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Chapter 2-7

ES 101 Chapter 2-7: EARTH 121 NOTES

Environmental Studies
Course Code

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Make up bulk of Earth’s crust with exception of Earth’s core
Magma: Parent material of igneous rocks
Ignis = FIRE
Igneous rock form as molten rock cools and solidifies
Magma is formed by partial melting.
Once magma is formed, a magma body buoyantly rises toward the surface because it is
less dense than the surrounding rocks.
Occasionally molten rock breaks through producing a volcanic eruption
Magma reaching Earth’s surface = LAVA
Sometimes magma is explosively ejected from a vent, producing a catastrophic eruption
Igneous rocks that form when magma solidifies at the surface → EXTRUSIVE (out
thrust) or VOLCANIC
- Abundant in western portions of Americas (Cascade Range and Columbia
Plateau) and oceanic islands (Hawaiian chain)
- Example: Andesite is a medium fine grained rock of volcanic
- Example: Pumice forms when large amounts of gas escape through lava to
generate a grey, frothy mass.
- Large % of voids make pumice float when placed in water...so flow lines visible
indicating some movement occurred before solidification was complete.
- Example: Basalt is a very dark green to black fine grained most common
volcanic rock composed primarily of pyroxene and calcium rich plagioclase
feldspar, with lesser amounts of olivine and amphibole.
- Many volcanic islands and the upper layer of oceanic crust consist of basalt
Magma that loses its mobility before reaching the surface crystallizes at depth.
Igneous rocks that form at depth → INTRUSIVE (into thrust) or PLUTONIC
- A body of plutonic rock = PLUTON
- Many are the remnants of magma chambers that once fed ancient volcanoes
- These rocks would not be exposed at the surface if portions of the crust were not
uplifted and the overlying rocks stripped away by erosion.
- Exposure occurs in western and Central BC
- Very ancient exposed throughout the Canadian Shield
- They are classified as either TABULAR or MASSIVE and by their orientations
with respect to the host rock
- Intrusive rocks are said to be discordant if they cut across existing rocks and
concordant if they form parallel to sedimentary layers
- Example: Diorite is coarse grained intrusive rock equivalent of andesite.

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- Primarily sodium - calcium plagioclase feldspar and amphibole with lesser
amounts of biotite.
- Hence, has a salt and pepper appearance.
- Distinguished from granite by absence of visible quartz crystals and a higher %
of dark silicate minerals
- Example: Gabro is the intrusive equivalent of basalt.
- It is very dark green to black in colour and composed primarily of pyroxene and
calcium rich plagioclase feldspar
- Not a common constituent of the continental crust, but a significant % of the
oceanic crust
- So large portions of magma found in the subsurface magma chambers that once
fed basalt flows eventually solidified at depths to form gabbero.
Generating Magma from solid rock
From melting in Earth’s mantle and plate tectonics play a major role
Greatest amount of activity occurs at divergent plate boundaries, in association with sea
floor spreading
Substantial amount of magma also produced at subduction zones (where oceanic
lithosphere descends into the mantle)
- Magma generated here contains components of mantle as well as subducted
crust and subducted sediments.
- Some appears deep in the mantle where it is not directly influenced by plate
*** Earth’s crust and mantle are composed primarily of solid, NOT molten, rock***
Outer core is a fluid, its iron rich material is very dense and remains deep within the
What is the source of magma that produces igneous activity?
- Magma originates essentially when solid rock, located in the crust and upper
mantle, melts. Magma generated from solid rock in most obvious way is to raise
the temperature above the rock’s melting point.
Role of Heat
- Temperatures get higher as you go deeper.
- It averages about 25°C per kilometre in upper crust
- Change in temperature with depth = GEOTHERMAL GRADIENT
- At high temperatures of 1200°C to 1400°C, rocks in the lower crust and upper
mantle are near their melting points, they are very hot but still essentially solid.
- Relatively small and limited magma produced by:
1) At subduction zones, friction generates heat as huge slabs of lithosphere slides
past one another.
2) Crustal rocks are heated as they descend into the mantle during subduction.

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3) Hot mantle can rise and intrude crustal rocks
- Vast bulk of magma forms without the aid of an additional heat source.
- Rock near its melting point can begin to melt if the confining pressure drops or if
volatiles are introduced
Role of pressure
- Pressure increases with depth
- Melting which is accompanied by an increase in volume, occurs at higher temperatures
at depth because of greater confining pressure.
- An increase in confining pressure causes a rise in the rock’s melting temperature.
- Reducing confining pressure lowers a rock’s melting temperature.
- When confining pressure drops enough, decompression melting is triggered. That
occurs when rock ascends as a result of convective upwelling, thereby moving into
zones of lower pressure.
- Even though mantle is a solid, it does flow at very slow rates over millions of years which
generates magma along ocean ridges where plates are drifting apart.
Role of Volatiles
- Water causes rock to melt at lower temperatures
- Effect is magnified by increased pressure
- “wet” rock buried at depth has a much lower melting temperature than does “dry” rock of
the same composition and under the same confining pressure
***In addition to a rock’s composition, it’s temperature, depth, and water content determine
whether it exists as a solid or liquid***
- They are generated in regions where cool slabs of oceanic lithosphere descend into the
- As an oceanic plate sinks, both heat and pressure drive water from the subducting
crustal rocks.
- These mobile volatiles migrate into the wedge of hot mantle that lies above.
- This process lowers the melting temperature of mantle rock sufficiently to generate some
- When enough mantle derived mafic magma forms, it will buoyantly rise toward the
- Mafic is from magnesium and ferric = IRON
- In a continental setting, mafic magma may pond beneath crustal rocks, which have a
lower density and are already near their melting temperatures.
- This pooling can result in some melting of the crust and formation of a secondary, silica
rich or felsic magma
- Felsic is from feldspar and silica
***Magma can be generated under 3 sets of conditions:
1) heat added by a magma body from a deeper source intruding and melting crustal rock
2) a decrease in pressure can result in decompression melting
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