Igneous Rocks: Forged By Fire

1
Igneous Rocks Forged By Fire
2

• Igneous rocks (from the Latin ignis, or fire)
  form as molten rock cools and solidifies.

- Igneous rocks and metamorphic rocks, derived
from igneous parents, make up about 95 percent
of the Earths crust.
- The mantle, which accounts for more than 82
percent of the Earths volume, is also composed
of igneous rock. Thus, Earth can be described as
a huge mass of igneous rocks covered with a thin
veneer of sedimentary rocks and having a
relatively small iron-rich core.
3
Where Do Igneous Rocks Come From?
Igneous rocks form from a material called magma,
which is created when intense heat and pressures
melt solid rock located in the crust and upper
mantle of the Earth.

• General characteristics of magma
• Parent material of igneous rocks
• Forms from partial melting of rocks inside Earth
• Magma that reaches the surface is called lava

4
How Does Magma Make Igneous Rocks?
- Igneous rocks can be described as intrusive and
plutonic (formed from magma inside the Earth), or
extrusive and volcanic (formed from lava above
the Earths surface).

• - Igneous rocks that form below the Earths
  surface are called intrusive igneous rocks (or
  plutonic). The word plutonic comes from Pluto,
  the name for the Greek god of the underworld.
• - They form when magma enters a pocket or
  chamber underground that is relatively cool and
  solidifies into crystals as it cools very slowly.

5
Characteristics of Magma

• Magma consists of three components
• A liquid portion, called melt, that is composed
  of mobile ions
• Solids, if any, are silicate minerals that have
  already crystallized from the melt
• Volatiles, which are gases dissolved in the melt,
  including water vapor (H2O), carbon dioxide
  (CO2), and sulfur dioxide (SO2)

6
Characteristics of Magma
- A major portion of all magma is silica, which
is a compound of silicon (the second most
abundant element on Earth), and oxygen (the most
abundant element on Earth). Magma also contains
gases, which expand as the magma rises.
- Magma that is high in silica resists flowing,
so expanding gases are trapped in it. Pressure
builds up until the gases blast out in a violent,
dangerous explosion.
- Magma that is relatively poor in silica flows
easily, so gas bubbles move up through it and
escape fairly gently.
7
Characteristics of Magma

• Role of heat
• - Temperature increases within Earths upper
  crust (called the geothermal gradient) average
  between 20oC to 30oC per kilometer
• - Rocks in the lower crust and upper mantle are
  near their melting points
• - Any additional heat (from rocks descending into
  the mantle or rising heat from the mantle) may
  induce melting

8
Characteristics of Magma

• Role of pressure
• - An increase in confining pressure causes an
  increase in a rocks melting temperature or
  conversely, reducing the pressure lowers the
  melting temperature

- When confining pressures drop,
decompression melting occurs
9
Characteristics of Magma

• Role of volatiles
• Volatiles (primarily water) cause rocks to melt
  at lower temperatures
• This is particularly important where oceanic
  lithosphere descends into the mantle

10
Characteristics of Magma
Sometimes as it is forming, magma can change its
composition. This may result in different types
of magma coming out in one volcanic event.

• - Assimilation Changing a magmas composition by
  the incorporation of foreign matter (surrounding
  rock bodies) into a magma

- Magmatic differentiation Separation of a melt
from earlier formed crystals to form a different
composition of magma
11
Geology and Chemistry Connect!

• When scientists began observing different
  compositions of lavas coming from various
  volcanic events, this led them to wonder if there
  was a way to use this information to learn more
  about the various igneous rocks that form.
• One scientist, N.L. Bowen, used this information
  to demonstrate that as a magma cools, minerals
  crystallize in a systematic fashion based on
  their melting points.
• Bowens reaction series, as it is called today,
  shows us temperature and mineral content are the
  main determinants of how crystal structures will
  change and evolve (and ultimately, which type of
  chemical composition an igneous rock will have).

12

• During crystallization (as the magma cools), the
  composition of the liquid portion of the magma
  continually changes. At the higher temperatures
  associated with mafic and intermediate magmas,
  the general progression can be separated into two
  branches the continuous branch and the
  discontinuous branch.

