Chapter 4Rocks & Igneous Rocks

Rock DefinitionA naturally occurring, solid

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

Appearance of a RockMineralogy refers to the relative proportions of the constituent minerals

Texture is determined by the size, shape and arrangement of the mineral crystals

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

Three Families of Rocks

Igneous- from magma/lava

Sedimentary-compaction/cementing of sediments

Metamorphic- changed by heat and pressure

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 RocksIgneous Rocks

Most Igneous Rock are Silicates

QuartzFeldspar

MicaPyroxene Amphibole

OlivineBasalt

Granite

Sedimentary Rocks

Rocks formed by consolidation of sediments

Classified by the type of sediments

Three Major Types of Sediments

Clastic Sediments are physically deposited particles derived from weathered rocks

Chemical Sediments include minerals carried in solution such as calcite and halite

Organic Sediments are composed of living materials (plant/animal material and fossils/shells)

From Sediments to Solid RocksLithification 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

Sedimentary Rocks

5 % by volume of the upper crust

75 % by exposed surface area of continents

Sedimentary RocksSedimentary 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 RocksMetamorphism 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 ContactMetamorphism

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 MetamorphismUltra-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 MineralizationSilicates 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)

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

Metamorphic Mineralization

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

Magma rises from melting plate and intrudes or extrudes in the crust

Rock Cycle

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

Tectonic plates interact

Metamorphism of subducted rock

Rock Cycle

Entire process starts over as plates interact

Rock Cycle

Igneous Rocks

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?

We Classify Igneous Rocksby Appearance

Mineralogy (Chemistry)

Texture

Igneous Rocks

We Classify Igneous Rocks by Genetics (Origin)

There are two classes of igneous rocks

Intrusive

Extrusive

Genetic Classification of Igneous Rocks

Intrusive igneous rocks crystallized from slowly cooling magma intruded within the Earth’s crust, such as granite and gabbro

Granite Intrusions

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

Extrusive Igneous RocksRocks 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 (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”

Mineralogy

There are four major divisions of igneous rocks based upon the content of elements in the minerals:

Felsic

Intermediate

Mafic

Ultramafic

Rich (high) in minerals containing silica

Poor (low) in iron and magnesium

They include: Granite Rhyolite

Felsic Igneous Rocks

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 DaciteDioriteAndesite

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

BasaltAndesiteDaciteRhyolite

Common Minerals of Igneous Rocks

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

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

When Do Rocks Melt?

Magma Differentiation

The process by which rocks of various compositions can arise from a

uniform parent magma

The Formation of

Magma Chambers

Partial meltingPartial melting

Partial meltingPartial melting

Less dense magmaLess dense magma

The Formation of

Magma Chambers

Partial meltingPartial melting

Less dense magmaLess dense magma

Magma risesMagma risesThe

Formation of Magma

Chambers

Partial meltingPartial melting

Less dense magmaLess dense magma

Magma risesMagma rises

Magma pools in Magma pools in magma chambermagma chamber

The Formation of

Magma Chambers

Magma DifferentiationOccurs because different minerals crystallize (solidify) at different temperatures

In other words, as the magma cools some minerals form first, some form last

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

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

Mafic

Magmas

Felsic Magmas

Partial Melting and the Origin of 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

PlutonsLarge igneous bodies formed at depth in the

Earth’s crust

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

Types of intrusive and extrusive igneous structures

Sill

DikeDike

Where Do Most Magmas Occur?Divergent Plate Margins

Convergent Plate Margins

Mantle Plumes/Hot Spots

Island Arc Plate Subduction

(Japanese Islands)

ContinentalPlate Subduction

(Mount St. Helens)

Hot SpotVolcanism

(Hawaii)

Generation of Magmas at Convergent Plate Margins

Subduction drags oceanic lithosphere (including a veneer of “wet” sediments) beneath the adjacent plate

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

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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

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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