Igneous Rocks



Igneous Rocks


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Igneous Rocks


Igneous Rocks

The Rock Cycle

Characteristics of magma

Igneous rocks form from molten rock (magma)

Characteristics of magma

Protolith of igneous rocks

Forms from partial melting of rocks

 Magma at the surface is called lava

Characteristics of magma

Rocks formed from lava are classified as extrusive, or volcanic rocks

Rocks formed from magma are termed intrusive, or plutonic rocks

The nature of magma

Consists of three components:

A liquid portion, called melt, made of mobile ions

Solids, if any, are crystallized silicate minerals

Volatiles, (dissolved gases), mostly water (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

Crystallization of magma

Texture = the size and arrangement of mineral grains

Igneous rocks are classified by


Mineral composition


Igneous textures

Texture describes the size, shape, and arrangement of interlocking minerals

Factors affecting crystal size

Rate of cooling

Slow rate promotes fewer and larger crystals

Fast rate forms many small crystals

Very fast rate forms glass

Types of igneous textures

Aphanitic (fine-grained) texture

Rapid rate of cooling

Microscopic crystals

May contain vesicles

Phaneritic (coarse-grained) texture

Slow cooling

Crystals can be seen


Aphanitic texture

Phaneritic texture

Types of igneous textures

Porphyritic texture

Minerals form at different temperatures as well as differing rates

Large crystals (phenocrysts) embedded in a matrix (groundmass)

Glassy texture

Very rapid cooling

Results in obsidian



 Porphyritic texture

Andesite porphyry

Porphyritic Texture

Glassy (vitreous) texture

An obsidian flow in Oregon

Types of igneous textures

Pyroclastic texture

Fragments ejected from violent volcanic eruption

Textures similar to sedimentary rocks

Pegmatitic texture

Exceptionally coarse grained

Late crystallization stages of granitic magmas


Igneous Compositions

Igneous rocks are primarily silicate minerals

Dark (ferromagnesian or mafic) silicates




Biotite mica

Igneous rocks are primarily silicate minerals

Light (felsic) silicates


Muscovite mica


Granitic versus basaltic compositions

Granitic composition

Composed of light-colored silicates

Felsic composition

High silica (SiO2)

Major constituents of continental crust

Granitic versus basaltic compositions

Basaltic composition

Composed of dark silicates and calcium-rich feldspar

Mafic composition

More dense than granitic rocks

Comprise the ocean floor as well as many volcanic islands – the most common volcanic rock



Other compositional groups

Intermediate (or andesitic) composition

At least 25% dark silicate minerals

Associated with explosive volcanic activity

Ultramafic composition

Rare composition, very high in magnesium and iron

Composed entirely of ferromagnesian silicates

Mineralogy of common igneous rocks

Silica content influences a magma’s behavior

Granitic magma (high silica)

Extremely viscous

Higher temperatures Molten as low as 700oC

Basaltic magma (low silica)

Fluid-like behavior

Pyroclastic rocks

Eruptive fragments


Tuff – ash-sized fragments

Welded tuff – ejected hot, “welds” on landing

Volcanic breccia – particles larger than ash (similar to sedimentary breccia)

Ash and pumice layers

Classification of igneous rocks

Origin of Magma

Several Factors Involved

Generating magma from solid rock

Partial melting crust and upper mantle

Role of heat

Temperature increases with depth in the upper crust (called the geothermal gradient, ~ 20oC to 30oC per km)

Estimated temperatures in the crust and mantle

Role of heat

Rocks in lower crust and upper mantle near melting points

Additional heat (from descending rocks or rising heat from the mantle) may induce melting

Role of pressure

Increase in confining pressure increases rock melting temperature and reducing the pressure lowers the melting temperature

When confining pressures drop, de-compression melting occurs

Heat and Pressure Affect Melting

Decompression melting

Role of volatiles

Volatiles (primarily water) lower rock melting temperatures

Particularly important with descending oceanic lithosphere


Evolution of magmas

A single volcano may extrude lavas exhibiting very different compositions

Bowen’s reaction series and the composition of igneous rocks

N.L. Bowen demonstrated that as a magma cools, minerals crystallize in order based on their melting points

Bowen’s reaction series

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

Composition changes due to removal of elements by earlier-forming minerals

The silica component of the melt becomes enriched as crystallization proceeds

Minerals in the melt can chemically react and change

Processes responsible for changing a magma’s composition

Magmatic differentiation

Separation of crystals from melt forms a different magma composition


Incorporation of foreign matter (surrounding rock bodies)

