What is magma and how and where does it form?
o Magma: melted rock in the earth – below the earth’s surface; melt +
crystals + bubbles
o Lava: melted rock exposed at the earth’s surface; melt + crystals +
o Magma is created by the melting of pre-existing rock in earth’s
interior (mantle and crust)
o Reaches the surface through fractures and erupts as lava or pyroclastic
List the different categories of volcanic rocks and explain the differences
between the magmas they came from.
o Dark colour – Light colour
o Mafic (less silica) intermediate intermediate Felsic (more
o Hot (1200-1400) Cool (600-1000) o Low viscosity High viscosity
o Non-explosive (lava) Explosive (pyroclastic)
o Basalt Andesite Dacite Rhyolite
Glass is a non-crystalline or amorphous, metastable solid formed by
quenching a melt.
o Cools so fast that a regular crystal lattice cannot form
As magma rise, they: crystallize, exsolve volatiles
o Volatiles (gases) dissolved exsolved
Explain what lava flows, fire fountains, lava bombs and volcanic ash are and
how they form.
o Magma is generated by melting of existing rock
o Primary Processes
Lava flows (mafic-intermediate)
Downslope, ponds in topographic lows
Outer crust cools and solidifies insulating the molten interior
Lava domes (felsic intermediate) Gravitational collapse of lava flows/domes (block and ash
Buoyant eruption column of ash
Pyroclastic fall: widespread distribution of ash, 10’s of kms