Igneous Rocks

1
Igneous Rocks

• pictures taken from
• http//csmres.jmu.edu/geollab/Fichter/IgnRx/Ighome
  .html

2
Formation

• Igneous rocks form when magma cools and mineral
  grains crystallize.
• Rocks that form from magma deep within the earth
  are called intrusive rocks.
• Magma that reaches the earths surface is called
  lava.
• Rocks that form from lava are called extrusive
  rocks.

3
Intrusive Rocks

• Intrusive Rocks
• cool slowly
• crystals have a long time to form
• crystals are large Phaneritic
• are exposed at the earths surface when an
  area is uplifted and the rock above is
  eroded away
• ex. granite

4
Intrusive Rocks

• Intrusive Rocks - Pegmatite
• cool slowly from a water-rich magma
• the water allows ions to quickly flow to
  crystallization sites
• crystals are extremely large gt 1 cm!
• often contain rare minerals

5
Extrusive Rocks

• Extrusive Rocks
• cool quickly
• crystals form very quickly
• crystals are too small to see Aphanitic
• ex. basalt

6
Extrusive Rocks

• Extrusive Rocks Glassy Texture
• cool extremely quickly
• crystals dont have time to form
• the rock has a glassy appearance
• ex. obsidian

7
Extrusive Rocks

• Extrusive Rocks Vesicular Texture
• cools quickly
• bubbles of gas were present in the lava
• the rock contains outlines of the gas bubbles
• ex. Vesicular basalt
• or scoria

8
Extrusive Rocks

• Extrusive Rocks Pyroclastic Rocks
• pieces of lava and/or rock are thrown from a
  volcano
• cools quickly pieces of hot ash are welded to
  one another
• ex. tuff

9
Porphyritic Texture

• What if a magma starts to cool slowly, but then
  something happens to make it finish cooling
  quickly?
• Porphyritic Texture
• large crystals embedded in a matrix of smaller
  crytals
• larger crystals Phenocrytsts
• smaller crystals Groundmass
• the rock is called a Porphyry

10
Magmas

• The mineral types found in an igneous rock are
  controlled by the chemical composition of the
  magma.
• Light colored igneous rocks Felsic
• contain a lot of nonferromagnesian silicates
• Dark colored igneous rocks Mafic
• contain a lot of ferromagnesian silicates

11
Magma Types
See Figure 3.7, page 52
12
Magma Types

• Ultramafic rocks
• contain no quartz
• mostly olivine
• black, greenish color
• Peridotite

13
Where does the magma come from?

• The earths crust and mantle are solid or
  plastic, but not molten!
• Some magma is believed to form during
  decompression melting.
• remember.
• Higher pressures push the mantle towards a solid
  phase.
• Lower pressures push the mantle towards the
  liquid phase.

14
Where does the magma come from?

• The earths crust and mantle are solid or
  plastic, but not molten!
• Some magma is believed to form during
  decompression melting.
• material rises via convection in the mantle
• as pressure decreases, melting point drops
• molten material continues to rise

15
Where does the magma come from?

• The earths crust and mantle are solid or
  plastic, but not molten!
• Some magma is believed to form during
  decompression melting.
• this occurs at divergent boundaries and hot
• spots
• forms mafic magmas

16
Where does the magma come from?

• Other magma is believed to form at subduction
  zones.
• friction generates heat
• water trapped in sediments lowers the melting
  point
• occurs at convergent boundaries
• forms felsic magmas

17
Magma Evolution

• As minerals crystallize from a magma, the
  chemistry of the resulting melt is changed.
• Different minerals crystallize at different
  temperatures
• Bowens Reaction Series
• See Figure 3.16 page 59

18
Bowens Reaction Series

• The earliest minerals to form (olivine,
  Ca-rich plagioclase) can settle out from the
  melt
• The melt then contains very little Fe or Mg.
• As time passes, more Si-rich minerals will
  form.
• Magmatic Differentiation
• As the melt crystallizes into rock, the
  composition of the magma changes greatly over
  time.

