TUKUL 2: Geology and Volcanology

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Volcanoes in activity

Strombolian volcano in eruption

Degassing from the crater with projection of scoriaceous material which creates the cone; emission of a lava flow bringing away scoriae fallen on its surface and avoiding the closing of the cone (breached cone). Piton de la Fournaise, Reunion Island, October 1977.

Piton De La Fournais, Reunion Island 1977

Spatter cone in eruption

Feeble degassing from a very fluid lava.

The projections fall close to the crater and, due to their fluidity, agglutinate to form a spatter cone with slopes steeper than those of a strombolian one. Ardoukoba, Republic of Djibouti, November 1978.

Ardoukoba, Republic of Djibouti, 1978

Lava flow output

Provided at a feeble emission rate, the lava exhibits at the vent yellow colours corresponding to at least 1100°.
Due to the cooling, the surface darkens very quickly while the inner mass can keep its heat during several kilometres of displacement. Etna, Italy, June 1985.

Etna, Italy, 1985

Pahoehoe lavas surface

Pahoehoe lavas with a low gas content move as a viscous liquid (honey or melted sugar).

They cover themselves by a deformable skin giving origin to spectacular features such as the roped lavas. Kilauea, Hawaii, May 1988.

Kilauea, Hawaii, 1988

Aa lavas surface

The aa lavas are relativity rich in gas concentred in their upper layers; they create a foam which is broken up when flowing.

It results a chaotic surface made by scoriaceous unstable blocks (clinkers) where it is very difficult to walk. Etna, Italy, June 1985.

Etna, Italy, 1985

Freatic explosion

When entering the sea, a lava-flow can trap some water under its mass in fusion and vaporize it.
The vapour generated under pressure by the process is able to release itself during so called freatic (or rootless) explosions throwing scoriaceous and incandescent fragments. Kilauea, Hawaii, April 1988.

Kilauea, Hawaii, 1988

Sommital explosion at Mount Etna (Italy)

An other type of freatic explosion can occur after the vaporization of the humidity contained in the crumbled materials obstructing the bottom of a crater. Explosive opening occurs unexpectedly, without precursor signs, and represents a significant danger for the tourists visiting the high areas of the volcano (nine casualties on September 11, 1979).

Etna, Italy, 1979

The Santiaguito dome (Guatemala) in degassing activity

It is a complex dome made up by various unities successively settled for several ten of years.

The viscosity of the lava had forbidden its flowing and obligated it to accumulate in situ (April 4, 1990).

Santiaguito, Guatemala, 1990

Initial phase of a nuée ardente (glowing avalanche)

In activity for more than ten years, the Unzen dome (Japan) has grown with very viscous lava. It has emitted many nuées ardentes developed from the crumbles of portions of its very unstable high areas which were dislocated and pulverized when releasing its gas (at several hundred degrees). On June 1991 such a nuée ardente killed about forty persons.

Mount Unzen, Japan 1991

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