Geology Reference
In-Depth Information
TABLE 4.1 Relationships among Magmatic Composition, Viscosity, Volcano Form, and Typical Volcanic Eruption
Products for Different Types of Volcanoes
Magma
Composition
Typical Eruption Products
Relative Viscosity
Volcano Form
Pyroclastic flows, lava flows, lateral
blasts, tephra, lava domes, gases,
lahars (especially if caldera-filling lakes
are suddenly drained)
Rhyolitic
(approximately
70-75% silica)
High (sticky) for lavas; gas-
charged pyroclastic flows (hot
ash clouds) can be very mobile
Smaller domes, local
flows, or large calderas
from explosive eruptions
Intermediate between rhyolitic and
andesitic: pyroclastic flows, lava flows,
lateral blasts, tephra, lava domes,
lahars, gases
Dacitic (approxi-
mately 65% silica)
Intermediate to high
Composite volcanoes,
domes
Andesitic (approxi-
mately 60% silica)
Intermediate
Conical composite
volcanoes
Lava flows, tephra, pyroclastic flows,
landslides, lahars, gases
Basaltic (approxi-
mately 50% silica)
Low (fluid)
Shields and rifts; cones
where formed of tephra
Lava flows, tephra, gases
Note that all volcanoes can erupt from central vents or from fracture zones on the flanks of the volcano. Calderas (volcanic collapse areas) can form in all
types of volcanoes, with their size controlled by the amount of new magma erupted.
