Topic 3 igneous rocks

Topic 3 – Igneous Rocks

Topic 3 - Igneous RocksOutline

IntroductionMagma and Lava

TextureComposition

Bowens Reaction SeriesChanges in Magma Composition

Igneous Rock ClassificationUltramafic Rocks

Mafic RocksIntermediate Rocks

Felsic RocksOther Igneous Rocks

Plutons

Introduction

Igneous rocks are

formed by the

crystallization of

molten rock material

called magma.

Igneous rocks are

one of the 3 main

rock types.

Sedimentary and

metamorphic rocks

are the other 2 types.

Recall the rock cycle.

2 main types of igneous rocks

• Plutonic– cool below the surface of the Earth

– intrusive igneous rocks

– common in mountain chains and continental areas

• Volcanic– cool at the surface of

the Earth– extrusive igneous

rocks– common along

continental margins of active tectonic plates

– Circum-Pacific “Pacific Ring of Fire”

– very significant in the ocean

– volcanic islands (Hawaii, Iceland, etc) and mid-oceanic ridges

Characteristics of igneous rocks

Rhyolite

(extrusive,

rapid cooling)

vs.

Granite

(intrusive,

slow cooling)

Extrusive igneous rocks form by the solidification of lava at the Earth’s surface. Intrusive igneous rocks are formed when magma solidifies within the crust or mantle.

Both types of igneous rocks are named and classified on the basis of rock texture and mineral assemblage.

Igneous rocks form by cooling and solidification of magma. Crystallization of mineral grains. Larger mineral grains are formed during slow cooling. Smaller mineral grains grow during rapid cooling.

Texture is very important in igneous rocks. Finely crystalline (volcanic) vs. coarsely crystalline (plutonic).

Igneous Features and Landforms

Magma and LavamagmaMolten rock, together with any suspended mineral grains and dissolved gases, that forms when temperatures rise sufficiently high for melting to happen in the crust or mantle. volcanoVent from which magma, solid rock debris and gases are erupted onto the Earth’s surface and into the Earth’s atmosphere. Term volcano comes from the name of the Roman god of fire: Vulcan. lavaMagma that reaches the Earth’s surface.

Will cover the following subtopics:

Key Characteristics of Magma

Types of Magma

Gases Dissolved in Magma

Temperature of Magma

Viscosity of Magma

Three key characteristics of magma:

(i) Range in Composition

(ii) High Temperatures

(iii) Ability to Flow

(i) Range in Composition

Silica (SiO2) always occurs. But in varying proportions.

Composition influences the viscosity or ability of the fluid to flow.

(ii) High Temperatures

Magma and lava can reach temperatures of 1400° C.

Or up to 1600° C in extreme cases.

(iii) Ability to Flow

Magma has the properties of a liquid.

Most magma is a mixture of crystals and liquid.

Often referred to as a “melt”.

Parent magma plays a significant role in determining the mineral

composition of an igneous rock. Four magma types:

Ultramafic

Mafic

Intermediate

Felsic

Four Types of Magma:

• Ultramafic Magma– silica content < 45 %, silica poor– 1300° to 1600° C– magma generates peridotite

• Mafic Magma– silica content = 45-52 %, silica poor– 1000° to 1300° C– generates basalt and gabbro – more Ca, Fe and Mg

• Intermediate Magma– silica content = 53-65 %,

intermediate silica content

– 700° to 1000° C

– generates andesite and diorite

• Felsic Magma– silica content > 65 %, silica rich

– 600° to 800° C

– generates rhyolite and granite

– considerable Na, K and Al

– little Ca, Fe and Mg

These four magmas are not formed in equal abundance…• 80 % of all magmas erupted by volcanoes is basaltic (e.g. Hawaiian volcanoes)• 10 % are andesitic (egs. Mt. St. Helens 1980 and Mt. Pinatubo 1991)• 10 % are rhyolitic (e.g. dormant Yellowstone NP: super/mega-volcano)• minor concentration of ultramafic magma (< 1 %)

Krakatau is an example of andesitic volcano. Indonesia, August 27, 1883. Island disappeared. Sound of the volcanic eruption was heard 4600 km away.

