1. Chapter 5 Igneous Rocks

2. Section 5.1 What are Igneous Rocks?
Objectives:
Summarize igneous rock formation
Describe the composition of magma
Identify the factors that affect how rocks melt and crystallize
Define:
Silicate
Lava
Igneous rock
Partial melting
Bowen’s reaction series
Fractional crystallization

3. I. Igneous Rock Formation
Magma – molten rock below Earth’s surface
Lava – magma that flows out onto Earth’s surface
Igneous rocks – form when magma cools and minerals crystallize

4. Rocks must be heated to temperatures of 800oC – 1200oC before they melt
Temperatures are present in nature in upper mantle and lower crust
Theory – heat came from remaining energy from Earth’s molten formation and head generated from the decay of radioactive elements

5. A. Composition of Magma
Type of igneous rock that forms depends on the composition of the magma
Magma – often slushy mix of molten rock, dissolved gases, mineral crystals
Common elements present in magma are same major elements that are in Earth’s crust:
Oxygen, Silicon, Aluminum, Iron, Magnesium, Calcium, Potassium, Sodium

6. Silica is most abundant
Silica has greatest effect on magma characteristics
Magma classification: based on amount of silica it contains
Basaltic –
Andesitic –
Rhyolitic -
Silica content affects melting temperature and impacts how quickly magma flows

7. Once magma is free of overlying pressure of rock layers around it  dissolved gases are able to escape into atmosphere
Chemical composition of lava is slightly different from chemical composition of magma from which it develops
Mt. Etna, Sicily, Italy

8. B. Magma Formation
Magma can be formed by melting Earth’s crust or by melting within the mantle
4 main factors involved in formation of magma: temperature, pressure, water content, mineral content
1. Temperature – generally increases with depth (geothermal gradient)
2. Pressure – increases with depth
Result of weight of overlying rock
As pressure increases  melting point increases
3. Water Content – changes melting point
As water content increases  melting point decreases

9. C. Mineral Content Different minerals have different melting points
Basalt (olivine + calcium feldspar + pyroxene) melts at higher temperatures than granite (quartz + potassium feldspar)
Granite has lower melting point than basalt because granite contains more water and minerals that melt at lower temperatures
In general, rocks that are rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon

10. D. Partial Melting
Not all parts of a rock melt at the same temperature because they contain different minerals
This explains why magma is often a slushy mix of crystals and molten rock
Partial melting – process whereby some minerals melt at relatively low temperatures while other minerals remain solid
as each group of minerals melts, different elements are added to the magma mixture  changing magma composition
If temperatures are not high enough to melt the entire rock  the resulting magma will have a different composition than that of the original rock
This is one way different igneous rocks form

11. II. Bowen’s Reaction Series
Bowen’s Reaction Serious – process that demonstrate as magma cools and crystallizes  minerals form in predictable patterns
2 main patterns / branches of crystallization:
Right – continuous, gradual change of mineral compositions in feldspar group
Left – abrupt change of mineral type in iron-magnesium groups

12. A. Iron-Rich Minerals Left branch
Undergo abrupt changes as magma cools and crystallizes
Olivine = 1st mineral to crystallize when magma that is rich in iron and magnesium begins to cool
When temp decreases enough for completely new mineral (pyroxene) to form  olivine that previously formed reacts with magma and is converted to pyroxene
As temp decreases further similar reactions produce minerals amphibole and biotite mica

13. B. Felspars Right branch
Plagioclase feldspars – undergo continuous change of composition
As magma cools – 1st feldspars to form are calcium-rich
As cooling continues, feldspars react with magma, and their calcium-rich compositions change to sodium-rich compositions
In some cases, such as when magma cools rapidly, the calcium-rich cores are unable to react completely with magma – result in zoned crystal

14. III. Fractional Crystallization
When magma cools  it crystallizes in reverse order of partial melting
1st minerals that crystallize from magma are last minerals that melted during partial melting
Fractional crystallization – similar to partial melting in that the composition of magma can change
Early formed crystals are removed from the magma and cannot react with it
As minerals form and their elements are removed from the remaining magma  it becomes concentrated silica

15. Questions arose from discovery of Bowen’s reaction series:
If olivine converts to pyroxene during cooling, why is olivine found in rock?
Hypothesis: under certain conditions, newly formed crystals are separated from magma, and chemical reactions b/w magma and minerals stop. Can occur when crystals settle at bottom of magma body and when liquid magma is squeezed from crystal mush to form 2 distinct igneous bodies with different compositions.
Peridotite = olivine + pyroxene
Olivine

16. Last 2 minerals to form are potassium feldspar and quartz
As fractional crystallization continues and more magma is separated from crystals, the magma becomes more concentrated in silica, aluminum, and potassium
Last 2 minerals to form are potassium feldspar and quartz
Potassium feldspar – one of most common feldspars in Earth’s crust
Quartz – often occurs in veins because it crystallizes while the last liquid portion of magma is squeezed into rock fractures

