Transcript Igneous Rock

Key Terms: igneous rock, extrusive, intrusive,
vesicular, silica, felsic, mafic, viscosity
Classifying Igneous Rocks
 Turn 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 rock
2
Igneous Rock: Origin
 Extrusive Rock
 Igneous rock formed from lava that erupted onto Earth’s
surface.
 Basalt is the most common extrusive igneous rock.
 Intrusive Rock
 Igneous rock that formed when magma hardened below
Earth’s surface.
 Granite is the most common intrusive rock.
3
Igneous Rock: Texture
 The 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
Glassy
(No visible
grain)
Fine
Grained
Coarse Grained
Rate of Cooling
Characteristics
Origin and
Occurrence
Extremely rapid cooling
No visable grains
(crystals), concoidal
(shell shaped) fracture
Extrusive
Surface of lava flows,
lava entering water,
volcanic vent debris
Obsidian
Rapid cooling
Very fine grained
crystals (need
magnification).
Extrusive
Interior of lava flows
Basalt
Visible interlocking
crystals of almost equal
size
Intrusive
magma
Granite
Both intrusive and
extrusive
May exist in fine of
coarse grained igneous
rocks
Extrusive.
Lava ejected out of a
volcano which cools
before it hits the ground.
Various fine or coarse
grained igneous rocks.
Diabase Porphyry
Uniform rate of slow
cooling from molten
rock
Porphyritic
Two stages of cooling
initially slow then more
rapidly (when extruded)
Vesicular
Rapid
Cooling
Visible interlocking
crystals of two distinct
sizes. Large coarse
grained and smaller fine
grained crystals
Generally fine grained
with many holes and
small grains of minerals
Stop
On the “Classifying Igneous Rock Origin and
Texture Activities” Worksheet, do numbers 1
through 5 on the front of the sheet.
Example
Pumice
and
Scoria
5
Igneous Rock: Mineral Composition
 The 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.
Stop
On 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 Shade
 The 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: Viscosity
 All 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
easily
Stop
On the “Classifying Igneous Rock: Mineral
Composition Activities” Worksheet, do
numbers 1 through 5.
8
Igneous Rock Identification
 Petrologists 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 here
 After 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
9
complete.