Summary 1. The Rock Cycle 2. Formation of Igneous Rocks 3.
Classification of Igneous Rocks
Slide 3
Rock Cycle All rock types can be converted into any other rock
type Formation of Sedimentary Rocks (weathering, erosion,
deposition, cementation) Formation of Igneous Rocks (subduction,
remelting, crystallization) Formation of Metamorphic Rocks (burial,
heat and pressure)
Slide 4
Igneous Rocks Formed from magma (liquid rock) Classified in two
common ways: 1)Crystal or grain size (relates to crystallization)
2)Chemical Composition (related to colour) Coarse grainedporphyry
Fine grained Intermediate Mafic UltramaficFelsic
Slide 5
Grain Size of Igneous Rocks Extrusive (Aphanitic) fine grained
Magma is erupted at the surface. (Volcanic) Minerals have little or
no time to crystallize Mineral grains are very small (often
invisible) Intrusive (Phaneritic) coarse grained Magma cools below
the surface. Minerals have time to grow slowly. Mineral grains are
larger and are easily visible. In some cases, crystals can grow
very large (Pegmatite).
Slide 6
Extrusive (Aphanitic) Rocks Fine grained rock that cooled
quickly on the Earths surface. Eruption at the surface causes rapid
cooling of lava which does not allow mineral crystals much time to
grow. Basalt, the most common extrusive rock, often erupts from the
bottom of oceans
Slide 7
Surface Textures of Hawaiian Basalts (Extrusive, Mafic Rocks)
Aa (Rubbley) Mostly solid when flowing Pahoehoe (Ropey) mostly
liquid when flowing) Hawaiian Volcano eruption
http://www.youtube.com/watch? v=WwBVG0Si7rs
Slide 8
Intrusive (Phaneritic) Rocks Course grained rock that cooled
slowly under the surface. More time is allowed for crystal growth.
Crystals are clearly visible to the eye. This is the most common
type of igneous rock. Also described as Plutonic Diapirs a body of
magma that rises through another rock (country rock) and then
eventually crystallizes as a pluton See Kehew, Fig 3-6
Slide 9
Igneous Rock Textures
Slide 10
Igneous Rock Terminology Intrusive Extrusive Pluton Dike
Laccolith Magma Batholith Sill Xenolith Ash Lava volcano
Slide 11
Igneous Intrusions
Slide 12
Igneous Rock Terminology Magma liquid rock that has not
solidified or been erupted. Lava recently erupted magma that has
yet to solidify Ash fine (often glassy) igneous rock that
solidifies rapidly (in the air) during an eruption and is deposited
on the surface. Intrusive coarse grained rock not erupted at
surface Extrusive - fine grained rock erupted at surface (Volcanic)
Pluton A large body of intrusive igneous rock they often form the
basis of continental crust.
Slide 13
Igneous Rock Terminology Dike a roughly vertical intrusion of
igneous rock that cuts through country rock Laccolith A large
igneous body that is intruded horizontally between layers of rock
often uplifting the rocks above Batholith A large body of igneous
rock Sill A large horizontal intrusion of igneous rock Xenolith a
large chunk of country rock that has fallen into a body of magma
(often the xenolith undergoes contact metamorphism) Volcano a
structure where magma is erupted
Slide 14
Formation of Magma How are rocks melted? 1. Heating Rocks
melted in the crust. Rocks carried back into the mantle by
subduction or heated from below by a Mantle Plume (Hot Spot) 2.
Depressurization Magma requires 10% more volume than the rock.
Often rocks remain solid only due to pressure a release in pressure
allows rocks to liquify into magma. 3. Partial Melting Only a
portion of a pluton is melted, resulting in a magma with a
different chemical composition than its parent rock (see Bowens
Reaction Series) Types of Volcanism Hawaiian Stratovolcano
Intermediate Types of magma? Ultramafic Mafic Intermediate Silicic
or Felsic
Slide 15
Chemical Composition of Magma Ultramafic Upper Mantle Less than
45% Silica (SiO 2 ) Very high Mg, Fe (20-30%) olivine and
pyroxenes, no Qtz Dark minerals (green, black) Peridotite Mafic
Oceanic Crust 45-55% Silica (SiO 2 ) High Mg and Fe (10 - 20%) Dark
minerals, no Qtz Basalt, Gabbro Intermediate Cordilleran Mountains
(ex Andes, Rockies) 55-63% Silica (SiO 2 ) 5-10% Mg + Fe Mix of
light and dark minerals, some Qtz Andesite, Diorite Silicic or
Felsic Continental crust Greater than 63% silica 0-5% Mg + Fe High
in Group 1 metals (K, Na, Light coloured minerals Orthoclase and
abundant Qtz Rhyolite, Granite
Slide 16
Classification of Igneous Rocks Remember that grain size and
chemistry are the two main factors when classifying igneous rocks.
