SHORT VOLCANOLOGICAL NOTES WITH EXPLANATIONS OF THE MOST USED DEFINITIONS

Classification of Eruptions

A simple classification based on the violence of the event is:

Effusive eruptions: characterized by low explosivity and emission of lava flowing along the flanks of the volcano.

Explosive eruptions: characterized by a very high explosivity; they form an eruptive column expanding on ascent due to turbulent mixing with atmospheric air; it has a typical mushroom or cauliflower shape. They are also called "Plinian eruptions" after Pliny the Elder, who died during the 79 AC eruption of Vesuvius, and Pliny the Younger, who described the same eruption.

Phreato-magmatic eruptions: characterized by an explosivity due to the interaction between magma and water. The eruptions can be also called by different names according to the volume of erupted products, their dispersal, and violence: Hawaiian, Strombolian, Vulcanian, Pelean, Plinian, Ultraplinian.

The products of eruptions

An effusive eruption produces mostly lava flows. Very viscous lava can form small hills, of circular shape, called domes. Magma is fragmented during explosive eruptions before being emitted under the form of pumice, scoria, bomb, and ash, generally called pyroclastic products. Pyroclastic products can be classified according to the mechanism of emplacement:

Surges and pyroclastic flows are generally called pyroclastic flows; they are among the most dangerous phenomena related with explosive volcanism, because of their high velocity of emplacement and high temperature.

Mudflows are often associated with explosive eruptions (they are also called with the indonesian name of Lahars). They form because of the accumulation of ash and loose material on the flank of a volcano. Rain, glacier ice, or water vapour, emitted during the eruption, can mobilize the material and channelize it in creeks and rivers transporting rocks as heavy as few tons, tree trunks, and destroy everything in its course.

The most violent explosive eruptions can modify the morphology of a volcano. There is both deposition of new material and destruction of part of the cone. Sometimes, the rapid drainage of magma can produce the collapse of a large circular area, called a caldera.

The particles of pyroclatic material have different names according to the size. Bombs and blocks have a diameter greater than 64 millimetres. The bombs are pieces of magma hurled from the crater still in the fluid state. Some bombs are thrown very high and cool during the flight, attaining an aerodynamic shape; the accumulation of bombs and blocks on the ground can give rise to spatter cones. Lapilli have a size between 2 and 64 millimetres; they can be crystalline or glassy. The term lapilli is often used instead of pumice or scoria, which possess a vesicular structure. The difference between scoria and pumices is given by the different chemical composition, instead of the size. The pumices have a light colour, whereas scoriae have generally a black or reddish-black colour. The deposition of these materials near the vent can form scoria-cones, cinder-cones or pumice-cones. Ash is made up of particles (mostly glass which may be fragments of vesicles walls) smaller than 2 millimetres. The pyroclastic products emitted by a volcano during explosive eruptions form, upon aggregation and consolidation, pyroclastic rocks. The generic name of tuff (fine tuff or coarse tuff) is given to a rock formed by particles with the size of ash. Pyroclastic breccia is a rock made up mostly by blocks and bombs. Lapillistone is a rock mostly made of lapilli.

Very thick lava flows (as well as thick and hot pyroclastic-flow deposits) often show columnar jointing due to fracturing of the rocks because of contraction upon cooling. The joints have a direction perpendicular to the flow. Degassing structures, called pipes, are often observed in pyroclastic-flow deposits emplaced at high temperature. These structures consist of a vertical alignment of coarse material without fine particles, because they were extracted by hot gases escaping from the deposit.


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