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

The Rock Cycle

The Formation of Igneous Rocks Eruption as lava or ash Forms magma chamber (solidifies or rises) EXPLANATION Melting occurs at depth (40-150 km) Magma rises because it is less dense Accumulates into rising magma body Partial melting of source 05.03.a1

The Origins of Magma The four main factors that influence the origins of magma are: * Temperature * Water Content * Pressure * Mineral Composition

Geologic Settings in which Magma Originates

Magma Formation and the Geothermal Gradient

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

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 05.05.c2

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 05.05.a1-3

Decompression melting The Formation of Magma INSTRUCTIONS TO STUDENTS Think about how melting occurs along a divergent boundary 05.06.c2 05.06.a1 Solid asthenosphere rises to fill gap Decompression melting

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 05.06.b4

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

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 05.07.c1 Melting during subduction before collision (but shuts off)

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

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 05.07.b1 Adding water melts mantle

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 05.07.a

The Formation of Magma Hot water circulates in ridge; added in pores and in minerals 05.07.d1 Water in minerals released at depth

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

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 05.11.b1

The Origins of Magma and Igneous Rocks This Concludes Volcanic Stones The Origins of Magma and Igneous Rocks