Igneous Rocks

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Igneous Rocks

Cool from the Molten State

Large Grain Size ----> Slow Cooling

Porphyritic Texture:
Large Crystals in Fine-grained Setting

Igneous Rock Classification

  1. How Much Silica?
    Account for Si
    • Excess - Rock Has Quartz
    • Just Enough to Form Other Silicates
    • Deficient - Silica - Poor Minerals (Like Olivine)
  2. What Feldspars?
    Account for Al, Ca, K, Na
    • Potash Feldspar KAlSi3O8
    • Plagioclase Series
  3. What Other Minerals Are Present?
    Account for Fe, Mg


K - Feldspar:

KAlSi3O8: Several Slightly Different Forms:

Plagioclase (Solid Solution)

Bowen's Reaction Series

The geologist N.L. Bowen found that minerals tend to form in specific sequences in igneous rocks, and these sequences could be assembled into a composite sequence. Bowen's series

No igneous rock ever displays the whole sequence. Igneous rocks display a slice across the sequence. Basalt, for example, typically has olivine and calcium plagioclase forming first, followed by pyroxene and more sodium-rich plagioclase. In granite, sodium plagioclase and biotite typically form first, followed by muscovite, potassium feldspar, and last of all quartz. The sketch below turns the series on its side. It's actually a more realistic view since successive minerals often form simultaneously.

Bowen's series

Bowen's Series and Igneous Rocks

Mineral Composition
Ca  Plagioclase Na  Plagioclase K - Feldspar Muscovite
Olivine Pyroxene Amphibole Biotite
Volcanic Rocks
(Rare)  Basalt  Andesite     


Plutonic Rocks
Dunite Gabbro  Diorite                 Granite
1200 C  Melting Point    700 C
Heavy  Density   Light
Mg, Fe  Rich In...   Si, Na, K 
Fluid  Lava Is... Viscous
Mild  Eruptions   Violent 
Type of Volcano
Shield Volcano Stratovolcano

Plug Dome

Rapid    Weathering Slow 
Usually Dark   Color Often Light 

Some Igneous Rocks Are Named on Textural Criteria:

Types of Volcanoes

types of volcanoes

Products of Eruptions

Environmental Hazards of Volcanoes

Greatest Earthquakes and Volcanic Eruptions

Nuee Ardente (French: Fiery Cloud) or Pyroclastic Flow

pyroclastic flow

  1. Gas Expands as Lava Rises
  2. Lava Breaks up into Fragments Supported by Escaping Gas
  3. Cloud Flows Downhill at 60-100 M.p.h. Temperature about 1000 C

How Calderas Form

caldera formation

Calderas form when volcanoes collapse. In some cases, violent explosive eruptions (left) can empty a magma chamber enough that the summit collapses. In other cases, magma may erupt on the flanks of a volcano or drain back to deeper levels, permitting the summit to subside (right). These caldera collapses are generally not violent.

Evolution of Volcanoes

An active volcanic landscape

volcanic landscape

A volcanic landscape after a million years or so

volcanic landscape after a million years

This figure shows some of the things that can happen to a volcanic area over time:


Principal Types of Intrusion

types of intrusions

Tabular and Irregular Intrusions

Tabular intrusions are sheetlike, and consist of dikes and sills. All others are irregular.

Concordant versus Discordant Intrusions

Concordant intrusions are parallel to layers in the rocks

Discordant intrusions cut across layers.

Structure of Batholiths

structure of a batholith

In the above diagram, non-batholithic rocks are shown in light blue.

Geological cross-sections often show batholiths as extending downward indefinitely. In fact, geophysical techniques and field observations suggest that most batholiths are thick lenses 10 km thick or so. Many batholiths have some of the former roof rocks still present as roof pendants. Isolated masses of rock that were trapped in the magma are called xenoliths (Greek for "foreign stone"). Most batholiths are composite and actually composed of many smaller intrusions, as shown here.

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Created 3 Feb 1997, Last Update 18 Sep 1997