IGNEOUS
INTRUSIONS
Ardnamurchan, W. Scotland
IGNEOUS INTRUSIONS
•Magma moves through joints, fractures and
between the crystals of the solid rock of the crust
and mantle.
•When it reaches its freezing temperature, it
crystallises.
•Dykes, sills and plutons are igneous bodies that
have cooled from magma beneath the surface.
•If the magma crystallises at depths of 20/30km it is
called a plutonic rock and will have large crystals.
•If the magma crystallises at a shallow depth below
the surface it will have small to medium crystals.
CHILLED AND BAKED
MARGINS
• When intrusions cool they will crystallise fastest
where they are in contact with the colder
country rock.
• Crystals on the edge of the intrusion will be
smaller than those in the centre.
• The outside edge with the smallest crystals is
called the CHILLED MARGIN.
• The country rock will be heated by the magma
next to it. The country rock will be baked by the
heat and may recrystallise. This is called the
BAKED MARGIN.
INTRUSIONS
• Intrusions crystallise within the country rock,
which can be igneous, sedimentary or
metamorphic.
• The magma will follow a route which is at the
least pressure, usually along fractures or cracks.
• If a sedimentary country rock has bedding
planes it is easy to see whether an intrusion of
magma has followed the bedding planes or cut
across them.
• If magma cuts across bedding planes it is called
a DISCORDANT intrusion.
• If magma follows bedding planes it is called a
CONCORDANT intrusion.
Dykes are
sheets of
igneous
rocks which
cut across
bedding
planes or
igneous or
metamorphic
foliation. Are
they
discordant or
concordant?
DISCORDANT
Lava flows
This dyke in Tenerife cuts
across the country rocks
which are basalt lava flows.
Chilled margin has small crystals which have weathered fast.
Gently dipping
Jurassic beds
Dolerite dyke
Baked margin of sandstone is hard
because it has recrystallised.
Ardnamurchan, N. W.
Scotland
Dyke in S. Arran
cutting through
red sandstone.
Chilled margin
in dyke,
probably
basalt.
Baked margin in
sandstones
0.75m
Dolerite dyke
Some dykes weather faster than the country
rock around.
Red
sandstone
baked
margin
dolerite
Corrie Shore, Arran
Describe the
baked margin
and say how it
has been
altered by the
intrusion of the
dolerite dyke.
Field sketch to show dyke on Corrie shore, Arran
Now add the title
1.22m
Baked
margin
Basalt
Chilled
margin
More resistant
dolerite
Recrystallised,
hard metaquarzite
Country
rock is red
sandstone
Closely spaced
cooling joints
This
dyke
cuts
across a
previous
intrusion
of
gabbro.
Rhum,
N.W.
Scotland
0.5m
Rhum dyke
Gabbro
Dolerite
dyke
Dyke at
Dyke at Blackwaterfoot Beach, Arran
Blackwaterfoot
The country
Beach, Arran
rock is red
sandstone
which is
usually
buried under
the sand.
Cooling
joints
2.5m
Dolerite in
the centre
of the
dyke
Series of silicic dykes cutting discordantly
across a metamorphosed sandstone.
CA
B
C
In what order were the dykes intruded?
Dykes cool
from the
edges to the
centre. As
they cool they
contract
producing
cooling joints.
These usually
run in two
directions at
90o parallel to
the cooling
surfaces.
Sills are igneous bodies which lie
parallel to bedding planes.
Edinburgh
Salisbury Crag dolerite sill
Sills
Bamburgh Castle, Northumbria, stands on a
sill of dolerite called the Great Whin Sill
dolerite
Carboniferous sandstone
Contact of
Great Whin
Sill with
sandstones,
below
Bamburgh
Castle
Slightly
recrystallised
bedding planes
dolerite
CHILLED
MARGIN
BAKED
MARGIN
Carboniferous
sandstones
Drumadoon Sill, Arran
Quartz feldspar
porphyry sill with
basalt margins
Columnar
jointing
40m
Scree made of
fallen columns
Red sandstones and
shales
Field sketch to show the Drumadoon Sill, NOW DRAW YOUR
Arran, showing the relationship of the sill OWN FIELD
SKETCH
with the country rock.
Basalt
Quartz feldspar porphyry
40m
Basalt
Country rock
of red
sandstones
BEDDING PLANES
Scree of fallen
columns
PLUTONS
•Plutons are large sheets of igneous rock, up to 510km thick, that cooled 20-30km below the surface.
•The rocks that form plutons are always coarsegrained because they cooled slowly. Plutons are
usually made of granite, diorite or gabbro.
Gabbro of the Cuillins, Skye.
Cuillins, Skye
Cuillins
Gabbro pluton, cut by dolerite dykes, seen outlined
in the snow.
Batholiths are made of many
separate plutons. The S.W.
England granite is a good
example.
30cm
Top of granite pluton at
Porthmeor, Cornwall
Granite with large
phenocrysts
Coarse crystals, with very large phenocrysts
Baked
in the granite
margin in
the local
slates.
Chilled margin of
fine crystals in the
granite
Igneous contact in Porthmeor Bay, Cornwall.
Plutons in N.W. Scotland
Gabbro pluton
in Rhum
Gabbro in
Skye
Basalt lava flows on
Eigg and Muck
Gabbro pluton of Ardnamurchan
Peggy’s Cove lighthouse, on granite pluton, Nova
Scotia, Canada
Equigranular granite, Nova Scotia, Canada
Pegmatite vein, in Peggy’s Cove
granite
Large grey xenoliths in granite, Ingonish,
Nova Scotia
THE END
Arran granite pluton from Kintyre