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Major extrusive volcanic landforms •Powerpoint on prezi.com •Print out resources •Go through each boundary and associated lava / eruption after fill in blank exercise for each type. •Pupils fill in table •The pupils cut and stick resources to construct a mind map linking the information together and giving examples and explanations. E.g. Oceanic hotpot (diagram) – shield volcano (photo / diagram) – Hawaiian eruption (diagram) –because of mafic lava due to low silicon content as oceanic crust –Low viscosity flows build large shield volcanoes. These are rarely explosive as low water content, but fountains of lava can be shot up to 1,000 feet or more by jets of gas. e.g. Mauna Loa •Pupils the answer exam question. How rocks melt Add heat: Increase temperature of rocks, usually via a mantle plume at a hot spot. Examples include the Hawaiian Islands (oceanic hot spot) and Yellowstone continental hot spot). Decrease pressure: Decrease the pressure on rock by removing overlying material. This is accomplished at spreading ridges associated with divergent plate boundaries. This occurs at all mid-ocean ridges, but is exposed only on Iceland. Add water: Adding water (or any other material) will decrease the melting temperature of rock. This process is important in subduction zones where water is released from the down going plate. Examples include the Andes in South America, the Cascade volcanoes in the U.S., the Aleutians in Alaska, and Japan and other islands in south east Asia. Divergent plate boundaries: Most volcanically active regions on Earth. Most occurs ________ ________ and is not directly observed by humans. Part of the mid-Atlantic ridge is exposed on _______. Characterized by basaltic volcanism. Dominated by lava flows, explosive eruptions are _______. Decompression of mantle material beneath spreading ridges causes partial melting of mantle material, resulting in _______. Also occur in continental rifts where divergent boundaries just starting to form (East African Rift) and where continental crust is being pulled apart (i.e. Colorado Plateau – Basin and Range boundary, Cedar City area) Divergent plate boundaries: Most volcanically active regions on Earth. Most occurs under oceans and is not directly observed by humans. Part of the mid-Atlantic ridge is exposed on Iceland. Characterized by basaltic volcanism. Dominated by lava flows, explosive eruptions are rare. Decompression of mantle material beneath spreading ridges causes partial melting of mantle material, resulting in basalts. Also occur in continental rifts where divergent boundaries just starting to form (East African Rift) and where continental crust is being pulled apart (i.e. Colorado Plateau – Basin and Range boundary, Cedar City area) Convergent plate boundaries • Subducting plates at convergent plate boundaries carry ______into the upper parts of the mantle. At about 150 Km depths, reactions occurring in the subducting plate release water into the overlying _________, thus lowering the melting temperature of mantle material. The mantle undergoes ________melting producing basalt. This basaltic magma rises to the base of the continental crust. This magma has a temperature (1200°C-1400°C) much higher than the melting temperature of the crust (650°C-750°C), causing the crust to melt. • Melting of continental crust produces magmas with _________ compositions. Mafic basaltic magmas and felsic magmas are thought to mix, thus forming magmas of intermediate composition. Thus, subduction zone volcanoes erupt ___________ and Rhyolite lavas, with minor amounts of basalts. • Because water is released from the subducting slab at a constant depth (about 120 km), chains of ______________ tend to form along a line directly above the 120 km zone of melting. These volcanoes tend to be regularly spaced at about 70 km, and all are potentially eruptive at the same time. volcanoes / Mantle / water / partial / felsic / Andesite, Convergent plate boundaries • Subducting plates at convergent plate boundaries carry water into the upper parts of the mantle. At about 150 Km depths, reactions occurring in the subducting plate release water into the overlying mantle, thus lowering the melting temperature of mantle material. The mantle undergoes partial melting producing basalt. This basaltic magma rises to the base of the continental crust. This magma has a temperature (1200°C-1400°C) much higher than the melting temperature if the crust (650°C-750°C), causing the crust to melt. • Melting of continental crust produces magmas with felsic compositions. Mafic basaltic magmas and felsic magmas are thought to mix, thus forming magmas of intermediate composition. Thus, subduction zone volcanoes erupt andesite, and rhyolite lavas, with minor amounts of basalts. • Because water is released from the subducting slab at a constant depth (about 120 km), chains of volcanoes tend to form along a line directly above the 120 km zone of melting. These volcanoes tend to be regularly spaced at about 70 km, and all are potentially eruptive at the same time. • Rhyolite and andesite are often saturated with _______, so explosive eruptions are common. ____________ can also erupt as lava flows, and Rhyolite (if dry) can erupt to form ______ domes. Add occasional basalt flows to this mix, and we end up with complex _________ volcanoes. These are the classic cone-shaped volcanoes we are familiar with. Composite volcanoes consist of a mix of ___________materials and lava flows, having a wide range of compositions (basalt to rhyolite). • Rhyolite and andesite are often saturated with water, so explosive eruptions are common. Andesite can also erupt as lava flows, and rhyolite (if dry) can erupt to form lava