GEOL 1110 Lecture Notes - Lecture 6: Igneous Differentiation, Gabbro, Diorite
3 Jul 2018
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Igneous Rocks – Chapter 3
Igneous rocks form as molten rock (magma) cools and solidifies
General characteristics of magma
oForms from “partial melting” of rocks from inside the Earth
oMagma that reaches the surface is called lava
oThere are different viscosities
There are changes in its abilities of it to flow
The composition of the elements and minerals change it’s colors
and viscosities
Low viscosity = Flowing
Magma (lava) that solidifies on Earth’s surface create Extrusive Igneous Rocks
oVolcanoes and fissures
oLarge Lava Plateaus
Magma that solidifies in the interior of Earth’s crust form Intrusive Igneous
Rocks
oPlutonic rocks
oEx. The north shore mountains
The stuff above the Batholith/ Intrusive igneous rocks weathers
away/ erodes away, eventually revealing the rock to the surface
A body of Plutonic rock = pluton
oDifferent types of plutons
On the west coast there is an interior sedimentary basin
oThere were sets of little islands that eventually moved towards toward the
coast today, when BC subducted, they were too big to be subducted, and
they therefore formed the Rocky Mountains
Alberta used to be ocean front, but now they have a mountain chain
due to the Accretion of Terrain
oMore and more things continued to attach to the others and crashing into
one another, the subduction zone moved more and more to the west
eventually stopped where the Juan De Fuca plate is now
This is why we have different mineral compositions in different area
because they all had slightly different compositions
Igneous activity occurs at the greatest rates at:
o1. Divergent plate boundaries
Lack of pressure allows magma to upwell
o2. Convergent plate boundaries
Subduction and re-melting
When there is subduction, there is a different density due to
the composition of the ocean floor etc. it is less dense and
therefore it is forced upwards
Igneous rock is created through the melting of Earth’s solid crust
Intrusive Igneous rocks are found on the Earth’s surface through:
oUplifting of rock
oUncovering of rock
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oWeathering and erosion of overlying strata
Magma
Generating magma from solid rock
oProduced from partial melting of rocks in the upper mantle and the crust
oRole of heat
Temperature increases with depth in Earth’s upper crust
(geothermal gradient)
~20 degrees Celsius to 30 degrees Celsius per kilometer
oNot a linear temperature change
Temperature @ 100km depth =~1200-1400 degrees Celsius
There are some pockets that are hotter than others in the
Asthenosphere
Heat vs. Pressure
Rock in the lower crust and upper mantle are essentially solid regardless of the
increases temperature
Additional heat from ____ can melt this rock
oSubduction zone friction melting of the material on the outside edges,
which then can allow rock to descent more so
oSubduction zone rocks descend and are heated
oHot mantle rises and intrudes
Volatiles
Rocks found in the Earth’s crust are under pressure
oLoss in pressure (less confinement) and the addition of gasses or fluids
(volatiles) can cause them to melt
oThis is crucial in the subduction of materials, especially in the ocean
oWater, water vapor are both important
oRold of volatiles
Volatiles (gasses, primarily water vapor)
Important: oceanic lithosphere descends into the mantle
Water not present = 1200 degrees Celsius melting point
Water present = 1000 degree melting point
With the water, the rock melts more and therefore is able to
subduct deeper
Pressure
Role of pressure
oIncreased confining pressure= increase in rocks melting temperature or
conversely, reducing the pressure lowers the melting temp
More pressure = 1200 Degree melting point
Less pressure= 1000 melting point
oWhen confining pressures drop, decompression melting occurs
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oWhen there is more pressure, it is harder for heath to get through the
extremely dense material
oIf there is a low pressure in the earth’s crust (Divergent plate boundary) ,
there is a place for the magma to come out, if it has a low viscosity it can
flow out, if it has high viscosity, it will just fill the fissures and not flow to
the surface
Magma
Mafic magma (magnesium and iron (Ferrum)) very high melting point, dark
colour
oBuoyantly rises – enters continental crust
oBegins to pool below crust
Felsic Magma - Intrudes into crust – crust melts and forms Felsic magma
(feldspar and silica)
oForms chains that are difficult to break, this is why It doesn’t want to flow
easily because it is pulling other stuff behind it and it is slowed down
3 Things that generate Magma
o1. Heat added through subduction and contact with rock at greater depth
o2. Loss of pressure, reduced confinement
o3. Introduction of volatiles (gas, fluid)
Magma is composed of three parts
o1. Liquid = melt
o2. Solid some things are not heated up enough to melt so there are
chunks that are still solid
o3. Gas some rocks, teh gas is able to escape easily, other rocks
explode if the gas is unable to escape
oNot just melting rock!!! Also organic materials that have been subducted
from the ocean etc.
They will all make up lots of different rock types
oThere is a spectrum of different rocks that are created according to the
different things in the magma released
Things that are higher in the atmosphere are more felsic, the deeper it is, the
more mafic and ultra maphic are deep
oFelsic is the stuff that melts away earlier because it has a lower melting
point
oFelsic material upwells first
Melt
A liquid portion, called melt, that is composed of mobile ions (mainly O, Si, also
Al, K, Ca, Na, Fe etc)
This type of magma acts differently than other types of Magma
When it cools, it becomes more solid and less melt
As the magma cools, certain minerals start to form, and the rest is still somewhat
melted
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Document Summary
Igneous rocks form as molten rock (magma) cools and solidifies. General characteristics of magma: forms from partial melting of rocks from inside the earth, magma that reaches the surface is called lava, there are different viscosities. There are changes in its abilities of it to flow. The composition of the elements and minerals change it"s colors and viscosities. Magma (lava) that solidifies on earth"s surface create extrusive igneous rocks: volcanoes and fissures, large lava plateaus. Magma that solidifies in the interior of earth"s crust form intrusive igneous. The stuff above the batholith/ intrusive igneous rocks weathers away/ erodes away, eventually revealing the rock to the surface. A body of plutonic rock = pluton: different types of plutons. This is why we have different mineral compositions in different area because they all had slightly different compositions. Igneous activity occurs at the greatest rates at: 1. Lack of pressure allows magma to upwell: 2.
