1. Chapter 4
Rocks & Igneous Rocks

2. Rock Definition
A naturally occurring consolidated mixture of one or more minerals
e.g, marble, granite, sandstone, limestone

3. Rock Definition Must naturally occur in nature, cannot be man-made
Is a solid, not gas, ice or liquid
Composed of one or more minerals

4. Exceptions to the Definition
Some rocks are not formed of minerals
Noncrystalline, glassy volcanic rocks such as obsidian and pumice
Coal which is formed from compacted plant remains

5. Appearance of a Rock
The appearance of a rock is determined by two major factors
Mineralogy
Texture

6. Appearance of a Rock
Mineralogy refers to the relative proportions of the constituent minerals
Texture is determined by the size, shape and arrangement of the mineral crystals

7. Appearance of a Rock
Mineralogy and texture are related to how and where a rock was formed and what has happened to the rock since it was created

8. Three Great Families of Rocks
Igneous
Sedimentary
Metamorphic

9. Three Types of Rocks

10. Igneous Rocks
Rocks formed by the solidification of magma (molten rock)
Granite

11. Two Types of Igneous Rocks
Extrusive igneous rocks form when magma erupts at the surface (i.e., above ground) and rapidly cools
Intrusive igneous rocks form when magma intrudes into bedrock and slowly cools (i.e., below ground)

12. Igneous Rocks

13. Igneous Rock Texture
Intrusive rocks such as granite are coarse grained (can easily see the grains with the unaided eye)
Extrusive igneous rocks such as basalt are glassy or fine grained

14. Most Igneous Rock are Silicates
Quartz
Feldspar
Mica
Pyroxene
Amphibole
Olivine
Basalt

15. Granite

16. Sedimentary Rocks Rocks formed by consolidation of sediments
Classified by the type of sediments

17. Two Major Types of Sediments
Clastic sediments are physically deposited particles derived from weathered rocks
Chemical and biochemical sediments include minerals carried in solution such as calcite and halite

18. From Sediments to Solid Rocks
Lithification is the process of converting sediments into solid rocks
(will cover in detail in another lecture)

19. The Two Types of Lithification
Compaction – sediments are squeezed together by the weight of overlying sediments into a solid mass
Cementation – minerals precipitate around the sediments and bind them into a solid rock

20. Sedimentary Rocks

21. Sedimentary Rocks 5 % by volume of the upper crust
75 % by exposed surface area of continents

22. Sedimentary Rocks Sedimentary rocks commonly contain fossils
In fact, some sedimentary rocks are almost entirely composed of fossils
Can exhibit extensive horizontal layers called bedding

23. Composition
Sedimentary rocks can be composed of sediments created from any of three great families of rock (igneous, sedimentary, metamorphic)
Chemical cementation commonly includes calcite, gypsum and halite

24. Metamorphic Rocks
Metamorphic rocks are formed by the transformation of previously-existing rocks in the solid state due to increased temperature and pressure

25. Metamorphic Rocks
Metamorphism can change the mineralogy, texture and/or the chemical composition of a “parent rock” while maintaining its solid form
Note that the rock does not melt (temperature range 250 to 700 C)

26. Regional and Contact Metamorphism
Regional metamorphism occurs where high temperature and pressures occur over large region (plate tectonics)
Contact metamorphism is limited to smaller areas such as around a magma intrusion into bedrock

27. Regional Metamorphism

28. Contact Metamorphism

29. Other Forms of Metamorphism
Ultra-high pressure metamorphism occurs deep in the continental and oceanic crust
High-pressure, low-temperature metamorphism occurs where oceanic crust subducts under a continental plate
Shock metamorphism at impact sites

30. Metamorphic Mineralization
Silicates are the most common mineral in metamorphic rocks
Minerals can tell you the “grade”of metamorphism
Certain minerals are uniquely characteristic of metamorphic rocks (kyanite, staurolite, some garnets)

