Rocks

3 types of rocks

• igneous

• sedimentary

• metamorphic

1. Igneous rocks (fire-formed)= rock that solidified from a molten state

• majority of crustal rock (although it is often covered with sedimentary rocks)

• consists of silicate minerals w/ metallic elements

• “interlocking grains”

e.g. granite, basalt, obsidian, pumice

Magma: molten rock beneath Earth’s surface

GraniteBasalt

ObsidianPumice

Composition of igneous rocks

Felsic Mafic

Elements Si & Al Fe & Mg

Melting pt low temp. high temp.

Density low (light) high (heavy)

Color light dark

e.g. granite, diorite basalt, gabbro

Global Composition Distribution

Continents- Felsic

Oceans- Mafic

Digital Elevation Model (DEM)

Cooling and crystals

Slow cooling = bigger crystals

Fast cooling = smaller crystals

So, the cooling history of an igneous rock determines how big its crystals will be

Intrusive“Plutonic”

• cools slowly under the surface

• large crystals

• e.g. granite

Extrusive“Volcanic”

• cools quickly above the surface

• small crystals

• e.g. basalt

Felsic Intermediate Mafic

Intr

usiv

eE

xtru

sive

Rhyolite

Classifying igneous rocks

Andesite Basalt

Granite Diorite Gabbro

RhyoliteRhyolite

AndesiteAndesite

BasaltBasalt

GraniteGranite

DioriteDiorite

GabbroGabbro

YellowstoneYellowstone

Sierra NevadaSierra Nevada

AndesAndes

Ocean basementOcean basement

HawaiiHawaii

New ZealandNew Zealand

Intrusive features

batholith dike(vertical)

sill(horizontal)

Pluton: body of intrusive rock

Famous extrusive rocks

Famous intrusive rocks

2. Sedimentary rocks= formed through erosion, transportation,

deposition, compaction, cementation, and hardening of sediment

Sediment = fine-grained mineral matter transported by air, water or ice

• accumulates at low elevations to form strata (rock layers)

Sedimentary rocks

• majority of surface rock• often fossiliferous• e.g. sandstone, shale, limestone, coal

Classifying sedimentary rocks1. Clastic2. Chemical3. Organic

Classifying sedimentary rocks1. Clastic

- derived from weathered & fragmented rock

- boulder microscopic grain sizes- e.g. mudstone, shale, sandstone

Sandstone Shale

Classifying sedimentary rocks2. Chemical

- dissolved minerals - transported in solution & precipitated- e.g. limestone, evaporites

Limestone

Halite

Classifying sedimentary rocks3. Organic

- remains of dead organisms- e.g. coal

CoquinaCoal

Famous sedimentary rocks

3. Metamorphic rocks (change form)

• rocks changed by heat &/or pressure

• changes structure, appearance , composition, & texture of parent rock WITHOUT MELTING IT

• commonly at roots of mountains

e.g. slate, schist, gneiss

Metamorphic processes

3. Metamorphic rocks

limestone marble

sandstone quartzite

granite gneiss

shale slate schist

Somewhat famous metamorphic rocks

Rock Identification• Break into groups of 4, 5, or 6• Have one person come up and get a set of 3

rocks.• Use the flow chart on the following slide to

identify your rocks.• Determine the type of each rock (igneous,

sedimentary, or metamorphic).• Write your names, rock ID #’s, rock names, and

rock types on a sheet of paper to hand in.

Granite BasaltSandstone ShaleLimestoneGneissSchist

Can you see the mineral grains?

Are the grains interlocking, or cemented together?

Are there layers or stripes(foliation) to the grains? yes no

interlocking cemented

yes no

Is the rock “scaly” or “glittery”?

yes no

Is it very dark colored (mafic)?

yesno

Definitive test: Does it react to a weak acid by “fizzing”?

Ok substitute test for today: is it layered in flat sheets?

noyes

yes no

Double check: is it layered in flat sheets?

noyes

IncludeRock #Rock NameRock Type

Answer key

Granite

Basalt

SandstoneShale

Limestone

Gneiss

Schist 2

1

3

5

4

6

7