Revised 11 / 17 (Monroe 6th ed.)

Plate Tectonics, Volcanoes, and Plutons




Factors affecting magma generation

Spreading Center magmas

Subduction Zone magmas

Continental magmas

Formation of granitic magma

A new thought...

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The interior of the earth is hot

Subject to igneous activity throughout its history

Differentiation is the key!!

Heavies to the center, scum to the crust

Review of igneous rocks

Texture and composition

Felsic to intermediate to mafic to ultramafic

Different compositions represent different magmas

Different magma types are common to specific tectonic areas


Factors affecting magma generation

Heating - solid/liquid phase change

Depth of burial - an obvious source of heat

Friction - common in a subduction zone

Addition of water

"Wet" rock melts at a lower temperature than "dry" rock

Helps initiate partial melting in subduction zones

The subducting material is saturated with sea water

Mixes with the material and speeds up the melting process

Steam rises into the overlying lithosphere and asthenosphere and causes the formation of vast quantities of magma

Pressure relief melting - very important!

Magma occupies more space than rock does

Therefore, in order to melt, the rock needs room to expand

This may not be possible where the pressure is too great

That's why the interior of the earth is probably not liquid

A reduction in pressure can cause local melting of rock to form magma

This pressure drop can happen in several ways

Fracturing of overlying material

Common at spreading centers

Displacement of asthenosphere by descending lithosphere

Common in a subduction zone

What about at "hot spots?"

Asteroid impacts?


Spreading Center magmas

Review process of spreading

Differentiation of upper mantle

Where the final differentiation takes place prior to inclusion in the crust

Basaltic magma is the initial material to form from the mantle

Ultramafic (peridotite) is what is left over

Discuss ophiolites

Mafic (basaltic) magmas - High in iron, magnesium

Olivine - Mg2SiO4 to Fe2SiO4

Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6

Plagioclase - CaAlSi3O8 to NaAlSi3O8

Tectonic setting

Tensional environments

Normal faults

Lots of small earthquakes (hot, thin crust)

Also find basaltic magmas at other locations

"Hot Spots" - like Hawaii

On continents as sheet flows - massive eruptions from long fissures

Columbia River / Modoc Plateau basalt

Deccan Traps in India

Characteristics of basaltic magma

Very hot and fluid, but low in volatile content

Flows like water in many cases

Better run fast!

Shield volcanos

Pahoehoe vs. aa lavas

Eruptions relatively quiet, but spectacular


Fissure eruptions

Lava lakes

Halemaumau, Mauna Loa

Pillow basalt - indicator of submarine eruption

Columnar jointing - indicator of subaerial eruption

Vesicular basalt


Subduction Zone magmas

Review subduction process

Takes basaltic crust AND continental debris into the asthenosphere, and lots of water

Partial melting generates magmas

Different from magmas produced directly from the mantle

Intermediate (andesitic) magmas

Plagioclase - CaAlSi3O8 to NaAlSi3O8

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Quartz - SiO2

Tectonic setting

Compressional environments

Reverse faults

Lots of moderate to great earthquakes

Characteristics of andesitic magma

Medium in everything

Temperature, volatile content, density

Eruptions relatively explosive, VERY impressive

Mt. St. Helens is a SMALL example

Can be MUCH larger

Explosiveness due to high volatile content

Like shaking up a can of carbonated soda

Also due to "stickiness" of magma (high silica content)

Tends to plug up the works until the pressure builds to the breaking point

Like a log jam in a river

Composite cones (stratovolcanoes)

Layered flows and pyroclastics

Pyroclastic flows (nuée ardentes) can be very dangerous

Very hot and very fast

You CANNOT outrun one of these


These types of volcanoes often form high mountain peaks

Collect snow and ice

Melt when an eruption occurs

Major flooding downhill

Can pick up massive amounts of pyroclastic (and other) debris

Extensive damage and loss of life


Continental magmas

Commonly form at depth beneath the edges of the continents

Somehow associated with the subduction process?

The "final" purified product

Example - the Sierra batholith - with extensions north & south

Felsic (granitic) magmas

Potash Feldspar - KAlSi3O8

Quartz - SiO2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

Characteristics of granitic magma

Relatively cool

High in volatile content and high in silica

Eruptions very explosive, MOST impressive

Make huge features called "calderas"


Long Valley Caldera

Explosiveness again due to high volatile and silica content

Most felsic magmas cool as intrusive igneous rocks (fortunately!)

Granite is the common end result


Formation of granitic magma

Basalt is (relatively) easy to understand

Primary differentiate from the upper mantle

Granite is much tougher to explain

Some early theories...

Precipitated from seawater

When I was in Geo101

It just happened

Maybe a process called granitization in special cases

High grade metamorphics

It seem clear that granitic magmas are the logical end product of the differentiation process

High in silicon and oxygen and dissolved water (and other volatiles)

Low in the ferromagnesian elements and minerals

Most form at depth within the lithosphere

As they rise they lose water (as steam)

This raises the temperature at which they crystallize

Causes the magma to crystallize deep within the crust

Rarely does it reach the surface to erupt as an extrusive

This is probably a very good thing!

Basaltic magma has very little water

Any loss is relatively unimportant - it rises freely to the surface

Another factor relating to viscosity is the form of the silicate minerals

Shorter chains common in the mafic minerals

Long chains common in the felsic minerals

Like a log jam in a river - restricts the flow

Therefore, higher amounts of silica increases viscosity


A new thought...

There's been plenty of hot debate concerning the origin of granitic magmas

High grade metamorphic environment vs. injection from below

The way I see it: "The ultimate source of granitic magmas must be a metamorphic process unless you accept that they can be differentiated directly from the upper mantle" -GeoMan

Could granitic magmas be formed directly from the upper mantle

Beneath the central portions of the continental cratons

We know that ion differentiation can occur in the mantle

Even under the intense pressures found there

If complete differentiation is what we are looking for...

Heat & pressure from the overlying continental material could allow this

Would be a very slow process

Isostatic uplift following erosion, etc. would lower temperature and pressure

Initiate crystallization of a fundamentally felsic material

This would continually add additional felsic material to the roots of the continents, replacing what is lost to erosion

Help maintain the continents as topographic highs


Click here for more on elements and minerals common to the major magma types

Click here for more on magma and igneous rocks


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