Name
Date _
Class
lgneous Rocks
Guide for
Reading
I What
characteristlcs
are
usedto classity
igneous
rocks?
132. F
rock is anv rock that forms from magmaor lava.The name
f Bneous
I "igneous"comesfrom the Latin word ignis,tneaning"fire." Most
igneousrocksare madeof mineralcrystals.The only exceptionsto this
rule are the different types of volcanic glass-igneous rock that lacksa
crystal structure. Igneous rocks are classified according to their
origin, texture, and mineral composition.
GeologistsclassiS'igneousrocks accordingto where they formed.
Extrusive rock is igneous rock formed from lava that erupted onto
Earth'ssurface.Basaltis the most common extrusiverock. Igneousrock
that formed when magma hardenedbeneathEarth'ssurfaceis called
intrusive rock. Graniteis the most abundantintrusiverock.
The textureof an igneousrock dependson the sizeand shapeof its
mineral crystals.Igneousrocks may be similar in mineral composition
and yet havevery differenttextures.The texturemay be fine-grained,
coarse-grained,
porphyritic, or glassy.Rapid cooling lava forms finegrainedigneousrocks with small crystals.Slow cooling magma forms
coarse-grainedrock with large crystals.Intrusive rocks have larger
crystalsthan extrusiverocks.Some intrusiverocks have large crystals
scatteredon a background of much smaller crystals,creating a
porphyritic texture. Porphyritic rocks fbrm when magma cools in two
stages,
at first slowlyand then more rapidlyasthe magmamovesnearer
to the surface.Extrusiverocks have a fine-grainedor glassytexture.
Basalt,for example,is an extrusiverock whosecrystalsaretoo smallto be
seenwithout a microscope.
Lavathat is low in silicausuallyforms dark-coloredrocks,suchasbasalt.
Magmathat is high in silicausuallyformslight coloredrock,suchasgranite.
Granite,however,comesin manyshadesand colors,from dark to light gray,
red,and pink. Its colorchanges
alongwith its mineralcomposition.Granite
that is rich in reddishfeldsparis speckledpink. Granite that is rich in
hornblendeand dark micais a light graywith dark specks.
Many igneousrocksare hard, dense,and durable.As a result,people
havelong usedigneousrock for tools and building materials.Granite,
one of the most abundantigneousrocks,hasa long historyasa building
material.For example,ancientEgyptiansusedgranitefor statues.In the
United Statesin the late 1800sand early 1900s,granitewaswidely used
to build bridgesand public buildings.Igneousrocks such as basalt,
pumice,and obsidianalsohaveimportant uses.
TeachingResources
InsideEarth
o
!
o
=
=.
o
T
"g
5
Name
Class
lgneous Rocks
o Understanding Main ldeas
Fill in the lilanksin the tablebelow.
Origin
of lgneous
Rock
Resulting
Texture
Slowcoolingof magmafar beneath
Earth'ssurface
l.
Magmacoolingin two stages,
first
slowlyand then quickly
2.
Extremelyrapid coolingof lavain
which no crystalsform
3.
Rapidcoolingof lavain which tiny
crystalsform
4,
Answerthefollowingquesfiotts
on a seporate
sheetof paper.
5. Explainhow the silicacontentof molten materialaffectsthe color of ieneous
rocks.
6. What qualitiesof igneousrockshavelong madethem usefulfor toolsand
building materials?
7. Describeone useeachfor the igneousrocksgranite,basalt,and pumice.
a Building Vocabulary
Fill in tlrc blank to cornplete
eachstatement.
(d
I
q)
8. lgneousrock formedfrom lavaon Earth'ssurfaceis called
E
c
O
rock.
L
I
9. A rock with largecrystalsscattered
on a backgroundof much smallercrystals
hasa(n)
texture.
10.Igneousrock formed from magmabelowEarth'ssurfaceis called
rock.
InsideEarth
Teaching
ResourcesF. 133