Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Cool from the Molten State
- Volcanic -- Erupted on Surface
- Plutonic -- Solidify Within Earth
Large Grain Size ----> Slow Cooling
- Volcanic Rocks -- Fine Grained
- Plutonic Rocks -- Coarse Grained
Large Crystals in Fine-grained Setting
- Slow Initial Cooling
- Rapid Final Cooling
Igneous Rock Classification
- How Much Silica?
Account for Si
- Excess - Rock Has Quartz
- Just Enough to Form Other Silicates
- Deficient - Silica - Poor Minerals (Like Olivine)
- What Feldspars?
Account for Al, Ca, K, Na
- Potash Feldspar KAlSi3O8
- Plagioclase Series
- What Other Minerals Are Present?
Account for Fe, Mg
K - Feldspar:
KAlSi3O8: Several Slightly Different Forms:
Plagioclase (Solid Solution)
- Albite: NaAlSi3O8
- Anorthite: CaAl2Si2O8
- Any Mixture of the Two Is Possible
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.
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
Bowen's Series and Igneous Rocks
||K - Feldspar
||Si, Na, K
||Type of Volcano
Some Igneous Rocks Are Named on Textural Criteria:
- Scoria: Porous
- Obsidian - Glass
- Tuff - Cemented Ash
- Breccia - Cemented Fragments
- Pegmatite - Extremely Large Crystals
- Aplite - Sugary Texture, Quartz & Feldspar
- Porphyry - Fine Matrix, Large Crystals
Types of Volcanoes
Products of Eruptions
- Lava Flows
- Pyroclastic Debris
- Carbon Dioxide
Environmental Hazards of Volcanoes
- Ash Falls
- Building Collapse
- Crop Destruction
- Direct Damage (Colombia, 1985)
- Floods (Several Types)
- Lava Flows
- Falling Ejecta
- Blast (Mt. St. Helens, 1980)
- Nuee Ardente (St. Pierre, 1902)
- Gas (Lake Nyos, Cameroon, 1986)
Greatest Earthquakes and
Nuee Ardente (French: Fiery Cloud) or Pyroclastic Flow
- Gas Expands as Lava Rises
- Lava Breaks up into Fragments Supported by Escaping Gas
- Cloud Flows Downhill at 60-100 M.p.h. Temperature about
How Calderas Form
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
A volcanic landscape after a million years or so
This figure shows some of the things that can happen to a
volcanic area over time:
- Erosion of a volcano, leaving solidified magma in the
conduit as a volcanic neck.
- Collapse of a volcano in a catastrophic landslide, now
recognized as a common event in the lives of volcanoes.
- Collapse of a volcanic summit to form a caldera and
perhaps a "crater" lake.
- Erosion to leave lava-capped hills as mesas, or former
valley-filling flows as ridges.
Principal Types of Intrusion
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
- Sills are thin sheets of magma between layers.
- Laccoliths are blister-like intrusions, usually
near the surface, that form when magma pushes overlying
- Lopoliths are mega-sills, usually of gabbro or
diorite, that may cover hundreds of square kilometers and
be kilometers thick. They often have a concave structure
and are differentiated. That is, they take so long to
harden that heavy minerals have a chance to sink and
light minerals can rise.
Discordant intrusions cut across layers.
- Dikes are thin sheets of magma intruded into
fractures in the crust.
- Stocks or plutons are small irregular
- Batholiths are usually granitic and cover hundreds
or thousands of square kilometers.
Structure of Batholiths
In the above diagram, non-batholithic rocks are shown in light
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
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