Introduction to Geology GEO101004 Class 6: Igneous Rocks

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Introduction to GeologyGEO-101-004Class 6
Igneous Rocks
From http//volcano.und.nodak.edu
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Last Question of the Day

• Name one mineral that you have seen and/or used
  in the last two hours
• Graphite, muscovite, barium, gypsum, etc

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Question of the Day

• What is the major reason for some igneous rocks
  being coarse grained, and some being fine
  grained?

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Titan
Earth-like processes on Titan This image from
Huygens shows detail of a high ridge area
including the flow down into a major river
channel from different sources evidence of
fluvial activity
From http//www.nasa.gov/mission_pages/cassini/mul
timedia/pia07236.html
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And on Mars..
The Mars Exploration Rover Opportunity has found
a pitted, basketball-size meteorite made of iron
and nickel. This is the first ever meteorite seen
on the surface of the planet Metal-rich
meteorites are rare (making up only 2 of
meteorites on Earth). Such meteorites must come
from a destroyed planet that was big enough to
differentiate into a metallic core and a rocky
mantle
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General characteristics of magma

• Igneous rocks form as molten rock cools and
  solidifies
• General characteristics of magma
• Parent material of igneous rocks
• Forms from partial melting of rocks
• Magma at surface is called lava

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General characteristics of magma

• General characteristic of magma
• Rocks formed from lava extrusive, or volcanic
  rocks
• Rocks formed from magma at depth intrusive, or
  plutonic rocks

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Which of the following describes best the
difference between magma and lava?
04.03

• Upon eruption, magma becomes lava, the form of
  the magma that flows out of the volcano
• When magma gets to the top of the volcano it
  releases gas found within the magma, and the lava
  that escapes the volcano has therefore lost those
  gases.
• Lava is the name given to the ejected magma from
  a passive (fluid) volcano, but it is still called
  magma if it is erupted explosively, like at Mt.
  St. Helens.
• If the magma has no crystals or gases within it,
  it is called lava.
• Both 1 and 3
• Both 3 and 4
• All of these.

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General characteristics of magma

• The nature of magma
• Consists of three components
• Liquid portion melt
• Mobile ions, Si, O (readily combine to SiO2), Al,
  K, Ca, Na, Fe, Mg
• Solids, if any, are silicate minerals
• Volatiles dissolved gases in the melt,
  including water vapor (H2O), carbon dioxide
  (CO2), and sulfur dioxide (SO2)
• Remain in magma until it moves nearer the
  surface, or it crystallizes

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General characteristics of magma

• Crystallization of magma
• As a rock melts the ions begin to vibrate more
  rapidly, colliding with their neighbors
• The rock expands
• When the melt is hot enough the bonds break, and
  the rock becomes a liquid
• A liquid is composed of unordered ions moving
  randomly

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General characteristics of magma

• Crystallization of magma
• Cooling of magma results in the systematic
  arrangement of ions into orderly patterns the
  silicates (tetrahedra)
• The silicate minerals resulting from
  crystallization form in a predictable order
  ions lose their mobility and join a crystal
  network
• Earliest formed minerals have better developed
  crystal faces room to grow
• Later formed minerals fill the remaining space
• When all the melt has cooled igneous rock

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Igneous textures

• Texture is used to describe the overall
  appearance of a rock based on the size, shape,
  and arrangement of interlocking minerals
• Factors affecting crystal size
• Rate of cooling
• Slow rate fewer but larger crystals
• Fast rate many small crystals
• Very fast rate forms glass

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What is the most important factor for whether
magma cools slowly or quickly?
04.05

• Pressure of the environment higher P slower
  magma cooling.
• The presence or absence of volatiles (gases)
  less gases slower magma cooling.
• Temperature of the environment lower T slower
  magma cooling.
• The presence or absence of volatiles (gases)
  more gases slower magma cooling.
• Temperature of the environment higher T
  slower magma cooling.
• Pressure of the environment lower P slower
  magma cooling.

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Igneous textures

• Types of igneous textures
• Aphanitic (fine-grained) texture
• Rapid rate of cooling
• Crystal can only be seen under magnification
• May contain vesicles (holes formed by gas
  bubbles) vesicular texture

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Igneous textures

• Types of igneous textures
• Phaneritic (coarse-grained) texture
• Slow cooling
• Large, visible intergrown crystals
• Form deep in the Earth so are only exposed at the
  Earths surface after uplift and/or erosion

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Igneous textures

• Types of igneous textures
• Porphyritic texture
• Minerals form at different temperatures some
  crystals can become quite large before others
  start to form
• If environment changes (e.g. eruption) the
  remaining melt may cool rapidly
• Large crystals (phenocrysts) are embedded in a
  matrix of smaller crystals (groundmass)

