Since granite is a common rock with this composition, the term granitic is used to denote rocks of this composition - these rocks are also known as FELSIC (feldspar/silica) - the rocks are generally light in color , and significantly lighter than most rocks in the earth's crust (s.g. ~2.67) e.g. granite and rhyolite 21 CHAPTER 3 IGNEOUS ROCKS NAMING OF IGNEOUS ROCKS 3) From BRS, rocks of intermediate composition lie between basalt and granite - the minerals Amphiboles , and Na/K-Plagioclase are the main minerals of this group - rocks within this group are known as andesitic after the rock andesite - color and specific gravity lie between basalt and granite e.g. diorite and andesite Gradations in composition: granite - granodiorite - andesite - basalt - peridotite (ultramafic) 22 CHAPTER 3 IGNEOUS ROCKS PLATE TECTONICS AND IGNEOUS ROCKS ORIGIN OF MAGMA Partial Melting of Rocks - partial melting is the incomplete melting of a rock to produce magma - composition of the magma is different from that of the parent rock - generates rocks with higher silica content than parent rock 23 CHAPTER 3 IGNEOUS ROCKS PLATE TECTONICS AND IGNEOUS ROCKS ORIGIN OF MAGMA Heat source for melting rocks 1) heat from radioactive decay in the upper mantle/crust 2) geothermal heat (gradient of 20-300C/km) 3) heat transfer - from mantle plumes/convection cells 4) frictional heat (friction in subducting plates) 24 CHAPTER 3 IGNEOUS ROCKS FACTORS THAT INFLUENCE MELTING OF ROCKS 1) Temperature - usually between 800-1200OC 2) Rock Composition - granite melts ~ 800OC and basalt ~12000C (reflects silica content - BRS) 3) Pressure at Melting - a drop in pressure will cause hot rock to melt resulting in &quot;pressure relief melting&quot; without need for increased temperature - most important at spreading centers 4) Water Content of Rock - water lowers the melting point of magmas - dry granite melts at 700OC while wet granite (10-15% water) melts at 600OC 25 CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA SO WHEN DO ROCKS MELT?
(granite; is; a common rock with this composition) (these rocks; rhyolite; From BRS) (the term granitic; is used; to denote rocks of this composition) (this group - rocks within this group; are known; as andesitic; T:after the rock andesite - color and specific gravity lie between basalt and granite) (pressure relief melting&quot; without need for increased temperature - most important at spreading centers 4) Water Content of Rock - water; lowers; the melting point of magmas - dry granite melts) Context(pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([1702, 1848))):(3) Pressure at Melting - a drop in pressure; will cause; hot rock to melt resulting in &quot) Context(pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([1702, 1848))):(800OC and basalt ~12000C; reflects; silica content - BRS) Context(pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([1702, 1848))):(- partial melting; is; the incomplete melting of a rock to produce magma - composition of the magma) Context(- partial melting is pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([889, 1848))):(21 CHAPTER 3 IGNEOUS ROCKS NAMING OF IGNEOUS ROCKS 3; are; the main minerals of this group - rocks within this group are known as andesitic after the rock andesite - color and specific gravity lie between basalt and granite e.g. diorite and andesite Gradations in composition: granite - granodiorite - andesite - basalt - peridotite (ultramafic) 22 CHAPTER 3 IGNEOUS ROCKS PLATE TECTONICS AND IGNEOUS ROCKS ORIGIN OF MAGMA Partial Melting of Rocks - partial melting is the incomplete melting of a rock to produce magma - composition of the magma is different from that of the parent rock - generates rocks with higher silica content than parent rock 23 CHAPTER 3 IGNEOUS ROCKS PLATE TECTONICS AND IGNEOUS ROCKS ORIGIN OF MAGMA Heat source for melting rocks 1) heat from radioactive decay in the upper mantle/crust 2) geothermal heat (gradient of 20-300C/km) 3) heat transfer - from mantle plumes/convection cells 4) frictional heat (friction in subducting plates) 24 CHAPTER 3 IGNEOUS ROCKS FACTORS THAT INFLUENCE MELTING OF ROCKS 1) Temperature - usually between 800-1200OC 2) Rock Composition - granite melts ~ 800OC and basalt ~12000C (reflects silica content - BRS) 3) Pressure at Melting - a drop in pressure will cause hot rock to melt resulting in &quot;pressure relief melting&quot; without need for increased temperature - most important at spreading centers 4) Water Content of Rock - water lowers the melting point of magmas - dry granite melts at 700OC while wet granite (10-15% water) melts at 600OC)
pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([1702, 1848)) pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([1702, 1848)) pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([1702, 1848)) - partial melting is pressure relief melting & quot ; without need for increased temperature - most important at spreading centers 4 ) Water Content of Rock - water lowers,List([889, 1848))
granite these rocks the term granitic this group - rocks within this group pressure relief melting&quot 3) Pressure at Melting - a drop in pressure 800OC and basalt ~12000C - partial melting 21 CHAPTER 3 IGNEOUS ROCKS NAMING OF IGNEOUS ROCKS 3
is rhyolite is used are known without need for increased temperature - most important at spreading centers 4) Water Content of Rock - water will cause reflects is are
a common rock with this composition
From BRS
to denote rocks of this composition
as andesitic T:after the rock andesite - color and specific gravity lie between basalt and granite
lowers the melting point of magmas - dry granite melts
hot rock to melt resulting in &quot
silica content - BRS
the incomplete melting of a rock to produce magma - composition of the magma
the main minerals of this group - rocks within this group are known as andesitic after the rock andesite - color and specific gravity lie between basalt and granite e.g. diorite and andesite Gradations in composition: granite - granodiorite - andesite - basalt - peridotite (ultramafic) 22 CHAPTER 3 IGNEOUS ROCKS PLATE TECTONICS AND IGNEOUS ROCKS ORIGIN OF MAGMA Partial Melting of Rocks - partial melting is the incomplete melting of a rock to produce magma - composition of the magma is different from that of the parent rock - generates rocks with higher silica content than parent rock 23 CHAPTER 3 IGNEOUS ROCKS PLATE TECTONICS AND IGNEOUS ROCKS ORIGIN OF MAGMA Heat source for melting rocks 1) heat from radioactive decay in the upper mantle/crust 2) geothermal heat (gradient of 20-300C/km) 3) heat transfer - from mantle plumes/convection cells 4) frictional heat (friction in subducting plates) 24 CHAPTER 3 IGNEOUS ROCKS FACTORS THAT INFLUENCE MELTING OF ROCKS 1) Temperature - usually between 800-1200OC 2) Rock Composition - granite melts ~ 800OC and basalt ~12000C (reflects silica content - BRS) 3) Pressure at Melting - a drop in pressure will cause hot rock to melt resulting in &quot;pressure relief melting&quot without need for increased temperature - most important at spreading centers 4) Water Content of Rock - water lowers the melting point of magmas - dry granite melts at 700OC while wet granite (10-15% water) melts at 600OC