13
Bowens Reaction Series Explained
The continuous branch describes the evolution of
the plagioclase feldspars as they evolve from
being calcium-rich to more sodium-rich.
14
Bowens Reaction Series Explained

• The discontinuous branch describes the formation
  of the mafic minerals olivine, pyroxene,
  amphibole, and biotite mica.

15
Bowens Reaction Series Explained

• The weird thing that Bowen found concerned the
  discontinuous branch.
• At a certain temperature a magma might produce
  olivine, but if that same magma was allowed to
  cool further, the olivine would "react" with the
  residual magma, and change to the next mineral on
  the series (in this case pyroxene). Continue
  cooling and the pyroxene would convert to
  amphibole, and then to biotite.
• Mighty strange stuff, but if you consider that
  most silicate minerals are made from slightly
  different proportions of the same 8 elements (O,
  Si, Al, Fe, Ca, Na, K, Mg), all we're really
  doing here is adjusting the internal crystalline
  lattice to achieve stability at different
  temperatures. Really no big deal.

16
So, What Does This Mean For Igneous
Rocks?

• Understanding the way magma forms and solidifies
  helps us know WHY we have different textures and
  compositions for various igneous rocks.

17
Properties of Igneous Rocks

• Igneous rocks are composed primarily of silicate
  minerals
• Dark (or ferromagnesian) silicates

Biotite Mica

• Light (or nonferromagnesian) silicates

Quartz
Muscovite Mica
Feldspar
18
Properties of Igneous Rocks
19
Main Differences of Igneous Rocks

• Granitic composition
• Composed of light-colored silicates
• Felsic (feldspar and silica) in composition
• Contains high amounts of silica (SiO2)
• Major constituents of continental crust

• Granitic magmas are higher in silica and
  therefore more viscous than other magmas
• Because of their viscosity, they lose their
  mobility before reaching the surface and tend to
  produce large plutonic structures

20
Main Differences of Igneous Rocks

• Basaltic composition
• Composed of dark silicates and calcium-rich
  feldspar
• Mafic (magnesium and ferrum, for iron) in
  composition
• More dense than granitic rocks
• Comprise the ocean floor as well as many volcanic
  islands

• - Basaltic magmas form at mid-ocean ridges by
  decompression melting or at subduction zones
• - Large outpourings of basaltic magma are common
  at Earths surface

21
Main Differences of Igneous Rocks

• Other compositional groups
• Intermediate (or andesitic) composition
• Contain at least 25 percent dark silicate
  minerals
• Associated with explosive volcanic activity
• Ultramafic composition
• Rare composition that is high in magnesium and
  iron
• Composed entirely of ferromagnesian silicates

22
Physical Properties of Igneous Rocks
Texture in igneous rocks is determined by the
size and arrangement of mineral grains, which is
usually determined by when/how a rock and
crystals were formed.
23
Factors That Determine Crystal Size

• Rate of cooling
• Slow rate promotes the growth of fewer but larger
  crystals
• Fast rate forms many small crystals
• Very fast rate forms glass
• Amount of silica (SiO2) present
• Amount of dissolved gases

24
Types of Igneous Textures

• Aphanitic (fine-grained) texture
• Rapid rate of cooling of lava or magma
• Microscopic crystals
• May contain vesicles (holes from gas bubbles)
• Phaneritic (coarse-grained) texture
• Slow cooling
• Crystals can be identified without a microscope

Aphanitic Basalt
Phaneritic Diorite
25
Types of Igneous Textures

• Porphyritic texture
• Minerals form at different temperatures as well
  as differing rates
• Large crystals, called phenocrysts, are embedded
  in a matrix of smaller crystals, called the
  groundmass
• Glassy texture
• Very rapid cooling of molten rock
• Resulting rock is called obsidian

Porphyritic Andesite
Glassy Obsidian
26
Types of Igneous Textures

• Pyroclastic texture
• Various fragments ejected during a violent
  volcanic eruption
• Textures often appear to more similar to
  sedimentary rocks
• Pegmatitic texture
• Exceptionally coarse grained
• Form in late stages of crystallization of
  granitic magmas

Pyroclastic Tuff
Pegmatite