Classifying Igneous Rocks
IGNEOUS ROCKS
TEXTURE
Light-colored (felsic)
(quartz
Dark-colored (mafic)
olivine)
little quartz
Coarse-grained
Granite
Diorite
Gabbro
Fine-grained
Rhyolite
Andesite
Basalt
Glassy
Obsidian
Porous
Pumice
Scoria
orthoclase feldspar
plagioclase feldspar
***(quartz, olivine, orthoclase feldspar, & plagioclase feldspar are minerals)
Use the above chart to answer the following questions.
1.
What type of igneous rock is light-colored (felsic), with visible crystals of quartz and orthoclase feldspar ?
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2.
What type of igneous rock is a fine-grained and dark-colored (mafic) ? ________________________________________
3.
What is the difference between diorite and andesite ?
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4.
How are gabbro and granite different ?
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5.
Would it be easier to identify individual minerals of granite or rhyolite ? Explain why.
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6.
Which of the following igneous rocks would have the greatest amount of quartz: rhyolite, diorite, or basalt ?
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7.
What mineral, other than plagioclase feldspar, is very common in gabbro ? ____________________________________
8.
Circle the following igneous rocks that are extrusive (cools quickly on the surface creating fine-grained crystals).
basalt
9.
diorite
andesite
rhyolite
granite
What two basic characteristics are used to classify igneous rocks ?
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Classifying Igneous Rocks
• Igneous rocks (most abundant type of rock) are formed from the cooling of molten rock material. The heat (approximately
1,400 ° C) that melts the rock does not come from the core of the earth, rather it generally comes from the radioactive decay of
elements in the mantle (60-200 km below the surface). This molten rock material that is found below the surface is referred to as
magma. As soon the molten rock material reaches the surface it is called lava.
• In situations which magma cools into igneous rocks, the resultant rocks are categorized as intrusive. Due to the slow cooling
within the earth the mineral crystals are able to grow relatively large, course-grained.
• When lava cools relatively quickly (volcanoes), the minerals form small crystals and the igneous rock is categorized as extrusive,
fine-grained. Generally speaking the crystals are too small to see with the naked eye.
• A sub-category of extrusive igneous rock are the glasses. These rocks cool so fast that no crystals form. Obsidian and pumice
are examples of volcanic glass.
• Additionally, you may further categorize igneous rocks by their chemical composition. Recollect the 2 types of crustal
material are basalt and granite.
• Mafic- basaltic magma is more dense, dark-colored, makes up ocean crust, and creates lava
flows such as Hawaii.
• Felsic- granitic magma is less dense, light-colored, makes up continental crust, and creates
pyroclastic eruptions.
• Look at the examples of igneous rocks. List them by the following categories: intrusive,
extrusive, and glass
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I-5 ________________________
I-6 ________________________