1. CHAPTER 4 IGNEOUS ROCKS

2. Magma: The parent material of igneous rock
Lava
Extrusive – volcanic
Intrusive – plutonic
Nature of magma
Melt
Volatiles
From magma to crystalline rock
Crystallization

3. IGNEOUS TEXTURES Factors affecting crystal size
Rate at which magma cools
Amount of silica present
Amount of dissolved gases (volatiles) in the magma
Types of igneous textures
Aphanitic (fine-grained) texture
Vesicular
Phaneritic (coarse-grained) texture
Porphyritic
Phenocrysts
Groundmass
Glassy
Pyroclastic (fragmental)
Pegmatitic

9. ORIGIN OF MAGMA Generating magma from solid rock Role of heat
Role of pressure
Role of volatiles

11. ORIGIN OF MAGMA Generating magma from solid rock Role of heat
Role of pressure
Role of volatiles

13. ORIGIN OF MAGMA Generating magma from solid rock Role of heat
Role of pressure
Role of volatiles

15. MORE VOLATILES = LOWER MELTING TEMPERATURES

17. ADDING A SECOND MINERAL LOWERS MELTING POINT OF FIRST MINERAL

18. HOW MAGMAS EVOLVE BOWEN’S REACTION SERIES
and the composition of igneous rocks
Discontinuous reaction series
Continuous reaction series
Magmatic differentiation (crystal settling)
Assimilation and magma mixing

19. BOWEN’S REACTION SERIES

20. IGNEOUS ROCK COMPOSITIONS
Silica content is an indicator of composition
Composition categories
Granitic – felsic
Andesitic – intermediate
Basaltic – mafic
Peridotitic – ultramafic

21. NAMING IGNEOUS ROCKS
Felsic (granitic) igneous rocks – intrusive & extrusive
Granite
Ryolite
Obsidian
Pumice (vesicular)
Intermediate (andesitic) igneous rocks – intrusive & extrusive
Diorite
Andesite
Mafic (basaltic) igneous rocks – intrusive & extrusive
Gabbro
Basalt
Pyroclastic rocks – airborne
Tuff
Welded tuff
Volcanic breccia

24. OBSIDIAN - Glassy (cooled too rapidly to form crystals)
OBSIDIAN - Glassy (cooled too rapidly to form crystals). Photomicrographs are magnified about 27 times.

26. Pumice, a glassy rock containing numerous vesicles

27. Vesicles are small holes left by escaping gas bubbles.

29. Rhyolite, the aphanitic equivalent of granite, is less abundant.

31. Aphanitic (fine-grained).

32. PEGMATITE

34. Volcanic rock - angular rock fragments embedded in a light-colored matrix of ash.

35. Andesite porphyry - a common volcanic rock.

36. Photomicrograph of a thin section of andesite porphyry to illustrate texture. Notice that the few large crystals (phenocrysts) are surrounded by much smaller crystals (groundmass).

37. Because replacement was not complete, this feldspar crystal has a calcium-rich interior surrounded by zones that are progressively richer in sodium.

40. Basalt is aphanitic and a very common extrusive rock.

45. Outcrop of welded tuff (tan) interbedded with obsidian (black) near Shoshone, California. Caption:Tuff is composed mainly of ash-sized particles and may contain larger fragments of pumice or other volcanic rocks

48. Granite - one of the most common phaneritic igneous rocks.

54. Diorite - phaneritic igneous rock of intermediate composition.

55. Close up of diorite. The white crystals are plagioclase feldspar, and the black crystals are amphibole and biotite.

57. Gabbro, the phaneritic equivalent of basalt, is less abundant.