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Igneous textures

• Types of igneous textures
• Glassy texture
• Very rapid cooling of lava
• Resulting rock is called obsidian no
  crystalline structure
• Chain structure of high silicate content minerals
  can impede ionic transport and thus restrict
  crystal growth
• Granitic magmas (higher silicon content) more
  commonly form obsidian than basaltic
• Basaltic magmas form glassy texture when cooled
  rapidly

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Igneous textures

• Types of igneous textures
• Pyroclastic texture
• Fragmental appearance produced by violent
  volcanic eruptions
• Welded tuff
• Fragments solidified during flight, forming a
  rock on landing will not have interlocking
  crystals
• Often appear more similar to sedimentary rock

http//volcanoes.usgs.gov/
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Lava flows are typically finer grained than
intrusive igneous rocks. Why?
04.01

• Intrusive magma is cooler because it is well
  insulated by the surrounding rock.
• Intrusive magma flows onto the Earth's surface
  and cools very slowly, allowing many small
  mineral grains to grow.
• The extrusive magma cools quickly so the mineral
  grains do not have time to grow.
• The extrusive magma, because it is deep below the
  surface, cools very slowly producing very small
  mineral grains.

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Igneous textures

• Types of igneous textures
• Pegmatitic texture
• Exceptionally coarse grained
• Form in late stages of crystallization of
  granitic magmas
• Volatiles such as chlorine, water, fluorine, and
  sulfur enhance ionic transport and aid in crystal
  growth
• Great environment for rare minerals to form
  beryl, topaz

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Igneous compositions

• Igneous rocks are composed primarily of silicate
  minerals
• Dark (or ferromagnesian) silicates
• Olivine, pyroxene, amphibole, and biotite mica
• Rich in iron, magnesium and comparatively low in
  silicon
• Light (or nonferromagnesian) silicates
• Quartz, muscovite mica, and feldspars
• Greater amounts of potassium, sodium, and calcium
  rather than iron and magnesium

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Igneous compositions

• Granitic versus basaltic compositions
• Granitic composition
• Light-colored silicates
• Termed felsic (feldspar and silica) in
  composition
• High amounts of silica (SiO2) about 70
• May contain 10 dark silicate minerals (mica,
  amphibole)
• Major constituent of continental crust

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Igneous compositions

• Granitic versus basaltic compositions
• Basaltic composition
• Dark silicates and calcium-rich feldspar
• Termed mafic (magnesium and ferrum, for iron) in
  composition
• Higher density and darker than granitic rocks
• Comprise the ocean floor and many volcanic islands

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Igneous compositions

• Other compositional groups
• Intermediate (or andesitic) composition
• Composition between granitic and basaltic rocks
• Contain 25 or more dark silicate minerals
  (amphibole, pyroxene, biotite mica, plagioclase
  feldspar
• Associated with explosive volcanic activity on
  continent margins
• Ultramafic composition
• Rare composition that is high in magnesium and
  iron
• Composed entirely of ferromagnesian silicate
• Peridotite is believed to be the main constituent
  of the upper mantle

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Figure 4.7
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Igneous compositions

• Naming igneous rocks granitic rocks
• Granite
• Phaneritic
• Over 25 quartz (clear to light gray), about 65
  or more feldspar (not as glassy, white to gray or
  salmon pink)
• Minor muscovite mica and amphibole
• Very abundant - often associated with mountain
  building
• The term granite includes a wide range of
  mineral compositions
• May have porphyritic texture (large feldpar
  crystals)

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Igneous compositions

• Naming igneous rocks granitic rocks
• Rhyolite
• Extrusive equivalent of granite
• May contain glass fragments and vesicles
• Aphanitic texture
• May contain phenocrysts of quartz or potassium
  feldspar
• Less common and less voluminous than granite

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Igneous compositions

• Naming igneous rocks granitic rocks
• Obsidian
• Dark colored
• Glassy texture
• Unordered ions no crystal structure
• Native Americans used it to make arrowheads and
  cutting tools
• Pumice
• Volcanic
• Glassy texture
• Large amounts of gas escaping create a frothy
  appearance with numerous voids
• Sometimes will float on water (dangerous for
  shipping)

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Igneous compositions

• Naming igneous rocks intermediate rocks
• Andesite
• Volcanic origin (Andes, Pacific rim)
• Porphyritic texture (phenocrysts of plagioclase
  feldspar, amphibole).
• Minor quartz (compared to 25 for rhyolite)
• May look similar to rhyolite (distinguish using a
  microscope)
• Diorite
• Plutonic equivalent of andesite
• Coarse grained
• Absence of visible quartz (distinguishes it from
  granite)

