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Igneous Rocks
Dr. R. B. Schultz
Igneous Rocks
Igneous rocks form from molten rock (magma) crystallizing below earth's
surface or from volcanic activity. They commonly form at plate boundaries
and are commonly exposed in mountainous areas.
Igneous rocks form from crystallization of magma at depth (within the earth's
crust) or at the surface (from volcanic eruptions)
There are two (2) basic types or forms of igneous rocks:
1. Plutonic rocks = intrusive igneous rocks = igneous rocks that form from
cooling magma at depth
2. Extrusive igneous rocks = igneous rocks that form from volcanic activity (at
or near surface)
*Plutonic rocks are usually coarse-grained
*Extrusive rocks are usually fine-grained
These samples represent
igneous rocks which have
formed at depth within the
Earth, although each
exhibit different textures.
Igneous rock textures depend on cooling history
Intrusive textures:
1. Fine-grained texture (Aphanitic) -- due to fast cooling (at or near surface)
2. Coarse-grained texture (Phaneritic) -- due to slow cooling at depth
3. Porphyritic texture -- coarse crystals (phenocrysts) surrounded by finegrained matrix (groundmass)
forms due to initial slow cooling, then magma rising to (or close to) surface
and the remaining magma cooling quickly
Extrusive textures:
4. Glassy texture -- due to very rapid cooling -- magma cools so fast crystals
don't have time to form. Obsidian (volcanic glass) forms this way.
5. Vesicular texture -- full of rounded holes (vesicles) -- forms due to escape
of gas bubbles during cooling of lava. Pumice is a light-colored rock with this
vesicular texture.
6. Pyroclastic texture- chunks of molten material that fuse together
So, how does magma form?
• Magma forms in the lower crust and mantle in which high temperature melts
the rock and makes it into molten magma.
•
Rock composition, pressure, and water content influence the melting
temperature of rocks also.
•
When magma reaches the surface through a volcanic eruption, it is referred
to as lava.
Igneous Rock Textures
Coarse-grained
Glassy
Fine-grained
Vesicular
Porphyritic
Pyroclastic
lava
magma
Influences on rock melting temperature
Composition -- if rock contains a mixture of minerals it will melt
at a lower temperature compared to rocks that are monomineralic
Pressure -- high-pressure environments raise the melting
temperature of rocks
Water content -- rocks that contain some water melt at lower
temperatures than dry rocks
Magma composition
• The composition of the magma determines the composition
of the rock that forms
• Igneous rock classification scheme shows that rocks vary in
silica content (e.g. quartz-rich rocks have higher silica
content)
• Gabbros and basalts are low in silica, with bulk compositions
less than about 60% silica
• Granites and rhyolites are high in silica (bulk compositions
with higher than 60% silica)
Types of Igneous Intrusions
Igneous intrusions are rock bodies that form from crystallization of magma at depth
within earth's crust. They are categorized based on their shape and overall size.
They come in a variety of shapes and sizes:
Discordant: cut across pre-existing fabric of rock layers
Dikes are small igneous intrusions that cut across rocks into which the magma
intrudes. They are commonly sheet-like, only a few meters wide, but possibly
laterally extensive. Think of magma invading a vertical or near-vertical fracture in
rock. Igneous rock would fill the crack due to crystallization of magma. One would
call the rock body a dike.
Stocks are fairly large (10’s of miles) igneous intrusions that cut across pre-existing
rock layers. In size, they are on the order of an individual mountain peak.
Batholiths are huge igneous intrusions made of many stocks. Their size is on the scale
of an entire mountain range (100’s of miles).
Concordant: follow pre-existing fabric of rock layers without interruption of layering.
Sills are also small igneous intrusions. They are sheets of rock that, unlike dikes, are
parallel to pre-existing rocks. Think of magma invading sedimentary rocks by
spreading out between rock layers. That magma would cool to form a sill.
Laccoliths are rather large, mushroom-shaped intrusions that “puff up” in the center
due to gases.
Lopoliths are also large, inverted mushroom-shaped (spoon-shaped) intrusions that
“sag down” in the middle because of dense rocks.
Sill
Key Terminology
Plutonic
Intrusive
Extrusive
Volcanic
Texture
Phaneritic
Aphanitic
Porphyritic
Glassy
Vesicular
Pyroclastic
Magma
Lava
Bowen’s Reaction Series
Assimilation
Partial melting
Fractional crystallization
Discordant
Concordant
Dike
Stock
Batholith
Sill
Laccolith
Lopolith
Pertinent Web Sites
Ask a Geologist (USGS)
Have a geological question? Here is your chance to ask it.
Bob's Rock Shop
An interesting site for hobbyists and collectors.
Earth Resources Links (NAGT)
An extensive listing of Earth resources links arranged by topic from the National Association of Geology
Teachers (NAGT).
Electronic Volcano (Dartmouth College)
An excellent site with a wealth of information and numerous links to related sites.
Geology Central
One of the most extensive lists of geology related sites available online.
Granite Page
Everything you ever wanted to know about granites from Rob's Granite Page.
Igneous Rock Classification
A great site dealing with the classification and identification of igneous rocks hosted by James Madison
University, Virginia.
Igneous Rocks Links (NAGT)
An extensive listing of igneous rocks links arranged by topic from the National Association of Geology
Teachers (NAGT).
Igneous Rocks Tour
A good review of igneous rocks with photographs from California State University at Long Beach.
Igneous Rocks (UBC)
An overview of igneous rocks from the University of British Columbia.
Mineral and Rock Description (National Park Service)
Mineral and rock photographs and descriptions from the National Park Service.
Online Rock Tutorial from W. H. Freeman Company
An introduction to rock identification online tutorial.
Rock and Mineral Collecting (USGS)
Selected references on rocks, minerals, and gemstones from the United States Geological Survey (USGS).
Rock Classification and Identification
A good tutorial on the classification and identification of common rocks from the Ministry of Energy and Mines, British
Columbia, Canada.
Rock Cycle (Mineralogical Society of America)
The Mineralogical Society of America site offers a good, general description of the rock cycle directed toward
elementary school students.
Rock Cycle - University of British Columbia
A general introduction to the rock cycle.
Rockhounds Information Page
The Rockhounds Information Page offers many links to mineral and rock related Websites.
Rock Identification Tutorials
Rock identification tutorials (see lab notes) from Texas A & M University.
Rocks and Their Origins (UBC)
An online study of rocks and their origins at the University of British Columbia.
Rocks and the Rock Cycle Links (NAGT)
An extensive listing of rocks and the rock cycle links arranged by topic from the National Association of Geology
Teachers (NAGT).