Giant tsunami or sea wave that was 40m high crashed into Java. 36,000 people were killed.

20 km3 of volcanic debris was ejected into the atmosphere. Produced strangely coloured sunsets...green, blue, purple. Earth temperature dropped 0.5° C. Northern hemisphere had no summer for two years (snow in July).

It took 5 years for all of the dust to settle back to the ground. Significant climate change.

Gases Dissolved in Magma:

• Magma consists of three different phases:– solid: crystals (silicate minerals)

– liquid: molten material (mainly silicate)

– gas: volatiles

• small amounts of gas (0.2 to 3.0 % by weight) are dissolved in all magmas

• these gases strongly influence the properties of magma

• gas consists of – water vapour, carbon dioxide, nitrogen, chlorine, sulphur and argon

– principal gas is water vapour

– water vapour and CO2 constitute 98 % of all gases emitted dissolved in

magma

• effect of volatiles on the behaviour of magma and lava is important, generally speaking gases:– increase the fluid nature of lava (reduce viscosity)

• less polymerization

– magma melt at lower temperature• lower the melting point of the lava

– increase the likelihood of explosive eruption

• highly viscous (sticky) silica-rich lavas tend to trap gases… leading to explosions– gases are trapped at high pressures and temperatures beneath the

Earth’s surface

– when the gases reach near the surface they tend to expand explosively• therefore rhyolite lavas tend to erupt explosively

• these explosions produce great clouds of ash and dust, rather than runny lava flows

– e.g. Mt. St. Helens (1980)

Temperature of Magma:

• difficult to measure• no direct measurements have been taken beneath the surface• measured during volcanic eruptions (mafic volcanoes)

– use optical pyrometer for measurement 

• temperature average 1000° to 1200° C (basalt) • ash flows from Mt. St. Helens were 300 to 420° C two weeks after the

eruption!

Temperature & composition are linked:

1300-1600° C Ultramafic

1000-1300° C Mafic

700-1000° C Intermediate

600-800° C Felsic

Viscosity of Magma:

Viscosity is an internal property of a substance that offers resistance to flow. Water has a low viscosity. Porridge or spaghetti sauce has a higher viscosity.

Viscosity = 1/fluidity.

Low viscosity = high fluidity.

High viscosity = low fluidity.

Some magmas are very fluid, others are not. Basaltic lava in Hawaii has been measured at 16 km/h. i.e. low viscosity. This is rare.

Maximum speed 50-60 km/hr: confined flow, downhill.

Most magmas will move considerably slower. Meters per hour or day.

Two factors effect the viscosity of magma:

(a) temperature temp viscosity

(b) silica content silica viscosity

The higher the temperature, the lower the viscosity. Magma more readily flows at a higher temperature. As a lava cools from a volcano, the flow rate begins to slow down. It eventually slows to a complete halt.

The lower the silica content, the more easily the magma will flow. Basalts flow more freely than rhyolites. Thinner lavas. Better flowing magmas/lavas allow gas to escape. Therefore not as explosive. e.g. mafic.

If the magma has a high silica content, it is less runny (i.e. more viscous). Felsic lavas are thick and slow-moving. Therefore high silica = high viscosity (stickiness).

