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

ROCK REVISIONROCK REVISION

Focus on hand specimens for GL2 examFocus on hand specimens for GL2 examThree rock types:Three rock types:

IGNEOUSIGNEOUSSEDIMENTARYSEDIMENTARYMETAMORPHICMETAMORPHIC

Igneous rocksIgneous rocks

CRYSTALLINECRYSTALLINEMass of interlocking crystalsMass of interlocking crystalsCooled from molten magmaCooled from molten magmaAbove ground (EXTRUSIVE) cool rapidly Above ground (EXTRUSIVE) cool rapidly

– so have small crystals (crystals do not – so have small crystals (crystals do not have time to grow)have time to grow)

Underground (INTRUSIVE) are insulated Underground (INTRUSIVE) are insulated so cool slowly – so have large crystals so cool slowly – so have large crystals (crystals have a long time to form)(crystals have a long time to form)

CRYSTAL SIZESCRYSTAL SIZES

COARSE crystals bigger than 2mmCOARSE crystals bigger than 2mmSlow cooling, insulated, intrusiveSlow cooling, insulated, intrusive

MEDIUM crystals smaller than 2mm but MEDIUM crystals smaller than 2mm but bigger than 0.5mmbigger than 0.5mmSome insulation, probably an intrusion near Some insulation, probably an intrusion near

surface, or thick lava flowsurface, or thick lava flowFINE crystals smaller than 0.5mmFINE crystals smaller than 0.5mm

Surface cooling, fast, no insulation, extrusiveSurface cooling, fast, no insulation, extrusive

GraniteGranite

CRYSTALLINE

Credit: British Geological Survey

GRANITE

This is a very coarse granite – a pegmatite

1cm


GRANITE – labels?GRANITE – labels?

1cm

GRANITEGRANITE

Biotite mica, black, flaky mineral

Quartz, grey glassy mineral

Feldspar, white mineral

1cm

Mass of interlocking crystals, large sizeso slow coolingindicates anintrusive igneousrock

RHYOLITEMass of interlocking crystals, small sizeso rapid coolingindicates anextrusive igneousrock

flow banding


BASALT – labels?BASALT – labels?


BASALTBASALT

Individual crystals not visible to eye,indicates rapid cooling of extrusiveigneous rock

Overall dark colour to rock, showsvery little quartz is present, so a BASIC composition

n.b. Some basalts containholes called VESICLES. These formed as the lavaSolidified. Vesicles markwhere gases escaped from the cooling lava.


BASALTBASALT

Columnar jointing in basalt

DOLERITEDOLERITESlightly larger crystals, some seen withhand lens, shows cooling was fairly rapidprobably insulated by thickness of magma

Some lighter colouredminerals, shows someQuartz content of magma,still of basic composition


GABBROGABBRO

Coarse grain size – showing cooling was slow

Pale coloured mineral =mainly feldspar

Green-black mineral = augite


IGNEOUS SUMMARYIGNEOUS SUMMARY

Crystal size (use measurements/numbers)Crystal size (use measurements/numbers) link to cooling history link to cooling history

and hence and hence link to environment eg. Volcano or pluton link to environment eg. Volcano or pluton

and and plate tectonic environment (constructive / plate tectonic environment (constructive /

destructive)destructive)Mineralogy (named minerals)Mineralogy (named minerals)

Sedimentary rocksSedimentary rocks

FRAGMENTALFRAGMENTALTEXTURE - T.O.S.S.S …….TEXTURE - T.O.S.S.S …….Orientation of fragments (lined up)Orientation of fragments (lined up)Size of fragments (large – strong current)Size of fragments (large – strong current)Shape of fragments (rounded – long Shape of fragments (rounded – long

transport)transport)Sorting of fragments (well sorted to poorly Sorting of fragments (well sorted to poorly

sorted – dumped material)sorted – dumped material)

CONGLOMERATE – labels?CONGLOMERATE – labels?



CONGLOMERATECONGLOMERATE

n.b. Large fragments show - strong current

Variety of fragments

Fine grainedMATRIX

Large rounded fragment soLONG transport history

If poorly sorted - shows rapid dumping of material

If no orientation to fragments - shows rapid dumping of material/ no strong currents


SANDSTONE





Cross bedding – indicates current flow

SANDSTONERipples – beach environment



SANDSTONESANDSTONE

Fine grains indicate wind transportGrains are frosted under hand lens

Red colour indicates landdeposition, iron rusted toIron Oxide

Slightly coarser layer, small increase instrength of current

Cross beddingshows successive fronts of sand dune

Current flow direction


LIMESTONE

Fossil shells


How do you know it islimestone?

