How are igneous rocks formed?
What are types of igneous
rocks are there?
•
Magma (molten rock)
•
Rise through crust (less dense)
•
Extrusive (lava flows)
•
Cool
•
Intrusive
•
Crystallisation (reaches melting point)
(country rock, air or water)
(dykes,
sills,
batholiths)
IGNEOUS ROCKS
How can igneous rocks
be classified?
Where do Igneous
Rocks Form?
•
Colour
(felsic, intermediate, mafic, ultramafic)
•
Chemical composition (acidic, intermediate, basic, ultrabasic) •
•
Mineralogy
•
Texture
•
Structures
(Q, F, M, A, H , O)
(Crystalline, interlocking, randomly orientated,
grain size, equigranular, porphyritic)
(Xenolith, vesicular, amygdaloidal, flow
banding, columnar jointing)
Constructive plate margins (basalt)
•
Destructive plate margins (andesite)
•
Hot spots (basalt)
Baked margin
• small intrusions
• right angles to bedding
planes
• discordant
•1–2m
• chilled margin at edges
• small intrusions
• follow bedding planes
• concordant
• 1 – 100 m
• chilled margin at edges
• very large intrusions
• cuts across bedding
planes
• discordant
• many km’s
• chilled margin at edges
What am I ?
Dyke because I am
discordant (cut
across)
What am I ?
Sill because I am concordant (lay at
the same angle as the bedrock)
Investigation Into What Can Change Viscosity
Changing the Properties of Treacle
•
Collect appropriate equipment (water bath using bunsen
burner for heat)
•
Set up stand with two bosses in tray. Clamp one
treacle test-tube upright to one boss
•
Add component to second treacle test-tube. Clamp
upright to other boss
•
Using stop watch, turn both test-tubes upside down
(watch the bung and keep in place if necessary). Time
decent of both tubes – record in a simple table
How Can Igneous Rocks be Classified?
Fine
Grain Size
mm
Silicic
Intermediate
Basic
<0.5
Rhyolite
Andesite
Basalt
Medium
>0.5 - <2
Coarse
>2
Minerals
Present
Ultrabasic
Dolerite
Granite
Gabbro
Orthoclase and Plagioclase and
Olivine,
Plagioclase
Orthoclase
Augite,
Feldspar,
Feldspar,
Plagioclase
Quartz, Biotite Hornblende,
Feldspar
and Muscovite Biotite, Quartz
Increasing darkness
Decreasing SiO2
Peridotite
Olivine,
Augite
Mineralogy
• Felsic
• Acidic
Quartz, feldspar & mica
• Intermediate
• Intermediate
Quartz, feldspar, mica &
hornblende
• Mafic
• Basic
Feldspar, olivine, augite
• Ultramafic
• Ultrabasic
Olivine and Augite
Chemical Composition
• Acidic
> 65% SiO2
• Intermediate
65 - 52% SiO2
• Basic
52 - 45% SiO2
• Ultrabasic
<45% SiO2
Types of igneous rocks
1 mm
1 mm
1 mm
Igneous Rocks
Baked
margin
Chilled
margin
Chilled Margin Grain Size
Igneous Rock Textures
• Crystalline
• Interlocking crystals
• Randomly orientated
• Crystal shape:
euhedral (fully developed crystals)
subhedral (incomplete crystals)
anhedral (no crystal development)
• Equigranular – equal size
• Porphyritic – one crystal is much larger than the rest
Igneous Rock Textures
• Other crystalline textures/structures
• Xenolith (from stoping)
Lava Flows and Minor Intrusions
How can you tell the Difference ?
Lava Flows and Minor Intrusions
Summary
Lava Flow
Sill
Dyke
Chilled/Baked
Margin
Base only
Upper and Lower
Both Sides
Relative to Bedding
Planes
Concordant
Concordant
Discordant
Cross Section
Horizontal
Parallel to Beds
Gentle Dip
Parallel to Beds
Vertical
Steeply cross-cutting
Columnar Jointing
(may have weathered
surface)
Jointing
(may have weathered
surface)
Structures
Columnar Jointing
Pillows
Vesicles
Weathered Surface
(always)