Magma Rising from the Mantle

• consequence of rate of cooling– Intrusive rocks… cool slowly at

depth– Extrusive rocks… cool quickly

at the surface

• the size of the mineral grains– intrusive igneous rocks tend to

be coarse grained– extrusive igneous rocks tend to

be fine-grained

• photos show an example of a felsic extrusive (rhyolite) igneous rock and a felsic intrusive (granite) igneous rock

Texture of Igneous Rocks

• at least 6 different

textural terms:

– Phaneritic

– Aphanitic

– Glassy

– Porphyritic

– Vesicular or

Amygdaloidal

– Pyroclastic or

Fragmental

• the mineral grains are of such a size as to be visible with the unaided eye or the help of a hand lens

– magmas which cool slowly tend to form coarser grained rocks (phaneritic)

– because the crystals have enough time to grow

Phaneritic Texture

Aphanitic Texture

• grain size is so small that the individual grains can be seen only with a microscope

• magma cools very quickly• the crystals did NOT have enough time to grow (aphanitic)

Glassy Texture• The magma cooled so quickly, it did not have time to grow any

crystals… it just quenched… froze… glass (silica-rich).

• Magmas which cool extremely fast (volcanic) tend to form glassy rocks. Because the crystals do not have time to form at all.

• Extrusive igneous rocks that are largely or wholly glassy are called obsidian. Such felsic rocks display a distinctive fracture pattern on a broken surface. Fracture pattern consists of a series of smooth curved surfaces.

Porphyritic Texture

Some magmas are cool enough to have crystals floating around in them. If the whole lot is now cooled more rapidly, these early formed crystals will be preserved as phenocrysts.

phenocryst

Large mineral grains suspended in a finely crystalline groundmass.

Final product is said to be porphyritic.

Larger crystals set in a finer grained groundmass.

The name given to an intrusive igneous rock that contains unusually large

mineral grains (average grain diameter > 2 cm) is a pegmatite.

Pegmatites are found in eastern and northern Manitoba. Are mined for

cesium and tantalum (mineral name is tantalinite).

e.g. Bernic Lake Pegmatite, Eastern Manitoba. The Tanco Mine is a

tantalum-cesium-lithium producer located east of Winnipeg. The

pegmatite, which does not outcrop to surface, was originally discovered in

the 1920s during a diamond drill program.

Tantalum is a very useful metal with unique properties. The major uses of tantalum are in the electronics industry and for cutting tools. High quality capacitors are the major single use for tantalum. Tantalum carbide is used in production of hard metal alloys for cutting tools. Other tantalum alloys are important constituents of aero engines as well as in acid resistant pipes for the chemical industry. Tantalum pins are used for medical purposes such as hip-joint replacement, since tantalum is the only metal that is not rejected by bodily fluids. 

Cesium formate is a water clear, water soluble fluid with a specific gravity of 2.3g/cc (two and one third times the density of water). It is used in the oil industry as a drilling fluid where the properties of low viscosity, high SG and complete solution have significant benefits over traditional bentonite/barite drill muds in deep wells (> 4,575m/15,000 feet!).

Vesicular and Amygdaloidal Texture

• Some lavas contain gas bubbles which are preserved when they cool.

– gas is mostly water vapour or CO2.

– “froth” on top of the lava.

• These old gas bubbles are called vesicles. Rocks that possess numerous vesicles are termed vesicular.

• When the vesicles have been filled in later by other minerals they are called amygdules.– produces an amygdaloidal texture.– i.e. gas bubble vesicles or cavities

are filled with a solid mineral.

Pyroclastic or Fragmental Texture

Characterizes igneous rocks that are formed by explosive volcanic

activity. Made up of fragments from within the volcano or material that is

ripped-up from the edges of the volcano.

e.g. volcanic ash.

e.g. volcanic bomb: “tear-drop” shaped, erupted as globs of lava and

cooled/solidified upon descent.

Composition of Igneous Rocks

Magma has many elements within it. As magma cools, a number of

different minerals crystallize, all with different melting/freezing points.

As magma cools, minerals with high melting/freezing points crystallize

first. Followed by those with lower melting/freezing points.

Parent magma plays a significant role in determining the mineral

composition of an igneous rock.

Recall the earlier definition of magma types:

Ultramafic

Mafic

Intermediate

Felsic

Magma types are defined based on silica content. Magmas have a wide range of compositions and are also exposed to a wide range of temperatures, pressures and cooling conditions.

