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 Page 22 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 Page 23 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 Page 24 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 Page 25 FACTORS THAT INFLUENCE MELTING OF ROCKS 1) Temperature - usually between 800-1200 O C 2) Rock Composition - granite melts ~ 800 O C 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 700 O C while wet granite (10-15% water) melts at 600 O C 25 Page 26 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) (Rock Composition - granite melts; ~; 800 O C and basalt ~12000C; T:usually) (this group - rocks within this group; are known; as andesitic; T:after the rock andesite - color and specific gravity lie between basalt and granite e.g. diorite and andesite Gradations in composition) (- partial melting; is; the incomplete melting of a rock to produce magma - composition of the magma) Context(- partial melting is,List([855, 875))):(21 Page 22 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 Page 23 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 Page 24 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 Page 25 FACTORS THAT INFLUENCE MELTING OF ROCKS 1) Temperature - usually between 800-1200 O C 2) Rock Composition - granite melts ~ 800 O C 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 700 O C while wet granite (10-15% water) melts at 600 O C)
- partial melting is,List([855, 875))
granite these rocks the term granitic Rock Composition - granite melts this group - rocks within this group - partial melting 21 Page 22 NAMING OF IGNEOUS ROCKS 3) From BRS, rocks of intermediate composition lie between basalt and granite - the minerals Amphiboles, and Na/K-Plagioclase
is rhyolite is used ~ are known is are
a common rock with this composition
From BRS
to denote rocks of this composition
800 O C and basalt ~12000C T:usually
as andesitic T:after the rock andesite - color and specific gravity lie between basalt and granite e.g. diorite and andesite Gradations in composition
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 Page 23 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 Page 24 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 Page 25 FACTORS THAT INFLUENCE MELTING OF ROCKS 1) Temperature - usually between 800-1200 O C 2) Rock Composition - granite melts ~ 800 O C 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 700 O C while wet granite (10-15% water) melts at 600 O C