ROCK REVISION Focus on hand specimens for GL2 exam Three rock types: IGNEOUS SEDIMENTARY METAMORPHIC

Igneous rocks CRYSTALLINE Mass of interlocking crystals Cooled from molten magma Above ground (EXTRUSIVE) cool rapidly – so have small crystals (crystals do not have time to grow) Underground (INTRUSIVE) are insulated so cool slowly – so have large crystals (crystals have a long time to form)

CRYSTAL SIZES COARSE crystals bigger than 2mm Slow cooling, insulated, intrusive MEDIUM crystals smaller than 2mm but bigger than 0.5mm Some insulation, probably an intrusion near surface, or thick lava flow FINE crystals smaller than 0.5mm Surface cooling, fast, no insulation, extrusive

Granite Credit: British Geological Survey CRYSTALLINE

GRANITE 1cm This is a very coarse granite – a pegmatite Credit: British Geological Survey This is a very coarse granite – a pegmatite

GRANITE – labels? 1cm

GRANITE Biotite mica, black, flaky mineral Mass of interlocking crystals, large size so slow cooling - indicates an intrusive igneous rock Quartz, grey glassy mineral Feldspar, white mineral 1cm

RHYOLITE Mass of interlocking crystals, small size so rapid cooling indicates an extrusive igneous rock flow banding Credit: British Geological Survey

BASALT – labels? Credit: British Geological Survey

BASALT Individual crystals not visible to eye, indicates rapid cooling of extrusive igneous rock n.b. Some basalts contain holes called VESICLES. These formed as the lava Solidified. Vesicles mark where gases escaped from the cooling lava. Credit: British Geological Survey Overall dark colour to rock, shows very little quartz is present, so a BASIC composition

BASALT Columnar jointing in basalt

DOLERITE Slightly larger crystals, some seen with hand lens, shows cooling was fairly rapid probably insulated by thickness of magma Some lighter coloured minerals, shows some Quartz content of magma, still of basic composition Credit: British Geological Survey

GABBRO Coarse grain size – showing cooling was slow Pale coloured mineral = mainly feldspar Green-black mineral = augite Credit: British Geological Survey

IGNEOUS SUMMARY Crystal size (use measurements/numbers) link to cooling history and hence link to environment eg. Volcano or pluton and plate tectonic environment (constructive / destructive) Mineralogy (named minerals)

Sedimentary rocks FRAGMENTAL TEXTURE - T.O.S.S.S ……. Orientation of fragments (lined up) Size of fragments (large – strong current) Shape of fragments (rounded – long transport) Sorting of fragments (well sorted to poorly sorted – dumped material)

CONGLOMERATE – labels? Credit: British Geological Survey

CONGLOMERATE Large rounded fragment so LONG transport history Fine grained MATRIX Credit: British Geological Survey Variety of fragments n.b. Large fragments show - strong current If poorly sorted - shows rapid dumping of material If no orientation to fragments - shows rapid dumping of material/ no strong currents

SANDSTONE Credit: British Geological Survey

SANDSTONE Ripples – beach environment Cross bedding – indicates current flow Credit: British Geological Survey Credit: British Geological Survey

SANDSTONE Slightly coarser layer, small increase in strength of current Red colour indicates land deposition, iron rusted to Iron Oxide Cross bedding shows successive fronts of sand dune Current flow direction  Credit: British Geological Survey Fine grains indicate wind transport Grains are frosted under hand lens

LIMESTONE Credit: British Geological Survey Fossil shells

LIMESTONE How do you know it is limestone? Test it with dilute hydrochloric acid FIZZ = calcium carbonate Fossil content Broken up – strong current Aligned – current flow/direction Delicate preservation – lack of current -Type – environment (eg. Corals Clear, warm, shallow Marine, aerated sea Credit: British Geological Survey

SEDIMENTARY SUMMARY FRAGMENTAL TEXTURE - T.O.S.S.S ……. Orientation of fragments Size of fragments Shape of fragments Sorting of fragments

Metamorphic Rocks CHANGED rocks HEAT – causes new mineral growth PRESSURE – lines up the minerals PRODUCT depends on ORIGINAL rock Key words: CRYSTALLINE (shows rock has been heated severely); NEW MINERAL GROWTH (heat); ALIGNMENT (layers, shows pressure).

MARBLE Not many features to draw! Originally LIMESTONE FOSSILS and structures DESTROYED (streaks of colour remain?) FIZZES with dil HCl – still a carbonate – so there has been NO CHANGE TO MINERAL COMPOSITION Credit: British Geological Survey

GNEISS Crystalline, BANDED, shows high heat Credit: British Geological Survey Credit: British Geological Survey Crystalline, BANDED, shows high heat to almost MELT the original rock and then allow it to CRYSTALLISE as it cools Bands also show some PRESSURE to separate out the minerals into light and dark layers

“Shiny” SCHIST Layers of mica, give rock very smooth surface Credit: British Geological Survey Credit: British Geological Survey Shiny, reflective surfaces MICA – growth of new mineral (heat) in parallel layers (pressure) Garnet porphyroblast (larger crystal)

A few extra metamorphics That we will see in the Lake District! Visit walking through the metamorphic aureole of the Skiddaw Granite Slate Chiastolite slate/Andalusite hornfels Spotted rock

SLATE CLEAVES (splits) into layers Shows minerals have ALIGNED Due to intense PRESSURE Original rock: mudstone Credit: British Geological Survey

ANDALUSITE HORNFELS Original rock probably shale Minerals -long thin white Growth of new mineral so HEAT RANDOM – not lined up, so NO PRESSURE Reddish colour, iron staining These pieces are found in the footpath walking up to the Skiddaw Granite, ie. Walking through a metamorphic aureole around a granite pluton

“SPOTTED ROCK” Area of less spots Area of more spots RANDOM spots Credit: British Geological Survey Area of less spots Area of more spots RANDOM spots (looks like measles!) Growth of new mineral HEAT, but lacks pressure (not aligned) Original rock probably a Mudstone or shale Credit: British Geological Survey Please look at hand specimen – it is much darker than these!

Shale contains many minerals and produces a wide range of new minerals if heated during metamorphism Limestone has restricted composition (calcium carbonate) so can’t form anything other than calcium carbonate when metamorphosed! – UNLESS new minerals are introduced (by percolating fluids).

METAMORPHISM SUMMARY CRYSTALLINE – if enough HEAT NEW MINERAL – if HEATED ALIGNED – if enough PRESSURE Mix of both – alignment of new minerals – shows heat and pressure were both factors.

EXAM HINTS Examiners will chose something that has recognisable features (ie. Something to identify – minerals, shape, alignment) If asked to DRAW, only draw a small part of the rock, and get the features in! Give all diagrams lots of LABELS and/or ANNOTATIONS Don’t forget a SCALE Don’t forget the basics: Igneous/Sedimentary/Metamorphic? – and why? (Crystalline, fragmental, crystalline/new minerals/banded)