Earth Sciences (Rocks and Volcanoes 2)

Magma’s composition

proportion of silica

Viscosity

resistance to flow (runny to sticky)

More silica in magma

more strong silicon-oxygen bonds (even before
crystallization begins) = more resistance
to flow (higher viscosity)

Importance of dissolved gases in magma

With more of this, the more explosiveness there will be

Mafic/Basaltic Magma’s silica content and viscosity

50% silica content & least viscosity (thin/runny)

Mafic/Basaltic Magma’s gas content and explosiveness

0.5 - 2% gas, least explosivity

Mafic/Basaltic Magma’s temperature

1100 - 1200 celcius

Andesitic/intermediate Magma’s silica content and viscosity

60% silica content & inermediate viscosity

Andesitic/intermediate Magma’s gas content and explosiveness

3-4% gas and intermediate explosivity

Andesitic/intermediate Magma’s temperature

900-1000 celcius

Rhyolitic/Felsic Magma’s silica content and viscosity

70% silica content and greatest viscosity

Rhyolitic/Felsic Magma’s gas content and explosivity

4-6% gas content, greatest explosivitiy

Rhyolitic/Felsic Magma’s temperature

700 - 800 celcius

Pyroclastic materials

Ejected fragments from eruptions that range in size from dust to pieces several tonnes in weight

Welded Tuff

Hot ash falls and glassy shards fuse to form it

Scoria

bubbly mafic pieces

Pumice

bubbly felsic pieces, very low density (may float)

Pyroclastic Flow

consist of hot gases infused with hot ash and
larger rock fragments

• Rush down side of volcano at
speeds up to 200 km/hr\
• 100º - 800ºC
• Often the main cause of death
during volcanic activity

Lahar

a volcanic mudflow that's a mixture of water,
ash, rock fragments, and other materials that flow down a
volcano's slopes.

These flows can travel at speeds of 70-
80 km/hr.

Shield volcanoes

Broad, slightly domed-shaped
• Composed primarily of mafic lava
• Generally cover large areas, flows can travel
up to 150km
• Mauna Loa on Hawaii is a good example

Stratovolcanoes (composite volcanoes)

• Explosions cause pyroclastic debris
• Most are located adjacent to the Pacific Ocean (Ring of
Fire)
• Large, symmetrical volcano (hundreds of metres high &
several kilometres wide at base)
• Composed of interbedded lava flows and layers of ash, like mount fuji