Igneous Rocks part 1.pdf

Chapter 4
The Cornerstones
of Geology:
Rocks!
Strato-volcano: Anatahan, Marianas Arc

2
Educational Outcomes
 Identify common igneous, metamorphic, and
sedimentary rocks in hand samples.
 Explain the basis for igneous, metamorphic,
and sedimentary classification systems.
 Describe igneous, sedimentary and
metamorphic rock-forming processes based
on observations of rock textures,
structures, and composition.
 Relate rock-forming processes and rock
types to plate tectonics.

Petrology is the branch of geology concerned with
the compositions, structures, and origins of rocks.
Petrology is the study of the macroscopic and
microscopic mineralogical and chemical
composition of rocks.
Petrology is scientific study of rocks that deals
with their composition, texture, and structure;
What is Petrology?

4
Rock
 A rock is a physical mixture of
mineral grains (crystals)
 What controls which minerals are in a
rock?
 Abundance of elements
 T and P control mineral stability
 Processes forming the rock

5
Rock Cycle
 The rock cycle
describes the
interconnections
between geological
processes. Internal
forces build up
mountains; external
forces wear them
down.

6
Rock cont…
 Processes
forming the
rock
 Crystallising
from a magma
 Waves
depositing sand
on a beach
 Burial by a
tectonic plate
Rock Cycle

8
Rock Cycle cont…
 Igneous rocks – formed from
molten rock that freezes
 Weathering – at Earth’s
surface, igneous,
metamorphic, and
sedimentary rocks are
broken down:
 into fragments (grains), like
sand, gravel, and mud
 into dissolved ions, like Na ions
in a water
 Erosion – grains are moved by
wind, water, and ice.

9
Rock Cycle cont…
 Sedimentary rocks – made
from deposited fragments
and precipitated ions.
 Metamorphic rocks – buried
sedimentary, igneous, and
metamorphic rocks changed
by T, P without melting
 Most at convergent plate
boundaries
 More heat and metamorphic
rocks melt and become magma.

11
Igneous rocks
 Formed from the crystallization
from a magma
Magma?
 Usually a hot (650 to 1200°C) silicate
melt (liquid)
 Most of which consists of ions of
eight elements,
 Mixed with gases and suspended
crystals.

12
How do igneous rocks form?
 Melting stage
 Silicate minerals melt to make a liquid
 Rocks in lower crust or upper mantle melt
 Rising stage
 Emplacement stage
 Extrusive igneous rocks from lava on surface
 Volcanoes—violent or ―gentle‖ from point
 Fissure
 Intrusive igneous rocks from magma that
cools below Earth’s surface.

13
How to Generate Magma
 Heat rocks above their melting
point (e.g. increase depth of burial)
 Lower melting point by:
 decreasing pressure (e.g. depth)
 adding volatiles (e.g. water)

14
Why and where do rocks melt?
 Melting is related to plate tectonics
 Temperature has to be higher than melting
temperature of rock for pressure.
 Melting T-P diagrams
 Geothermal gradient – the rate of T change with
D in Earth.
 T increases with increasing depth in Earth.
 Temperature of the mantle is mostly lower than
melting T, so mantle is solid (although slow
convective flow!).

15
Why and where do rocks melt?
 Pressure-temperature diagram

16
Melting
Two main ways in which melting occurs:
 Decompression – deep, hot rock moves up
without losing heat
 Occurs in upper mantle at spreading centres
 Occurs where hot deep mantle rises at hot spots.
 Hydration – add water to lower melting T of
mantle material
 Sinking oceanic crust at convergent boundaries

17
Melting cont…
Decompression Hydration

18
Formation of Igneous Rocks
Movie - Intro to Igneous Rocks

19
Rising Stage
 Buoyancy – hot magma rises because
it is less dense than surrounding
rocks
 Rising magma may be contaminated
by ―country‖ rocks.

20
How does magma rise?
 Along cracks – magma
forces its way up
 Balloon – magma up
crack, then expands to
make large intrusion.
 Assimilation of rock
that melts.
Different ways in
which a magma can
ascend through the
crust

21
Emplacement stage
 Magma may stop below the surface –
intrusive
 cools slowly to form large crystals
 Magma that reaches the surface is
lava (extrusive).
 cools quickly to form small crystals or glass
 may be low viscosity and ―gentle‖ (non-
explosive)
 may be high viscosity and explosive.
 different volcano types from different
behavior of magma.

