Chapter 4 new textbook Igneous Rocks

1
Chapter 4 (new textbook) Igneous Rocks

• Igneous rocks form as molten rock within or below
  the crust and then cool.
• Magma present below the Earths surface.
• When these cool, they become intrusive (plutonic)
  igneous rocks. When intrusive igneous rocks are
  exposed, it is usually either because of erosion
  or faulting.
• Lava erupted on the Earths surface. When
  these cool, they become extrusive or volcanic
  rocks.

2
2

• Magma can be either completely or partly molten,
  depending on temperature, pressure, and
  composition.
• Most magmas consist of a liquid phase (melt), a
  solid phase (crystallized silicate minerals), and
  volatiles (pressurized gases). The liquid phase
  of a magma consists of randomly moving ions.
• As temperature decreases, the ionic movement
  slows, allowing the formation of chemical bonds
  (crystallization of minerals).

3
3

• The simple silica tetrahedra of olivine form
  first, followed by the single chain of tetrahedra
  of pyroxenes.

4
4
As hot, bouyant magmas rise through the crust, as
the temperature/pressure de-creases, minerals
start to crystallize. If mag-ma pauses
crystals may settle to bottom.
Decreasing Temperatures
5
5
In an area of vigorous igneous activity, younger,
hotter plutons may overtake older, cooler plutons
magma mixing. This is why there are so many
different types of igneous rocks. Magmas rise by
Assimilation melting of crustal rock or by
stoping.
6
6

• Igneous Rocks are classified by texture and
  mineral composition.
• Texture is the size, shape, and arrange-ment of
  randomly-oriented crystals.
• Texture is affected by cooling history, silica ,
  gases (as fluids because of high pressure.
• Cooling history (and crystal size) is affected by
  crustal depth, pressure conditions, fluid
  content.
• Gas content is related to origin of magma.

7
7

• Magma composition is related to plate tec-tonic
  setting and history of pluton as it rised through
  the crust.
• Mafic igneous (Gabbro/Basalt) magmas occur within
  ocean basins along rift (divergent) zones, hot
  spots (Hawaii), and where continental crust is
  thinned.
• Felsic igneous (Granite/Rhyolite) magmas occur
  where subsurface igneous activity has
  partially-melted continental crust.
• Intermediate igneous (Diorite/Andesite) occur in
  areas of magmatic mixing.

8
8

• Igneous Associations
• Phaneritic - Aphanitic
• Gabbro basalt Dark silicate minerals Ca
  plagioclase feldspars, little or no quartz.
  Higher temperatures.
• Diorite andesite 25 dark silicates, Na
  plagioclase feldspars, small of quartz
• Granite rhyolite Quartz and orthoclase (K)
  feldspar 10 dark silicates (biotite
  amphibolite). Lower temperatures.
• Silica (quartz) content influences igneous
  eruptive style. Mafic (dark) magmas/lavas less
  viscous flow easily. Felsic (light)
  magmas/lavas more viscous freeze up are
  more explosive.

9
9

• Where new
• oceanic crust is
• formed, study is
• difficult, so we
• study old ocea-
• nic crust, called
• ophiolites,
• preserved as
• fault slices in
• areas of Conti-
• nental collision.

10
10
Phaneritic intrusive igneous rocks
11
11
As oceanic plate separates, new mafic material
from the mantle fills the gap, yielding basalts
(surface) gabbros (sub-surface).
12
12

• Intermediate igneous rocks usually form in
  association with convergent (sub-duction) zones.
  Descending oceanic crust oceanic sediments are
  partially melted, producing intermediate magmas.

13
13
Piedmont Granites in Ga., SC, NC formed with
Late Paleozoic Era collision of Africa North
America. Heat from partially-melted oceanic
crust melts granitic continental crust.
14
14
Mafic igneous rock textures
15
15
Diabase in shallow intrusion, Norcross, GA
16
16
Felsic igneous intrusion
17
17
Granite, Porphyritic texture
18
18
Obsidian, glassy texture
19
19
Lava Lakes, Lava Fountains features of
Hawaiian basaltic eruptions.
Lava Lake Lava Fountain
20
20
Small spatter vent Small lava tube
Basaltic features in the Aden Basalts, New Mexico.
21
21
Vesicular basalt
22
22
Vesicular texture - Scoria
23
23
Pyroclastic volcanic rocks are the result of
violent, explosive eruptions from either a
caldera-type super-volcano or a composite
volcano. From the Valles Caldera near Los Alamos,
NM.
24
Porphyritic, welded ash flow tuff, 1 b.y. old,
Thunderbird Rhyolite, Franklin Mts., El Paso, TX
24
25
25
Small igneous intrusion, Red Bluff Granite
26
26
Pegmatitic texture (see page 115)
As magma solidifies, remaining liquid is
squeezed out migrates into surrounding
fractures. The presence of higher water content
unusually large crystal growth. Oddball
small (Be, Li) large ions (U) also migrate with
the water.
27
27

• More classification details
• Granites best known igneous rocks because of
  abundance, exposure, and natural beauty, when
  polished.
• Quarry areas include Elberton, GA, Barre, VT,
  Mt. Airy, NC, and St. Cloud, MN.

Generalized map showing relative sizes of Stone
Mt., Elberton, and other NE Ga. Granites. Stone
Mt. granite approx. 32 sq. mi.. Elberton
granite approx. 320 sq. mi.. http//www.egaonli
ne.com/images/gmap.jpg
28
28
Minerals at bottom of Bowens Reaction
Series are more stable, less susceptible to
chemical weathering. As granites are composed of
(usually) 25 quartz 65 feldspar (orthoclase
Na plagioclase) muscovite, they are resistant
to weathering. Examples of large granite bodies
Stone Mt., El Capitan, Mt. Rushmore, Pikes Peak,
etc. (see pg. 122). Granite is commonly used to
describe a wide range of light colored igneous
rocks.
Because of the viscosity, true rhyolite lava
flows are scarce, i.e., they dont travel far
from their eruptive centers. Because of the
lower temp. viscosity, felsic eruptions tend to
be explosive, producing pyroclastic ash flows.
29
Where Does Molten Rock Originate?
29

• Normal temperature increases with depth
  Geothermal Gradient. Local variations due to
  thickness of crust or presence of igneous
  plutons.
• Example Rio Grande Rift - crustal thinning,
  higher heatflow.
• Role of Pressure decrease in confining pressure
  lowering of melting temperature, as in Rio
  Grande Rift example above.
• Role of Volatiles Water and other fluids/gases
  under pressure mobilize ions lowering of
  melting temperature.
• Example In subduction zone oceanic crust
  oceanic sediments seawater more volatiles,
  lower melting point. Also, lighter minerals
  (bottom of Bowens Reaction Series) melt first
  Partial Melting.
• Causes heat added pressure decreases
  volatiles increase.

30
30

• How magmas evolve (change).
• Lab experiments by N.L. Bowen, early 20th
  century, demonstrated systematic crystallization
  of minerals in a basaltic melt.
• Different presentation of Bowens Reaction Series
  on p. 117, Fig. 4.7.
• Magmatic differentiation crystal settling. If
  melt moves to another location, chemistry will be
  different from parent magma.