It is possible for the same magma to generate two completely different igneous rocks because its composition can change as a result of the sequence in which minerals crystallize, settle, assimilate and mix.

Will examine two key topics related to how crystals form:

(A) Bowen’s Reaction Series

(B) Change in Magma Composition

(i) Magma Mixing

(ii) Crystal Settling

(iii) Assimilation

Bowen’s Reaction SeriesCanadian born scientist N.L.Bowen. First recognized the importance of “magmatic differentiation by fractional crystallization”. Term used to describe the sinking of dense, early crystallized minerals to the bottom of a magma chamber thereby forming a solid mineral layer covered by melt. Minerals do not crystallize at the same time. Bowen suggested that a single magma could crystallize into both basalt (mafic rock) and rhyolite (felsic rock) because of fractional crystallization. Theoretically correct, but does not occur in significant volumes in nature. In nature, the crystallization of a basaltic magma occurs too quickly for the melt to become dominantly rhyolitic. Bowen proved that specific minerals crystallize from magma at different times under different temperature conditions. He studied this in the laboratory and also through observations in the field.

He proposed a mechanism, which is now called the “Bowen’s reaction series”, to account for the derivation of intermediate and felsic rocks from a basaltic (mafic) magma.

Important part of his

theory is that once a

mineral is

crystallized from a

magma it can still

chemically react

with the liquid

magma in order to

form new minerals.

Bowen identified 2

types of reactions:

Discontinuous

Continuous

Discontinuous Reaction Series

Discontinuous reactions led

to the formation of

completely different

minerals as the magma

cooled and reacted with the

crystallized minerals.

Minerals associated with

this process are olivine,

pyroxene, amphibole and

biotite mica (in order of

decreasing temperature of

formation).

This branch of the Bowen’s

reaction series is called the

“discontinuous branch”.

For ferromagnesian minerals, olivine crystallizes first as the magma cools.

Leaves a melt or magma enriched in SiO2 because olivine has 40 % SiO2,

while a typical basaltic magma has 50 % SiO2.

As the temperature drops past a certain point, pyroxene will start to form. Solid olivine reacts with the melt to form a more silica-rich mineral, pyroxene. If the cooling is at a very slow rate, all of the olivine will react to form pyroxene.

This is called a discontinuous reaction series.

Early formed minerals form entirely new compounds through reaction with the remaining liquid.

Reaction converts one mineral into another mineral.

The reaction is not always complete.

e.g. olivine may have a rim of pyroxene which would indicate an incomplete reaction.

Continuous Reaction Series

Ca

Na

Na Na

Na

Ca

Ca

In contrast, continuous

reactions led to the gradual

chemical change of a

specific mineral group: the

plagioclase feldspars.

They continuously change

from Ca-rich plagioclase to

Na-rich plagioclase, as the

temperature decreases.

Plagioclase grains in many igneous rocks have concentric zones of

differing compositions. Inner layers are Ca-rich, outer layers are Na-rich.

Bowen pointed out the significance of these zoned crystals.

Also, he observed that the main plagioclase associated with basalt is Ca-

rich, while in contrast the main plagioclase associated with rhyolite is Na-

rich.

This branch of the Bowen’s reaction series is called the “continuous

branch” or the “continuous reaction series”.

Both branches of the

Bowen’s reaction series

meet at a common mineral.

Potassium feldspar or “K-

spar”.

Continue to form

muscovite mica.

Finally quartz.

As the temperature of the

magma cools even further.

Then the crystallization of

the magma is complete.

Change in Magma CompositionBowen showed that magmas change composition during their cooling

history. Three mechanisms by which a magma can change

composition:

Mixing with

other

magmas.

Crystal

settling.

Assimilation of

surrounding

rock material.

Magma Mixing

Felsic magma Mafic magma

Intermediate magma

Obvious.

Two different magmas

are mixed.

Forms a third magma.

