GLG 101

Chapter 4

Volcanism

volcanism: processes whereby magma and its gases are extruded onto the Earth's surface

volcanic gases: in most cases water vapor

Lava flows:

pahoehoe lava (ropy surface)

aa lava (rough, jagged angular surface)

pillow lava (submarine flow)

Pyroclastic material:

ash (very fine, <2.0 mm)

lapilli (fine, 2 to 64 mm)

bomb, block (>64 mm)

Volcanoes:

crater: more or less circular central depression, <1km in diameter

caldera: large crater with steep slopes

shield volcanoes: large, gentle slope, mafic comp., low viscosity

cinder cones: mostly pyroclastic material, small size (max. 400 m high), large crater

composite volcanoes: Stratovolcanoes, composed of pyroclastics and lava flows (intermediate), lahars (mud flows)

lava dome: felsic comp., relatively small, explosive eruptions (nuee ardente)

Distribution of volcanoes:

well defined zones or belts (e.g. Circum-pacific belt, Mediterranean belt)

at or near mid-ocean ridges

hot spots

Plate tectonics and igneous activity:

plate boundaries

divergent plates (spreading ridges)

colliding plates ( subduction zones)