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?
Lava flows
DISCORDANT
This dyke in Tenerife cuts across the country rocks which are basalt lava flows.
Gently dipping Jurassic beds
Baked margin of sandstone is hard because it has recrystallised.
Chilled margin has small crystals which have weathered fast.
Ardnamurchan, N. W. Scotland
Dolerite dyke
Dyke in S. Arran cutting through red sandstone.
Baked margin in sandstones
Dolerite dyke0.75m
Chilled margin in dyke, probably basalt.
Some dykes weather faster than the country rock around.
Corrie Shore, Arran
dolerite
Red sandstone baked margin
Describe the baked margin and say how it has been altered by the intrusion of the dolerite dyke.
Now add the title
Baked margin
Country rock is red sandstone
1.22m
Chilled margin
Closely spaced cooling joints
More resistant dolerite
Basalt
Field sketch to show dyke on Corrie shore, Arran
Recrystallised, hard metaquarzite
Rhum dyke
Gabbro
Dolerite dyke
0.5mThis dyke cuts across a previous intrusion of gabbro.
Rhum, N.W. Scotland
Dyke at Blackwaterfoot Beach, Arran
Dyke at Blackwaterfoot Beach, Arran
Dolerite in the centre of the dyke
Cooling joints
The country rock is red sandstone which is usually buried under the sand.
2.5m
Series of silicic dykes cutting discordantly across a
metamorphosed sandstone.
CA
BC
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.
SillsSills are igneous bodies which lie parallel to bedding planes.
Salisbury Crag dolerite sill
Edinburgh
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
dolerite
Carboniferous sandstones
Slightly recrystallised bedding planes
CHILLED MARGIN
BAKED MARGIN
Drumadoon Sill, Arran
Red sandstones and shales
Quartz feldspar porphyry sill with basalt margins
Columnar jointing
40m
Scree made of fallen columns
Quartz feldspar porphyry
Basalt
Basalt
Scree of fallen columns
Country rock of red sandstones
40m
Field sketch to show the Drumadoon Sill, Arran, showing the relationship of the sill with the country rock.
BEDDING PLANES
NOW DRAW YOUR OWN FIELD SKETCH
•Plutons are large sheets of igneous rock, up to 5-10km thick, that cooled 20-30km below the surface.
•The rocks that form plutons are always coarse-grained because they cooled slowly. Plutons are usually made of granite, diorite or gabbro.
PLUTONS
Gabbro of the Cuillins, Skye.
CuillinsCuillins, Skye
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.
Granite with large phenocrysts
Top of granite pluton at Porthmeor, Cornwall
30cm
Coarse crystals, with very large phenocrysts in the granite
Chilled margin of fine crystals in the granite
Baked margin in the local slates.
Igneous contact in Porthmeor Bay, Cornwall.
Plutons in N.W. Scotland
Gabbro pluton in Rhum
Gabbro pluton of Ardnamurchan
Gabbro in Skye
Basalt lava flows on Eigg and Muck
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
Arran granite pluton from Kintyre
THE END