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Chapter 4: Types of eruptions
Volcanic eruptions are classified according to their degree of eruptive violence. The violence associated with each type of volcano is a reflection of the
materials ejected. However, it is important
to notice that volcanoes that erupt
with different degrees of violence may
produce similar cones, so the shape
of the cone is not a guide to the kind of
eruption, except that the most gentle-
sided volcanoes are connected with
the least violent eruptions and also
contain only low-viscosity basaltic lava.
A stratovolcano commonly exhibits more
than one kind of eruptive violence in its
history, or even within the same eruption.
Icelandic or fissure type
The icelandic type of eruption rarely produces a volcanic cone, nor does lava flow out of a single pipe or vent. Instead, this type of eruption displays a flow of
lava out of a fissure, so that a long split develops
in the crust, and the lava simply pours out of
it over a great length. These kinds of eruptions are most common at spreading boundaries, and most occur under water; but a few also occur on land, particularly in Iceland.
Iceland is entirely built of flood basalts. The largest eruption from the active Laki fissure was in 1783. Lava fountains occurred along its length, while lava poured out over the surrounding landscape, covering some 565 square kilometres and amounting to 12 cubic kilometres of new material.
Large fissure eruptions can pour out unimaginable floods of runny basalt, which can cover a landscape as fast as water. They have built up landscapes so immense that it is hard to realise that they are the result of a volcanic
(Below) Fissure eruptions occur where tectonic plates pull apart and probably also where some hot spots reach the surface.
(Below) The Columbia River has cut through some of the flood basalts in the northwestern United States, producing a gorge.