IGNEOUS ROCKS How are igneous rocks formed? Colour Chemical composition Mineralogy Texture What are types of igneous rocks are there? Where do Igneous Rocks Form? Constructive plate margins (basalt) Destructive plate margins (andesite) Hot spots (basalt) How can igneous rocks be classified? Magma Rise through crust Cool Crystallisation Extrusive Intrusive (molten rock) (less dense) (country rock, air or water) (reaches melting point) (lava flows) (dykes, sills, batholiths) (felsic, intermediate, mafic, ultramafic) (acidic, intermediate, basic, ultrabasic) (Q, F, M, A, H, O) (Crystalline, interlocking, randomly orientated, grain size, equigranular, porphyritic) Structures (Xenolith, vesicular, amygdaloidal, flow banding, columnar jointing)

small intrusions right angles to bedding planes discordant 1 – 2 m chilled margin at edges small intrusions follow bedding planes concordant 1 – 100 m chilled margin at edges Baked margin very large intrusions cuts across bedding planes discordant many km’s chilled margin at edges

What am I ? Dyke because I am discordant (cut across)

What am I ? Sill because I am concordant (lay at the same angle as the bedrock)

Investigation Into What Can Change Viscosity Changing the Properties of Treacle Collect appropriate equipment (water bath using bunsen burner for heat) Set up stand with two bosses in tray. Clamp one treacle test-tube upright to one boss Add component to second treacle test-tube. Clamp upright to other boss Using stop watch, turn both test-tubes upside down (watch the bung and keep in place if necessary). Time decent of both tubes – record in a simple table

How Can Igneous Rocks be Classified? Grain Size mm SilicicIntermediateBasicUltrabasic Fine<0.5RhyoliteAndesiteBasalt Medium>0.5 - <2Dolerite Coarse>2GraniteGabbroPeridotite Minerals Present Orthoclase and Plagioclase Feldspar, Quartz, Biotite and Muscovite Plagioclase and Orthoclase Feldspar, Hornblende, Biotite, Quartz Olivine, Augite, Plagioclase Feldspar Olivine, Augite Increasing darkness Decreasing SiO 2

Mineralogy Felsic Intermediate Mafic Ultramafic Acidic Intermediate Basic Ultrabasic Quartz, feldspar & mica Quartz, feldspar, mica & hornblende Feldspar, olivine, augite Olivine and Augite

Chemical Composition Acidic Intermediate Basic Ultrabasic > 65% SiO % SiO % SiO 2 <45% SiO 2

Types of igneous rocks 1 mm

Igneous Rocks Baked margin Chilled margin

Chilled Margin Grain Size

Igneous Rock Textures Crystal shape:euhedral (fully developed crystals) subhedral (incomplete crystals) anhedral (no crystal development) Crystalline Interlocking crystals Randomly orientated Equigranular – equal size Porphyritic – one crystal is much larger than the rest

Igneous Rock Textures Other crystalline textures/structures Xenolith (from stoping)

Lava Flows and Minor Intrusions How can you tell the Difference ?

Lava Flows and Minor Intrusions Summary Lava FlowSillDyke Chilled/Baked Margin Base onlyUpper and LowerBoth Sides Relative to Bedding Planes Concordant Discordant Cross Section Horizontal Parallel to Beds Gentle Dip Parallel to Beds Vertical Steeply cross-cutting Structures Columnar Jointing Pillows Vesicles Weathered Surface (always) Columnar Jointing (may have weathered surface) Jointing (may have weathered surface)