Magma mixing

Involves two magmas intruding one another

Two distinct magmas may produce a composition quite different the originals

Assimilation and magmatic differentiation

Partial melting and magma formation

Incomplete melting of rocks is known as partial melting (silica rich minerals melt first)

Formation of basaltic magmas

Most originate from partial melting of ultramafic rock in the mantle

Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones

Decompression melting

Formation of basaltic magmas

As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature

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

Formation of andesitic magmas

Interactions between mantle-derived basaltic magmas and more silica-rich rocks in the crust generate magma of andesitic composition (assimilation)

Andesitic magma may also evolve by magmatic differentiation (loss of early olivine and pyroxene)

Partial melting and magma formation

Formation of granitic magmas

Most likely the end product of an andesitic magma

Granitic magmas are higher in silica and therefore more viscous

Because of viscosity, mobility lost before reaching surface (rhyolite rarer than basalt)

Tend to produce large plutonic structures


Magmatic Differentiation

Mineral resources and igneous processes

Many important sources of metals are produced by igneous processes

Igneous mineral resources can form from

Magmatic segregation – separation of heavy minerals in a magma chamber

Hydrothermal solutions - Originate from hot, metal-rich fluids that are remnants of the late-stage magmatic process


Origin of hydrothermal deposits


The Nature of Volcanic Eruptions

 Factors determining the “violence” or explosiveness of a volcanic eruption

Composition of the magma

Temperature of the magma

Dissolved gases in the magma

 The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption

Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity materials flow with great difficulty)

Factors affecting viscosity

Temperature  -  Hotter magmas are less viscous

Composition  -  Silica (SiO2) content

                        Higher silica content = higher viscosity

                        (e.g., felsic lava such as rhyolite)

Factors affecting viscosity continued

Lower silica content  = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt)

Dissolved Gases

Gas content affects magma mobility

Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure

The violence of an eruption is related to how easily gases escape from magma

Factors affecting viscosity continued

            In Summary

Fluid basaltic lavas generally produce quiet eruptions

Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions


Materials extruded from a volcano

Lava Flows

Basaltic lavas are much more fluid

Types of basaltic flows

Pahoehoe lava (resembles a twisted or ropey texture)

Aa lava (rough, jagged blocky texture)

Dissolved Gases

One to six percent of a magma by weight

Mainly water vapor and carbon dioxide

A Pahoehoe lava flow

A typical aa flow

Pyroclastic materials – “Fire fragments”

            Types of pyroclastic debris

Ash and dust  -  fine, glassy fragments (<2 mm)

Pumice  -  porous rock from “frothy” lava

Lapilli  -  walnut-sized material  (2-64 mm)

Cinders  -  pea-sized material

Particles larger than lapilli (>64 mm)

Blocks  -  hardened or cooled lava

Bombs  -  ejected as hot lava


A volcanic bomb


Global Volcanic Effects


General Features

Opening at the summit of a volcano

Crater  -  steep-walled depression at the summit, generally less than 1 km in diameter

Caldera  -  a summit depression typically greater than 1 km in diameter, produced by collapse following a massive eruption

Vent – opening connected to the magma chamber via a pipe

Types of Volcanoes

Shield volcano

Broad, slightly domed-shaped

Composed primarily of basaltic lava

Generally cover large areas

Produced by mild eruptions of large volumes of lava

Mauna Loa on Hawaii is a good example


Shield Volcano

Types of Volcanoes continued

Cinder cone

Built from ejected lava (mainly cinder-sized) fragments

Steep slope angle

Rather small size

Frequently occur in groups

Sunset Crater – a cinder cone near Flagstaff, Arizona


Types of volcanoes continued

Composite cone (Stratovolcano)

Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)

Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)

Composed of interbedded lava flows and layers of pyroclastic debris


A composite volcano

Mt. St. Helens – a typical composite volcano

 Mt. St. Helens following the 1980 eruption

Composite cones continued

Most violent type of activity (e.g., Mt. Vesuvius)

Often produce a nueé ardente

Fiery pyroclastic flow made of hot gases infused with ash and other debris

Move down the slopes of a volcano at speeds up to 200 km per hour

May produce a lahar, which is a volcanic mudflow


A size comparison of the three types of volcanoes

 A nueé ardente on Mt. St. Helens

St. Pierre after Pelee’s Eruption

   Other volcanic landforms

Pyroclastic flows

Associated with felsic & intermediate magma

Consists of ash, pumice, and other fragmental debris

Material is ejected a high velocity

e.g., Yellowstone Plateau


Steep walled, roughly circular, depressions at the summit of a volcano

Size generally exceeds 1 km in diameter

Formed by the collapse of the structure

Caldera Formation

Type 1 Empty magma chamber under volcano leads to collapse (like Crater Lake)

