Download Igneous rocks

Chapter 3
Igneous Rocks, Intrusive Activity,
and the Origin of Igneous Rocks
Photo credit: G. Mattioli
The Rock Cycle
• A Plate Tectonic Example
Igneous Rocks
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Igneous Rock Textures
Identification of Igneous Rocks
Varieties of Granite
Chemistry of Igneous Rocks
Fundamental Questions
• How are rocks sampled in the field and analyzed in the lab
to determine their chemical, modal, and mineralogical
composition?
• What do these analyses tell us about the composition of
magmatic rocks?
• How can the data be presented to elucidate compositional
patterns and contrasts?
• How do we classify magmatic rocks to convey meaningful
petrogenetic information on the origin and evolution of the
magma from which they solidified?
The Rock Cycle
Intrusive Rocks in Northern Victoria Land, Antartica
Plate Tectonics and the Rock Cycle
Igneous Rocks: Terminology
• Igneous rocks are formed as a result of cooling
and crystallization from a magma.
• Magma is molten rock (fluid), rich in silica
(SiO2), which contains dissolved volatiles (e.g.
CO2 and H2O).
• Lava is magma extruded on or very near the
Earth’s surface. Most lavas have been
significantly degassed en route to the surface.
Classification of Igneous and Volcanic Rocks
• Based on hand specimen fabric
• Based on field relationships and textures
• Based on mineralogy and chemistry
– Color Index
Classification Schemes I
• Based on Fabric
– Phaneritic: rocks with mineral grains that are
large enough to be identified by eye. Texture is
typical of slowly cooled intrusive rocks.
– Aphanitic: rocks with grain too small to be
identified by eye. Texture is most common in
rapidly solidified extruded magma and
marginal facies of shallow intrusions.
Classification based on Field Relations and Textures
• Extrusive or volcanic rocks: typically aphanitic or glassy. This means
that they are generally fine grained in texture. Grains are typically 0.5
to 1 mm. Common example is basalt.
– Many varieties are porphyritic. This means that the grain size is bimodal,
with a fine grained matrix surrounding larger grains that are called
phenocrysts. Common example is andesite.
• Intrusive or plutonic rocks: typically phaneritic. This means that they
are generally coarse grained and this texture is often quite uniform.
Grains can range in size but are often clearly visible to the naked eye
(>2-3 mm). Common example is granite.
– Amphiboles and biotites are commonly altered to chlorite. Muscovite
found in some granites, but rarely in volcanic rocks. Perthitic feldspar,
reflecting slow cooling and exsolution, is widespread.
Rhyolite Hand Specimen
Granite Hand Specimen
More on Fabric Classification
• Porphyritic texture: magmatic rocks with bimodal grain
size distributions.
– Larger grains are called phenocrysts
– Smaller grains constitute the groundmass or matrix
– Porphyritic aphanitic rocks are more common than porphyritic
phaneritic rocks
• Glassy or vitric texture: rocks that contain variable
proportions of glass.
– Holocrystalline rocks: wholly composed of crystals
– Vitrophyric rocks: porphyritic rock with phenocrysts in a glassy
matrix
Magmatic Intrusion
Deeply Eroded Intrusions
GRANITE INTRUSION
SHALE COUNTRY ROCK
Torres del Paine, Chile
Coarse Grained Intrusive Rock Texture
Potassium Feldspar (stained yellow)
Photomicrograph - Phaneritic Texture
Interlocking grains
Photo credit: C.C. Plummer
Matrix or groundmass
Plagioclase Feldspar
phenocrysts
Andesite Hand Specimen
Interlocking grains
with uniform size
Diorite Hand Specimen
Classification based on Mineralogy & Chemistry
• Felsic rocks: mnemonic based on feldspar and
silica. Also applies to rocks containing abundant
feldspathoids, such as nepheline. GRANITE
• Mafic rocks: mnemonic based on magnesium and
ferrous/ferric. Synonymous with ferromagnesian,
which refers to biotite, amphibole, pyroxene,
olivine, and Fe-Ti oxides. BASALT
• Ultramafic rocks: very rich in Mg and Fe.
Generally have little feldspar. PERIDOTITE
• Silicic rocks: dominated by quartz and alkali fsp.
Sometimes referred to as sialic (Si + Al).
Color Index
• Defined as the modal proportion of darkcolored minerals in a rock. Should really be
based on the proportion of ferromagnesian
minerals as feldspars may range in color.
– Leucocratic: 0-30% mafics
– Melanocratic: 60-100% mafics
Gabbro Hand Specimen
Fined grained mafic volcanic rock
Basalt Hand Specimen
Summary Classification for Igneous Rocks
Igneous Rock Identification: Summary
Intrusive Bodies
• Shallow Intrusive Structures
– Sills
– Dikes
– Volcanic necks
• Intrusive Rocks That Crystallize at Depth
• Abundance and Distribution of Plutonic Rocks
• Sierra Nevada Batholith
• Pegmatites
San Cristobal Volcano, Nicaragua
Photo Credit: G. Mattioli
Volcanic Neck and Dike
Aerial Photo of Shiprock, New Mexico
From: http://www.ngdc.noaa.gov/seg/hazard
Dikes vs. Sills
Sills: Concordant structures
Parallel to pre-existing layers
Dikes: Discordant structures
Cross-cut pre-existing layers
2 m Dike in Dominica, West Indies
Photo Credit: G. Mattioli
Dominica Dike along Strike View
Photo Credit: G. Mattioli
Dikes in northern Victoria Land, Antartica
Edinburgh Sill
From: http://www.ngdc.noaa.gov/seg/hazard
Magmatic
Diapirs
A diapir is a dome
that is cored by plastic
material, in this case,
partially molten rock.
Concept first applied
to salt domes.
Coalescing
Diapirs
and
Plutons
Rise because of
buoyancy-magma
is lower density
than rocks.
But siliceous
magmas have
high viscosity
or resistance
to flow, which
makes eruption
difficult without
high gas contents.
Pegmatite Outcrop
Sierra Nevada Batholith