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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 AS terms
13
2 4
What % Si02??
455266
Silicic, intermediate, mafic and ultramafic % silica
567
8
9
10
HEAT
a
b
d
c
g
h
ie
f11. What type of plate margin is shown? (a)
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?
14. What is the composition of the magma at e?
15. What process in the asthenosphere produces magma e? p_____ _______
16. What is the name for the lava erupted at g?
17. What two minerals would you be most likely to find in the rock at i?
Divergent or constructive
lithosphere
mafic or basaltic or gabbro(ic)artial melting
basalt or MORB (ask!)
olivine and pyroxene
Bowen’s reaction series18 19 20
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24
What do we mean by the continuous series (25) and the discontinuous series (26) ?
27
27 27 27
The magma composition goes from A to B to C to D
This change in composition is because the magma becomes relatively depleted in mafic minerals and therefore relatively saturated in silica
29 3031 32
(mineral name)This rock formed layer of olivine crystals (or whatever) is called a cumulate. 28
These are zoned plagioclase crystals.
The core will be more calcium rich.
The rim will be more sodium rich.
(explain…)
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34
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In these pictures the paler (leucocratic) melt has been “squeezed out” by mafic minerals settling in the magma – this is called…
FILTER PRESSING36
Can you explain why the “squeezed out” melt is leucocratic??
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 is an… amphibole.37 38
This is gabbro. So what is the main mafic mineral in here? (shown close up on the right)
Augite…. which is a… pyroxene.39 40
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… STOPING41
42
(or is assimilated)43
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
magma 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
44
Pinatubo’s eruption is thought to have been caused by this process
Where on this slide might magma mixing take place? (45)
vesicular amygdaloidal
What igneous textures are shown here?
46 47
It is easy to confuse amygdales and phenocrysts.
Which is which in this thin section, and how can you tell?
amygdales48
phenocrysts49
Very vesicular rocks (called pumice) can be very light!50
What igneous texture is shown here? (larger phenocrysts in smaller “groundmass” of crystals)
porphyritic51
It forms due to….
different stages / rates of cooling
porphyritic
52 53
And the opposite of porphyritic? (crystals of more-or-less the same size)
equigranular54
This tells you that the crystals all…
…cooled at a similar rate / speed
equigranular
55 56
You know what this rock is…
obsidian…but what does it tell you about the cooling rate?
Super-quick (no time for crystals to form)
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Small crystal size… but what size specifically? …smaller than… 1mm Forms in extrusive rocks60 61
And what specific crystal size? 1-5mm
Hypabyssal rocks formed in sills and dykes will cool a bit more slowly – so will have medium-grained texture – like this microgranite and this dolerite.63
64
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Above 5mm and the texture is called…coarse-grained … and forms in intrusive rocks …specifically those formed deep underground: plutonic
64 6566
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…)
pegmatite67
Special texture often found in viscous volcanic rocks: flow banding68
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79 - basalt
Tectonic setting for different volcanic rocks – and WHY??
80 - andesite
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8283
Rock types?? Tectonic setting?
84
85
Angular clasts (‘bits’)
Ash matrix
elongated clasts show sense of ‘flow’ (not always present)
Deposited from pyroclastic flow …so what is it?
ignimbrite85
A few angular clasts (but not many – mostly ash)
…so what is it?
tuff86
Ash matrix
Lots of volcanic ‘bombs’, often rounded
…so what is it?
agglomerate87