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IGNEOUS INTRUSIONS

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.

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IGNEOUS INTRUSIONS

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  1. IGNEOUS INTRUSIONS Ardnamurchan, W. Scotland

  2. 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.

  3. 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.

  4. 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.

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

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

  7. 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

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

  9. Some dykes weather faster than the country rock around. Describe the baked margin and say how it has been altered by the intrusion of the dolerite dyke. Red sandstone baked margin dolerite Corrie Shore, Arran

  10. Field sketch to show dyke on Corrie shore, Arran Recrystallised, hard metaquarzite Now add the title More resistant dolerite 1.22m Baked margin Country rock is red sandstone Basalt Closely spaced cooling joints Chilled margin

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

  12. Dyke at Blackwaterfoot Beach, Arran Dyke at Blackwaterfoot Beach, Arran The country rock is red sandstone which is usually buried under the sand. Cooling joints Dolerite in the centre of the dyke 2.5m

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

  14. 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.

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

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

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

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

  19. NOW DRAW YOUR OWN FIELD SKETCH Field sketch to show the Drumadoon Sill, Arran, showing the relationship of the sill with the country rock. Basalt Basalt Quartz feldspar porphyry 40m Country rock of red sandstones BEDDING PLANES Scree of fallen columns

  20. PLUTONS • 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. Gabbro of the Cuillins, Skye.

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

  22. Batholiths are made of many separate plutons. The S.W. England granite is a good example. Granite with large phenocrysts 30cm Top of granite pluton at Porthmeor, Cornwall

  23. Coarse crystals, with very large phenocrysts in the granite Baked margin in the local slates. Chilled margin of fine crystals in the granite Igneous contact in Porthmeor Bay, Cornwall.

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

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

  26. Equigranular granite, Nova Scotia, Canada

  27. Pegmatite vein, in Peggy’s Cove granite

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

  29. THE END Arran granite pluton from Kintyre

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