Different composition

from the parent magma.

Mafic magma mixing with felsic magma. Produces an intermediate magma.

Crystal Settling

crystal(e.g. olivine)

magma chamber

country rock

One of the mechanisms by which a magma can change composition.

Involves physical separation of minerals by crystallization and

gravitational settling. Does occur in magmas.

However, not as widespread as what Bowen first theorized.

Involves physical separation of minerals by crystallization and

gravitational settling. Occurs in large magma chambers, or large calm

volumes of magma.

Fractional Crystallization

Assimilation

magma chamber

country rock

inclusion

One of the mechanisms by which a magma can change composition.

Assimilation is a process whereby a magma reacts with pre-existing

rock, called “country rock”, with which it comes in contact.

Country rock is partially or completely melted by the instrusive body.

Blocks of country rocks can be observed at the margins of the instrusive

body and these are termed inclusions.

Fragments of rock dislodged by upward-moving magma could remain as inclusions.

Many inclusions are broken or wedged off the walls of the magma chamber and incorporated into the molten magma.

Effect on the bulk composition of the magma is generally thought to be minimal. Only a limited amount of rock can be included. Inclusions tend to reduce the temperature of the magma and therefore speed up the process of crystallization.

Relative age determination in the presence of included fragments. One of the fundamental principles of historical geology.

“Whenever two rock masses are in contact, the one containing pieces of the other will be the younger of the two”. Lyell 1830.

Inclusions in an igneous rock represent older parent rock material, while the igneous rock that contains the inclusions is younger.

Igneous Rocks Classification

• igneous rocks are formed from the cooling of molten matter, two

main kinds

Plutonic

– liquid material (magma) both forms and cools within the Earth

– intrusive igneous rocks

Volcanic

– liquid material (magma) forms within the Earth but erupts (lava)

and cools at the Earth’s surface

– extrusive igneous rocks

• most igneous rocks classified on the basis of composition and

texture

• usually 2 names given for one rock with the same composition…

different texture

– extrusive name (basalt) … aphanitic

– intrusive name (gabbro) … phaneritic

• furthermore, igneous rocks are classified on silica content:

– Low Silica = Ultramafic Mafic

– Intermediate Silica = Intermediate

– High Silica = Felsic

Ultramafic Rocks

peridotite

Composed largely of ferromagnesian

silicate minerals.

Peridotite is an ultramafic rock that

contains mostly olivine, lesser

amounts of pyroxene and minor

amounts of plagioclase feldspar.

Dark green or black in colour. Note:

olivine is a green mineral.

Upper mantle origin. Very rare at the

surface. Rare in rocks younger than

2.5 billion years.

Peridotite. Mostly made up of olivine minerals (green).

Very hot temperatures associated with ultramafic magma (up to 1600° C). 1300° to 1600° C. These high temperatures are not common today. But were more typical of the Earth’s past. An example of an ultramafic rock is kimberlite. Host rock for diamonds. Named after Kimberly, South Africa. Kimberlite rock is generally unstable at the Earth’s surface and tends to weather very rapidly compared to the surrounding host or country rock. Many kimberlite pipes are therefore at the base of a lake or swamp in Canada. Very difficult to prospect for and find.

Use geophysical (magnetic) surveys and geochemical trace element sampling to help pinpoint these valuable rocks. Latter involves sampling glacial overburden and identifying indicator mineral trains (pyrope garnet, chrome diopside, ilmenite). Diamond mines in the NWT are mining kimberlite pipes.

e.g. Kimberlite– host rock for diamonds– originate 100 to 300 km

below the surface, upper mantle

– carrot-shaped pipe, no more than a few hundred meters in diameter

Diamond mining in Russia is big business, boasting to have some of the clearest diamonds in the world. Picture shows a general view of one of Russia's biggest kimberlite pipes, Mir, near the town of Mirny in western Yakutia region, August 30, 2001. Mir, discovered in 1955, was the first kimberlite pipe of the former Soviet Union. The kimberlite pipes located in Yakutia remain Russia's main source of diamonds.

