Intrusive Igneous Rocks/Activity
I.
Intrusive igneous rock
A.
Form from magma that solidifies underground.
1.
2.
B.
Rate of cooling determines crystal size
a.
slow cooling = larger crystals
b.
rapid cooling = smaller crystals
Crystal size or texture classifies igneous rocks
Evidence that suggests that intrusive rocks solidify underground from
magma
1.
Mineralogically and chemically = to volcanic rocks (Rhyolite and
Granite).
2.
Volcanic rocks small grained/intrusive are larger grained.
3.
Experiments have shown that most minerals in intrusive rocks
require high temp. and high pressure, indicating deep burial.
4.
Preexisting solid rock (country rock) appears to have been
forcibly broken by an intruding liquid. Country rock = any older
rock into which an igneous body intruded.
5.
Country rock appears “baked” or metamorphosed close to
contact point.
6.
Rock types of the country rock often match xenoliths –
fragments of rock that are distinct from the body of igneous
rock, in which they are enclosed.
7.
Intrusive rocks next to country rock have chill zones – finer –
grained rocks indicating rapid cooling because of loss of heat to
surrounding rock.
II.
Intrusive Igneous Activity
A.
Magma emplaced at depth
B.
Underground igneous body is called a pluton.
C.
Plutons are classified according to
1.
form at depth or shallow
2.
particular shape (tabular of massive)
3.
does it follow layering of country rock or not (discordant,
concordant)
4.
D.
is body large or small
Shallow intrusive structures
1. Form at depth of probably <2km
2. These form the “plumbing systems” of volcanoes.
3. These tend to be small (compared to
those at depth).
4. Tend to chill quickly because they are near the surface. This makes
them fine grained.
Dike – tabular or sheet-like discordant intrusive structure (deep or
E.
shallow). Injected into fractures that cut across rock layers.
Discordant = the body is not parallel to any layering of the country
rock. Range from <1cm to >1km. Horizontal or vertical depending on
strata.
Sill – tabular intrusive structure that is
F.
concordant or parallel to any planes or layering in the country rock.
1.
Formed when magma is injected along sedimentary bedding
surfaces.
2.
Occur shallower because they must lift overlying layers.
3.
Ex. Palisades Sill New York City
4.
Sill resembles buried lava flows (horizontal to layers).
Evidence that a sill is not a lava flow
a.
lava flows contain void spaces of escaped gas.
b.
Only rock below lava flow shows metamorphic
alteration.
G.
c.
Sills will contain fragments of overlying alteration rock.
d.
‘Baked’ zones both above and below sills.
Laccolith
1.
Forms the same way as a sill
2.
Forms when more viscous magmas, collect as a lens shaped
mass that arches overlying strata upward (detected by the
dome it creates). Opposite to laccolith is a lopolith.
III.
Large intrusive igneous bodies(plutons)
Large, discordant and form deeper.
Classified according to the area of rock exposed at the surface.
A.
Stock
1.
Small discordant pluton with an outcrop area (the area exposed) of less
than 100 sq. km.
B.
Batholith
1.
a massive discordant pluton almost always composed of granite.
IV.
2.
greater than 100 square miles of it are exposed to the atmosphere.
3.
forms from numerous coalesced plutons called diapers.
4.
irregular roof
How magmas of different composition form
A.
Differentiation and Bowen’s Reaction series.
1.
The process by which different ingredients separate from an
originally homogenous mixture.
2.
Shows that those minerals with the highest melting
temperatures crystallize from the cooling magma before those
with lower melting points. This is called
Crystal settling = the downward movement of minerals that are
denser than the magmas from which they crystallized.
B.
Partial Melting – produces most magmas.
1.
When rock is heated those minerals with the lowest melting
temperatures melt first (quartz and potassium feldspars).
2.
These melted minerals can separate from rock and collect into
pockets and form diapers.
3.
This can explain how a magma high in silica can form from a
mafic parent rock.
4.
The lower part of the continental crust is a plausible source for
felsic magma by partial melting.
C.
Assimilation – process called stoping where xenoliths are broken off
and incorporated into the magma changing its composition (forms
intermediate magma).
D.
Mixing of magma – two magmas might meet or merge within the crust
to combine to form intermediate magma (convergent boundaries).
V.
Explaining igneous activity by plate tectonics
A.
Generation of Basaltic Magma
1.
Believed to be generated at divergent boundaries by partial
melting of the athenosphere, which is mainly composed of the
ultramafic rock peridotite (olivine,pyroxene, and plagioclase).
2.
Process of basaltic magma goes as follows
a.
the athenosphere moves up and out – the crust splits
causing a reduction in pressure and causes melting.
b.
if only part of the mantle melts and most of the olivine is
left behind, you have a basaltic magma.
B.
Generation of Granitic Magma
1.
Believed to occur at convergent boundaries where there is
partial melting of the lower crust.
2.
Continental crust contains the high amount of silica needed for
a silica rich magma.
3.
The partial melting of a subducting oceanic plate supplies heat
to the lower crust. As the plate descends it begins to melt. As
the mafic material melts it rises and is trapped underneath the
crustal plate causing the felsic crustal plate to melt- called
magmatic underplating. The melted silica rich magmas work
their way upward in diapirs to higher levels and solidify as a
pluton (batholith).
IV.
Classification of Igneous Rocks
A.
Magma that solidifies and forms at depth is called
intrusive or plutonic igneous rock.
1.
Coarse – grained (slow cooling)
a. grains > than 1 mm.
2.
porphyritic – two stage cooling process. Large
crystals called phenocrysts are embedded in a
matrix of fine- grained crystals.
C.
D.
Those that cool and crystallize above ground called extrusive or volcanic rocks
1.
fine grained
2.
no grains = glass
Mineralogically (chemically) equivalent rocks
Intrusive
Extrusive
Granite _______ Rhyolite
Diorite _______ Andesite
Gabbro _______ Basalt