Igneous Rock ClassificationIgneous Rock Classification

Dr. David Steer

Rock cycleRock cycle

Analysis TimeAnalysis Time

Given:Bag rocks

Group these rocks in some manner

Grouping itemsGrouping items

Group these objects in some logical way.

Color (Proxy for silica content)

Size (Proxy for grain size)

Grouping itemsGrouping items

Group these objects in some logical way.

Analysis timeAnalysis time

Given:Bag rocks

Group these rocks in some manner

Rock CycleRock Cycle

Igneous RocksIgneous Rocks Volcanic (extrusive) igneous rocks

Cool fast, small crystalsform when molten rock (magma) in the earth’s interior rises to the surface through pipes or fractures in the crust.

Igneous RocksIgneous RocksPlutonic (intrusive) igneous rocks

Cool slow, large crystals

form when magma cools within the earth. Igneous rocks that cool below the earth's surface are termed plutonic (or intrusive) igneous rocks.

Classification of Igneous rocksClassification of Igneous rocks Texture size of crystals most important

Cooling rates– Surface, fast

Small crystals

– Below surface slow Larger crystals

Which formed at surface?

Both are the same chemistry

Rhyolite Granite

a) b)

Chemical CompositionChemical Composition Color

Indicative of chemistry and temperatures of formation– Dark colors

High temperatures (1000 to 1200 oC) Low silica content

– Light colors Lower temperatures High silica content

Gabbro Granite

Which is the high silica rock?

Both formed below the surface

a) b)

TextureTexture So explain this!!!!

Six common Igneous RocksSix common Igneous Rocks

Volcanic Rocks

Silica Content

Minerals Present (in order of abundance)

Plutonic Rocks

Basalt low

pyroxene, olivine, feldspar, & amphibole

Gabbro

Andesite intermediate feldspar, amphibole, pyroxene, biotite mica

Diorite

Rhyolite high feldspar, quartz, muscovite mica, & amphibole

Granite

Solidifying Temperature 500 C1000 C

Lighter C

olor

Increasing Grain Size

Draw a concept map that guides you through classifying an igneous rock.

Use these terms to: Classify Igneous Rocks

Basalt Color Cool QuickCool Slow Dark Extrusive Gabbro Granite High SilicaLarge Grain Low Silica LightPlutonic Texture RhyoliteSmall Grain

Identify the rock type associated with the diagram below. Identify the rock type associated with the diagram below.

a. andesite porphoryb. gabbroc. basaltd. obsidian

1) Plot 12) Plot 23) Plot 34) Plot 4

Question

5) Imagine that all minerals found in rocks were the same color. What information would you no longer be able to infer?

a. texture b. cooling rate

c. composition

5) Imagine that all minerals found in rocks were the same color. What information would you no longer be able to infer?

a. texture b. cooling rate

c. composition

Analysis TimeAnalysis Time

Given:Bag rocks

Identify the rocks