22
Genetic Igneous Rock Classifications
 Extrusive:
 extruded at the earth’s
surface as lava or
pyroclastic material
 Intrusive:
 intruded within the
earth’s crust
Intrusion of volcanic
dike (dark brown) into
limestones (white).
Adamello, Italy

23
Igneous rocks classification
 Texture:
 size, shape,
arrangement
of crystals
 Composition:
 chemical or
mineral
components

24
Classification
 Texture – general appearance of
the rock -- crystal size and
orientation
 Crystals large enough to see with
naked eye form when magma
 cools slowly below the surface
(intrusions = intrusive). Produced by
a few crystal seeds grow to a large
size before growing into each other.

25
Classification
 Texture – general appearance of
the rock -- crystal size and
orientation
 Microscopic crystals – too small to see
easily with hand lens; form when
magma cools rapidly (on or near the
surface-extrusive).
 Produced by many very small crystal seeds
that increase in size before growing into
each other.

26
Factors Controlling Texture of Igneous Rocks
 Rate of cooling (most
important)
 Amount of silica (SiO2)
present
 Amount of dissolved gases

27
Major Rock-Forming Silicates
Olivine
Pyroxene
Amphibole MAFIC
Biotite
-----------------------------
Muscovite
Orthoclase FELSIC
Plagioclase
Quartz

28
Classification cont…
Classification
of common
igneous rocks

29
Partial Melting
 Partial melting of a rock makes a magma that is higher in
felsic elements and lower in mafic elements than the
original rock.
 Ultramafic rock (mantle) partially melts to form mafic
magma.
 Rises to make oceanic crust (basalt)
 Mafic rock partially melts to form intermediate magma.
 Rises to make continental crust
 Intermediate rock partially melts to form felsic magma
 Felsic magma partial melts to felsic magma
 Felsic magmas crystallizes in the crust, or makes felsic lava.

30
Crystal Fractionation
 Crystals form,
then sink if
denser than
melt.
 Mafic minerals
sink and magma
becomes lower
in Fe and Mg
and richer in
felsic elements
(Si, K, Na).
 Mafic magma can change to
an intermediate magma
 Occurs in large intrusions at
base of crust

31
Crystal Fractionation cont…

32
Intrusive processes
 Magma stops ascending within crust and
cools slowly  coarse grained rock
 Batholith – very large volume ( 10 – 50 km wide
by 50 – 200 km long)
 usually composed of several plutons (steep sided
intrusions, 1 – 5 km by 5 – 10 km long) which may have
different chemical compositions (granitic to dioritic)
 Dikes – filled fractures within country rock
 Sills – magma intrudes along fractures and
forces its way along layers
 Volcanic neck – lava that solidified within
feeder pipe of volcano

34
Extrusive processes
 Gentle eruptions -- no explosions, just lava
flows or gently erupting lava fountains
 Low viscosity, mainly mafic (rich in MgO
and FeO) lavas -- low SiO2 content
 Common at divergent margins, hot spots
 Produce Shield volcanoes – very large,
gently sloping sides built by lava flows, 25 –
100 km across by 5 – 20 km high – Hawaii,
Iceland, Galapagos Islands

35
Shield volcano: Mauna Loa, Hawaii
Extrusive processes cont…

37
 Violent eruptions – explosions (magma has
dissolved H2O)
 Mainly intermediate to felsic lavas (andesite – rhyolite)
higher in SiO2
 Magma becomes water saturated as crystals settle out
during magma ascent
 Bubbles of water form; H2O gas expands to higher volume –
large pressure increase within magma chamber (like CO2
bubbles in a soda can)
 Water pressure greater than weight of rocks above –
overpressured – results in
 Explosion (like pulling pop-top on soda can)
 Rock and magma fragments thrown into air
(pyroclastic material)

38
A pyroclastic flow: One of the biggest dangers

39
Roman victims of a pyroclastic
flow at Pompeii, Italy, 79 AD

40
 Violent eruptions – explosions (magma
has dissolved H2O)
 Composite volcanoes – 5 – 30 km across, 5
– 10 km tall
 steep sides built from alternating layers
of fragments and lava above subduction
zones Andes, Mt. Fuji
 Calderas – huge eruptions with surface
collapse (Lake Toba, Sumatra, Indonesia)

41
A pyroclastic flow: One of the biggest dangers

Igneous Rocks part 1.pdf

More Related Content

Similar to Igneous Rocks part 1.pdf

More from MuuminCabdulle

Recently uploaded

Igneous Rocks part 1.pdf