Igneous Rocks

1. Igneous Rocks

2. Standards • Classify matter in a variety of ways • Describe the composition and structure of Earth’s materials, including: • the major rock types (i.e., sedimentary, igneous, metamorphic) and their formation

3. Igneous Rocks • Rocks formed from magma • Make up 95% of Earth’s crust, much of which is covered by thin layer of sedimentary rocks

4. Formation of Igneous Rocks • Form from the solidification of magma (molten rock) • Hot temperatures in the aesthenosphere (600 - 1400 C) cause rock to melt • As rock heats up, it becomes less dense and rises • When it cools and solidifies, either underground or above ground, it becomes igneous rock

5. Classification of Igneous Rocks • Classification is based on: • Where rock is formed • Texture (which is directly affected by where rock is formed) • Mineral content (what minerals are in rock)

6. Environment where Rock is Formed • Two main groups of igneous rocks: • Intrusive (plutonic) • Extrusive (volcanic)

7. Intrusive (Plutonic) • Named after Pluto, god of the underworld • Forms from solidification of magma underground • Forms 1 kilometer or deeper below surface of Earth • Ex: granite

9. Granite Photo: http://hsc.unm.edu/medicine/integrative_med/brian_shelley_biosketch.shtml

10. Extrusive (Volcanic) • Named after Vulcan, god of fire • Forms from eruption and solidification of magma (lava) on surface of Earth • Forms less than 1 kilometer deep • Ex: basalt

12. Textures • Rock texture refers to the size, shape and arrangement of mineral grains

13. Intrusive Texture • Intrusive magmas cool slowly, over hundreds of thousands of years, due to insulating properties of surrounding rocks. • This allows time for crystals to grow = larger crystals. • These rocks are medium to large grained, called Phaneritic • Ex: granite – most abundant rock in Earth’s crust

14. Intrusive Texture

15. Extrusive Texture • Extrusive magmas (lavas) cool rapidly. • There is little time for crystal growth = very small or micro crystals. • These rocks are fine grained, called aphanitic • Ex: basalt

16. Extrusive Texture

17. Extrusive Texture • Some extrusive lavas cool so rapidly there is no time for crystal growth • In this case they have a glassy texture • A glassy texture can be either massive (like obsidian [volcanic glass]) or frothy (like pumice)

18. Obsidian Pumice

19. Extrusive Texture • Some extrusive rocks can contain holes that are called vesicles. • Vesicles form when gases (air bubbles) escape from the cooling rock leaving behind a cavity.

20. Vesicular Basalt

21. ExtrusiveTexture • Some extrusive rocks form a porphyry (or porphyritic texture) – large crystals, called phenocrysts, are present in a fine-grained groundmass. • Chocolate chip cookie analogy: chocolate chips are like the phenocrysts and cookie dough is like the groundmass.

22. Porphyritic Texture

23. Color Classification • Based on mineralogy • Felsic rocks – igneous rocks that are light colored. They are rich in minerals that contain lots of silica. • Mafic rocks – dark colored igneous rocks. They are rich in iron and magnesium • Intermediate – rocks midway between felsic and mafic. • Ultramafic – very dark rocks, containing very little silica.

24. Igneous Rock Pairs • Igneous rocks are classified in pairs, in which members of the pairs contain identicalminerals, but have differenttextures (intrusive vs. extrusive) Color Intrusive Extrusive Felsic Granite Rhyolite Intermediate Diorite Andesite Mafic Gabbro Basalt Ultramafic Peridotite

25. Felsic Granite Rhyolite

26. Intermediate Diorite Andesite

27. Mafic Gabbro Basalt

28. Ultramafic Peridotite