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Tuff
Tuff
A rock made from volcanic ash. Volcanic activity produces a large amount of ash that settles over the landscape. It is particularly thick close to the vent of a volcano. After an eruption has ended, the ash begins to consolidate, and a later covering with lava or more ash will turn the first ash layer into a soft rock called tuff. This rock could be thought of as a sediment because it is deposited, but the material is entirely igneous and it forms only at the time of
an eruption.
V
Vent
An opening in the ground through which magma erupts or volcanic gases are emitted.
When an eruption takes place through a pipe-like vent, the force of the explosion may tear the rock in the walls of the vent and widen it (see: Crater). The widening vent is
a reason eruptions tend to get more powerful as they go on.
(See also: Fumarole.)
Vesicle
A bubble in a volcanic rock originally created by air trapped in the molten lava. (See also: Amygdule and Basalt.)
Viscous, viscosity
A term describing the thickness
or stickiness of a liquid. It is applied to lava that is acidic
and moves slowly. Rhyolite and andesite are viscous lavas. Basalt, which is a runny lava, is said to have a low viscosity.
Volcanic
Anything from, or of, a volcano. Volcanic rocks are extrusive igneous rocks (for example, basalt andesite and rhyolite) that cool as they are released at the Earth’s surface – including those formed underwater. They typically have small crystals due to rapid cooling.
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Volcanic activity
Volcanoes are often put in categories based on how active they appear to be. There are
three categories: active, dormant and extinct.
 Volcanic activity – Popocatepetl in Mexico during a phase of low activity.
 Volcanic activity – Mount Pinatubo,
Philippines, erupting in 1991.