  Factors that determine the eruption intensity:  Composition, magma temp, dissolved gasses  Types of lava:lava  Pahoehoe : resembles rope braids  Aa : rough, jagged blocks  Pyroclastic materials:  Ash and dust : fine, glassy fragments  Pumice : from frothy lava  Lapilli : walnut sized  Cinders: pea-sized  Particles larger than lapilli:  Blocks: hardened lava  Bombs : ejected as hot lava  Factors that determine the eruption intensity:  Composition, magma temp, dissolved gasses  Types of lava:lava  Pahoehoe : resembles rope braids  Aa : rough, jagged blocks  Pyroclastic materials:  Ash and dust : fine, glassy fragments  Pumice : from frothy lava  Lapilli : walnut sized  Cinders: pea-sized  Particles larger than lapilli:  Blocks: hardened lava  Bombs : ejected as hot lava 1 2: Volcanic Eruptions Kilauea lava ocean entry, (2:15) Kilauea flow Image: USGS Volcanic eruption at Fimmvörðuháls in Iceland Image: Olikristinn, CC-A-SA 3.0 Mayan Volcano, Philippines, USGS, C.G. Newhall

  Shield: Broad, slightly domed, large  Primarily made of basaltic lava (fluid)  Cinder Cone: built from ejected fragments  Composite Cone (Stratovolcano):  Large, most adjacent to Pacific Ocean  Violent, produce pyroclastic flow of hot gases  May produce a lahar-volcanic mudflow 2 Volcano Types Kilauea, Nandaro, CC-A-SA 3.0 Lassen Volcanic NP Calif, FanaticTRX, CC-A-SA 3.0 Mt. Adams Wash., Benjamin Zingg, CC-A-SA 3.0

  Cocos subducting beneath N. American plate  Large earthquakes in subduction zones  Two plates grind, sometimes “lock” together  Lower plate (6 cm/yr) pulls upper plate down  Hundreds of years stress builds before quake  Stress builds up to breaking, then springs back  Ocean floor lifts and produces earthquake  Sometimes tsunami produced  Volcano chain associated with subduction  Produced when: subducting oceanic crust melts  Causes metamorphism  Releases water, triggers mantle melting  Resulting magma rises to surface as lava  Forming volcanoes  Cocos subducting beneath N. American plate  Large earthquakes in subduction zones  Two plates grind, sometimes “lock” together  Lower plate (6 cm/yr) pulls upper plate down  Hundreds of years stress builds before quake  Stress builds up to breaking, then springs back  Ocean floor lifts and produces earthquake  Sometimes tsunami produced  Volcano chain associated with subduction  Produced when: subducting oceanic crust melts  Causes metamorphism  Releases water, triggers mantle melting  Resulting magma rises to surface as lava  Forming volcanoes Case: Mexican Subduction Zone Mexico Subduction Zone, (slides 2-4)

  Most subduction zones: quakes occur at interface between plates  At depths from surface to 600 km called “ Benioff Zone ” Benioff Zone  This locates the subducting plate  Unusual: few Mexico quakes below 100 km  Most subduction: inland volcanic arc parallel to trench  S. Mexico: volcanic arc bends 15° away from trench  Most subduction zones: giant quakes about every 100 yrs  Southern Mexico: large quakes near coast  Slow-slip events or silent earthquakes occur  Month long single event, cannot be felt  Seismic stations found subducting slab beneath Mexico  Nearly horizontal, i.e. flat slab subduction Geological Feature

  30 mya: normal subduction produces coastal volcanoes  29 mya: oceanic plate resists downward pull  Remains against overlying continental crust  This extinguishes coastal volcanic arc  19 mya: subducted slab far reaches loses buoyancy  Subducted slab begins to sink  Volcanic activity resumes but inland  Arc rolls back toward Pacific coast  7 mya: subducted oceanic plate breaks  Present: flat subduction continues  Reaches inland 250 km  Normal earthquakes occur in this region Flat Slab Subduction