Bibliografia - Rochas Magmaticas Pamela Gore
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Pamela J. W. Gore
Georgia Perimeter College
Origin
"fire-formed rocks"
Crystallize from molten material:
• Magma - below the Earth's surface
• Lava - erupts onto the Earth's surface through a volcano or crack (fissure)
Lava cools more quickly because it is on the surface.
Cooling Rates
Cooling rates influence the texture if the igneous rock:
• Quick cooling = fine grains
• Slow cooling = coarse grains
Igneous rocks are classified on their texture and their composition.
Igneous textures:
• Glassy - instantaneous cooling
o Obsidian = volcanic glass
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Obsidian
• Aphanitic - fine grain size (< 1 mm); result of quick cooling
Rhyolite
o Basalt
o Rhyolite
o Andesite
o
• Phaneritic - coarse grain size; visible grains (1-10 mm); result of slow cooling
Granite - polished
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o Granite
o Diorite
o Gabbro
o
• Pegmatitic - very large crystals (many over 2 cm)
o Granite pegmatite or pegmatitic granite
• Porphyritic- Mixture of grain sizes caused by mixed cooling history; slow cooling
first, followed by a period of somewhat faster cooling.
o Terms for the textural components:
Phenocrysts - the large crystals Groundmass or matrix - the finer crystals surrounding the large
crystals. The groundmass may be either aphanitic or phaneritic.
o Types of porphyritic textures:
Porphyritic-aphanitic
Porphyritic-phaneritic
o Origin: mixed grain sizes and hence cooling rates, imply upward movement of
magma from a deeper (hotter) location of extremely slow cooling, to either:
a much shallower (cooler) location with fast cooling (porphyritic-
aphanitic), or
a somewhat shallower (slightly cooler) location with continued fairly
slow cooling (porphyritic-phaneritic).
o Rock name = porphyry
Granite porphyry or porphyritic granite (porphyritic-phaneritic) -
phenocrysts usually potassium feldspar
Granite porphyry
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Andesite porphyry or porphyritic andesite (porphyritic-aphanitic) -
phenocrysts usually hornblende (amphibole)
Rhyolite porphyry or porphyritic rhyolite (porphyritic-aphanitic)
Rhyolite
Porphyry
• Vesicular - contains tiny holes called vesicles which formed due to gas bubbles in
the lava or magma. Very porous. May resemble a sponge. Commonly low density;
may float on water.
o Vesicular basalt - basalt with a vesicles, which may be quite large. Sometimes
lined with crystals to form geodes.
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Vesicular basalt
Vesicular basalt with olivine phenocrysts, building stone at Hawaiian Volcano
Observatory, Big Island of Hawaii
o Pumice - light in color; white to gray; may be glassy or dull. Fully riddled with
holes. Very sponge-like. Floats. Used as an abrasive. (Pumice stone, Lava
Soap).
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Pumice
Pumice
o Scoria - dark in color; brown, black, or dark red; similar to vesicular basalt but
is fully riddled with holes to form a spongy mass. (May find in barbecue grills
as lava rock).
Scoria
• Pyroclastic or Fragmental - pieces of rock and ash come out of a volcano and get
welded together by heat. May resemble rhyolite or andesite, but close examination
shows pieces of fine-grained rock fragments in it. May also resemble a sedimentary
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conglomerate or breccia, except that rock fragments are all fine-grained igneous or
vesicular.
o Tuff - made of volcanic ash
o Volcanic breccia - contains fragments of fine-grained igneous rocks that are
larger than ash.
Pyroclastic rock
Composition of Igneous Rocks
Igneous rocks can be placed into four groups based on their chemical compositions:
1. Sialic (or granitic or felsic)
1. Dominated by silicon and aluminum (SiAl)
2. Usually light in color
3. Characteristic of continental crust
4. Forms a stiff (viscous) lava or magma
5. Rock types include:
1. Granite
Granite
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2. Rhyolite
Rhyolite
6. Minerals commonly present include:
1. potassium feldspar (generally pink or white)
2. Na-plagioclase feldspar (generally white)
3. quartz (generally gray or colorless)
4. biotite
5. amphibole?
