Igneous Processes
GLG 101 - Physical Geology
Bob Leighty
These notes and web links are your primary “lecture” content in this class.
Additionally, various articles are assigned each week to supplement this “lecture”
information. I believe you’ll have enough information to reference without having
to purchase a costly textbook.
These lecture notes are very similar to the ones I use in my traditional classes.
You’ll find they are loaded with imagery and streamlined text that highlight the
most essential terms and concepts. The notes provide a framework for learning
and, by themselves, are not meant to be a comprehensive source of information.
To take advantage of the global knowledge base known as the Internet, I have
included numerous hyperlinks to external web sites (like the Wikipedia, USGS,
NASA, etc.). Follow the links and scan them for relevant info. The information
from linked web sites is meant to supplement and reinforce the lecture notes –
you won’t be responsible for knowing everything contained in them.
As a distance learning student, you need to explore and understand the content
more independently than in a traditional class. As always, I will help guide you
through this learning adventure. Remember, email Dr. Bob if you have any
questions about today’s lecture (rleighty@mail.mc.maricopa.edu).
Leave no questions behind!
Explore and have fun!
Igneous
Processes
Heat Within the Earth
 The Earth has a lot of trapped heat (from radioactive decay)
colder
shallow
Temperature
hotter
lower T & P
X
Temperature increases
with depth
Depth
higher T & P
deep
Increasing pressure downward typically
keeps rocks from melting
Igneous
Processes
Heat Within the Earth
 Some areas have different geothermal gradients
0
0
Temperature (°C)
1000
Depth
(km)
50
Lower gradients (“cold”) <20C/km (continental interiors)
Higher gradients (“hot”) >40C/km (active volcanic areas)
Igneous
Processes
How Does Solid Rock Melt?
 Solid rock melts (become magma) when it is heated above a
certain temperature
0
Temperature (°C)
1000
0
Depth
(km)
geothermal
gradient
solid rocks
magma
50
melting temperature
(increases with depth)
Igneous
Processes
How Does Solid Rock Melt?
 Melting temperature is controlled by the composition of the rock
0
0
Depth
(km)
50
1000
Temp (°C)
geothermal
gradient
magma
solid rocks
granite melting T
(lower)
basalt melting T
Igneous
Processes
How Does Solid Rock Melt?
 Melting temperature is also controlled by the presence of water
(water lowers the melting T – makes it easier to melt)
0
Temp (°C)
0
Depth
(km)
geothermal
gradient
1000
magma
solid rocks
50
“wet” basalt melting T
(lower)
“dry” basalt melting T
Igneous
Processes
Different Ways Rocks Can Melt
 Rocks can melt by:
1) increasing T
2) decreasing P
0
Temp (°C)
0
Depth
(km)
geothermal
gradient
3) adding water
1000
magma
solid rocks
50
 Partial melting: only part of the rock melts
Igneous
Processes
Magmatism & Plate Tectonics
Divergent Margins
(mid-ocean ridges, continental rifts)
 Plates pull apart, lets mantle rocks rise & melt
(decompression melting)
 Forms oceanic crust
 Example: Mid-Atlantic Ridge, Red Sea,
East Pacific Rise
Igneous
Processes
Magmatism & Plate Tectonics
Intraplate “Hot spots”
 Rocks in a mantle plume rise &
melt (decompression melting)
 Intense heating melts overlying
lithosphere
 Example: Hawaiian Islands (oceanic crust),
Yellowstone (continental crust)
Igneous
Processes
Magmatism & Plate Tectonics
Convergent Margins
(subduction zones)
 Hydrous (“wet’) melting above
down-going slab
 Volcanic arcs
 Most continental crust forms
in this way
 Example: Andes, Cascades
Igneous
Processes
Rising Magma
 Magma will start to rise due to a buoyancy contrast with
surrounding rock
 Magma forms diapirs (magma balloons)
 Differentiation: magmas may change as they rise
Igneous
Processes
Magma Differentiation
 Magma mixing - magmas may mix & contaminate each other
Igneous
Processes
Magma Differentiation
 Assimilation - contamination of a magma by wall rock
Igneous
Processes
Magma Differentiation
 Assimilation - contamination of a magma by wall rock
Igneous
Processes
Magma Differentiation
 Assimilation - contamination of a magma by wall rock
Igneous
Processes
Magma Differentiation
 Crystallization – Depending on a magma’s composition, certain
minerals will begin to crystallize as the magma cools
Mafic minerals
Igneous
Processes
Magma Differentiation
 Crystal settling – Heavy, early-formed crystals (like olivine, rich
in Fe, Mg, & Ca) sink & form layers
Igneous
Processes
WWW Links in this Lecture
> Radioactive decay - http://en.wikipedia.org/wiki/Radioactive_decay
> Geothermal gradient - http://science.enotes.com/earth-science/geothermal-gradient
> Magma - http://en.wikipedia.org/wiki/Magma
> Partial melting - http://en.wikipedia.org/wiki/Magma#Partial_melting
> Oceanic crust - http://en.wikipedia.org/wiki/Oceanic_crust
> Mid-Atlantic Ridge - http://en.wikipedia.org/wiki/Mid-Atlantic_Ridge
> Red Sea - http://en.wikipedia.org/wiki/Red_Sea
> East Pacific Rise - http://en.wikipedia.org/wiki/East_Pacific_Rise
> Divergent boundary - http://en.wikipedia.org/wiki/Divergent_boundary
> Hot spot - http://en.wikipedia.org/wiki/Hotspot_%28geology%29
> Mantle plume - http://en.wikipedia.org/wiki/Mantle_plume
> Hawaiian Islands - http://en.wikipedia.org/wiki/Hawaiian_Islands
> Yellowstone - http://en.wikipedia.org/wiki/Yellowstone_Caldera
> Convergent boundary - http://en.wikipedia.org/wiki/Convergent_boundary
> Volcanic arc - http://en.wikipedia.org/wiki/Volcanic_arc
> Continental Crust - http://en.wikipedia.org/wiki/Continental_crust
> Andes - http://en.wikipedia.org/wiki/Andes
> Cascades - http://en.wikipedia.org/wiki/Cascade_Range
Igneous
Processes
WWW Links in this Lecture
> Buoyancy - http://en.wikipedia.org/wiki/Buoyancy
> Magma differentiation - http://en.wikipedia.org/wiki/Igneous_differentiation
> Magma mixing - http://en.wikipedia.org/wiki/Igneous_differentiation#Magma_mixing
> Magma assimilation - http://en.wikipedia.org/wiki/Igneous_differentiation#Assimilation
> Crystallization -
http://en.wikipedia.org/wiki/Igneous_differentiation#Fractional_crystallization_of_igneous_rocks