Week 4
Igneous Rocks and Structures
Week 4
Igneous Rocks and Structures
Click
on the link below to hear a mini-lecture audio segment.
Igneous Rocks
- Igneous rocks are classified in terms of mineral content and texture.
- Igneous rocks containing the lighter colored silicate minerals high in Si+ and Al+ are called Felsic rocks.
- Igneous rocks containing the dark colored silicate minerals high in iron and magnesium are called Mafic rocks.
- The mineral content of igneous rocks is a function of the composition of the original magma from which the rock crystallizes, and the minerals present (in melted form) in the magma at the time of crystallization.
- The texture of an igneous rock is a function of its cooling history.
- If a magma cools quickly, the resulting rock displays fine grained Aphanitic texture, in which the crystal grains are too small to see with the naked eye.
- If a magma cools slowly , the resulting rock displays coarse grained Phaneritic texture, in which the individual grains are visible to the naked eye.
- If a magma has
a mixed cooling history, the resulting rock displaysPorphyritic
texture, in which coarse grained phenocrysts are
embedded in a fine grained matrix.
Granite; Quartz, feldspar, mica, plagioclase
Phaneritic texture
Rhyolite; Quartz, feldspar, mica, plagioclase
Aphanitic texture
Diorite; Quartz, plagioclase, amphibole, pyroxene, mica
Phaneritic texture
Andesite; Quartz, plagioclase, amphibole, pyroxene, mica
Aphanitic texture
Gabbro; Plagioclase, amphibole, pyroxene, olivine
Phaneritic texture
Basalt; Plagioclase, amphibole, pyroxene, olivine
Aphanitic texture
Put
on your 3D glasses and see the gallery of igneous rocks when you click the link
below.
The Bowen's Reaction Series
Each of the common silicate minerals crystallize at a characteristic temperature. The minerals also crystallize in a specific order, called the Bowen's Reaction Series. The Bowen's Reaction Series is composed of a discontinuous series containing discrete minerals, and a continuous series which contains a single mineral group with variable chemical composition.
If we walk through the crystallization of a magma, we can see how the Bowen's Reaction Series works.
Bowen's Reaction Series may be used to predict what types of igneous rocks occur in different situations and at different temperatures.
Igneous Structures
Extrusive Structures: Volcanoes
- Extrusive structures form in a variety of structures that are closely related to the type of magma being erupted, the tectonic environment, and the style of eruption.
Click
on the link below to hear a mini-lecture audio segment.
- For the most part volcanoes are closely linked to tectonic activity, or interaction between plates. This is shown in the diagram below in which volcanic activity is plotted on the map of the world. The locations of volcanic activity correspond with tectonic plate boundaries to a remarkable degree.
- When plates move apart, oceanic crust is formed by igneous activity:
- When plates move together, volcanism usually occurs at along the zone of convergence:
Shield Volcanoes
Shield Volcanoes are gently sloped large volcanoes that form on or near ocean basins. The eruption in shield volcanoes is relatively quiet and the mafic magma that is erupted forms the rock basalt.
Lava flows from shield volcanoes are often ropey (pahoehoe) like the image below.
Lava flows from shield volcanoes can also be sharp and irregular (Aa) like the image below.
Composite Volcanoes
Composite Volcanoes are moderately sloped volcanoes of moderate size that are formed by alternating layers of erupted ash and lava. The eruptions vary from quiet (producing the lava flows) to violent (producing the ash deposits). The rock type formed in composite volcanoes is typically andesite. Composite volcanoes are the most common of all volcanoes.
Composite volcanoes may erupt quietly or violently, as shown below.
Composite volcanoes may leave ash and tuff deposits like these.
Cinder Cones
Cinder Cones are small steeply sloped volcanoes that display violent eruptions of rhyolitic rock, ash, and glass. These are the rarest of the volcanoes.
Cinder cones often eject volcanic bombs, like the one shown below.
Volcanic characteristics:
Intrusive Structures
Igneous rocks that form below the surface of the Earth are called intrusive rocks and they display characteristic structures, that are related to the location of intrusion and to rock type (although less so than is the case with extrusive volcanic structures).
BathoIith
Stock
LaccoIith
SilI
Dike
The eruption of a volcano can be devastating. The ash and volcanic bombs lofted into the atmosphere in the 1815 eruption of Mt. Tambora, in Indonesia, caused the destruction of nearly 100 villages in the neighboring area and caused worldwide cooling. Fortunately, we can predict major eruptions with some accuracy and evacuate populations. In the exercise linked below, you will use data to predict the eruption of a volcano.
Igneous Rock Identification
In this week's lab you will identify unknown igneous rocks by determining their texture and mineral content. In the links below you will be asked to identify unknown rocks in the same manner.
Important Web Sites
Visit the web sites below for more information on Igneous rocks and structures.
http://geology.csupomona.edu/alert/igneous/ignrxs.htm
http://geology.csupomona.edu/alert/igneous/ignrxs.htm