19
Chapter 3 Vocab for Quiz

• Magma Pegmatite
• Lava Felsic
• Aphanitic Mafic
• Phaneritic Bowens Reaction Series
• Porphyritic Magmatic Differentiation
• Glassy
• Pyroclast

20
(No Transcript)
21
Igneous Activity

• pictures taken from
• http//volcanoes.usgs.gov/Products/Pglossary/

22
Igneous Activity

• Volcanoes
• Igneous Intrusions

23
Volcanic Eruptions

• Nature of the Eruption controlled by
• viscosity of lava Silica Content!!
• Mafic lava little Si are very fluid
• Pahoehoe flow
• Aa flow
• Lava Tubes

24
Volcanic Eruptions

• Nature of the Eruption controlled by
• viscosity of lava Silica Content!!
• Felsic lava lots of Si form short, thick
  flows
• Silica tetrahedra chains and networks form
• Dissolved gases cause explosive eruptions!
• Pyroclastic Eruptions
• Ash
• Bombs

25
Volcanoes

• Viscosity controls Morphology
• Shield Volcanoes
• Mafic Lavas Low viscosity Basalts
• ex. Hawaii

26
Volcanoes

• Viscosity controls Morphology
• Cinder Cones
• Gas-rich Mafic Lavas (usu.) Low viscosity,
  but explosive! Basalts
• ex. Pisgah Crater, Amboy Crater, Paricutin

27
Volcanoes

• Viscosity controls Morphology
• Composite Cones (Stratovolcanoes)
• Feslic Lavas high viscosity rhyolites
  pyroclastic material
• ex. Cascade Range, Mt. Fuji, Mt. Pinatubo,
  Vesuvius

28
(No Transcript)
29
Pyroclastic Eruptions

• Deadly!!!
• Nuée Ardente
• Lahar

30
Nuée Ardente

• Pyroclastic flow of hot gases and ash
• Dense, hugs the ground
• Flows very quickly
• 1902 St. Pierre, Martinique

31
Lahar

• Destructive mudflow
• Pyroclastic debris becomes saturated with water
• Rainfall
• Rapid glacier/snow melting

32
Igneous Intrusions

• Plutons igneous rock bodies at depth
• Tabular vs. Massive
• Discordant vs. Concordant
• Batholith large, massive igneous intrusion

33
Igneous Intrusions

• Plutons igneous rock bodies at depth
• Tabular vs. Massive
• Discordant vs. Concordant
• Dike tabular discordant intrusion

34
Igneous Intrusions

• Plutons igneous rock bodies at depth
• Tabular vs. Massive
• Discordant vs. Concordant
• Sill tabular concordant intrusion

35
Igneous Intrusions

• Plutons igneous rock bodies at depth
• Tabular vs. Massive
• Discordant vs. Concordant
• Laccolith tabular concordant intrusion, it
  pushes up the layers of rock above it

36
(No Transcript)
37
Unit 3 Test Next Class

• Intrusive vs. Extrusive Rocks
• Rock textures
• Mafic vs. Felsic Rocks
• Magmas
• Volcanoes
• Types of Volcanoes
• Nuée Ardentes and Lahars
• Igneous Intrusions
• 40 multiple choice
• 10 point essay

38
Geopardy
39
Igneous Textures - 100

• This texture forms when an extrusive rock cools
  so quickly that no crystals form.
• glassy

Back to Geopardy Main Page
40
Igneous Textures - 200

• This extrusive texture forms when pieces of lava
  and/or rock are thrown from a volcano.
• Pyroclastic

Back to Geopardy Main Page
41
Igneous Textures - 300

• What is the difference between a phaneritic and
  an aphanitic rock? What does the texture tell
  you about the cooling history of the rock?
• Phaneritic can see crystals, cooled slowly
• Aphanitic cant see crystals, cooled quickly

Back to Geopardy Main Page
42
Igneous Textures - 400

• Under what conditions does a pegmatite form? (be
  very specific!)
• A water rich magma cools slowly. The presence of
  magma allows ions to move quickly through the
  magma. Crystals grow to a very large size.

