Chapter 5, Section 1

1) How are igneous rocks formed?
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Igneous rocks are formed from the crystallization of magma as it cools.

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Lava is magma that flows out onto Earth’s surface.

3) What are extrusive igneous rocks?
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Extrusive rocks are fine-grained igneous rocks that cool quickly on the Earth’s surface.

4) What are intrusive igneous rocks?
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Intrusive rocks are coarse-grained igneous rocks that cool slowly beneath the Earth’s surface.

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Magma is a slushy mixture of molten rock, gases, and mineral crystals.
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Elements found in magma include oxygen, silicon, aluminum, and iron

6) Why is silica an important magma compound?
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Because silica is the most abundant compound in magma, it has the greatest effect on magma characteristics.

7) What are the 3 classes of igneous magma, and how…
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The 3 classes of magma are basaltic, andesitic, and rhyolitic.
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They are classified according to their silica content.

8) At what temperatures do rocks melt?...
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Rocks melt at temperatures between
800 ° C and 1200 ° C.
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These temperatures are found in the upper mantle and lower crust.

9) State the four factors that affect magma formation…
(a)
(b)
(c)
(d) temperature—increases with depth in the
Earth’s crust
Pressure—as pressure on a rock increases, its melting point also increases
Water content—as water content increases, melting point decreases
Mineral composition—oceanic crust is rich in iron and magnesium and melts at higher temperatures than continental crust, which is rich in aluminum and silicon

10) Explain why magma is often a slushy mix of crystals…
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Because different minerals have different melting points, not all parts of a rock melt at the same time.
Thus, this partial melting is why magma is slushy.

11) What is fractional crystallization…

When magma cools, it crystallizes in the reverse order of partial melting.
Fractional crystallization is the process by which the first minerals to crystallize from magma are the last to melt during partial melting.

12) Name 4 properties by which geologists classify igneous rocks.

Intrusive or Extrusive

Mineral Content

Grain Size

Texture

13) Describe the mineral composition of the 3 major groups..
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
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Felsic—high silica content, contain quartz and feldspar crystals (ex. Granite)
Mafic—low silica content, rich in iron and magnesium (ex. Gabbro)
Intermediate—moderate amount of silica, contains biotite, amphibole, pyroxene (ex.
Diorite)

14) Why do some igneous rocks have large mineral grains…


When magma cools slowly beneath Earth’s surface, large mineral crystals may form.
These are intrusive rocks.
Extrusive rocks have no visible grains because they are made from lava that cooled too quickly for crystals to form.



Porphyritic texture forms when slowly cooling magma suddenly cools rapidly.
This forms a rock with large, well-formed crystals surrounded by finer-grained crystals of the same mineral.

16) What characteristics of igneous rocks make them useful…
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
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Interlocking grain textures give them strength
They are resistant to weathering
Granite is one of the most durable igneous rocks.

17) Which minerals are often found in veins of ore?
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Gold, silver, lead, copper, and quartz are often found in veins.
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Usually, these veins are found inside igneous intrusions.

18) What is pegmatite, and give an example.

Pegmatites are veins of extremely large-grained minerals, such as garnets, which occur as crystals.



Kimberlites are rare, ultramafic rocks, which are a variety of peridotite.
They form under very high pressure and often contain diamonds.
Ultramafic rocks are very high in iron and magnesium.