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Igneous compositions

• Naming igneous rocks basaltic rocks
• Basalt
• Volcanic origin
• Aphanitic texture or porphyritic (with
  phenocrysts of feldspar or olivine)
• Composed mainly of pyroxene and calcium-rich
  plagioclase feldspar, lesser amounts of olivine
  and amphibole
• Most common extrusive igneous rock
• Upper layers of oceanic crust, many volcanic
  islands

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Igneous compositions

• Naming igneous rocks mafic rocks
• Gabbro
• Intrusive equivalent of basalt
• Phaneritic texture consisting of pyroxene and
  calcium-rich plagioclase
• Significant of the oceanic crust
• Formed underground reservoirs that fed basaltic
  lava flows

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Igneous compositions

• Naming igneous rocks pyroclastic rocks
• Composed of fragments ejected during a volcanic
  eruption
• Varieties
• Tuff ash-sized fragments
• Volcanic breccia particles larger than ash
• Flows speeds approaching 100 km/h

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Which of the following rocks is likely to have
the most quartz within it and why?
04.04

• Granite intrusive rock that formed from cooling
  of relatively high silica magma.
• Rhyolite extrusive rock that formed from
  cooling of relatively low silica magma.
• Diorite intrusive rock that formed from the
  cooling of relatively intermediate silica magma.
• Granite intrusive rock that formed from cooling
  of relatively intermediate silica magma.
• Basalt extrusive rock that formed from cooling
  of relatively low silica lava.
• Basalt extrusive rock that formed from cooling
  of relatively high silica lava.

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Origin of magma

• Generating magma from solid rock
• Earths crust and mantle are composed primarily
  of solid rock why does rock melt?
• Role of heat
• Temperature increases in the upper crust
  (geothermal gradient) average between 20oC to
  30oC per kilometer (1200-1400oC at 100 km)
• Rocks in the lower crust and upper mantle are
  near their melting points
• Any additional heat may induce melting (friction)
• Hot mantle rocks can rise and intrude crustal
  rocks

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Origin of magma

• Role of pressure
• Increases in confining pressure cause an increase
  in a rocks melting temperature
• When confining pressure drops, decompression
  melting occurs

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Origin of magma

• Role of volatiles
• Volatiles (primarily water) cause rocks to melt
  at lower temperatures
• Important factor where oceanic lithosphere
  descends into the mantle
• Heat and pressure drive water from subducting
  rocks
• Lowers melting temperature of mantle rocks

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Evolution of magmas

• A single volcano may extrude lavas exhibiting
  very different compositions

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Evolution of magmas

• Bowens reaction series
• Minerals crystallize in a systematic fashion
  based on their melting points

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Evolution of magmas

• Bowens reaction series
• During crystallization, the composition of the
  liquid portion of the magma continually changes
• When about one third of the magma has
  crystallized the melt will be nearly depleted in
  Fe, Mg, and Ca (constituents of first formed
  minerals)
• Remaining portion of magma is richer in silica
• If the solid portion of the magma is removed, the
  melt will crystallize to for a more felsic rock

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Evolution of magmas

• Bowens reaction series
• If the solid portion of the magma remains in
  contact with the melt, the mineral will
  chemically react and evolve into the next mineral
  in the sequence
• This arrangement of minerals is known as Bowens
  reaction series

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Evolution of magmas

• Processes responsible for changing a magmas
  composition
• Magmatic differentiation
• Separation of a melt from earlier formed crystals
• Crystal settling early formed dense minerals
  sink to the bottom of a magma chamber
• Remaining melt will form a rock with a different
  composition to the parent magma

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Evolution of magmas

• Processes responsible for changing a magmas
  composition
• Assimilation
• Changing a magmas composition by the
  incorporation of surrounding rock bodies into a
  magma
• Magma mixing
• One magma body intrudes another

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Evolution of magmas

• Partial melting and magma formation
• Incomplete melting of rocks is known as partial
  melting
• Low temperature minerals melt first (quartz)
• Formation of basaltic magmas
• Most originate from partial melting of ultramafic
  rock in the mantle at oceanic ridges
• Large outpourings of basaltic magma are common at
  Earths surface

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Evolution of magmas

• Partial melting and magma formation
• Formation of andesitic magmas
• Produced by interaction of basaltic magmas and
  more silica-rich rocks in the crust
• May also evolve by magmatic differentiation
• Granite may be the end product of the
  crystallization of an andesitic magma or partial
  melting of silica rich continental rocks
• Granitic magmas usually lose their mobility
  before reaching the surface and tend to form
  large plutonic structures
• If they reach the surface they form large
  explosive eruptions (Mount St. Helens)