Objectives • Compare and contrast intrusive and extrusive igneous rocks. What are igneous rocks? • Describe the composition of magma. • Discuss the factors that affect how rocks melt and crystallize. – igneous rock – lava – extrusive – intrusive Vocabulary – partial melting – fractional crystallization – Bowen’s reaction series
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Objectives Compare and contrast intrusive and extrusive igneous rocks. What are igneous rocks? Describe the composition of magma. Discuss the factors.
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Objectives• Compare and contrast intrusive and extrusive
igneous rocks.
What are igneous rocks?
• Describe the composition of magma.
• Discuss the factors that affect how rocks melt and crystallize.
Composition of Magma• Magma is often a slushy mix of molten rock,
gases, and mineral crystals.
What are igneous rocks?
• The elements found in magma are the same major elements found in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na).
Origins of Magma• In the laboratory, most rocks must be heated
to temperatures of 800°C to 1200°C before they melt.
What are igneous rocks?
• These temperatures are found in the upper mantle and lower crust.
• Scientists theorize that the remaining energy from Earth’s molten formation and the heat generated from the decay of radioactive elements are the sources of Earth’s thermal energy.
– Mineral content also impacts how magma is formed as different minerals have different melting points.
– In general, oceanic crust is rich in iron and magnesium and therefore melts at higher temperatures than continental crust, which contains higher levels of silicon and aluminum.
– For rocks to melt, the right combination of temperature, pressure, and composition must be present.
– When magma cools, it crystallizes in the reverse order of partial melting—the first minerals to crystallize from magma are the last minerals to melt during partial melting.
– Fractional crystallization is the process wherein different minerals form at different temperatures.
– Geologists hypothesize that under certain conditions, newly formed crystals can be separated from magma.
– This stops the chemical reactions between the magma and the minerals.
– Crystal separation can occur when crystals settle to the bottom of the magma body, and when liquid magma is squeezed from the crystal mush to form two distinct igneous bodies with different compositions.
Texture• Often, it’s easier to observe the sizes of mineral
grains than it is to observe their shapes.
Classifying Igneous Rocks
• Many mineral grains have interlocking edges.
• As the grains crystallize from magma, they grow together and form irregular edges.
• During fractional crystallization, the minerals that form early in the process float in a liquid and have space in which to grow distinct crystal shapes.
– A rock that has a porphyritic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.
– Porphyritic textures indicate a complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly.
– Some important metallic elements that are not included in common minerals are gold, silver, lead, and copper.
– These elements, along with the dissolved silica, are released at the end of magma crystallization in a hot, mineral-rich fluid that fills cracks and voids in the surrounding rock.
– This fluid solidifies to form metal-rich quartz veins, such as the gold-bearing veins.
Section 5.1 Main Ideas• Igneous rocks are formed by the cooling and
crystallization of magma. Intrusive rocks form inside Earth’s crust, and extrusive rocks form on Earth’s surface. Extrusive rocks, which cool more rapidly than intrusive rocks, are generally more fine grained.
• Magma is a slushy mix of molten rock, gases, and mineral crystals. The elements found in magma are the same major elements found in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na).
Section 5.1 Study Guide
Section 5.1 Main Ideas• Silica (SiO2) is the most abundant compound in magma.
Magmas are classified as basaltic, andesitic, and rhyolitic, based on the amount of SiO2 they contain.
• Different minerals melt and crystallize at different temperatures in the processes of partial melting and fractional crystallization. Minerals crystallize from magma in a sequential pattern known as Bowen’s reaction series.
Section 5.1 Study Guide
Section 5.2 Main Ideas• Igneous rocks are classified as felsic, mafic, intermediate,
and ultramafic, depending upon their mineral compositions. Felsic rocks such as granite are light-colored, have high silica contents, and contain quartz and feldspars. Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium. Intermediate rocks have moderate silica levels. Ultramafic rocks have low silica contents and very high levels of iron and magnesium. Igneous groups can be further identified by crystal size and texture.
• Early forming minerals may have well-shaped crystals, while later-forming minerals have irregular shapes. Porphyritic textures contain both large and small crystals.
Section 5.2 Study Guide
Section 5.2 Main Ideas• Igneous rocks such as granite are often used as building
materials because of their strength, durability, and beauty.
• Valuable ore deposits and gems are often associated with igneous intrusions. Ores of rare elements such as lithium and beryllium are found in veins of extremely large-grained minerals called pegmatites. Diamonds are found in rare types of igneous intrusions known as kimberlites.
Section 5.2 Study Guide
1. The process wherein different minerals form at different temperatures is called ____.
a. partial melting
b. Bowen’s reaction series
c. fractional crystallization
d. crystal separation
Multiple Choice
Chapter Assessment
Partial melting is when some minerals melt at low temperatures while others remain solid. Bowen’s reaction series describes in what order minerals form. Crystal separation is a hypothesis that answers questions about Bowen’s reaction series.
2. Andesitic magma’s SiO2 concentration is ____.
a. 30 percent c. 60 percent
b. 45 percent d. 70 percent
Basaltic magma has an SiO2 concentration of 50 percent. Rhyolitic magma has an SiO2 concentration of 70 percent.
Multiple Choice
Chapter Assessment
Multiple Choice
3. What minerals are associated with the left branch of Bowen’s reaction series?
a. iron-magnesium c. intrusive
b. feldspars d. felsic
Chapter Assessment
Feldspars are represented on the right branch of Bowen’s reaction series. Intrusive describes igneous rocks that cool slowly beneath Earth’s surface. Felsic rocks are one classification of igneous rocks.
Multiple Choice
4. What is the texture of ultramafic rocks?
a. glassy c. coarse-grained
b. fine-grained d. very coarse-grained
Chapter Assessment
Ultramafic rocks include peridotite and dunite which both have a coarse grain.
Multiple Choice
5. Extrusive igneous rocks generally have small mineral grains because ____.
a. they cool slowly.
b. the minerals in extrusive rocks do not form crystals under any condition.
c. they cool quickly.
d. there is too much water in the magma.
Chapter Assessment
By quickly cooling, the minerals in extrusive igneous rocks do not have enough time to form large crystals.
Short Answer
6. What characteristics make igneous rocks useful as building materials?
Chapter Assessment
Short Answer
7. What causes a porphyritic texture to form in certain rocks?
Chapter Assessment
True or False
8. Identify whether the following statements are true or false.
______ Diamonds are found in kimberlites.
______ Fe-Mg minerals undergo a continuous change
according to Bowen’s reaction series.
______ Silica is the most common compound in magma.
______ A rock that melts at 1000ºC on the surface of Earth may not melt until 1200ºC at a depth of 100 km.
______ Fractional crystallization occurs in the reverse