Mafic Rocks

gabbro

basalt

Temperatures 1000° to 1200° C.

Low viscosity (i.e. runny).

Generally dark colour.

Relatively dense.

Basalt (extrusive).

Gabbro (intrusive).

e.g. Hawaii.

basalt

Crystallize from mafic magmas. Silica

content 45-52 %. Large proportion of

ferromagnesian minerals.

Basalt is the most common extrusive

igneous rock. Basalt lava flows dominate

the sea bed (ocean crust).

Basalt. Mafic extrusive rock. Aphanitic texture.

Gabbro. Mafic intrusive rock. Phaneritic texture.

Intermediate Rocks

andesite

diorite

Intermediate silica content. 700° to 1000° C.

Medium viscosity (i.e. a bit runny).

Intermediate colour. Equal amounts of

dark and light coloured minerals.

Andesite (extrusive).

Diorite(intrusive).

Syenite and monzonite are similar to granite,

but less quartz.

e.g. Mount St. Helens produced

andesitic lava. Close to felsic in

composition: dacitic (62-63 %

silica).

andesite

diorite

Intermediate in composition: 53 to 65 %

silica. Mostly plagioclase feldspar (light

minerals) with amphibole or biotite (dark

minerals). “Salt and pepper” appearance.

Andesite is common in the Cascade

Range and Andes Mountains. Diorite is

fairly common in the continental crust. Not

as common as granite.

Andesite. Intermediate extrusive rock.

Hornblende phenocrysts. Porphyritic texture.

Diorite. Intermediate intrusive rock.

Salt and pepper appearance. Phaneritic texture.

Felsic Rocks

granite

rhyolite

Silica-rich rocks. 600° to 800° C. High viscosity (i.e. not runny, sticky). Light colour. Relatively low density. Rhyolite (extrusive). Granite (intrusive). e.g. Yellowstone National Park. Both granite and rhyolite are derived from felsic magmas. Silica content of > 65 %.

rhyolite

granite

Felsic igneous rocks consist of the following minerals:- potassium feldspar (K-spar)- Na-rich plagioclase feldspar- quartz- some biotite - rare amphiboles

 Considerable Na, K and Al. Little Ca, Fe and Mg. Granites are common in the PC Shield areas of Canada. Many granitic rocks in northern and north-eastern Manitoba. Granites are the most common intrusive igneous rock.

Other Igneous Rocks

(i) Pegmatites

Very coarsely crystalline igneous rocks. Mentioned

in the previous section. Minerals > 1 cm.

Most pegmatites consist of the same minerals as

granite. K-spar, plagioclase and quartz. Spatially

associated with granite plutons. Thought to

represent the minerals formed from the remaining

fluid and vapour phases that existed after the granite

had crystallized. Water-rich vapour phase contains

rare elements, such as cesium and lithium.

e.g. Bernic Lake Pegmatite, E. MB. Tanco mine.

Will briefly discuss three other varieties of igneous rocks: (i) Pegmatites,

(ii) Volcanic Tuff and Breccia, (iii) Obsidian and Pumice.

Pegmatite (light

coloured rock)

exposed in the

Black Hills,

South Dakota.

Gem minerals,

such as

tourmaline, are

observed in

some

pegmatites.

(ii) Volcanic Tuff and Breccia

Fragmental material erupted from volcanoes which eventually turns into

rock. Collective term used for these igneous rocks is “pyroclastic”.

Pyroclastic rock is a rock formed from fragments that are ejected during a

volcanic eruption.

Tuff is used to describe volcanic ash-

sized material that becomes a rock. < 2

mm in diameter (ash is < 2 mm in

diameter). Use a prefix to describe the

composition. e.g. rhyolitic tuff.

