Si02??
66
7
52
6
5
45
1
3
2
4

3. 10
9
8

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. 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. 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. These are zoned plagioclase crystals.
33
The core will be more calcium rich.
34
35
The rim will be more sodium rich.
(explain…)

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. See http://geologicalintroduction.baffl.co.uk/?p=3 for some good igneous pics etc

11. 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

12. 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.

13. 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

14. 42
43
(or is assimilated)

15. 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

16. Where on this slide might magma mixing take place? (45)

17. 46
vesicular
47
amygdaloidal
What igneous textures are shown here?

18. 48
amygdales
49
phenocrysts
It is easy to confuse amygdales and phenocrysts.
Which is which in this thin section, and how can
you tell?

19. Very vesicular rocks (called pumice) can be very light!
50

20. 51
porphyritic
What igneous texture is shown here?
(larger phenocrysts in smaller “groundmass” of crystals)

21. porphyritic
It forms due to….
52
53
different stages / rates of cooling

22. 54
equigranular
And the opposite of porphyritic?
(crystals of more-or-less the same size)

23. equigranular
This tells you that the crystals all…
56
…cooled at a similar rate / speed
55

24. 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)

25. 59
Small crystal size… but what size specifically?
60
61
…smaller than… 1mm Forms in extrusive rocks

26. 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

27. Above 5mm and the texture is called…
65
coarse-grained … and forms in intrusive rocks
64
66
…specifically those formed deep underground: plutonic

28. 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

29. Special texture often found in viscous volcanic rocks:
68
flow banding

30. 70
69
71
72
73

31. 74

32. 76
75

33. 77

34. 78

35. Tectonic setting for different volcanic rocks – and WHY??
79 - basalt
80 - andesite

36. 83
82
Rock types?? Tectonic setting?
81

38. 85
84

39. Deposited from pyroclastic flow …so what is it?
elongated clasts show sense of
‘flow’ (not always present)
Angular clasts (‘bits’)
Ash matrix
85
ignimbrite

40. …so what is it?
86
tuff
A few angular clasts
(but not many – mostly ash)

41. Lots of volcanic ‘bombs’, often rounded
Ash matrix
…so what is it?
87
agglomerate