domes. Add occasional basalt flows to this mix, and we end up with complex composite volcanoes. These are the classic cone-shaped volcanoes we are familiar with. Composite volcanoes consist of a mix of pyroclastic materials and lava flows, having a wide range of compositions (basalt to rhyolite). Hotspot Volcanoes • These are volcanoes that form over hot _______ of material rising from the base of the mantle (mantle plumes). Mantle plumes interact with either oceanic ________ or continental lithosphere. In either case, the plume heats one part of the lithosphere and a volcano forms above the plume. As _______ move, different parts of the lithosphere are carried over the plume, resulting in a ______ of volcanoes, of which only one is _______. Volcanoes along a hot spot chain become older away from the active volcanic center. Hotspot Volcanoes • These are volcanoes that form over hot plumes of material rising from the base of the mantle (mantle plumes). Mantle plumes interact with either oceanic lithosphere or continental lithosphere. In either case, the plume heats one part of the lithosphere and a volcano forms above the plume. As plates move, different parts of the lithosphere are carried over the plume, resulting in a chain of volcanoes, of which only one is active. Volcanoes along a hot spot chain become older away from the active volcanic center. Oceanic hotspots: • Where a mantle plume interacts with oceanic lithosphere • Only basaltic magmas are produced • Basalts have low viscosities and tend to have low water contents. • Lava flows are the primary product of eruptions • Low viscosity flows build large shield volcanoes. • Cinder cones develop near vents by fountaining lava and very minor pyroclastic eruptions. Oceanic hotspots: • Where a mantle plume interacts with oceanic _____________ • Only basaltic magmas are produced • Basalts have low _________and tend to have low water contents. • Lava flows are the ________ product of eruptions • Low viscosity flows build large __________ volcanoes. • Cinder cones develop near vents by ____________lava and very minor pyroclastic eruptions. shield /lithosphere / fountaining / viscosities / primary Continental hotspots: With continental hotspots, melting of continental crust results in the development of huge felsic magma chambers. Pressure builds up within the chamber until the crust above the chamber breaks. With a sudden release of pressure, the chamber rapidly empties itself in massive eruptions, and the overlying crust collapses into the magma chamber forming large calderas. These are “super volcanoes” and represent the largest eruptions known. No eruptions of this type have occurred during recorded history. These volcanoes produce massive eruption columns, and pyroclastic flows that can extend hundreds of miles. Measurable ash falls can cover the earth, and material suspended in the atmosphere can contribute to planetary cooling. Continental hotspots: With continental hotspots, melting of continental crust results in the development of huge _______ magma chambers. Pressure builds up within the chamber until the crust above the chamber breaks. With a sudden release of pressure, the chamber rapidly empties itself in massive eruptions, and the overlying crust _________into the magma chamber forming large calderas. These are “super volcanoes” and represent the largest eruptions known. ____ eruptions of this type have occurred during recorded history. These volcanoes produce massive eruption _________ , and pyroclastic flows that can extend hundreds of miles. Measurable ash falls can cover the earth, and material suspended in the atmosphere can contribute to planetary_________. columns / No/ felsic / cooling / collapses / Types of Lava Rhyolite Felsic Andesite Intermediate Basaltic Basaltic Viscosity Silicon Flow rate Temperature Materials erupted Frequency of eruption Type of volcano Example Plate boundary type •Slow • Medium •fast •Long dormant periods •Regular / continuous •Time to time •Lava domes / Calderas •Composite •Shield / fissure / cinder •Mainly Lava •Tephra •Tephra / lava •70% •60% •50% •900 •11000 •10000 Continental hot spots / continental – Oceanic continental – Oceanic convergent / Oceanic hot spots / oceanic – oceanic Types of Lava Rhyolite Felsic Andesite Intermediate Basaltic Basaltic Viscosity Very Increasing Runny Silicon 70% 60% 50% Flow rate slow Medium fast Temperature 900 degrees 10000 11000 Materials erupted Tephra / pyroclastics Tephra / pyroclastics lava Mainly Lava Frequency of eruption Long dormant periods Time to time Regular / continuous Type of volcano Example Lava domes / Calderas composite Shield / fissure / Ash cone Plate boundary type Continental hot spots / continental – Oceanic convergent continental – Oceanic hot spots / Oceanic convergent oceanic – oceanic / continental – divergent continental divergent In what ways does volcanic activity vary in relation to the type of plate margin along which it occurs? (10 marks) GEOG 3 June 11 Mark scheme Level 1 (1-4 marks) (Midpoint 3) Simple statements of variation of volcanic activity between plate margins. No specific detail or elaboration provided; or activity at one margin discussed well. Level 2: (5-8 marks) (Midpoint 6) Specific statements of a range of variations. Elaboration that demonstrates good understanding of the interrelationships between type and frequency of volcanic activity at plate margins. May be use of case studies to support. Level 3 (9-10 marks) (Midpoint 9) A fully developed answer, with good elaboration of a range of variations between plate margins. A rounded answer with a full comparison (most of features given in table above) of the two main types of plate margin. Good use of case studies, though not a requirement.