Geology AS terms

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2 4

What % Si02??

455266

Silicic, intermediate, mafic and ultramafic % silica

567

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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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What do we mean by the continuous series (25) and the discontinuous series (26) ?

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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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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

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(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?

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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

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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

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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

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

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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?

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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