Igneous(Rock(Iden-fica-on“M2(Lab#3”

(Lab(#(6(in(Lab(Manual)!

Minerals)in)my)Lab)KitColor,)size,)shape)may)vary)in)your

kit(Numbers)facing)away)on)purpose,)

No)special)order)

Lets%look%at%the%“different”%onesfirst:*Obsidian)(like)black)broken)glass,)some4mesbrown)in)it)*Pumice)(probably)your)lightest)rock,)vesicular,usually)glassy)when)you)look)with)a)magnifyingglass)*Tuff)(oAen)looks)like)a)sedimentary)rock)made)upof)par4cles)–)the)“par4cles”)are)volcanic)ash)fusedtogether)

Obsidian(((((((((((((((((Pumice(((((((((((((((((((((((((((Tuff

Pumice'(le*)'vs.'Rhyolite'(right)

Pumice:(Very(light((may(float(inwater!!)(and(you(will(probably

observe(a(glassy(texture(if(you(use(amagnifying(lens.

Note:&These&two&rocks&are&the&same&type&ofIgneous&rock,&just&different&varia=ons.&The&one&onthe&le@&is&o@en&considered&“Pegma=c”,&but&wewill&consider&it&phaneri=c&for&ID&purposes.

Rocks&compared&to&I.R&Diagram&in&TextYou$can$use$the$classifica/on$chart$in$the$Lab$manual$toID$the$rocks.$Many$students$find$it$easier$to$separate

them$by$texture$and$Light/Dark$color$and$compare$it$tothe$Text$Diagram$to$ID$them.$Here$I$have$separated$my

rocks$so$they$line$up$with$the$diagram$in$the$text.

Gabbro:'This'rock'is'o.en'very'darkand'large'crystal'grains'may'be'hardto'see'(o.en'shiny!).'They'have

been'highlighted'in'red'below.'Youcan'use'a'magnifying'lens'to'help

you'see'them.

Andesite:)Though)this)rock)is)phaneri4c)intexture,)it)may)be)pegma4c)(and)thereforeconfusing).)The)red)arrows)show)large

crystals)(pegma4c))in)a)matrix)of)aphani4c(fine))crystals)(yellow)arrow).)Rhyolite

samples)can)also)be)pegma4c.