31. Metamorphic Mineralization
Contact Ultra-high pressure Regional High-press, low-temp

32. Proportions of the Rock Types

33. A Few More Facts… Igneous processes create new rock
Metamorphic and sedimentary processes re-work old rock into a new form
Silicate mineral are the most common minerals found in all types of rocks

34. Common Minerals Found in Rocks

35. Five Very Different Locations

41. Rock Cycle

42. Rock Cycle

43. Rock Cycle
Subduction of an oceanic plate beneath a continental plate uplifts a volcanic mountain range
Magma rises from melting plate and intrudes or extrudes in the crust

44. Rock Cycle
Magmas cool to make igneous rock
Basalt versus granite

45. Rock Cycle
Weathering and erosion creates sediments

46. Rock Cycle
Sediments are carried to the oceans and lakes
Lithification

47. Rock Cycle
Metamorphism of subducted rock
Tectonic plates interact

48. Rock Cycle
Entire process starts over as plates interact

49. Igneous Rocks

50. Igneous Rocks How do igneous rocks differ from one another?
Where do they form?
How do rocks solidify from a melt (magma)?
Where do rock melt?

51. We Classify Igneous Rocks by Appearance
Mineralogy (Chemistry)
Texture

52. Igneous Rocks

53. We Classify Igneous Rocks by Genetics (Origin)
There are two classes of igneous rocks
Intrusive
Extrusive

54. Genetic Classification of Igneous Rocks
Intrusive igneous rocks crystallized from slowly cooling magma intruded within the Earth’s crust, such as granite and gabbro

55. Granite Intrusions

56. Genetic Classification of Igneous Rocks
Extrusive igneous rocks crystallized from rapidly cooling magma extruded on the surface of the Earth as lava or erupted as pyroclastic material, such as basalt

57. Extrusive Igneous Rocks
Rocks formed from the cooling of lavas extruded onto the Earth’s surface or onto ocean floors
Rocks formed by the cooling of pyroclastic material, such as fragmented pieces of magma and material erupted into the air

58. We Can Also Classify Igneous Rocks by Composition
Chemistry
Mineralogy

59. Chemistry
Modern classification of igneous rock is based upon the silica (SiO2) content
The silica content is determined by the silicate minerals that occur in the rock (i.e., the minerals contain SiO2)
The percentage of silica ranges from about 40% to about 70%
Rocks are referred to as “silica rich” or “silica poor”

60. Mineralogy
There are four major divisions of igneous rocks based upon the content of elements in the minerals:
Felsic
Intermediate
Mafic
Ultramafic

61. Felsic Igneous Rocks
Rich (high) in minerals containing silica Poor (low) in iron and magnesium They include: Granite Rhyolite

62. Intermediate Igneous Rocks
Intermediate in composition between felsic and mafic igneous rocks
Less silica, more Fe & Mg than felsic
More silica, less Fe & Mg than mafic They include: Granodiorite Dacite Diorite Andesite

63. Mafic Igneous Rocks Poor (low) in minerals containing silica
Rich (high) in iron and magnesium They include: Gabbro Basalt

64. Ultramafic Igneous Rocks
Very uncommon on the Earth’s surface
Very poor (lower) in minerals containing silica
Consist primarily of mafic minerals (olivine, pyroxene) The most common ultramafic rock is: Peridotite

66. Felsic Intermediate Mafic Granite Granodiorite Diorite Gabbro Rhyolite
Dacite
Andesite
Basalt

68. Common Minerals of Igneous Rocks

69. When Do Rocks Melt? Melting starts at ~700o C
When the temperature exceeds the melting point of the rock or some minerals within the rock
Minerals melt at different temperatures

70. When Do Rocks Melt? Three Factors Affecting Melting of Rocks
Pressure: Increased pressures raises melting points
Water Content: Increased water content lowers melting points
Composition: Felsic minerals melt at lower temperatures than mafic minerals

