Chapter 4 Rocks and Igneous Rocks (.pdf)

Chapter 4 

Rocks & Igneous Rocks

Rock Definition 

A naturally occurring consolidated mixture of 

one or more minerals 

e.g, marble, granite, sandstone, limestone

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

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

Appearance of a Rock 

The appearance of a rock is determined by 

two major factors 

Mineralogy 

Texture


Mineralogy refers to the relative proportions of the 

constituent minerals 

Texture is determined by the size, shape and 

arrangement of the mineral crystals


Mineralogy and texture are related 

to how and where a rock was 

formed and what has happened to 

the rock since it was created

Three Great Families of Rocks 

Igneous 

Sedimentary 

Metamorphic

Three Types of Rocks

Igneous Rocks 

Rocks formed by the solidification of magma 

(molten rock) 

Granite

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)

Igneous Rocks

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

Most Igneous Rock are Silicates 

Quartz 

Feldspar 

Mica 

Pyroxene 

Amphibole 

Olivine 

Basalt

Granite

Sedimentary Rocks 

Rocks formed by consolidation of sediments 

Classified by the type of sediments

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

From Sediments to Solid Rocks 

Lithification is the process of converting sediments 

into solid rocks 

(will cover in detail in another lecture)

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

Sedimentary Rocks


5 % by volume of the upper crust 

75 % by exposed surface area of continents


Sedimentary rocks commonly contain fossils 

In fact, some sedimentary rocks are almost entirely 

composed of fossils 

Can exhibit extensive horizontal layers called 

bedding

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

Metamorphic Rocks 

Metamorphic rocks are formed by the transformation 

of previously-existing rocks in the solid state due to 

increased temperature and pressure

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)

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

Regional Metamorphism

Contact Metamorphism

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

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)

Metamorphic Mineralization 

Contact Ultra-high pressure Regional High-press, low-temp

Proportions of the Rock Types

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

Common Minerals Found in Rocks

Five Very Different Locations

Rock Cycle

Rock Cycle

Subduction of an 

oceanic plate beneath 

a continental plate 

uplifts a volcanic 

mountain range 

Rock Cycle 

Magma 

rises from 

melting 

plate and 

intrudes or 

extrudes in 

the crust

Magmas cool 

to make 

igneous rock 

Basalt versus 

granite 

Rock Cycle

Weathering and 

erosion creates 

sediments 

Rock Cycle

Sediments 

are carried to 

the oceans 

and lakes 

Lithification 

Rock Cycle

Rock Cycle 

Metamorphism of subducted rock 

Tectonic plates interact

Entire process starts 

over as plates interact 

Rock Cycle



How do igneous rocks differ from one another? 

Where do they form? 

How do rocks solidify from a melt (magma)? 

Where do rock melt?

We Classify Igneous Rocks 

by Appearance 

Mineralogy (Chemistry) 

Texture


We Classify Igneous Rocks 

by Genetics (Origin) 

There are two classes of igneous 

rocks 

Intrusive 

Extrusive

Genetic Classification of 


Intrusive igneous rocks crystallized from slowly 

cooling magma intruded within the Earth’s crust, 

such as granite and gabbro

Granite Intrusions

Genetic Classification of 


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

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

We Can Also Classify Igneous 

Rocks by Composition 

Chemistry 

Mineralogy

Chemistry 

Modern classification of igneous rock is based 

upon the silica (SiO 2) content 

The silica content is determined by the silicate 

minerals that occur in the rock (i.e., the minerals 

contain SiO 2) 

The percentage of silica ranges from about 40% to 

about 70% 

Rocks are referred to as “silica rich” or “silica poor”

Mineralogy 

There are four major divisions of igneous rocks 

based upon the content of elements in the 

minerals: 

Felsic 

Intermediate 

Mafic 

Ultramafic

Felsic Igneous Rocks 

Rich (high) in minerals containing silica 

Poor (low) in iron and magnesium 

They include: Granite 

Rhyolite

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

Mafic Igneous Rocks 

Poor (low) in minerals containing silica 

Rich (high) in iron and magnesium 

They include: Gabbro 

Basalt

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

Felsic Intermediate Mafic 

Granite Granodiorite Diorite Gabbro 

Rhyolite 

Dacite 

Andesite 

Basalt

Common Minerals of Igneous Rocks

When Do Rocks Melt? 

Melting starts at ~700 o C 

When the temperature exceeds the melting point 

of the rock or some minerals within the rock 

Minerals melt at different temperatures

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

Magma Differentiation 

The process by which rocks of 

various compositions can arise from a 

uniform parent magma

Partial melting 

The 

Formation of 

Magma 

Chambers

Less dense magma 


The 

Formation of 

Magma 

Chambers

Magma rises 



The 

Formation of 

Magma 

Chambers

Magma pools in 

magma chamber 

Magma rises 



The 

Formation of 

Magma 

Chambers

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

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

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

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

Bowen’s Reaction Series

Evidence of Fractional 

Crystallization in the Palisades Sill

Evidence of Fractional 

Crystallization in the Palisades Sill 

first olivine next pyroxene pyroxene finally plagioclase 

& plag. Feldpar finishes

Partial melting of upper mantle: 

e.g. at divergent spreading 

centers 

Partial melting of continental 

crustal rocks 

Partial Melting 

and the Origin of Magmas 

Mafic 

Magmas 

Felsic 

Magmas

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

Rising Magma Wedges Open and 

Fractures Overlying Country Rock

Overlying rocks may bow up

Magma melts surrounding rock

…changing the composition of the 

magma

Blocks of the Overlying Country 

Rocks (Xenoliths) May Break Off and 

Sink into the Magma

Plutons 

Large igneous bodies 

formed at depth in the 

Earth’s crust

Types of Plutons 

Batholith: Massive, discordant intrusive body 

covering at least 100 km 2 

Stock: Massive, discordant intrusive body 

covering less than 100 km 2 

Dike: Tabular, discordant intrusive body 

Sill: Tabular, concordant intrusive body

Types of intrusive and extrusive 

igneous structures

Sill

Dike

Where Do Most Magmas Occur? 

Divergent Plate Margins 

Convergent Plate Margins 

Mantle Plumes/Hot Spots

Island Arc 

Plate Subduction 

(Japanese Islands)

Continental 

Plate Subduction 

(Mount St. Helens)

Hot Spot 

Volcanism 

(Hawaii)

Generation of Magmas at 


Subduction drags oceanic 

lithosphere (including a veneer 

of “wet” sediments) beneath 

the adjacent plate

Generation of Magmas at 


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

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

? 

Idealized Section 

of an Ophiolite 

Suite

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

Chapter 5 

Volcanoes

Chapter 4 Rocks and Igneous Rocks (.pdf)

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