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Earth Materials: Chp 3: Igneous Rocks

Earth Materials: Chp 3: Igneous Rocks. Divergent Boundary. Convergent Boundary. Convergent Boundary. Magma: molten rock below Earth’s surface Lava: molten rock at the surface. Composition. Type of magma Silica Location Viscosity Colour content

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Earth Materials: Chp 3: Igneous Rocks

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  1. Earth Materials:Chp 3: Igneous Rocks

  2. Divergent Boundary Convergent Boundary Convergent Boundary

  3. Magma: molten rock below Earth’s surface • Lava: molten rock at the surface

  4. Composition Type of magma Silica Location Viscosity Colour content felsic = silisic >65% cont’l cr thick light Intermediate 53–65% cont-ocean mafic = basaltic 45-52% ocean-cont ultra mafic <45% ocean thin dark

  5. Pressure solid liquid Temperature How magma forms • 1. Lower pressure • Caused by decrease in pressure as plates move apart at divergent boundaries High pressure High pressure Low pressure

  6. Pressure

  7. 2. Higher Temperatures at magma plumes: localized heating caused by increased radioactivity or the “blanket effect” (large thick continental plates insulate resulting in heat build up) Continental plate Magma plume

  8. Magma Plume

  9. 3. Water (yes water) lowers the melting temperature of rocks wet dry rock pressure solid liquid Water leaks down here 1100 C temperature 1500 C Cont’l plate Oceanic plate

  10. Water leaks down here

  11. Magma Melts: wet vs. dry

  12. Modification of Magma = how magma becomes more felsic/silisic 1. Fractional Crystallization (crystal settling) • High temeprature minerals high in Mg, Fe; low in SiO2 form first. • The minerals then sink/settle out of the magma (crystal settling). The magma becomes more felsic/silsic as the plume rises.

  13. Bowen’s Reaction Series Crystallize first

  14. Magma Modification: Fractional Crystalization

  15. 2.Wall Rock Assimilation • As hot magma rises it melts and mixes with silisic/felsic country rock become more silisicous. • Xenolith = fragment of country rock that falls into the magma and doesn’t melt (that’s how diamonds reach the surface!) Silisic country rock Mafic

  16. Magma Modification: Wall Rock Assimilation

  17. Xenoltih

  18. 3. Magma mixing • When two magmas of different compositions (more/less silisic) join and mix to form an intermediate mixture. • rare

  19. Igneous Rock TYPES: 1. Intrusive (plutonic) • Solidify slowly underground • Large crystals (visible to naked eye) – PHANERITIC texture 2. Extrusive (volcanic) • Solidify quickly at or near surface • Small crystals = aphanitic texture

  20. Igneous Rock Types Extrusive aphanitic PHANERITIC Intrusive

  21. TEXTURE 1. PHANERITIC (coarse) (5 rocks) • Minerals can be seen without magnification • Slow cooling = larger crystals • Intrusive • 4 intrusive rocks ID with chart 1b Pegmatic: very large crystals (+5 mm) (1 rock = pegmatite) • Very slow cooling • Usually granitic composition

  22. Granite

  23. Granite

  24. Diorite + Dalmation Rock

  25. Gabbro

  26. Peridotite

  27. Pegmatite

  28. 2. aphanitic (fine): minerals too small to be seen without magnification (need lens/scope) • Fast cooling • Extrusive • 3 extrusive rocks ID with chart

  29. 2a. Volcanic glass (obsidian) cools veery fast • 2b. Vesicular (scoria & pumice) rocks that freezes before the gas can escape leaving numerous small vesicles (cavities) = “frozen milkshake” • 2c. Pyroclastic = fragmental texture (Volcanic breccia & welded tuff) • Material blown out of volcano, settles and become rock

  30. Composition Texture Felsic Mafic Vesicular Pumice Scoria/vesicular (bubbly) “styrofoam” basalt Glassy Obsidian (volcanic glass,usually black) Pyroclastic Volcanic Breccia (welded cinders, blocks & bombs) Tuff/Welded Tuff (welded ash)

  31. 2d. Porphyritic (porphyry) • Large crystals (phenocrysts) surrounded by a small crystals (groundmass) • Forms by: • magma cools slowly at depth as an intrusive body forming initially the large crystals • Then magma is quickly extruded to the surface where it cools quickly freezing the large crystals in a fine ground mass

  32. Rhyolite

  33. Andesite

  34. Basalt

  35. Volcanic Glass = Obsidian(Glassy Texture)

  36. Pumice (Vesicular Texture)

  37. Scoria = Vesicular Basalt (Vesicular Texture)

  38. Welded Tuff (Pyroclastic Texture) Look for layering that formed as ash was laid down

  39. Volcanic Breccia = “Dog Puke”(Pyroclastic Texture)

  40. Porphyry = Choc-chip cookie(Porphyritic Texture)

  41. Therefore all igneous rocks are classified by: • 1. composition (ultra-mafic felsic) = minerals present • 2. texture (PHANERITIC aphanitic)

  42. HW • Do WS 3.2 • Do Lab 4.1

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