Grain Size reflects the rate at which magma cools there are two
major classifications intrusive and extrusive Chemistry reflects
the colour of the minerals and is ultimately related to the
geological source of the rocks (oceanic, continental, mix (at
subduction zones))
Slide 17
Igneous Rock Classification
Slide 18
Igneous Rock Classification Exercise Materials: 1) Pencil
crayons 2 shades of green, white, gray (regular pencil), black, red
or pink 2) Igneous rock diagram sheet.
Slide 19
Igneous Rock Classification Intrusive (Plutonic) Extrusive
(Volcanic) Mineral Percentage Continental Crust Oceanic Mantle
Crust
Granite most common intrusive, course-grained Rhyolite rare
extrusive, fine-grained rocks Rhyolite Volcanoes very explosive -
Yellowstone Park (no cone only a massive 75 km wide caldera) Felsic
(a.k.a, silicic) Magmas Cool (
Intrusive Felsic Igneous Rock Granite (Silicic, Phaneritic):
Poor in: Fe, Mg, Ca Rich in: Silica (>63%) Rich in: Al, K, Na
Light Coloured Coarse Grained Pegmatites show coarse grained
crystals Orthoclase, Quartz, Amphibole, Micas
http://www.youtube.com/watc h?v=nWNE_f6HqAk
Slide 24
Extrusive Felsic Igneous Rock Rhyolite (Silicic, Aphanitic):
Poor in: Fe, Mg, Ca Rich in: Silica (>63%) Rich in: Al, K, Na
Light Coloured usually grey Fine Grained Often Ash (Welded Tuff)
may contain obsidian Orthoclase, Quartz, Amphibole, Micas
http://www.youtube.com/watc h?v=nWNE_f6HqAk
Slide 25
Felsic Volcanism Mantle plumes intrude through the continental
crust Magma is very silicic, highly gaseous, low temperature and
pressure builds for millenia The volcanism is highly explosive The
Yellowstone caldera is 75 km across
Slide 26
Intermediate Rocks Andesite (common) Extrusive, Fine-grained,
intermediate rocks are found in subduction zones Diorite Intrusive,
Course-grained. If intrusive, Dikes and Sills more common
Intermediate Magmas Vary (700 - 1000 o C) Produce lavas and ash and
therefore can be unpredictable. Lower temperature magmas are gassy
and explosive (ex. Mt. St. Helens and Vesuvius)
Slide 27
Extrusive Intermediate Igneous Rock Andesite (Intermediate,
Aphanitic): High in: Ca, Al Rich in: Silica (55-65%) Median amounts
of: Al, K, Na, Mg, Fe Colour mixture of light and dark Fine Grained
and Porphyritic Orthoclase and Plagioclase, low Quartz, low
pyroxene, amphibole, Micas
Slide 28
Intrusive Intermediate Igneous Rock Diorite (Intermediate,
Phaneritic): High in: Ca, Al Rich in: Silica (55-65%) Median
amounts of: Al, K, Na, Mg, Fe Colour mixture of light and dark Fine
Grained - Orthoclase and Plagioclase, low Quartz, Amphibole, low
pyroxene, micas
Slide 29
Types of Volcanoes - Intermediate Composite Volcanoes
(Stratovolcanoes) - interayered lava flows and pyroclastic deposits
- ranges from intermediate to felsic in composition
(andesite-dacite-rhyolite) - in volcanic arcs above subduction
zones Composite Volcanoes The more felsic these volcanoes, the more
explosive. They can be subdivided into: Strombolian Vulcanian
Plinian Peleean
Slide 30
Intermediate Volcanism
Slide 31
Stratovolcanoes Pyroclastic flows A mixture of ash, small lava
bombs and hot gases that can race down the side of a volcano at
speeds of up to 200 km an hour this the biggest killer in most
volcanic eruptions. Lahars pyroclastic material melts the glaciers
on the sides of mountains forming a rapidly moving mudflow this was
the greatest killer in the Cerro Arenal eruption in Columbia. Lava
- Slow moving andesitic lava flows.
