Volcanoes and volcanism *Volcanoes represent venting of the Earth’s interior *Molten magma rises within the Earth and is erupted either quietly (lavas) or violently (pyroclastics)

Quiet vs. violent activity *Quiet eruptions tend to produce lava flows, which are not so dangerous *Explosive eruptions produce fragmental, or pyroclastic, material; these are dangerous *Two controls on explosivity are (1) the silica content and (2) the gas content of the magma Basalt: 50% SiO 2, gas-poor Andesite: 60% SiO 2, gas-rich Rhyolite: 70% SiO 2, gas-rich *Magmas with higher silica contents are more viscous

Global distribution of volcanoes

Magma generation at mid- ocean ridges In these zones, the mantle rises and melts, producing magma of silicate composition the magma continues to rise, and erupts mainly as basaltic lava flows

Magma generation at hot spots Magmas at hot spots are derived from deep within the mantle The magmas are fed by deep mantle plumes which are stationary relative to the drifting tectonic plates

Magma generation at subduction zones During subduction, the subducted oceanic plate is heated as it plunges into the mantle At a depth of km, melting begins, and volcanoes are produced which parallel the subduction zone Andesitic magmas are typical of these volcanoes

Plate tectonics and volcanism

Volcanic hazards of North America “Active” volcanoes have erupted at least once in the past 10,000 years The most active volcanoes (in red) are those associated with subduction zones

Volcanic hazards of Canada Canada has “active” volcanoes (black triangles) which pose a potential threat in B.C. Another major hazard is ashfall from explosive eruptions of Cascade volcanoes in Washington state

Volcano types

Volcano types: cinder cones Cinder cones are volcanoes which erupt only during one episode They are explosive, but small in size The cone is a pile of pyroclastic debris which piles up at the angle of repose

Volcano types: cinder cones The cinders are generally of basaltic composition The eruptive activity typically lasts a few months or years

Cinder cones: Parícutin Parícutin volcano in Mexico is a classic cinder cone The region contains many cinder cones It consists of both pyroclastics and lava

Parícutin - lava flows These images shows the development of lavas in 1943 and in Red areas show new lava flows

Parícutin - five views taken from Luhr and Simkin (1993) The eruption was preceded by about 1½ months of felt seismicity The eruption began in a farmer’s field on 20 February 1943 It erupted for a comparatively long (?) time ( )

Parícutin Here is a photo of the volcano showing the classic form of cinder cones In the foreground is the obviously distressed farmer, Dionisio Pulido

Parícutin This is a view of the volcano in March 1944 In the foreground, note the flat-lying lava flows from the volcano lava

Parícutin The partly unfinished towers of San Juan Parangaricutico surrounded by 1944 lava flows from the volcano Note how the lava fills, but does not destroy, the church

Parícutin Note how the percentage of pyroclastic material declines steadily with time… while the opposite is observed for lava The daily mass eruption rate also declines steadily

From: العرض من Volc1 to Volc 6 موجودة ضمن عروض أخرى على الموقع الموضح أعلاه على شبكة الأنترنت. آمل من الطلاب تصفح الموقع أيضا