2. IGNEOUS INTRUSIONS Ardnamurchan, W. Scotland

3. 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.

4. 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.

5. 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.

6. Dykes are sheets of igneous rocks which cut across bedding planes or igneous or metamorphic foliation. Are they discordant or concordant? Lava flows DISCORDANT

7. This dyke in Tenerife cuts across the country rocks which are basalt lava flows.

8. 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

9. Dyke in S. Arran cutting through red sandstone. Baked margin in sandstones Dolerite dyke 0.75m Chilled margin in dyke, probably basalt.

10. 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.

11. 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

12. Rhum dyke Gabbro Dolerite dyke 0.5m This dyke cuts across a previous intrusion of gabbro. Rhum, N.W. Scotland

13. 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

14. Series of silicic dykes cutting discordantly across a metamorphosed sandstone. C A B C In what order were the dykes intruded?

15. Dykes cool from the edges to the centre. As they cool they contract producing cooling joints. These usually run in two directions at 90 o parallel to the cooling surfaces.

16. Sills Sills are igneous bodies which lie parallel to bedding planes. Salisbury Crag dolerite sill Edinburgh

17. Bamburgh Castle, Northumbria, stands on a sill of dolerite called the Great Whin Sill dolerite Carboniferous sandstone

18. Contact of Great Whin Sill with sandstones, below Bamburgh Castle dolerite Carboniferous sandstones Slightly recrystallised bedding planes CHILLED MARGIN BAKED MARGIN

19. Drumadoon Sill, Arran Red sandstones and shales Quartz feldspar porphyry sill with basalt margins Columnar jointing 40m Scree made of fallen columns

20. Quartz feldspar porphyry 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

21. 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.

22. Cuillins Cuillins, Skye Gabbro pluton, cut by dolerite dykes, seen outlined in the snow.

23. 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

24. 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.

25. Plutons in N.W. Scotland Gabbro pluton in Rhum Gabbro pluton of Ardnamurchan Gabbro in Skye Basalt lava flows on Eigg and Muck

26. Peggy’s Cove lighthouse, on granite pluton, Nova Scotia, Canada

27. Equigranular granite, Nova Scotia, Canada

28. Pegmatite vein, in Peggy’s Cove granite

29. Large grey xenoliths in granite, Ingonish, Nova Scotia

30. Arran granite pluton from Kintyre THE END