Objectives

1. Summarize igneous rock formation. Describe the composition of magma. Identify the factors that affect how rocks melt and crystallize. Section 5.1 Objectives What are igneous rocks?

2. Igneous rocks are the rocks that form when molten material cools and crystallizes. Section 5.1

3. Section 5.1 Igneous Rock Formation Lava is magma that flows out onto Earth’s surface. Igneousrocks form when lava or magma cools and minerals crystallize.

4. Section 5.1 Igneous Rock Formation The type of igneous rock that forms depends on: composition dissolved gases mineral crystals.

5. Section 5.1 Composition of magma Magma is classified as basaltic, andesitic, or rhyolitic, based on the amount of silica it contains. Silica affects melting temperature and impacts how quickly magma flows.

6. Section 5.1 Magma formation The four main factors involved in the formation of magma are: temperature pressure water content mineral content.

7. Section 5.1 What trend do you see in this graph?

8. Section 5.1 Pressure increases with depth. The increased pressure on a rock also increases the rock’s melting point. As water content increases, the melting point decreases. What would these graphs look like?

9. Section 5.1 The process whereby some minerals melt before others is called partialmelting. Looking at the picture, what melts first?

10. Section 5.1 When magma cools, the first minerals that crystallize are the last minerals that melted during partial melting. This process is called fractionalcrystallization. Simply stated, this is the opposite of partial melting!

11. Classify different types and textures of igneous rocks. Recognize the effects of cooling rates on the grain sizes in igneous rocks. Describe some uses of igneous rocks. Section 5.2 Objectives Classification of Igneous Rocks

12. Section 5.2 Classification of Igneous Rocks Classification of igneous rocks is based on mineral composition, crystal size, and texture.

13. Section 5.2 Classification of Igneous Rocks When magma cools and crystallizes below Earth’s surface, intrusive rocks form. • Magma that cools and crystallizes on Earth’s surface forms extrusive rocks. Igneous rocks are classified by their mineral compositions.

14. Section 5.2 Basaltic rocks are dark-colored, have lower silica contents, and contain mostly. Granitic rocks are light-colored, have high silica contents, and contain mostly quartz or other light colored minerals Rocks that have a composition of minerals that is somewhere between basaltic and granitic are called intermediate rocks. Ultrabasic rocks contain only iron-rich minerals such as olivine and pyroxene and are always dark.

15. Section 5.2 Rock type can be determined by estimating relative percentages of minerals in the rocks.

16. Section 5.2 Texture Texture refers to the size, shape, and distribution of the crystals or grains that make up a rock. When lava flows on Earth’s surface, it cools quickly and there is not enough time for large crystals to form. The resulting extrusive igneous rocks have crystals so small that they cannot be seen without magnification.

17. Section 5.2 A porphyritic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.

18. Section 5.2 The spongy appearance that results from gas bubbles trapped in lava is called vesicular texture.

19. Section 5.2 Scientists use a special microscope, called a petrographicmicroscope, to help them identify minerals by examining the mineral grains in sections of rock thin enough for light to pass through.

20. Section 5.2 Classification of Igneous Rocks Valuable ore deposits often occur within igneous intrusions.

21. Section 5.2 Veins of extremely large-grained minerals that can contain ores are called pegmatites.

22. Section 5.2 Classification of Igneous Rocks Diamonds and other minerals that can form only under very high pressure are found in rare, ultrabasic rocks known as kimberlites.

23. 5 Study Guide Key Concepts • Ores often occur in pegmatites. Diamonds occur in kimberlites. • Some igneous rocks are used as building materials because of their strength, durability, and beauty.