Chapter 5Igneous Rocks

Section 5.1What 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

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

• 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

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

• 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

• 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

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 content1. 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

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

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

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

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

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

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

• 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.

OlivinePeridotite = olivine + pyroxene

• 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

Section 5.2Classification 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

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)

A. Classification of Igneous Rocks

• According to mineral composition1. Basaltic Rocks – dark-colored, lower

silica content, contain mostly plagioclase and pyroxene (gabbro)

2. Granitic Rocks – light-colored, high silica contents, contain mostly quartz, potassium feldspar, plagioclase feldspar (granite)

3. Intermediate Rocks – mineral composition somewhere in between basaltic and granitic, consist mostly of plagioclase feldspar and hornblende (diorite)

4. Ultrabasic – only iron-rich minerals, such as olivine and pyroxene – always dark (peridotite)

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

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)

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)

Basalt w/ olivine

Andesite w/ amphibole (horneblend)

Rhyolite w/ white feldspar & dark gray quartz

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

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

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

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

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

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

D. Igneous Rocks in Construction

• Especially useful as building materials– Interlocking grain texture = strong– Many minerals resistant to weathering

(granite)