What are Igneous Rocks????

1. What are Igneous Rocks???? Form when magma cools and minerals crystallize

2. Composition of magma • Type of Igneous rock depends on the composition of the magma • Silica is the most abundant compound and has the greatest effect on magma characteristics • 3 classifications of magma based on the amount of silica it contains • Basaltic • Andesitic • Rhyolitic

3. Chemical composition of lava is a little bit different than the magma it came from • Dissolved gases escape into the atmosphere

4. Magma Formation • Two ways of formation • Melting earth’s crust • Melting within the mantle • Four factors • Temperature • Increases with depth within earth’s crust • Geothermal gradient • Pressure • Also increases with depth • Result of the weight of overlying rock • Pressure increases, melting point increases

5. Water content • Changes the melting point of the rocks • Water content increases, melting point decreases • Mineral content • Different minerals have different melting points • Basalt – high melting point • Granite – lower melting point • Contains more water • Rocks high in iron and magnesium – high melting point • Rocks high in silicon – low melting point

6. Partial Melting • Process whereby some minerals melt at relatively low temperatures while other minerals remain solid • New elements are added during the process • Changing the chemical composition • One way different types of igneous rocks are formed

7. Bowen’s Reaction Series: minerals form in predicatable patterns

8. Bowen’s Reaction Series • Iron- rich minerals (left side) • Go through abrupt changes as magma cools • Feldspars (right side) • Lighter minerals • Continually change chemical composition as magma cools

9. Classification of Igneous Rocks • Intrusive rocks • Magma cools and crystallizes below the earth’s crust • Magma injected into surrounding rock

10. Extrusive rocks • Cools and crystallizes on the Earth’s surface • Lava flow of flood basalts • Classified by: • Mineral composition • Physical properties • Grain size • texture

11. Classification of Igneous rocks • Based on mineral compositions • Basaltic • Dark colored • Lower silica content • Granitic rocks • Light colored • High silica content • Intermediate • Falls between basaltic and granitic • Feldspar and hornblende • Ultrabasic • Contain only iron rich minerals • Always dark

12. Basaltic • Gabbro

13. Granitic • granite

14. Intermediate • Feldspar • Hornblende

15. Ultra basic • Pyroxene

16. Crystal size and cooling rates • When lava cools on the earth’s surface • Cools quickly • Not enough time for large crystals to form • Rhyolite – hard to see crystals without magnification • Sometimes no crystals form • When magma cools beneath earth’s surface • Cools slowly • Large crystals form

17. Porphyritic Rocks • Porphyritic texture • Large crystals surrounded by finer crystals of same or different minerals • Complex cooling • First – slow cooling – big crystals • Then quicker as the magma moves closer to the earth’s surface (or erupted onto the surface) – small crystals porphyritic granite

18. Vesicular rocks • Vesicular texture • Big holes making the rock look spongy • Dissolved gases escape when pressure lessens • Leaving vesicles behind

19. Thin sections • Easier to observe igneous rock composition when viewed in a thin strip under a microscope hornblende olivine

20. Igneous rocks in resources • Veins • Fluid left during magma crystallization contain high levels of water, silica, and leftover minerals • Fluid fills in the cracks of surrounding rocks • Solidifies into mineral-rich quartz veins

21. Pegmatites • Veins of very large-grained minerals • Ores of rare elements form • Can produce beautiful crystals

22. Kimberlites • Veins in ultrabasic rock • Diamonds • Most likely form deep in the crust or mantle • Intrude quickly upward • Forming long, narrow, pipe like structures

23. Obsidian • Normally extrusive • Forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. • The result is a volcanic glass with a smooth uniform texture that breaks with fracture

24. Rhyolite • Light-colored • Fine-grained • Extrusive igneous rock • Typically contains quartz and feldspar minerals.

25. Biotite Granite • intrusive, most commonly occurring in batholiths

26. Hornblende • greenish-black to black • extrusive

27. Basalt • Basalt is dark-colored and fine-grained • extrusive

28. Gabbro • Gabbro is dark, medium- to coarse-grained • Intrusive igneous rock