Igneous Rocks Chapter 4. Igneous Rocks Igneous rocks form as molten rock (magma) cools and solidifies General characteristics of magma Parent material.

Igneous RocksChapter 4

Igneous Rocks Igneous rocks form as molten rock

(magma) cools and solidifies General characteristics of magma

• Parent material of igneous rocks• Forms from partial melting of rocks• Magma at surface is called lava

General Characteristics of Magma Rocks formed from lava = extrusive,

or volcanic rocks Rocks formed from magma at depth

= intrusive, or plutonic rocks

General Characteristics of Magma, cont’d

Magma consists of three components: Liquid portion = melt Solids, if any, are silicate minerals Volatiles = dissolved gases in the melt,

including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

General Characteristics of Magma, cont’d

Crystallization of magma Cooling of magma results in the

systematic arrangement of ions into orderly patterns

Silicate minerals result from crystallization in a predictable order

Texture - size and arrangement of mineral grains

Igneous Textures Texture - overall appearance of a

rock based on the size, shape, and arrangement of interlocking minerals

Several factors affect crystal size

Igneous Textures Factors affecting crystal size

• Rate of cooling• Slow rate = fewer but larger crystals• Fast rate = many small crystals• Very fast rate forms glass

• % of silica (SiO2) present• Presence of dissolved gases

Types of Igneous Textures Aphanitic (fine-grained) texture

• Rapid rate of cooling• Microscopic crystals• May contain vesicles (holes from gas

bubbles)

Types of Igneous Textures Phaneritic (coarse-grained) texture

Slow cooling Large, visible crystals

Types of Igneous Textures Porphyritic texture

Minerals form at different temperatures Large crystals (phenocrysts) are

embedded in a matrix of smaller crystals (groundmass)

Types of Igneous Textures Glassy texture

Very rapid cooling of lava Rock is called obsidian or pumice

Types of Igneous Textures Pyroclastic

texture• Fragmental

appearance produced by violent volcanic eruptions

• Often appear more similar to sedimentary rocks

Types of Igneous Textures Pegmatitic

texture Exceptionally

coarse grained Forms in late

stages of crystallization of granitic magmas

Igneous Compositions

Igneous rocks are composed primarily of silicate minerals Dark (or ferromagnesian) silicates

• Olivine, pyroxene, amphibole, and biotite mica

Light (or nonferromagnesian) silicates• Quartz, muscovite mica, and feldspars

Igneous Compositions, cont’d

Granitic versus basaltic compositions Granitic composition

• Light-colored silicates• Termed felsic (feldspar and silica) in

composition• High silica (SiO2) content• Major constituent of continental crust

Igneous Compositions, cont’d

Granitic versus basaltic compositions Basaltic composition

• Dark silicates and Ca-rich feldspar• Termed mafic (magnesium and ferrum, for

iron) in composition• Higher density than granitic rocks• Comprise the ocean floor and many

volcanic islands

Igneous Compositions, cont’d

Other compositional groups Intermediate (or andesitic) composition

• Contain 25% or more dark silicate minerals• Associated with explosive volcanic activity

Ultramafic composition• Rare composition that is high in magnesium

and iron• Composed entirely of ferromagnesian

silicates

Igneous Compositions, cont’d Silica content as an indicator of

composition• Crustal rocks exhibit a considerable range--

45% to 70%

Silica content influences magma behavior

• Granitic magmas = high silica content and viscous

• Basaltic magmas = much lower silica content and more fluid-like behavior

Naming Igneous Rocks

Granitic rocks Granite

• Phaneritic• Over 25% quartz, about 65% or more

feldspar• Very abundant - often associated with

mountain building• The term granite includes a wide range of

mineral compositions

Naming Igneous Rocks, cont’d

Granitic Rocks Rhyolite

• Extrusive equivalent of granite• May contain glass fragments and vesicles• Aphanitic texture• Less common and less voluminous than

granite

Naming Igneous Rocks, cont’d Granitic rocks

• Obsidian• Dark colored• Glassy texture

• Pumice• Volcanic• Glassy texture• Frothy appearance with numerous voids

Naming Igneous Rocks, cont’d Intermediate rocks

Andesite• Volcanic origin• Aphanitic texture

Diorite• Plutonic equivalent of andesite• Coarse grained (phaneritic texture)

Naming Igneous Rocks, cont’d Basaltic rocks

• Basalt• Volcanic origin• Aphanitic texture• Composed mainly of pyroxene and calcium-rich

plagioclase feldspar• Most common extrusive igneous rock

Naming Igneous Rocks, cont’d Mafic rocks

• Gabbro• Intrusive equivalent of basalt• Phaneritic texture consisting of pyroxene and

calcium-rich plagioclase• Significant % of the oceanic crust

Naming Igneous Rocks, cont’d Pyroclastic rocks

• Composed of fragments ejected during a volcanic eruption

• Varieties• Tuff = ash-sized fragments• Volcanic breccia = particles larger than ash

