1

Factors that determine the eruption

intensity: Composition, magma temp, dissolved

gasses Types of 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

2: Volcanic Eruptions

Kilauea lava ocean entry, (2:15)

Kilauea flowImage: USGS

Volcanic eruption at Fimmvörðuháls in IcelandImage: Olikristinn, CC-A-SA 3.0

Mayan Volcano, Philippines, USGS, C.G. Newhall

2

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

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

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” 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