"Igneous Rock Identification Worksheet - Lawrence High School"

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Name: ______________________
REGENTS EARTH SCIENCE
Igneous Rock Identification
As you now know, rocks are composed of minerals or a combination of minerals. Rocks are
categorized into types based on the way in which they form. Igneous rocks form as molten, mineral-
rich material cools (or, you might say, “freezes”) as it rises toward earth’s surface. Igneous rocks are
classified based on two main characteristics- mineral composition and mineral grain size
(texture). These characteristics, in turn, signify a particular environment of formation. Herein lies
the key: if you know the rock, you know the past environment! Remember, rocks form the
sentences and paragraphs of earth’s language. Using your senses and the Scheme for Igneous
Rock Identification found in your reference tables, you will be able to first classify then identify the
environment of formation of a variety of different igneous rocks.
PROCEDURE
First, take some time to familiarize yourself with the flow of the identification chart. The chart is read
by “plotting” two major physical characteristics- color and texture. The outline below may be helpful
as a guide:
Although color is a poor Indicator for minerals, igneous
rocks are typically composed of a combination of 7
major minerals with specific coloration. As a result, color
turns out to be very useful for identifying composition.
EXTRUSIVE
GLASSY
GREEN
(volcanic)
no visible crystals
(ultramafic)
form above the
surface, may have
FINE
DARK(mafic)
gas pockets
crystals < 1mm
Decision 1:
Decision 2:
90% dark/10 light
(vesicles)
COLOR
TEXTURE
INTERMEDIATE
COARSE
(andesitic)
crystals 1-10mm
INTRUSIVE
50/50
(plutonic)
form below
LIGHT (felsic)
VERY COARSE
the surface
10% dark/90 light
crystals > 10mm
Environments of Formation
The composition and density of igneous rocks determine where they are formed on the earth. As
you already know, plutonic rocks form below the surface (big crystals), while volcanic rocks form at
or above the surface (fine or glassy texture).
Low Density/Light Color
High Density/Dark Color
Felsic
Mafic/Ultra Mafic
CONTINENTAL
INTERMEDIATE
OCEANIC
MANTLE
Rocks form at the
Rocks form where ocean
Rocks form in the
Rocks form
surface or beneath
crust and continent crust
ocean or beneath the
in the mantle
the surface of the
meet or collide (Andes Mtns)
ocean crust.
Land (continent).
Name: ______________________
REGENTS EARTH SCIENCE
Igneous Rock Identification
As you now know, rocks are composed of minerals or a combination of minerals. Rocks are
categorized into types based on the way in which they form. Igneous rocks form as molten, mineral-
rich material cools (or, you might say, “freezes”) as it rises toward earth’s surface. Igneous rocks are
classified based on two main characteristics- mineral composition and mineral grain size
(texture). These characteristics, in turn, signify a particular environment of formation. Herein lies
the key: if you know the rock, you know the past environment! Remember, rocks form the
sentences and paragraphs of earth’s language. Using your senses and the Scheme for Igneous
Rock Identification found in your reference tables, you will be able to first classify then identify the
environment of formation of a variety of different igneous rocks.
PROCEDURE
First, take some time to familiarize yourself with the flow of the identification chart. The chart is read
by “plotting” two major physical characteristics- color and texture. The outline below may be helpful
as a guide:
Although color is a poor Indicator for minerals, igneous
rocks are typically composed of a combination of 7
major minerals with specific coloration. As a result, color
turns out to be very useful for identifying composition.
EXTRUSIVE
GLASSY
GREEN
(volcanic)
no visible crystals
(ultramafic)
form above the
surface, may have
FINE
DARK(mafic)
gas pockets
crystals < 1mm
Decision 1:
Decision 2:
90% dark/10 light
(vesicles)
COLOR
TEXTURE
INTERMEDIATE
COARSE
(andesitic)
crystals 1-10mm
INTRUSIVE
50/50
(plutonic)
form below
LIGHT (felsic)
VERY COARSE
the surface
10% dark/90 light
crystals > 10mm
Environments of Formation
The composition and density of igneous rocks determine where they are formed on the earth. As
you already know, plutonic rocks form below the surface (big crystals), while volcanic rocks form at
or above the surface (fine or glassy texture).
Low Density/Light Color
High Density/Dark Color
Felsic
Mafic/Ultra Mafic
CONTINENTAL
INTERMEDIATE
OCEANIC
MANTLE
Rocks form at the
Rocks form where ocean
Rocks form in the
Rocks form
surface or beneath
crust and continent crust
ocean or beneath the
in the mantle
the surface of the
meet or collide (Andes Mtns)
ocean crust.
Land (continent).
COMPLETE THE CHART ON THE BACK USING THIS SCHEME AND YOUR OBSERVATIONS
Name: ______________________
SCORE: _____ /20
COLOR
TEXTURE
ENVIRONMENT
ROCK
INTRUSIVE or
(Dark w/green,
(Glassy, Fine, Coarse,
(Mantle, Ocean,
ROCK NAME
TYPE
EXTRUSIVE
Dark, Intermediate,
Very Coarse,
Intermediate,
Light)
Vesicular/Non)
Continental)
1.
Igneous
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
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