Revised 11 / 17 (Monroe 6th ed.)
Including...
Factors affecting magma generation
Click here for more on elements and minerals common to the major magma types
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Click here for a tectonic summary from GeoTours
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
Different compositions represent different magmas
Different magma types are common to specific tectonic areas
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?
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
Fountains
Fissure eruptions
Lava lakes
Halemaumau, Mauna Loa
Pillow basalt - indicator of submarine eruption
Columnar jointing - indicator of subaerial eruption
Vesicular basalt
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
Lahars
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
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"
Yellowstone
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
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
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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