chilled and baked margins

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Chilled and Baked Margins• The specification states that

you need to be able to:

• explain the origin and nature

of chilled and baked margins 

and metamorphic aureoles. • distinguish between intrusive

and extrusive igneous rocks.

• Explain the differences

between rocks formed by

these methods

 – Sill

 – Lava flow

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Chilled and Baked Margins• When an intrusion of hot magma occurs the

surrounding rocks will be heated up and baked.• This is a kind of metamorphism and it is called a

baked margin.

• The magma at the edge of the intrusion will cooldown quickly and so will form smaller crystals 

than in the middle.• This is called a chilled margin.

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Metamorphic Aureole

• Batholiths heat a much

larger zone of country

rock so that a

metamorphic aureole is

100's of metres toseveral kilometres.

• The rocks are altered by

contact metamorphism to produce a

metamorphic aureole. 

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Distinguishing between intrusive

and extrusive igneous rocks.•

Write down as many differences asyou can think of.

• Grain size.

 – Extrusive will be fine grained or glassy.

 –

Intrusive from fine grained to coarsegrained.

• Relationship to the country rock.

 – Extrusive parallel to beds.

 – Intrusive concordant if a sill but often

discordant (dykes and batholiths).

• Extrusive may have vesicles.

• Extrusive may have weathered

surfaces.

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Distinguishing between a sill and

a lava flow

• Look at the diagram above which shows a sill and a lavaflow.

• Look carefully at the diagram and note down as manydifferences as you can.

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Distinguishing between a

sill and a lava flow•

Baked margins: – Sill will have 2.

 – Lava flow will have 1

• Both will have 2 chilled

margins.• Grain size:

 – The lava flow will be fine orglassy.

 – The sill will be fine or medium.• The sill may occasionally cut

across the beds.

• Lava flow should remain

concordant.

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Distinguishing between a

sill and a lava flow• The lava flow may have

vesicles towards the top.

• The lava flow may have aweathered (reddened?) topsurface.

• The sill may have xenolithsfrom the country rock(included fragments)

• A xenolith is a bit of thecountry rock that has fallen

into the intruding magmabecause of the forcefulnature of the intrusion.

• There may be clasts of thelava flow in the next

(younger bed).