Volcanic breccia is used to describe

volcanic lapilli- sized (2-64 mm in

diameter) and block- or bomb-sized (> 64

mm in diameter) material that becomes a

rock. Larger sized material than tuff.

obsidian

Scoria

Pumice

(iii) Obsidian and Pumice

Varieties of volcanic glass.

Obsidian was mentioned earlier.

Black and conchoidal fracture pattern

of glass.

Pumice contains numerous vesicles

or “bubbles”. Looks like an Aero

chocolate bar. Porous.

Gas escapes through lava. Forms a

“froth” which solidifies into pumice.

Pumice (felsic). Scoria (mafic).

Plutons

Magma that forms a pluton did not

originate where we now find the

body. Originated much deeper into

the crust and mantle. Was intruded

upwards into the surrounding rock.

Plutons are classified into categories:

Dike

Sill

Batholith

Stock

Laccolith

All bodies of intrusive igneous rocks, regardless of shape and size are

called plutons. Named after Pluto, the Greek god of the underworld.

Pluton is defined as an intrusive igneous body.

DikeTabular, parallel-sided sheets of intrusive igneous rock.

Cuts across layering of intruded rock. Discordant: boundaries that cut across layering of the country rock.

Dike forms when magma squeezes into a fracture. Mostly small bodies (1 to 2 m across). Some greater than 100 m across. Later cools to fill the fissure.

A dike can be a conduit for magma to travel to the surface and be erupted by a volcano as a lava flow. This is termed a “volcanic pipe”.

Examples of volcanic necks: Le Puy, France and Shiprock, New Mexico (with radiating dikes).

Sill

Tabular, parallel-sided sheets of intrusive igneous rock that are parallel

to the layering of the intruded rock. Concordant: boundaries that are

parallel to the layering of the country rock.

Thicknesses are mostly a meter or less. Some are hundreds of meters

thick. e.g. Palisades Sill, West Side of Hudson River, NY.

Commonly occurs

together with a series of

dikes (dike “swarms”).

Does not push the crust

upwards into a dome.

If a dome develops, then

the body is called a

laccolith.

Batholith A very large, igneous body of irregular shape that cuts across the layering of the rock it intrudes. Largest kind of pluton.

Mostly granite. Some 1000 km long and 250 km wide. Most 20 to 30 km thick.

Well known batholith exposed in Yosemite National Park (El Capitan), California. Cliff is 900 m high. Tallest unbroken cliff in the world. Most are composite masses. Comprise a number of separate intrusive bodies of slightly differing composition. Important mineral resources in batholiths include gold and copper deposits. Mineral rich solutions move through the cracks (fractures) in the granite and the ore minerals are precipitated into these pore spaces.

Emplacement of batholiths is somewhat

analagous to salt dome or diapir emplacement.

Less dense material rises upwards and

laterally displaces the overlying country rock.

In the case of a batholith, the

magma could also melt or fragment

the country rock, as well as laterally displacing the overlying rock.

xenolith

Emplacement of a Batholith:

Stoping: Magma is injected into fractures and planes between layers in

the country rock. Blocks of country rock are detached and engulfed in

the magma, thus making room for the magma to rise farther.

Some of the engulfed blocks might be assimilated, and some might

remain as inclusions: xenoliths.

Granitic rocks, Yosemite

National Park, CA.

Part of the Sierra Nevada

batholith: 640 km long

and up to 110 km wide.

Near vertical cliff.

El Capitan.

Rises > 900 m above the

valley floor. Highest

unbroken cliff in the

world.

Stock

A small, irregular body of intrusive

igneous rock, smaller than a

batholith. Cuts across the layering

of the intruded rock. No larger than

10 km in diameter. Could be a

companion body to a batholith or

even the top of an eroded batholith.

Laccolith

A lenticular pluton intruded parallel

to the layering of the intruded rock,

above which the layers of the

invaded country rock have been

bent upward to form a dome. Dome

is recognized as an elevated area

on the Earth’s surface.

Mantle Melting and Plate Tectonics