71. Magma Differentiation
The process by which rocks of various compositions can arise from a uniform parent magma

72. The Formation of Magma Chambers
Partial melting

73. The Formation of Magma Chambers
Less dense magma
Partial melting

74. The Formation of Magma Chambers
Magma rises
Less dense magma
Partial melting

75. The Formation of Magma Chambers
Magma pools in
magma chamber
The Formation of Magma Chambers
Magma rises
Less dense magma
Partial melting

76. Magma Differentiation
Occurs because different minerals crystallize (solidify) at different temperatures
In other words, as the magma cools some minerals form first, some form last

77. Fractional Crystallization
The process by which crystals forming in a cooling magma are segregated from the remaining liquid
In a simple scenario, the crystal settle to the floor or adhere to the walls of the magma chamber

78. Fractional Crystallization
The elements (such as Fe and Mg) used to create the newly formed crystals are now no longer available for creating new minerals
Therefore the chemical composition of the magma slowly changes as new minerals are continually formed and the available matter is selectively used up

79. Bowen’s Reaction Series
Experiments that determined the sequence of crystallization of minerals from a gradually cooling mafic (basaltic) magma
Conducted by Norman L. Bowen prior to 1916
These experiments totally rewrote our understanding of igneous rock formation

80. Bowen’s Reaction Series

81. Evidence of Fractional Crystallization in the Palisades Sill

82. Evidence of Fractional Crystallization in the Palisades Sill
first olivine next pyroxene pyroxene finally plagioclase & plag. Feldpar finishes

83. Partial Melting and the Origin of Magmas
Partial melting of upper mantle: e.g. at divergent spreading centers
Partial melting of continental crustal rocks
Mafic
Magmas
Felsic

84. Magmatic Stoping: Making Room for the Intrusion of Magma
Wedging open overlying rock
Breaking off large blocks of rock (remnants of which are called xenoliths)
Melting of surrounding country rock

85. Rising Magma Wedges Open and Fractures Overlying Country Rock

86. Overlying rocks may bow up

87. Magma melts surrounding rock

88. …changing the composition of the magma

89. Blocks of the Overlying Country Rocks (Xenoliths) May Break Off and Sink into the Magma

90. Large igneous bodies formed at depth in the Earth’s crust
Plutons
Large igneous bodies formed at depth in the Earth’s crust

91. Types of Plutons
Batholith: Massive, discordant intrusive body covering at least 100 km2
Stock: Massive, discordant intrusive body covering less than 100 km2
Dike: Tabular, discordant intrusive body
Sill: Tabular, concordant intrusive body

92. Types of intrusive and extrusive igneous structures

93. Sill

94. Dike

95. Where Do Most Magmas Occur?
Divergent Plate Margins
Convergent Plate Margins
Mantle Plumes/Hot Spots

96. Island Arc
Plate Subduction
(Japanese Islands)

97. Continental
Plate Subduction
(Mount St. Helens)

98. Hot Spot
Volcanism
(Hawaii)

99. Generation of Magmas at Convergent Plate Margins
Subduction drags oceanic lithosphere (including a veneer of “wet” sediments) beneath the adjacent plate

100. Generation of Magmas at Convergent Plate Margins
The release of volatiles lowers the melting point of the adjacent mantle, causing fluid-induced melting to form a mafic magma, which becomes more intermediate in composition as it rises through the overlying crust

102. x

103. Ophiolite Suites
Unusual assemblages of rocks found on land that had characteristics of seafloor
Composed of deep-sea sediments, basaltic lavas and mafic igneous intrusions
Fragments of ocean crust moved onto land by plate tectonics

104. Idealized Section of an Ophiolite Suite?

105. Generation of Igneous Rocks at Divergent Plate Margins
Some of the mafic magma cools in massive magma chambers to form massive gabbros
Some of the magma is intruded as sheeted dikes
These dikes are feeder for basaltic lava flows which form pillows as they extrude beneath the ocean

107. Chapter 5
Volcanoes