Download document

Fig. 03.01
Fig. 03.02
Magma: molten rock + volatiles beneath the
Earth’s surface.
Lava: molten rock on the Earth’s surface
that has lost at least some of its volatiles.
Magma is usually silicate in composition:
~50-70 wt% SiO2
The rest is made up of Al2O3, Fe2O3, FeO,
Na2O MgO, K2O, CaO, P2O5, TiO2
A continuous range of magma chemistries
is observed.
Partial Melting
Rocks rarely melt to 100%.
Rocks contain several minerals – the one
with the lowest melting point melts first and
that with the highest melts last.
Liquid may be removed along grain
boundaries.
This can create a magma of a different
composition than the source material.
Fig. 03.06
Table 03.01
Felsic
Intermediate
Mafic
Ultramafic
~70 wt% SiO2
~60 wt% SiO2
~50 wt% SiO2
<50 wt% SiO2
(Komatiite)
Fig. 03.07a
Fig. 03.07b
Fig. 03.07c
Fig. 03.07e
Fig. 03.07f
General Rules
Felsic = light-colored (Si-, Al-, alkali-rich).
Ultramafic and Mafic = dark-colored (Fe, Mg,
Ca rich, ferromagnesian mineral-rich).
Magmatic temperatures range from ~600˚C to
>1,500˚C.
The Exception that Proves the Rule - Obsidian
Obsidian = felsic (quenched glass), but black.
General Textures & Crystallization
Rate of crystallization dictates mineral size:
slow cooling = large crystals; fast cooling =
small crystals or a quench glass.
General Textures
Extruded or erupted magmas or shallowly emplaced magma
bodies cool quickly  many small, interlocking crystals with
some interstitial quench glass.
Intrusions of magma deeper within the crust are insulated.
Although the margin of the magma next to the wallrock is chilled
(small grain size) and the wallrock is baked, the interior of the
magma cools much more slowly = large grain size (phaneritic).
Deep intrusive igneous rocks are give the generic term “plutonic”.
Fig. 03.05a
Plutonic Rock (Granite)
Fig. 03.05b
Plutonic Rock (Granite) in Thin Section
Fig. 03.03
Plutonic Rock (Granite) in the Field
Textural Terms
APHANITIC: individual crystals are so small they cannot be
discerned with the naked eye. Common in eruptive and shallow
intrusive igneous rocks.
Textural Terms
PHANERITIC: crystals can be seen, common in interiors
of thick flows and deeper intrusions.
Textural Terms
PORPHYRITIC:
large crystal
(PHENOCRYSTS) in
a fine-grained
groundmass. Reflects
two cooling stages –
slow cooling (in a
magma chamber) to
form the phenocrysts,
followed by rapid
cooling as would occur
upon eruption.
Textural Terms
PEGMATITIC: contains very large
(cm-size) crystals.
Crystal Shapes
EUHEDRAL: wellformed crystals with
many crystal faces
developed.
Crystal Shapes
SUBHEDRAL: crystals with only a few
crystal faces developed.
Crystal Shapes
ANHEDRAL: crystals with no crystal faces
developed.
Magma Viscosity
Magma viscosity is dependent upon
composition and temperature:
Mafic << Felsic
Hot << Cool
Silica affects viscosity because of polymerization or linking of SiO2 tetrahedra.
The more silica-rich the more felsic the
magma is and the stickier it is.
Magma Viscosity
All magmas contain gases, but felsic magmas are
generally more gas-rich - explosive.
When gases escape, can form vesicular lavas,
e.g., vesicular basalt:
Pumice = “froth” = felsic.
Intrusive Rocks
Intrude older rocks (generically known as
“countyrock” – can be igneous, sedimentary, or
metamorphic) and bake them = contact
metamorphism.
Intrusive Rocks
Intrusive rocks recognized by:
Coarse grain size, interlocking crystals, typically
lacking a fabric (oriented texture).
Baked contacts (country rock) and chill zone
(finer grain size) at the edges of the intrusion.
Inclusions of country rock = “xenoliths”.
They cross-cut features in the country rock.
Veins protrude outward into country rock.
Fig. 03.04
Ship Rock, New Mexico
Fig. 03.08b
Fig. 03.13
Fig. 03.12
Fig. 03.14
Granite Batholith, Sierra Nevada
Fig. 03.10
Intrusion Types
How to distinguish a sill from a flow:
• baked contacts above and below;
• vesicles (gas bubbles) in flow often
filled in with flows;
• smaller dikes present above sill
intruding the overlying countryrock.
Changing a Magma Composition
Fractional crystallization via crystal settling:
Changing a Magma Composition
Mixing of two compositionally distinct magmas:
Changing a Magma Composition
Assimilation of country rock:
Changing a Magma Composition
Partial melting of the source rock to different
degrees.
Plate Tectonics &
Igneous Rocks
Divergent Plate Margins –
lithosphere and asthenosphere;
pressure-release, almost all
mafic magmatism.
Plate Tectonics & Igneous Rocks
Intraplate: Hawaii,
Yellowstone, LIPs
– mantle plume
activity.
Fig. 03.24
Convergent Plate Margins: more
intermediate and felsic
magmatism, especially if
volcanoes are built on continents.
Plate Tectonics &
Igneous Rocks
Fig. 03.26
Origin of Granite: magmatic underplating.