Origin of Magma Generating magma from solid rock

• Role of heat• Temperature increases in the upper crust

(geothermal gradient) average between 20oC to 30oC per kilometer

• Rocks in the lower crust and upper mantle are near their melting points

• Additional heat may induce melting

Origin of Magma, cont’d• Role of pressure

• Increases in confining pressure increases a rock’s melting temperature

• When confining pressures drop, decompression melting occurs

• Role of volatiles• Volatiles (primarily water) cause melting at lower

temperatures• Important factor where oceanic lithosphere

descends into the mantle

Evolution of Magmas A single

volcano may extrude lavas exhibiting very different compositions

Bowen’s Reaction Series Minerals crystallize in a systematic

fashion based on their melting points

During crystallization, the composition of the liquid portion of the magma continually changes

Evolution of Magmas, cont’d Processes responsible for changing a

magma’s composition• Magmatic differentiation

• Separation of a melt from earlier formed crystals

• Assimilation• Changing a magma’s composition by incorporating

surrounding rock bodies into a magma

• Magma mixing• Two chemically distinct magmas may produce a

composition quite different from either original magma

Partial Melting and Magma Formation

Incomplete melting of rocks is known as partial melting

Formation of basaltic magmas• Most originate from partial melting of

mantle rocks at oceanic ridges (decompression melting) or at subduction zones (presence of water)

• Large outpourings of basaltic magma are common at Earth’s surface

Partial Melting and Magma Formation, cont’d Formation of andesitic magmas

• Produced by interaction of basaltic magmas and more silica-rich rocks in the crust

• May also evolve by magmatic differentiation

Partial Melting and Magma Formation, cont’d Formation of granitic magmas

• Most likely form as the end product of crystallization of andesitic magma

•Granitic magmas are more viscous than other magmas – tend to lose their mobility before reaching the surface•Produce large plutonic structures

End of Chapter 4

1. Intrusive magma is cooler because it is well insulated by the surrounding rock.

2. Intrusive magma flows onto the Earth's surface and cools very slowly, allowing many small mineral grains to grow.

3. The extrusive magma cools quickly so the mineral grains do not have time to grow.

4. The extrusive magma, because it is deep below the surface, cools very slowly, producing very small mineral grains.

04.01 Lava flows are typically finer grained than intrusive igneous rocks. Why?

1. The pressures that different minerals are formed in metamorphic rocks

2. Which minerals are recrystallized in a sedimentary rock3. The temperatures at which different minerals crystallize out of

a melt4. All of the above5. None of the above

04.02What does Bowen’s Reaction Series describe?

1. Upon eruption, magma becomes “lava,” the form of the magma that flows out of the volcano

2. When magma gets to the top of the volcano, it releases gas found within the magma, and the lava that escapes the volcano has therefore lost those gases.

3. Lava is the name given to the ejected magma from a passive (fluid) volcano, but it is still called “magma” if it is erupted explosively, like at Mt. St. Helens.

4. If the magma has no crystals or gases within it, it is called “lava.”

5. Both 1 and 36. Both 3 and 47. All of these.

04.03Which of the following describes best the difference between magma and lava?

1. Granite; intrusive rock that formed from cooling of relatively high silica magma.

2. Rhyolite; extrusive rock that formed from cooling of relatively low silica magma.

3. Diorite; intrusive rock that formed from the cooling of relatively intermediate silica magma.

4. Granite; intrusive rock that formed from cooling of relatively intermediate silica magma.

5. Basalt; extrusive rock that formed from cooling of relatively low silica lava.

6. Basalt; extrusive rock that formed from cooling of relatively high silica lava.

04.04Which of the following rocks is likely to have the most quartz within it and why?

1. Pressure of the environment – higher P = slower magma cooling.

2. The presence or absence of volatiles (gases) – less gases = slower magma cooling.

3. Temperature of the environment – lower T = slower magma cooling.

4. The presence or absence of volatiles (gases) – more gases = slower magma cooling.

5. Temperature of the environment – higher T = slower magma cooling.

6. Pressure of the environment – lower P = slower magma cooling.

04.05What is the most important factor for whether magma cools slowly or

quickly?

1. the changing of a rock from one set of minerals to another2. the compaction of metamorphic rocks3. the melting of sedimentary rocks4. the cooling of magma5. Both c and d are correct.

04.06Igneous rocks are produced largely by ________.

1. cools so fast that mineral grains cannot crystallize and grow2. cools so slowly that only one mineral is formed3. is composed of basalt4. is a rhyolitic type5. cools at an extremely high temperature

04.07Glassy igneous rocks form when the magma _______.

1. resistant to weathering and is an important component of sand in river beds and beaches

2. a main constituent of many igneous rocks3. a main constituent of many sedimentary rocks4. the most stable of all minerals at Earth’s surface

temperatures and pressures5. all of the above

04.08

Quartz is ________.