17. Section 5.2 Classification of Igneous Rocks
Objectives:
Classify different types and textures of igneous rocks.
Recognize the effects of cooling rates on the grain sizes in igneous rocks.
Describe some uses of igneous rocks.
Define:
Intrusive rock
Extrusive rock
Basaltic rock
Granitic rock
Texture
Porphyritic texture
Vesicular texture
Pegmatite
kimberlite

18. I. Mineral Composition of Igneous Rocks
Broad classification: intrusive or extrusive
Intrusive – when magma cools and crystallizes below Earth’s surface
If magma is injected into surrounding rock = igneous intrusion
Crystals = generally large enough to see without magnification
Extrusive – when magma cools and crystallizes on Earth’s surface
A.K.A. – lava flows or flood basalts
Crystals = small and difficult to see without magnification
Classified by mineral compositions, physical properties (grain size, texture)

19. A. Classification of Igneous Rocks
According to mineral composition
Basaltic Rocks – dark-colored, lower silica content, contain mostly plagioclase and pyroxene (gabbro)
Granitic Rocks – light-colored, high silica contents, contain mostly quartz, potassium feldspar, plagioclase feldspar (granite)
Intermediate Rocks – mineral composition somewhere in between basaltic and granitic, consist mostly of plagioclase feldspar and hornblende (diorite)
Ultrabasic – only iron-rich minerals, such as olivine and pyroxene – always dark (peridotite)

20. II. Texture Rocks differ in sizes of grains or crystals
Texture – refers to size, shape, and distribution of crystals or grains that make up a rock
Rhyolite = fine-grained
Granite = coarse-grained
Difference in crystal size explained by fact that one rock is extrusive and other is intrusive

21. A. Crystal Size and Cooling Rates
Lava flows on Earth’s surface – cools quickly and not enough time for large crystals to form = extrusive igneous rocks (rhyolite)
Sometimes cooling occurs so quickly that crystals do not form at all = volcanic glass (obsidian)
Magma cools slowly beneath Earth’s surface = sufficient time for large crystals to form
Intrusive igneous rocks can have crystals larger than 1 cm (granite, diorite, gabbro)

22. B. Porphyritic Rocks
Porphyritic texture – characterized by large, well-formed crystals surrounded by finer-grained crystals of same mineral or different minerals
Indicate complex cooling history during which a slowly cooling magma (forming large crystals) suddenly began cooling rapidly (remaining magma forms small crystals)
Rhyolite w/ white feldspar & dark gray quartz
Andesite w/ amphibole (horneblend)
Basalt w/ olivine

23. C. Vesicular Rocks
Magma contains dissolved gases that escape when the pressure on magma lessens
If lava is thick enough to prevent gases bubbles from escaping  holes (vesicles) are left behind
Rock that forms looks spongy
Vesicular texture – spongy appearance
Examples: pumice, vasicular basalt

24. III. Thin Sections
Usually easier to observe sizes of mineral grains than it is to identify the mineral
to identify minerals  geologists examine samples called thin sections
Thin section – slice of rock (2cm x 4cm x .033mm thick)
So thin light can pass through it
Petrographic microscope – observe mineral grains b/c they exhibit distinct properties
Properties allow geologists to identify minerals present in rock
Example: feldspar grains – show a distinct banding called twinning
Example: quartz grains – appear wavy as the microscope stage is rotated
Example: calcite crystals – become dark (extinguish) as stage is rotated

25. IV. Igneous Rocks as Resources
Cooling and crystallization history of igneous rocks sometimes results in unusual but useful minerals
Can be used in many fields: construction, energy production, jewelry making

26. A. Veins
Ores – minerals that contain useful mineral that can be mined for a profit
Often occur w/in igneous intrusions
Sometimes occur as veins
Fluid left during magma crystallization contains high levels of silica and water and any leftover elements that were not incorporated into the common igneous minerals
Some important metallic elements not included in common minerals = gold, silver, lead, copper
These + dissolved silica = released at end of magma crystallization in hot, mineral-rich fluid that fills cracks and voids in surrounding rock
Fluid solidifies to form metal-rich quartz veins

27. B. Pegmatites Pegmatites – veins of extremely large-grained minerals
Ores of rare elements (lithium) (beryllium) form in pegmatites
Can produce beautiful crystals
Minerals grow to voids and retain their shapes because these veins fill cavities and fractures in rock
Mount Rushmore

28. C. Kimberlites Variety of peridotite
Most likely form deep in crust or in mantle at depths of 150 – 300km
Because diamond and other minerals present in kimberlites can form only under very high pressure
Hypothesis: magma is intruded rapidly upward toward surface  forms long, narrow, pipe-like structures
Extend many km into crust & only 100 – 300m diameter

29. D. Igneous Rocks in Construction
Especially useful as building materials
Interlocking grain texture = strong
Many minerals resistant to weathering (granite)