G103
Week Three
Igneous Rocks
and Processes
Click on the link below to hear a mini-lecture audio segment.
Igneous Rocks
Igneous Rock Formation
- Although we think of volcanic eruption as a rare and violent event igneous rocks are forming constantly in various places all over the world.
Hawaii has several active lava flows.
Iceland is growing a few centimeters each year due to volcanism
- Igneous rocks form when a magma (a melted rock body) moves towards the surface of the earth and crystallizes below or above the ground.
- We will begin our examination of Igneous rocks with a look at the structures an processes that produce them.
Igneous Processes 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.
- For the most part volcanoes are closely linked to tectonic activity, or interaction between plates.
- 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.
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.
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.
Volcanic characteristics:
Volcanic Eruption Features
Volcanic eruptions produce a variety of special textures and materials that are ejected from the volcano, especially if the eruption is violent.
These include:
- Ash is a fine grained cloud of volcanic dust and glass that forms layered deposits. After the Eruption of Mt. Saint Helens in 1980 ash deposits from the volcano were detected up to 1,000 miles from the site.
- Pyroclasts are small rock or glass particles that are ejected from the volcano, often high into the sky. These may include pumice, obsidian (volcanic glass), and welded ash.
Volcanic Bombs
Pumice
- Obsidian and pillow shaped formations called pillow basalts also forms when lava is erupted into cold seawater.
- Lava flows can vary significantly also. Lava flows which are mostly solidified and pushed forward in broken rubble is called Aa.
AA lava.
- Lava flows that are formed when fully liquid lava crystallizes quickly into a ropy surface are called Pahoehoe.
Pahoehoe lava
Put on your 3D glasses and click the link below.
Volcanic Hazards
The prediction of volcanic eruptions is important because, unlike earthquakes, you cannot provide engineering solutions to offset the potential damage. Volcanoes are so violent that they cannot be withstood by building stronger buildings and structures. Evacuation is the only recourse. In the exercise below, you will be asked to observe and active volcano and predict its eruption.
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
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
In this weeks lab you will identify unknown igneous rocks by evaluating their texture and minerals content. In the exercises linked below you will do virtual identification of unknown igneous rocks.
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.
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