Slide 32
Mafic Rocks Basalt (very common) Extrusive, Fine-grained, Mafic
rock forms oceanic crust, Shield Volcanoes and Basalt Floods Gabbro
Intrusive, Course-grained. If intrusive, Dikes and Sills more
common Mafic Magmas Hot (>1000 o C) Non-Viscous (runny, flows
easily) Dry (no H 2 O or C0 2 ) See Kehew, Fig 3-40
Slide 33
Extrusive Mafic Igneous Rock E.g., Basalt (Mafic, Aphanitic):
Poor in: Si(45-55%) Poor in: K, Na Rich in: Mg, Fe, some Ca Rich
in: Al, K, Na Dark Coloured Fine Grained Plagioclase, Amphibole or
Pyroxene, maybe Olivine, no Quartz, dark mica Pillow Lava
Slide 34
Intrusive Mafic Igneous Rock E.g., Gabbro (Mafic, Phaneritic):
Poor in: Si(45-55%) Poor in: K, Na Rich in: Mg, Fe, some Ca Rich
in: Al, K, Na Dark Colured Coarse Grained Plagioclase, Amphibole or
Pyroxene, maybe Olivine, no Quartz, dark mica
Types of Volcanoes - Mafic 1) Shield Volcanoes - low viscosity
basaltic magmas - lava lakes and flows common - not explosive -
flood basalts and submarine pillow lavas are basaltic, too Shield
Volcanoes 2) Cinder Cones - variable composition, often basaltic -
usually a single batch of magma - steep cones formed by cinders
piled around vent at angle of repose associated with shield
volcanoes Cinder Cones
Slide 39
Mafic Volcanism, Hawaii Mantle Hot Spot Volcano See Kehew,
3-8
Slide 40
Hawaii 10.3 12.0 20.6 Midway 27.2 19.9 43.4 42.4 48.1 55.2 56.2
59.6 0-5 Current Plate Motion Hawaii A long chain of inactive
volcanoes Island ages, millions of years
Slide 41
Shield Volcanoes Mafic Magma Low-viscosity Non-explosive
eruptions Gentle slopes Covering large areas
Slide 42
Shield Volcanoes
Slide 43
Another method of classifying volcanos
Slide 44
Types of Eruptions (Mafic Volcanism) Types of Eruptions Lava
floods Lava fountains Fissure eruptions Rock Textures (Table 3-1)
Aphanitic Porphyritic Vesicular Glassy (Obsidian)
Slide 45
Mafic Sill: Intruded between layers Mafic magma is less viscous
and hotter so Does not form plutons but Cuts along layers (Sills)
or even across layers (Dikes) Also Baked Zones of adjacent country
rock and Chill Zones within the intrusion
Slide 46
Mafic Volcanic Features Columnar Basalt The Giants Causeway (N.
Ireland) Lava flows reaching the ocean, Kilauea, Hawaii Pumice
basalt with gas bubbles
Slide 47
Types of Volcanoes - Felsic Dome Volcanoes - similar in
composition and often related to composite volcanoes - usually more
viscous (more silicic) magma type (dacite/rhyolite) - pyroclastic
flows abundant - explosive eruptions common Dome Volcanoes
Continental Calderas - in continental regions the result of the
largest types of volcanic eruptions - large, shallow, silicic magma
chambers empty catastrophically with unparalleled violence; roof of
magma chamber collapses into emptied portion of magma chamber. The
Yellowstone caldera is over 100 km across. Continental
Calderas
Slide 48
Types of eruptions Felsic (Silicic) Uncommon usually form at
continental hot spots such as Yellowstone. Extraordinarily violent
due to the high gas content and low vicosity of the lava. See the
videos Yellowstone Supervolcano video http://www.youtube.com/
watch?v=Ap_YUwdiy8I http://www.youtube.com/ watch?v=Ap_YUwdiy8I
http://www.youtube.com/ watch?feature=endscreen&
v=aVUx1JtT-5I&NR=1 http://www.youtube.com/
watch?feature=endscreen& v=aVUx1JtT-5I&NR=1
Slide 49
Types of Eruptions (Intermediate) Higher gas content due to
water content in subducted crust and higher gas content of
continental crust Formation of Composite Volcanos (Mt. Fuji, Mt.
