1. The Origins of Magma and Igneous Rocks
Introduction to Geology
Volcanic Stones
The Origins of Magma and Igneous Rocks

2. The Rock Cycle

3. The Formation of Igneous Rocks
Eruption as lava or ash
Forms magma chamber (solidifies or rises)
EXPLANATION
Melting occurs at depth ( km)
Magma rises because it is less dense
Accumulates into rising magma body
Partial melting of source
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4. The Origins of Magma
The four main factors that influence the origins of magma are:
* Temperature * Water Content
* Pressure * Mineral Composition

5. Geologic Settings in which Magma Originates

6. Magma Formation and the Geothermal Gradient

7. The Formation of Magma EXPLANATION
Line slopes down to the right because higher temperatures are needed to melt a rock under higher pressure
Position of line depends on the composition of source rock (farther left for more felsic compositions)
Point B is same pressure as A, but at a higher temperature so plots in liquid field
Point C is at same temperature as B, but at higher pressure so plots in solid field
Point D will have solid rock and magma
INSTRUCTIONS TO STUDENTS
(after most of slide) Observe this graph and think about how changing conditions could cause a rock starting at point A and point C to melt
OBSERVATIONS
Increase temperature for both A and C
Decrease pressure for C
Decrease pressure and increase temperature for C

8. The Formation of Magma INSTRUCTIONS TO STUDENTS
Observe what happens when a hot rock at point C is uplifted so rapidly it does not have time to cool off
EXPLANATION
A hot rock at point C may melt as the pressure decreases (point B)
For decompression melting to occur, rock must be hot and uplifted faster than it can cool off
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9. The Formation of Magma Solid Pressure tends to hold lattice together
Liquid
INSTRUCTIONS TO STUDENTS
(after top parts of slide) Think about how changing temperature or pressure could cause melting
NOTES
You may introduce this subject by using the more familiar example of temperature versus humidity. Draw a temperature-humidity graph and ask how conditions could change to change to move you from a comfortable region to one that is too hot and sticky.
Increasing temperate vibrates atoms and can break bonds
Melting Rocks
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10. Decompression melting
The Formation of Magma
INSTRUCTIONS TO STUDENTS
Think about how melting occurs along a divergent boundary
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05.06.a1
Solid asthenosphere rises to fill gap
Decompression melting

11. The Formation of Magma Pillow basalt Sheeted dikes of basalt Gabbro
EXPLANATION
Pillow basalts: erupted into water
Sheeted dikes: vertical dikes of finely crystalline basalt
Gabbro: magma chamber (base of gabbro is base of oceanic crust)
Mantle: Ultramafic rocks
Mantle
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12. The Formation of Magma Decompression Crustal Heating 05.06.c
INSTRUCTIONS TO STUDENTS
Think about how melting occurs in a continental rift, both in the mantle and in the crust
EXPLANATION
Melting in the mantle caused by decompression melting
Melting in the crust depends on composition, but mafic mantle-derived magma can melt felsic crust
Decompression
Crustal Heating

13. The Formation of Magma Few volcanoes Felsic rocks (granite)
Burial and heating of crustal rocks
INSTRUCTIONS TO STUDENTS
Think about how melting occurs during a continental collision
EXPLANATION
Descending crust gets hotter
If it gets hot enough, partial melting occurs, which is aided by water released during metamorphism
Magmas typically do not reach the surface because of the high water content
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Melting during subduction before collision (but shuts off)

14. The Formation of Magma Oceanic island Rising mantle plume
Crustal melting and caldera in continent
EXPLANATION
Mantle plume spreads out along the boundary, causing melting of overlying lithosphere
In oceans, hot spots may form oceanic islands
On continents, mafic magma erupts as basalt
On continents, hot spots erupt explosively if composition is felsic or intermediate
Huge basalt flows on continents

15. The Formation of Magma Mostly felsic and intermediate rocks
Mafic magma melts crust
INSTRUCTIONS TO STUDENTS
Think about how melting occurs along an ocean-continent convergent boundary
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Adding water melts mantle

16. The Formation of Magma Mostly mafic and intermediate rocks
EXPLANATION
As plate subducts, temperature and pressure increase
Water supplied by the breakdown of water-bearing minerals
Water lowers the melting temperature, causing melting of overlying asthenosphere (mantle wedge)
Add water to hot mantle
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17. The Formation of Magma
Hot water circulates in ridge; added in pores and in minerals
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Water in minerals released at depth

18. Mantle Melting and Plate Tectonics
The Formation of Magma
To view this animation, click “View” and then “Slide Show” on the top navigation bar. 18

19. The Formation of Magma Subduction beneath continent
Observe these settings where large magma chambers form, and propose causes of melting for each site
Subduction beneath continent
Continental collision
Mid-ocean ridge
INSTRUCTIONS TO STUDENTS
Observe these settings where large magma chambers form, and propose causes of melting for each site
EXPLANATION
Oceanic hot spot: decompression melting
Mid-ocean ridge: decompression melting
Subduction zones: increase in water and increase in temperature
Continental hot spot: decompression melting and crustal melting by heating
Continental collision: mostly increase in temperature
Oceanic hot spot
Continental hot spot
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20. The Origins of Magma and Igneous Rocks
This Concludes
Volcanic Stones
The Origins of Magma and Igneous Rocks