6. muscovite?
2. Intermediate (or andesitic)
1. Intermediate in composition between sialic and mafic
2. Rock types include:
1. Andesite (aphanitic)
2. Diorite (phaneritic)
Diorite
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3. Minerals commonly present include:
1. plagioclase feldspar
2. amphibole
3. pyroxene
4. biotite
5. quartz
3. Mafic (or basaltic)
1. Contains abundant ferromagnesian minerals (magnesium and iron silicates)
2. Usually dark in color (dark gray to black)
3. Characteristic of Earth's oceanic crust, Hawaiian volcanoes4. Forms a runny (low viscosity) lava
5. Also found on the Moon, Mars, and Venus
6. Rock types include:
1. Basalt (aphanitic)
Basalt
2. Gabbro (phaneritic)
Gabbro
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3. Diabase - texture intermediate between basalt and gabbro;
characteristic of Early Mesozoic dikes in eastern North America.
7. Minerals commonly present include:
1. Ca-plagioclase feldspar
2. pyroxene
3. olivine
4. amphibole
4. Ultramafic
1. Almost entirely magnesium and iron silicates (ferromagnesian minerals)
2. Rarely observed on the Earth's surface3. Believed to be major constituent of Earth's mantle
4. Commonly found as xenoliths in basaltic lavas
5. Rock types include:
1. Peridotite (phaneritic)
1. dominated by olivine - the birthstone is Peridot , which gives its
name to Peridotite
Peridotite
6. Minerals commonly present include:
1. Olivine is dominant. (Olivine is olive green).
2. may have minor amounts of pyroxene and Ca-plagioclase
Other types of igneous rock:
Syenite
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A polished syenite called larvikite with centimeter- to inch-scale gray to blue plagioclase
crystals. The industrial name for the rocks is "blue pearl". Photographed in an above-ground
cemetery in New Orleans, LA
Bowen's Reaction Series
Bowen's Reaction Series is a listing of the most common silicate minerals in igneous rocks,
in the order in which they crystallize from a magma, as the temperature decreases and the
magma cools.
Bowen's Reaction Series also lists the minerals in the order in which they melt with
increasing temperatures.
Minerals higher on the reaction series crystallize before minerals lower on the reaction
series. The earlier-formed minerals react with the magma to form minerals lower on the
series. The process ends when the magma has completely crystallized. The composition of
the resulting igneous rock depends on the composition of the magma.
Visit this interactive web site to see the minerals of Bowen's Reaction Series (in order)
http://www.cat.cc.md.us/courses/eas101/unit1/brs.html.
Bowen's Reaction Series has two branches. They are:
1. Discontinuous reaction series, from olivine to biotite,.
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2. Continuous reaction series, from Ca plagioclase to Na plagioclase.
The discontinuous reaction series involves the dark-colored ferromagnesian minerals:
1. olivine2. pyroxene
3. amphibole
4. biotite.
As a magma cools, olivine crystallizes first. The olivine crystals react with the remaining
magma to form pyroxene. Pyroxene reacts with the magma to form amphibole. Amphibole
reacts with the magma to form biotite. Each successive mineral, from olivine to biotite, has a
different composition and a different silicate crystal structure. As crystallization
proceeds, the crystal structures become more complex (olivine has an isolated tetrahedral
structure, pyroxene has a single chain structure, amphibole has a double chain structure,
and biotite has a sheet structure). The series of minerals is called discontinuous because a
series of different minerals is formed, each with a different crystal structure.
The continuous reaction series involves the plagioclase feldspars. Plagioclase feldspars
are an example of a "solid solution series", exhibiting gradations in chemical and physical
properties. Chemically, this series consists of two "end members":
1. albite or Na plagioclase (NaAlSi3O8), the sodium "end member", and
2. anorthite or Ca plagioclase (CaAlSi2O8), the calcium "end member".
There is a continuous chemical and physical gradation between the two end members.
(Various plagioclase mineral names are given, based on the percentages of calcium and
sodium present, including anorthite, bytownite, labradorite, andesine, oligoclase, and albite).