St. Helens, Etna, Vesuvius) Mt. St. Helens eruption, 1980
http://www.youtube.com/wat ch?v=-H_HZVY1tT4
http://www.youtube.com/wat ch?v=gmwylbF3-CA
Slide 50
10 most active volcanos and dangers related to volcanos 10 most
active volcanos: http://www.youtube.com/watch?v=4aYQixhdWY4
http://www.youtube.com/watch?v=4aYQixhdWY4 Lake Nyos Videos
(volcanic gas release):
http://www.youtube.com/watch?v=eHvPI_pYZBs&list=LPEB
BnueOle7g&index=6&feature=plcp
http://www.youtube.com/watch?v=eHvPI_pYZBs&list=LPEB
BnueOle7g&index=6&feature=plcp
http://www.youtube.com/watch?v=pVMcSvG5Mpg Nevada del Ruiz,
Columbia (Lahar volcanic mudflow)
http://www.youtube.com/watch?v=3XMS-quxdGg Pyroclastic Flow
http://www.youtube.com/watch?v=yvG_N7eqMWk
Slide 51
Location of Igneous Rock Production Sea-Floor Spreading Zones
Mid-Ocean Ridges Iceland Subduction Zones (Crustal Melting)
Cordilleran Mountains (Andes Mtns., Vesuvius) Island Arcs (Japan,
Aleutian Islands) Mid-Plate Hot Spots Hawaii (mid-ocean)
Yellowstone (mid-continent)
Slide 52
Sea Floor Spreading (Mid-Ocean Ridge) Magma is constantly
erupted from the sea floor at mid-ocean ridges. (ex. Mid Atlantic
ridge) Mostly intrusive basalt Dark coloured lavas Forms the bulk
of oceanic crust which is denser and thinner than continental
crust
Slide 53
Subduction Zones Ocean Crust is forced under the continental
crust and melts Oceanic crust melts due to heat and release of H 2
O Mafic magma mixes with felsic continental crust to make
intermediate rocks Often massive stratovolcanoes are formed
Slide 54
Mantle Plumes or Hot Spots Due to convection in the upper
Mantle, plumes of hot mantle rise through the crust and produce a
variety of volcanic features: Island Arcs (Hawaiian Islands) with
shield volcanoes Continental Flood Basalts (Deccan Traps)
Yellowstone style continental Hot Spots
Slide 55
Slide 56
1200 o C 1000 o Bowens Reaction Series Two series of minerals
formed during crystallization of magma Intrus. Extrus. Gabbro
Basalt Diorite Andesite Granite Rhyolite Low Silica Magma High
Silica Magma Frame- Double Single Isolated work Sheet Chain Chain
Temperature of Crystallization 750 o
Slide 57
Bowens Reaction Series Illustrates the relationship between the
cooling magma and the crystallization of the minerals contained in
the rock. Rocks on the right side of this chart are rich in calcium
and sodium Rocks on the left side represents iron-rich minerals.
They cool and create quartz.
Slide 58
Bowens Dilemma Problem The Mantle is the source of most igneous
rocks. So, if the mantle is made of ultramafic rocks how come
continental crust is felsic? And how can felsic rocks be produced
from an ultramafic source? ANSWER Think of the Bowens Reaction
Series
Slide 59
Partial Melting Different minerals melt at different
temperatures. As the temperature of a rock rises, minerals melt in
the reverse order in which they cooled the Bowens reaction series
in reverse! Even if the rock was mafic or ultra mafic, the magma
produced can be felsic if the temperatures only rise to 700-800C
This explains why continental rocks are felsic they are partially
melted oceanic crust
Slide 60
Terminology Felsic or Silicic silicate minerals, magmas and
rocks enriched in lighter elements such as oxygen, aluminum,
sodium, and silicon.
Slide 61
Terminology Mafic iron and magnesium enriched minerals are
found in these rocks. They are dark in colour. Phaneritic Grains in
the rock can be seen by the naked eye.
Slide 62
Peridotite Ultrabasic Rare and part of the mantle Course
grained Green in colour
Slide 63
Obsidian Glassy (volcanic glass) Glass is produced when any
rock is cooled immediately Can be green or black Extrusive Often
used by native cultures to produce cutting tools or arrows
Slide 64
Slide 65
Pumice or Scoria Glassy (Frothy) Light (Pumice) to dark
(Scoria) in colour depending on its impurities. Solidified foam
Extrusive Can float on water