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. 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)
Bowen's Reaction Series
Professor Norman L. Bowen summarized results of experiments
done early in the 1900’s on crystallization of granitic
magmas. These experiments showed that there is a
sequence of minerals that crystallize as the temperature of
magma is lowered:
Start with a collection of molten magma and progressively cool
it. Minerals will crystallize (solidify) in a definite sequence.
Bowen’s Reaction Series represents that sequence that has
implications for other types of rocks as well, although it is
only used to determine the crystallization sequence in a
molten magma (intrusive igneous rocks).
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.
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