12
Igneous Rocks and the Geologic
History of Your Community
Think About It Date
Page U14 Page #
· How are igneous
rocks similar to and
different from the
sedimentary rocks that
you have seen?
Activity 2
Investigate Part A Date
Page U14 Page #
1a. List ways you
can divide these
igneous rocks into
groups.
Light / Intermediate / Dark2a. List the rocks that
you placed in each
category.
2b. Describe the
problems you had trying
to categorized them.
Rock # / Name1
2
3
4
5
6
4a. Use the chart to
name each sample.
4b. How do geologists
classify igneous rocks?
4c. Describe any
similarities/differences
between your
classification and that of
geologists.
4d. What is the
advantage of
classifying rocks?
5a. Does rhyolite form at
or below the Earth’s
surface? Explain.
5b. Does gabbro form at
or below the Earth’s
surface? Explain.
Igneous Rocks and the Geologic
History of Your Community
Investigate Part B Date
Page U14 Page #
Rocktype / Locations / Age
1a. Are there any
igneous rocks
described in the legend?
1b. What are the most
common igneous
rocks in your area?
1c. Is the landform
made of igneous rock?
Igneous Rocks and the Geologic
History of Your Community
Digging Deeper Date
Pages U17-20 Page #
Igneous rock rock formed when lava or magma cools and hardens (crystallizes)
Magma molten (melted) rock material from inside the Earth
The temperature of magma ranges from about 650°C to 1200°C
Mineral a solid material that is made of many chemical elements and has a specific crystal structure
They are the building blocks of igneous rocks
Silicate a mineral made up of silicon and oxygen
Igneous rocks form from cooling magma
Earth’s temperature increases with depth
Less dense magma is less dense than the surrounding rock, so it is forced upward toward the surface
Mineral grains as magma cools, atoms in the liquid rearrange themselves into new crystals called mineral grains
As the mineral grains grow together, rocks form
Intrusive igneous rock formed deep in the
igneous rock Earth when magma cools and crystallizes
usually occurs when magma cools and becomes solid before it reaches the surface
Large crystals when magma cools very slowly (coarse-grained) under Earth’s surface, the crystals have plenty of time to grow to be large
Intrusive igneous rocks are found at Earth’s surface after erosion has removed the rocks and soil above them
Lava magma that reaches the Earth’s surface and flows out
Extrusive igneous rocks that form as igneous rock lava cools and crystallizes at or
near Earth’s surface
Small crystals when magma is at the Earth’s (fine-grained) surface and cools very rapidly, there is not enough time for the crystals to grow to be large
http://www.classzone.com/books/earth_science/terc/content/investigations/es0603/es0603page05.cfm?chapter_no=investigation
Glassy texture results when the lava cools so quickly that no crystals have time to form (obsidian)
Light in color white, light gray, or pink
They are lighter in color because they are low in iron and magnesium
Dark in color dark green, gray, black or brown
They are darker in color because they are high in iron and magnesium
Classification igneous rocks are classified by the type of magma that formed them
Granite forms when magma that is high in silica does not flow very easily
The magma usually cools before it reaches the Earth’s surface
Rhyolite extrusive igneous rock
Basalt an extrusive igneous rock
It’s the most common rock on the Earth’s crust, because it is the major rock of the oceanic crust
Andesite an extrusive rock
It has an intermediate color between light and dark
Pumice volcanic rock that has bubble holes, with only thin walls between the holes
(pumice floats on water)
Activity 2
Check Your Understanding Date
Page U20 Page #
1. Describe the
difference between
intrusive and extrusive
igneous rock.
2. How do the two main
types of igneous rocks
form?
3. Explain why some
rocks are light in color
and others are dark in
color.
Igneous Rocks and the Geologic
History of Your Community
Understanding and Applying Date
Page U21 Page #
1a. Identify
igneous rocks.
Rocks / LightIntermediate
Dark / Glassy
Fine
Coarse / Intrusive
Extrusive / Name
1
2
3
4
5
6