Igneous Rocks

ESA Logo

fossils and expeditions...

Igneous Rocks

source http://www.columbia.edu/~vjd1/igneous.htm

basalt lava flow

Basic features of Igneous Rocks
Bowens reaction series
Types of intrusive igneous rock environments
Types of extrusive igneous rock environments
Some common igneous rocks and descriptions
Igneous rocks named on textural criteria
Common igneous textures
    see also E-Learning -  Igneous Rocks...   
    see also Processes - Volcanic Processes...
    see also Processes - Glossary of Volcanic Terms...
    see also Petrology - Igneous Rock Classification...
    see also Petrology - Origins of Magma and Magmatic Differentiation...

igneous rocks animation
animation of igneous processes

Basic Features of Igneous Rocks

Of the three principal rock types (igneous, sedimentary, and metamorphic), igneous rocks can be thought of as "primary" rocks because they crystallise from a liquid.
Sedimentary and metamorphic rocks may be thought of as derivative rocks.

Igneous rocks are rocks formed from the crystallization of a liquid (molten rock). Igneous rocks may be divided into three -  categories.
The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt:
crysatal size and composition  with cooling

In addition to texture, igneous rocks may are classified according to their chemical composition.
The most general classification is based on the relative abundance in a rock of felsic (feldspar and silica-quartz) minerals vs mafic( magnesium and ferrum or iron) minerals.

identification by crystal size, felsic / mafic composition and colour
Identification by crystal size, felsic / mafic composition and colour

igneous rock identification based on percent composition of most common minerals
A more detailed identification chart based on felsic / mafic mineral composition

rock cycle

Bowen's Reaction Series

can be thought of as an idealized order of crystallization of a cooling magma.
However, not all of these minerals will be crystallised together in the same rock.

  bowen's reaction series

another view of Bowen's Reaction Series

This system appears straight forward but as magmas cool their chemical composition changes as higher temperature elements drop out of the solution as minerals.
So for example a mafic magma can turn into a felsic magma as it cools...

mafic magma turns to felsic magma as it cools

Types of Intrusive Igneous Rock Environments

types of igneous intrusions

Types of Extrusive Igneous Rock Environments

types of extrusive igneous rock environments

Some common igneous rocks and descriptions

Intrusive Igneous Rocks
Extrusive Igneous Rocks

Igneous rocks named on textural criteria:

click on the links below to see an photo and a description of theses rocks...

Common igneous textures:

Allotriomorphic-granular (gabbroic) texture:
 is a feature of phaneritic rocks in which all of the minerals have anhedral shapes; it is characteristic of some mafic and ultramafic rocks, such as dunite and pyroxenite.
Hypidiomorphic-granular (granitic) texture:
igneous texture in which most of the mineral grains are subhedral. Typical of granite, granodiorite, quartz monzonite, etc.
Porphyritic texture:
rocks in which larger grains are contained in a finer grained matrix. May occur in either intrusive or extrusive rocks, but it is most common in extrusive rocks such as basalt, andesite, dacite, and rhyolite.
Pyroclastic texture:
produced by explosive volcanism, broken clasts of mineral, lithic clasts, and glass shards are common. The clasts may be either sorted or unsorted. Common in tuffs.
Intergranular texture:
plagioclase laths with interstitial pyroxene grains that are smaller than the plagioclase; commonly found in basalts.
Intersertal texture:
Small feldspars with glass or altered glass interstitial to the feldspars; common texture of basalts.
Ophitic texture:
Pyroxene grains partially or completely surround plagioclase laths; common texture of gabbros and basalts.
Trachytic texture:
Subparallel feldspars formed during flow in volcanic rocks.
Spinifex texture:
Interlacing olivine or pyroxene, a texture formed by quenching in komatiites.

Other features seen are: