Differences Between Intrusive and Extrusive Rocks

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Chilled Margin

the edge of the igneous intrusion, which is closest to the cold country rock, cools more rapidly than the rest of the intrusion and forms the chilled margin

the chilled margin may only be a few centimentres to several metres wide

the chilled margin has fine grained crystals because it cooled quickly

Baked Margin

the area of country rock adjacent to a minor intrusion gets altered by the heat from the intrusion and is called the baked margin

the rocks are recrystallised by the heat, so they have been contact metamorphosed

the rocks in the baked margin have a sugary texture and and become harder and lighter in texture

the bakend margin may be a few centimetres to a few metres wide, depending on the size of the intrusion. The larger the intrusion, the wider the baked margin

Metamorphic Aureole

Batholiths, as major intrusions, heat a much larger zone of country rock so that a metamorphic aureole can be hundreds of metres to several kilometres wide. The rocks in the aureole are altered by contact metamorphism to produce new metamorphic rocks. If the batholith is intruded into a country rock of shale, then a clear sequence of changes takes place:

closest to the intrusion, where the heat is greatest for longer, the rocks will be totally recrystallised to form hornfels

further away from the intrusion the rocks will be recrystallised to form a rock containing the metamorphic mineral andalusite.

in the zone just inside the metamorphic aureole, where the heat was least, recrystallisation is partial so that a spotted rock forms.

The width of a metamorphic aureole will depend on the size and temperature of the intrusion, the dip of the sides of the intrusion and the composition of the surrounding rocks

How would you distinguish intrusive sills from extrusive lava flows?

Sills have:

two chilled margins due to contact with cool country rock, both above and below

two baked margins, both above and below, because the sill heats all the country rock around it

xenoliths from rocks above and below the sill as the sill rips up material

may show differentiation of magma if the sill is thick and cools slowly

medium average crystal size as hypabyssal and cooled slowly

dolerite as the most common rock

rare vesicles

no fragments in the sill of overlying rocks

a regular upper surface

no weathering at upper surface

Lava flows have:

only one clear chilled margin below, while the top may be chilled or made of scoria

one baked margin below the lava flow

no xenoliths from above the flow but possibly some below

may have flow banding ot phenocrysts, may be aligned parallel to the direction of flow

fine average crystal grain size as volcanic and cooled rapidly

basalt as the most common rock

vesicles common in the upper part, may become amygdales when infilled by minerals

lava fragments in overlying sedimentary rocks as a result of erosion and deposition

irregular upper surface made up of scoria or rubble

a reddened surface or even an ancient soil if the flow was chemically weathered.

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