Igneous Rocks: Types, Textures, and Formation

1. Introduction to Rocks Chapter 5 Earth Science 11

2. What is a Rock? What do you think a rock is? • A group of minerals bound together some way • Example: lava hardens into rocks, sand and clay harden into rocklike materials

3. Types of rocks

4. Types of Igneous Rocks 1. Plutonic (Intrusive) • Formed underground from cooled magma • Has distinct mineral grains • Plutonic rocks are seen at the surface, after the rocks covering them are worn away. Granite exposed by weathering

5. Types of Igneous Rock 2. Volcanic (Extrusive) • Formed from magma that pours onto Earth’s surface during volcanic eruptions (lava). • May also be formed out of volcanic dust or ash. • Lack distinct mineral grains.

6. Plutonic vs. Volcanic Plutonic with distinct mineral grains Gabbro Volcanic that lacks distinct mineral grains Basalt

7. Types of Magma 1. High-Silica (felsic) • Thick and slow flowing magma • Forms rocks that have light-coloured minerals • Most plutonic rocks are felsic Granite • Contains light coloured mineral such as quartz and feldspar

8. Types of Magma 2. Low-Silica (mafic) • Lower percentage of silica • Hotter, thinner, more fluid, darker-colour • Forms rocks that contains mostly dark minerals Basalt • Contains dark minerals such as hornblende, augite, and biotite.

9. Textures of Igneous Rock Rocks are not only group by their mineral composition but by their texture. Texture depends on: • Size of the crystals • Shape of the crystals • And arrange of its mineral crystals

10. Textures of Igneous Rock Igneous rock texture ranges from: glassy-smooth coarse-grained Obsidian Granite

11. Textures of Igneous Rock Crystal size most important factor affecting texture. • Affected by how fast the magma hardens • Magma within the crust hardens slowly = larger crystals • Magma on Earth’s surface hardens rapidly = tiny crystals • Very rapid magma harden = no crystal development