Earth Science 3.2

1. Earth Science 3.2 Igneous Rocks

2. Igneous Rocks • Remember from the Rock Cycle that igneous rocks form when magma or lava cools and hardens • Different types of igneous rocks form when this hardening occurs depending on a variety of factors.

3. Igneous Rocks • The word igneous comes from the Latin ignis, which means fire • Fiery volcanic eruptions are responsible for some types of igneous rocks. • Still others are formed deep inside the Earth’s surface.

4. Igneous Rocks • Intrusive Igneous Rocks • Rocks that form when magma hardens beneath Earth’s surface are called intrusive igneous rocks.

5. Igneous Rocks • Magma consists mainly of the elements • silicon and oxygen, • plus aluminum, • iron, • calcium, • sodium, • potassium, • and magnesium.

6. Igneous Rocks • Magma also contains some gases including water vapor. • These gases are kept within the magma by the pressure of the surrounding rocks. • Because magma is less dense it slowly works it’s way toward the surface.

7. Igneous Rocks • As magma rises, it cools slowly, allowing elements to combine and form minerals. • Gradually, the minerals grow in size forming a solid mass of interlocking crystals. • Granite is a common intrusive igneous rock.

8. Extrusive Igneous Rocks • Extrusive Igneous Rocks: • Remember that when magma reaches the surface, we call it lava. • Lava is similar to magma except that most of the gases have escaped. • When lava hardens, the rocks that are formed are called extrusive igneous rocks “This is because they are extruded onto Earth’s surface”

9. Classifications of Igneous Rocks • Many different types of igneous rocks come from these intrusive and extrusive processes. • Texture and composition are the two most important characteristics to look at when we classify igneous rocks.

10. Classifications of Igneous Rocks • Texture describes the appearance of an igneous rock based on it’s size, shape and arrangement of it’s interlocking crystals. • Composition classes of igneous rocks are based on the proportions of light and dark materials in the rock.

11. Classifications Igneous Rocks • Course Grained Texture • The rate of cooling strongly affects the textures of igneous rocks. • Slow cooling results in the formation of large crystals. • Igneous rocks with large crystals exhibit a course-grainedtexture.

12. Classifications Igneous Rocks Fine Grained Texture • If cooling occurs rapidly instead, the ions in the melted material loose their motion and quickly combine. • This results in large numbers of tiny crystals that all compete for the available ions. • Rapid cooling of magma or lava results in rocks with small interconnected grains. • These rocks are said to be fine grained texture.

13. Classifications Igneous Rocks Glassy texture • When lava spews onto Earth’s surface rapidly, there may not be enough time for the ions to arrange themselves into a network of crystals. • Solids produced this way are made of randomly distributed ions. • Such rocks have a glassy texture.

14. Classifications Igneous Rocks Porphyritic Texture • A large body of magma located deep within the Earth may take tens of thousands of years to harden. • Minerals that crystallize from the magma do not form at the same rate or at the same time. • It is possible for some crystals to get quite large while others are just starting to form.

15. ClassificationsIgneous Rocks Porphyritic Texture • The resulting rock can have large crystals, called phenocrysts, while the surrounding rock has a fine grain. • These rocks have a porphyritic texture. • They are a mixture of large and fine grains intermingled.

16. Classification of Igneous Rocks Granite Composition: • Igneous rocks in which light colored silicate mineral quartz and feldspar are the main ingredient are said to have a granite composition. • In addition to quartz and feldspar, most granite rock contains about 10% dark silicate minerals.

17. Classification of Igneous Rocks Basaltic Compositions • Rocks that contain many dark silicate minerals and plagioclase feldspar have a basaltic composition. • Basaltic rocks are rich in iron and magnesium. • Because of the iron, they are usually darker and denser than granite rocks. • The most common basaltic rock is basalt.

18. Classification of Igneous Rocks • Rocks with a composition between granitic and basaltic rock have an andesitic composition. • This group of rocks is named after the common volcanic rock andesite. • Andesitic rocks contain at least 25 % dark silicate minerals.