- when rocks are heated to their melting point - the confining pressure around hot rock is reduced - the water content of the rock is increased (for &quot;dry&quot; rocks) 26 CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA Formation of Basaltic Magma - partial melting of peridotite - reduction in pressure is the more important control - as magma rises, pressure decreases, resulting in the lowering of melting temperature 27 CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA Formation of Granitic Magma - partial melting of water rich rocks subjected to increase temperature - the water content is the most important control - as the magmas rises, escape of water, pressure decreases, thus lowering the melting point - the magma often solidifies in the sub-surface 28 CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA Evolution of magma 1) partial melting (composition, temperature, confining pressure and water content) 2) magmatic differentiation 3) assimilation 29 CHAPTER 3 IGNEOUS ROCKS DISTRIBUTION OF IGNEOUS ROCKS Rift zones - occur where lithospheric plates separate (divergent zones)- exclusively basaltic magmas - fissure eruption but occasional vent eruptions - magma originates from partial melting of peridotite - landforms include extensive flood basalt (seafloor), oceanic ridges and shield volcanoes 30 CHAPTER 3 IGNEOUS ROCKS DISTRIBUTION OF IGNEOUS ROCKS Subduction zones - occur where lithospheric plates converge - associated with andesitic and granitic magmas - vent-type eruption but occasional fissure eruptions - eruptions are explosive - magma originates from the partial melting of descending water rich ocean crust and continental sediments 31 CHAPTER 3 IGNEOUS ROCKS DISTRIBUTION OF IGNEOUS ROCKS Subduction zones - landforms include volcanic chains or Island arcs with andesitic and rhyolitic volcanoes n ocean/ocean subduction (Aleutian Is.)
(rocks; are; heated to their melting point) (the water content; is; the most important control) (pressure; decreases; ) (lithospheric plates; separate; divergent zones) (the magmas; rises; ) (landforms; include; volcanic chains or Island arcs with andesitic and rhyolitic volcanoes) Context(landforms include,List([1719, 1736))):(occasional fissure eruptions; are; explosive - magma originates from the partial melting of descending water rich ocean crust and continental sediments) Context(landforms include,List([1719, 1736))):(Aleutian; Is; ) (water rich rocks; subjected; to increase temperature) (magma; rises; ) (the magma; solidifies; L:in the sub-surface 28 CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA Evolution of magma 1; CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA Evolution of magma 1) partial melting; T:often)
landforms include,List([1719, 1736)) landforms include,List([1719, 1736))
rocks the water content pressure lithospheric plates the magmas landforms occasional fissure eruptions Aleutian water rich rocks magma the magma
are is decreases separate rises include are Is subjected rises solidifies
heated to their melting point
the most important control
divergent zones
volcanic chains or Island arcs with andesitic and rhyolitic volcanoes
explosive - magma originates from the partial melting of descending water rich ocean crust and continental sediments
to increase temperature
L:in the sub-surface 28 CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA Evolution of magma 1 CHAPTER 3 IGNEOUS ROCKS ORIGIN OF MAGMA Evolution of magma 1) partial melting T:often