Name:
Date: 12/2/14
Hour:
Igneous, rocks have been melted at some time and then
hardened to become solid again. When melted rock material
cools and hardens, it may form crystals, depending on how
fast it cools. How fast the rock material cools depends on
where it cools. If melted rock cools deep within the Earth,
the resulting rocks are called intrusive igneous rocks. They
cool over thousands or millions of years, giving crystals
long periods of time to grow. The more slowly they cool,
the larger the crystals grow. Intrusive rocks have crystal
sizes, or grain sizes, that are larger than sand-sized and,
under rare circumstances, may be larger than a penny. If the
melted rock materials cooled on or near the Earth’s surface,
they are called extrusive igneous rocks. If extrusive igneous
rocks have crystals, they are smaller than sand-sized.
However, some extrusive igneous rocks cool so fast that
crystals do not form at all like obsidian.
Sometimes when extrusive igneous rocks are
cooling, volcanic gases bubble through the melted rock
material much like water vapor bubbles through boiling
water. When the rock hardens, these bubbles may become trapped to produce holes in the final product like pumice or
scoria. The holes tend to make the rock light in weight.
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 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:
Review your minerals before continuing the lab by clicking here now!-Mineral review
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Refer to the chart to complete the following Data Table and answer the questions.
Common Igneous Rocks
Felsic
(Aluminum)
Low
Light colored
or red hued
Element Composition
Density
Color
Mafic
(Iron or Magnesium)
High
Dark colored
or green hued
Rock Type
Crystal Size
Intrusive
Coarse
(larger than sand)
Granite
Gabbro
Extrusive
Fine
(smaller than sand)
Rhyolite
Basalt
Extrusive
none
Extrusive
none
Extrusive
none
Extrusive
none
Obsidian (nearly the same chemically as granite)
Commonly called volcanic glass; usually black, but may be red, gray or
brown; breaks in shell-like/glass like fractures
Pumice (nearly the same chemically as granite)
Made of silky glass fibers full of tiny pores. Millions of pores make pumice
so light that some floats in water
Olivine a common mineral in basalt and the igneous rocks of the oceanic crust. Green or yellow green,
crystals smaller than a grain of sand.
Scoria volcanic rock with gas pockets sinks in water. It is often made of the same chemicals as
basalt or andesite. Red or maroon, crystals smaller than a grain of sand.
Complete this link before continuing with the lab—Click here-The Virtual Rock Lab Interactivity. - Wiley
DATA TABLE Igneous Rocks
COLOR
TEXTURE
ENVIRONMENT
(Dark w/green,
(Glassy, Fine,
Rock
Intrusive or
(Mantle, Ocean,
Crystal Size
Dark,
Coarse,
Name
Extrusive
Intermediate,
Intermediate,
Very Coarse,
Continental)
Light)
Vesicular/Non)
Generally light
Large
4.
pink
Large
Black
6.
Smaller than
Blackish green 1.
sand-sized
Smaller than
Generally light
2.
sand-sized
colored
Smaller than
Green or
7.
sand-sized
yellow green
Smaller than
sand-sized,
Red or light
8.
many large gas
gray
pockets
Smaller than
sand-sized,
Gray to tan in 3.
Many very
color
Pumice
small gas
pockets
Dark green to
Glass like
brown to black 5.
texture
color
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Click this link and identify the following characteristics of igneous rocks.
1. a. List two intrusive igneous rocks. _______________________________________________
b. Did these two types of igneous rocks cool relatively fast or slowly? __________________
2. a. Do all extrusive igneous rocks contain crystals?
b. Explain your answer.
_______________________________________
_____________________________
_____________________________
3. Why do some igneous rocks have bubble holes?
____________________________________________
_________________________________
4. Which graph correctly shows the relative positions
of the igneous rocks granite, rhyolite, and pumice?
Graph_____
5. Granite and obsidian are similar
chemically. How are they different?
______________________
______________________
6. What igneous rock cooled deep
beneath the Earth’s surface and has
a dark blackish-green color?
__________________________
http://en.wikipedia.org/wiki/Medicine_Lake_Volcano
After the lab click this link to see if you know
your igneous rocks!
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