Back to Geopardy Main Page
43
Igneous Textures - 500

• This texture describes a rock that cooled at two
  different rates. Give the name of the texture,
  the name of each part of the rock, and an
  explanation of how it cooled.
• Porphyritic
• Phenocrysts Groundmass
• Cooled slowly and then more quickly

Back to Geopardy Main Page
44
Magmas - 100

• What is a mafic magma?
• A magma that will form dark colored igneous rocks
  containing ferromagnesian silicates.

Back to Geopardy Main Page
45
Magmas - 200

• What do you call an intrusive rock that formed
  from a felsic magma?
• granite

Back to Geopardy Main Page
46
Magmas - 300

• Why can magma form at a subduction zone?
• The water present in the sediments being
  subducted lowers the melting point of the rocks.
  The heat produced by friction melts the rock.

Back to Geopardy Main Page
47
Magmas - 400

• What is decompression melting?
• Decompression Melting rising asthenosphere
  melts because of the decrease in pressure

Back to Geopardy Main Page
48
Magmas - 500

• Describe the process of Magmatic Differentiation
• Different minerals crystallize from a magma at
  different temperatures. Once crystals form, the
  ions and atoms that make up the crystal are
  removed from the melt. The chemistry of the melt
  therefore changes over time.

Back to Geopardy Main Page
49
Volcanoes - 100

• The largest volcanoes on earth are this type
• Shield volcanoes

Back to Geopardy Main Page
50
Volcanoes - 200

• Felsic magmas produce this type of volcano
• Composite cones

Back to Geopardy Main Page
51
Volcanoes - 300

• This type of mafic lava flow has a smooth, ropy
  appearance
• Pahoehoe

Back to Geopardy Main Page
52
Volcanoes - 400

• Why does a lahar occur?
• Pyroclastic debris (ash) becomes saturated with
  water (from rainfall or melting ice or snow),
  causing a devastating mudflow

Back to Geopardy Main Page
53
Volcanoes - 500

• Explain the connection between the type of lava
  extruded by a volcano and the volcanos shape
• Silica Content -gt Viscosity

Back to Geopardy Main Page
54
Igneous Intrusions - 100

• All igneous rock bodies that formed at depth are
  called
• Plutons

Back to Geopardy Main Page
55
Igneous Intrusions - 200

• An enormously large intrusive igneous rock body,
  such as the one partially exposed in Yosemite
  valley, is called a
• Batholith

Back to Geopardy Main Page
56
Igneous Intrusions - 300

• laccolith

Back to Geopardy Main Page
57
Igneous Intrusions - 400

• A concordant, tabular intrusion.
• Sill

Back to Geopardy Main Page
58
Igneous Intrusions - 500

• Identify each of the following

B
A
D
C
E
Back to Geopardy Main Page
59
Mixed Bag - 100

• This type of mafic lava flow has a rough,
  irregular surface.
• Aa

Back to Geopardy Main Page
60
Mixed Bag - 200

• What do you call an intrusive rock that formed
  from a mafic magma?
• gabbro

Back to Geopardy Main Page
61
Mixed Bag - 300

• The rock that forms from this type of magma
  contains no quartz. This rock, called
  peridotite, is made up of mostly olivine.
• Ultramafic

Back to Geopardy Main Page
62
Mixed Bag - 400

• What is a Nuée Ardente?
• A dense flow of hot pyroclastic debris and gas
  that moves quickly down the flanks of a composite
  cone.

Back to Geopardy Main Page
63
Mixed Bag - 500

• For each of the following, identify the type of
  lava that will be extruded and the type of
  volcano that will form.
• Subduction Zone
• Divergent Plate Boundary
• Hot Spot
• Subduction zone felsic magmas produce
  composite cones
• Divergent Plate Boundary mafic magmas produce
  shield volcanoes ( cinder cones)
• Hot Spots mafic magmas produce shield volcanoes
  ( cinder cones)

Back to Geopardy Main Page