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% SiO2, gas-poor

Andesite: 60% SiO2, gas-rich

Rhyolite: 70% SiO2, 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 80-120 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 1951-52

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 (1943-1951)

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:http://www.meteo.mcgill.ca/195-250/2008/ppt/volcanoes.ppt

من ضمن Volc1 to Volc 6العرض موجودةعلى أعاله الموضح الموقع على أخرى عروض . الموقع تصفح الطالب من آمل األنترنت شبكة

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