Type 2 Magma chamber empties laterally through lava tubes (like Mauna Loa)

Type3 Magma chamber produces ring fractures and empties with explosive eruption producing silica-rich deposits and very large caldera (like Yellowstone, and Long Valley)

Crater Lake, Oregon is a good example of a caldera

Long Valley Caldera

Eruptions from 3.8 to 0.8 Ma (from basalt to rhyolite) ended with Glass Mountain eruption (0.76 Ma) that formed ring-structure caldera and produced the Bishop Tuff

More recent Mono-Inyo Crater system (0.4 to present, also from basalt to rhyolite) includes Mammoth Mountain (made of a series of lava domes and flows)

Currently active, probably reflecting smaller magma bodies


Long Valley Caldera

Bishop Tuff

Mono-Inyo Craters Activity

Explanation of Mono-Inyo Craters Trend


Fissure eruptions and lava plateaus

Fluid basaltic lava extruded from crustal fractures called fissures

May travel as much as 90 km from source

e.g., Columbia River Plateau individual flows accumulate to more than a km thick

Fissure Eruptions

The Columbia River basalts


Lava Domes

Bulbous mass of congealed lava formed post eruption

Most are associated with explosive eruptions of gas-rich magma

Volcanic pipes and necks

Pipes are conduits that connect a magma chamber to the surface


A lava dome on Mt. St. Helens

Kimberlite Pipe


Volcanic pipes and necks continued

Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone



Formation of a volcanic neck

Shiprock, New Mexico – a volcanic neck

Volcanic Neck

Plutonic igneous activity

Most magma is emplaced at depth in the Earth

An underground igneous body, once cooled and solidified, is called a pluton

Classification of plutons


Tabular (sheetlike)

Massive (massive refers to shape, not size)

Classification of plutons continued

Orientation with respect to the host (surrounding) rock

Discordant – cuts across sedimentary rock units

Concordant – parallel to sedimentary rock units

Types of intrusive igneous features

Dike – a tabular, discordant pluton

Sill – a tabular, concordant pluton (e.g., Palisades Sill in New York), uplifts overlying rock, so must be a shallow feature


Similar to a sill

Lens or mushroom-shaped mass

Arches overlying strata upward


A sill in the Salt River Canyon, Arizona


Intrusive igneous features continued


Largest intrusive body

Discordant and massive

Surface exposure of 100+ square kilometers (smaller bodies are termed stocks)

Frequently form the cores of mountains like our Sierra Nevada Batholith

Intrusive Igneous Structures

Batholith Distribution


Plate tectonics and igneous activity

Global distribution of igneous activity is not random

Most volcanoes are located within or near ocean basins (e.g. circum-Pacific “Ring of Fire”)

Basaltic rocks are common in both oceanic and continental settings, whereas granitic rocks are rarely found in the oceans

Distribution of some of the world’s major volcanoes

Igneous activity along plate margins

Spreading centers

The greatest volume of volcanic rock is produced along the oceanic ridge system

Mechanism of spreading

Lithosphere pulls apart

Less pressure on underlying rocks

Results in partial melting of mantle

Large quantities of basaltic magma are produced

Igneous activity along plate margins

Subduction zones

Occur in conjunction with deep oceanic trenches

Descending plate partially melts (fluxed with water)

Magma slowly moves upward

Rising magma can form either

An island arc if in the ocean

A volcanic arc if on a continental margin    

Subduction zones

Associated with the Pacific Ocean Basin

Region around the margin is known as the “Ring of Fire”

Most of the world’s explosive volcanoes are found here

Intraplate volcanism

Activity within a tectonic plate

Intraplate volcanism continued

Associated with plumes of heat in the mantle (perhaps from the core-mantle boundary, de-compression melting after rise)

Form localized volcanic regions in the overriding plate called a hot spot

Produces basaltic magma sources in oceanic crust (e.g., Hawaii)


Volcanic Activity

Volcanic Activity



A hot spot affecting a tectonic plate


Key Terms Chapter 6

Volcano (shield, stratovolcano, composite cone, cinder cone)




Pyroclastic flows


Fractional melt, fractionation, fractional crystallalization


Volcanic and plutonic rocks

Pluton (batholith, stock, laccolith, sill, dike)




Source : http://www2.bakersfieldcollege.edu/moldershaw/Ch%206%20Web%20Notes.doc

Web site link: http://www2.bakersfieldcollege.edu/moldershaw/

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