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