Key Terms: igneous rock, extrusive, intrusive, vesicular, silica, felsic,

mafic, viscosity

Classifying Igneous RocksTurn to page 48 in your textbook.Igneous rocks are classified according to

their:Origin

Where the rock formed (i.e. above or below Earth’s surface).

Texture Determined by the size and shape of the mineral

grains that make up the rock.Mineral Composition

Determine the color and shade of the rock2

Igneous Rock: OriginExtrusive Rock

Igneous rock formed from lava that erupted onto Earth’s surface.

Basalt is the most common extrusive igneous rock.

Intrusive RockIgneous rock that formed when magma

hardened below Earth’s surface.Granite is the most common intrusive rock.

3

Igneous Rock: TextureThe texture of an igneous rock depends on the

Size of the mineral grains.Shape of the mineral grains.

The grain size created in an igneous rock is determined by the rate of cooling.The faster the cooling the smaller the mineral

grains. Fast cooling = Extrusive Rock (fine or no visible grain).

The slower the cooling the larger the mineral grains. Slow cooling = Intrusive Rock (coarse grain).

4

Igneous Rock: Texture Data Table

Texture Rate of Cooling Characteristics Origin and Occurrence

Example

Glassy(No visible

grain)

Extremely rapid coolingNo visable grains

(crystals), concoidal (shell shaped) fracture

ExtrusiveSurface of lava flows, lava entering water, volcanic vent debris

Obsidian

FineGrained

Rapid coolingVery fine grained

crystals (need magnification).

ExtrusiveInterior of lava flows Basalt

Coarse GrainedUniform rate of slow cooling from molten

rock

Visible interlocking crystals of almost equal

sizeIntrusive

magma Granite

PorphyriticTwo stages of cooling

initially slow then more rapidly (when extruded)

Visible interlocking crystals of two distinct

sizes. Large coarse grained and smaller fine

grained crystals

Both intrusive and extrusive

May exist in fine of coarse grained igneous

rocks

Various fine or coarse grained igneous rocks.

Diabase Porphyry

VesicularRapid

Cooling

Generally fine grained with many holes and

small grains of minerals

Extrusive.Lava ejected out of a volcano which cools

before it hits the ground.

Pumiceand

Scoria

StopOn the “Classifying Igneous Rock Origin and Texture Activities” Worksheet, do numbers 1 through 5 on the front of the sheet.

5

Igneous Rock: Mineral CompositionThe composition of the igneous rock is

determined by the percentages of the different minerals that make up the rock.

The color and shade of an igneous rock is determined by the mineral composition of the rock.

StopOn the “Classifying Igneous Rock Origin and Texture Activities” Worksheet, do number 6 and the Sharpen your Skills activity on the back of the sheet. Turn in your worksheet when complete, then continue onto the next slide.. 6

Igneous Rock: Color and ShadeThe color of the igneous rock is determined

by the impurities in the magma/lava mixture.More importantly, the shade of the igneous

rock is determined by the amount of silica contained in the rock.High in silica = light shade

The term used to describe the chemistry of an igneous rock with a high silica content is felsic.

Low in silica = dark shade The term used to describe the chemistry of an

igneous rock with a low silica content is mafic.

7

Molten Rock: ViscosityAll liquids have a viscosity (resistance to flow)The viscosity of any liquid is the how the liquid

resists flow.Magma and lava are both molten (liquid) rock.

The viscosity of the molten rock is determined by the amount of silica contained in the molten mixture.

High in silica content = High Viscosity = thick and resists flow.

Low in silica content = Low Viscosity = thin and flows easilyStop

On the “Classifying Igneous Rock: Mineral Composition Activities” Worksheet, do numbers 1 through 5. 8

Igneous Rock IdentificationPetrologists have organized the information

we’ve discovered in this power point to create a chart to help identify igneous rocks.

You can link to the chart hereAfter linking to and studying the chart:

Link to the extrusive igneous rocks page. Read through the page, and view the igneous rock samples.

Link to the intrusive igneous rocks page. Read through the page, and view the igneous rock samples. Stop

On the “Classifying Igneous Rock: Mineral Composition Activities” Worksheet, do number 6. Turn in your worksheet when complete.

9