Igneous Rocks Worksheet printable pdf download

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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).

Igneous Rocks Worksheet

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