Ca-plagioclase is the first to crystallize. It reacts with the melt to become more sodium
rich. (If reaction is not complete, a zoned plagioclase crystal results which has a calcium-rich
center and sodium-rich edges).
This series of plagioclase minerals is called continuous because all of the plagioclase
minerals have the same crystal structure. The minerals differ primarily in the proportions
of calcium and sodium present.
During the last stages of crystallization, potassium feldspar (KAlSi308) crystallizes.
Muscovite may also form. If the remaining melt contains excess silica, quartz will crystallize.
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Bowen's Reaction Series helps us to understand why certain minerals tend to occur
together in igneous rocks. For example, the mafic rocks, basalt and gabbro tend to contain
olivine, pyroxene, and calcium-rich plagioclase feldspar. These are all minerals which
crystallize at high temperatures. As another example, felsic or sialic rocks such as granite
and rhyolite tend to contain quartz, potassium feldspar, sodium-rich plagioclase feldspar, and
sometimes muscovite. These are minerals which crystallize at lower temperatures. The
minerals that ultimately form are controlled by the initial composition of the magma.
Bowen's Reaction Series also helps us to understand why certain minerals do NOT occur
together in igneous rocks. For example, olivine and quartz are unlikely to occur in the same
igneous rock, because olivine is a high temperature mineral, and quartz is a low temperature
mineral.
Bowen's Reaction Series also shows us that the range of igneous rocks, from ultramafic
to sialic (or felsic), can be produced by the same original mafic magma . The magma
changes as it cools. As a magma cools, the early-formed crystals may settle to the bottom of
the magma chamber . This would produce a rock type at the bottom of the magma chamber
that is dominated by early-formed minerals such as olivine, pyroxene, and calcic plagioclase
(a mafic or ultramafic rock). The remaining melt would be enriched in silica (relative to its
original composition), and may continue moving upward toward the earth's surface. Crystal
settling may occur again, producing an intermediate rock. As the last remaining melt moves
toward the Earth's surface, it will crystallize to produce a sialic or felsic rock. The removal of
crystals from the magma by settling (or other processes) is known as fractional
crystallization.
The formation of several different rock types from one initial magma, through separation of
earlier-formed crystals, causing the magma to evolve to become more silica-rich, is known as
magmatic differentiation. Magmatic differentiation can produce a variety of types of
igneous rocks through evolution of the original parent magma.
Memorizing Bowen's Reaction Series
You need to memorize Bowen's Reaction Series. It helps to use a mnemonic device (or
phrase using the initial letters of each mineral) to help remember the names of the minerals
in order. Remember the Y-like shape. For the discontinuous reaction series and the minerals
in the "trunk" of the Y, you would have the following first letters of mineral names: O P A B K
M Q. (K is for potassium feldspar, which is abbreviated with the chemical symbol, K.) Try to
make up your own mnemonic device or phrase, but here is one that one of my students
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made up. I don't like it, but I remember it. Old People Are Bad; Keep Mother Quiet. Hopefully
you can make up a better one.
You have to know the series AND understand the concepts of how Bowen's Reaction Series
relates to melting and to crystallization, and to the origin of igneous rocks of various
composition.
Plutons
Subsurface igneous bodies
Named for Pluto, Roman god of the underworld
• Concordant Plutons
Oriented parallel to surrounding layered rocks. Formed from magma that is injected
between the layers of the surrounding rock.
o sill
o laccolith
• Discordant Plutons
Oriented at an angle to surrounding layered rocks. Magmas cut through the existing
layering.
o dike - less than 1 cm (less than 0.5 inch) to 1 km (0.6 mi)
o stock - 1 km2 (less than 1 mi2) to 100 km2 (40 mi2)
o batholith - more than 100 km2 (40 mi2)
Xenoliths
When magma moves upward through pre-existing rocks, some of the surrounding rocks are
melted and become assimilated into the magma. In other cases, blocks of the surrounding
rocks are broken off and carried along by the magma, unmelted. These blocks are called
xenoliths, a word meaning "foreign rocks". The rocks that make up the xenoliths would be
carried along without melting if the temperature of the magma is not high enough to melt the
rock (for example, if the minerals in the rock were higher on Bowen's reaction series).