Test it with dilute hydrochloric acidFIZZ = calcium carbonate

Fossil content -Broken up – strong current-Aligned – current flow/direction-Delicate preservation – lack of current-Type – environment (eg. Corals

Clear, warm, shallowMarine, aerated sea

LIMESTONELIMESTONE


SEDIMENTARY SUMMARYSEDIMENTARY SUMMARY

FRAGMENTALFRAGMENTALTTEXTURE - T.O.S.S.S …….EXTURE - T.O.S.S.S …….OOrientation of fragmentsrientation of fragmentsSSize of fragmentsize of fragmentsSShape of fragmentshape of fragmentsSSorting of fragmentsorting of fragments

Metamorphic RocksMetamorphic Rocks

CHANGED rocksCHANGED rocksHEAT – causes new mineral growthHEAT – causes new mineral growthPRESSURE – lines up the mineralsPRESSURE – lines up the mineralsPRODUCT depends on ORIGINAL rockPRODUCT depends on ORIGINAL rockKey wordsKey words: :

CRYSTALLINECRYSTALLINE (shows rock has been heated (shows rock has been heated severely); severely);

NEW MINERAL GROWTHNEW MINERAL GROWTH (heat); (heat); ALIGNMENTALIGNMENT (layers, shows pressure). (layers, shows pressure).

MARBLEMARBLE

Not many features to Not many features to draw!draw!

Originally LIMESTONEOriginally LIMESTONE FOSSILS and structures FOSSILS and structures

DESTROYED (streaks of DESTROYED (streaks of colour remain?)colour remain?)

FIZZES with dil HCl – still FIZZES with dil HCl – still a carbonate – so there a carbonate – so there has been NO CHANGE has been NO CHANGE TO MINERAL TO MINERAL COMPOSITIONCOMPOSITION


GNEISSGNEISS

Crystalline, BANDED, shows high heatto almost MELT the original rock andthen allow it to CRYSTALLISE as itcools

Bands also show some PRESSUREto separate out the minerals into light and dark layers



““Shiny” SCHISTShiny” SCHIST

Shiny, reflective surfacesMICA – growth of new mineral(heat) in parallel layers (pressure)

Layers of mica, give rock verysmooth surface

Garnet porphyroblast(larger crystal)



A few extra metamorphicsA few extra metamorphics

That we will see in the Lake District!That we will see in the Lake District!Visit walking through the metamorphic Visit walking through the metamorphic

aureole of the Skiddaw Graniteaureole of the Skiddaw GraniteSlateSlateChiastolite slate/Andalusite hornfelsChiastolite slate/Andalusite hornfelsSpotted rockSpotted rock

SLATESLATE

CLEAVES (splits) CLEAVES (splits) into layersinto layers

Shows minerals Shows minerals have ALIGNEDhave ALIGNED

Due to intense Due to intense PRESSUREPRESSURE

Original rock: Original rock: mudstonemudstone


ANDALUSITE HORNFELSANDALUSITE HORNFELS

Minerals -long thin whiteGrowth of new mineralso HEATRANDOM – not linedup, so NO PRESSURE

Reddish colour, ironstaining

These pieces are found in the footpath walking up to the Skiddaw Granite, ie. Walking through a metamorphic aureole around a granite pluton

Original rock probably shale

““SPOTTED ROCK”SPOTTED ROCK”

RANDOM spots (looks like measles!)Growth of new mineralHEAT, but lacks pressure(not aligned)

Area of lessspots

Area of morespots

Please look at hand specimen – it is much darker than these!

Original rock probably aMudstone or shale



Shale contains many minerals and Shale contains many minerals and produces a wide range of new minerals if produces a wide range of new minerals if heated during metamorphismheated during metamorphism

Limestone has restricted composition Limestone has restricted composition (calcium carbonate) so can’t form anything (calcium carbonate) so can’t form anything other than calcium carbonate when other than calcium carbonate when metamorphosed! – UNLESS new minerals metamorphosed! – UNLESS new minerals are introduced (by percolating fluids).are introduced (by percolating fluids).

METAMORPHISM SUMMARYMETAMORPHISM SUMMARY

CRYSTALLINE – if enough HEATCRYSTALLINE – if enough HEATNEW MINERAL – if HEATEDNEW MINERAL – if HEATEDALIGNED – if enough PRESSUREALIGNED – if enough PRESSURE

Mix of both – alignment of new minerals – Mix of both – alignment of new minerals – shows heat and pressure were both shows heat and pressure were both factors.factors.

EXAM HINTSEXAM HINTS Examiners will chose something that has Examiners will chose something that has

recognisable features (ie. Something to identify recognisable features (ie. Something to identify – minerals, shape, alignment)– minerals, shape, alignment)

If asked to DRAW, only draw a small part of the If asked to DRAW, only draw a small part of the rock, and get the features in!rock, and get the features in!

Give all diagrams lots of LABELS and/or Give all diagrams lots of LABELS and/or ANNOTATIONSANNOTATIONS

Don’t forget a SCALEDon’t forget a SCALE Don’t forget the basics: Don’t forget the basics:

Igneous/Sedimentary/Metamorphic? – and why? Igneous/Sedimentary/Metamorphic? – and why? (Crystalline, fragmental, crystalline/new (Crystalline, fragmental, crystalline/new minerals/banded)minerals/banded)

Documents

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