Volcanic products (2)

• Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation.

• Individual eruptive fragments are called pyroclasts ("fire fragments").

TEPHRA

1)ASH :-

• Very fine-grained fragments (< 2 mm).

• Dominated by broken glass shards.

CLASSIFICATION ON THE BASIS OF SIZE

2) LAPILLI :- • Pea- to walnut-size pyroclasts (2 to 64 mm).

• Often look like cinders.

• Rounded spheres known as accretionary lapilli are formed in water rich eruptions.

3) BLOCKS AND BOMBS :-

• Fragments >64 mm.

• Blocks are ejected as solid fragments with angular shapes.

• PELE'S TEARS :- Lapilli-size fragments of basaltic lava may cool quickly while airborne, to form glassy teardrop-shaped lapilli called Pele's tears.

• PELE’S HAIR :- During strong winds, the molten fragments that can be drawn out into fine filaments are called Pele's hair.

• SPATTER :- They are lapilli- to bomb-size fragments produced by Non-explosive Hawaiian-type eruptions.

SOME SPECIAL TYPES OF TEPHRA

• PUMICE :- They are produced from vigorous gas escape in felsic lavas.

• RETICULITE :- They are produced from similar gas escape in mafic lava.

• SCORIA :- A denser mafic rock containing a smaller abundance of relatively large vesicles.

• Scoria-fall deposits :- These are derived from Strombolian eruptions of scoria cones. The deposit is composed of basaltic to andesitic vesiculated pyroclasts (scoria) lying near the eruptive vent.

• Pumice-fall deposits :- These are derived from Plinian eruptions of stratovolcanoes. The deposit is composed of highly vesiculated dacitic to rhyolitic pyroclasts (pumice) which can be distributed for hundreds of square kilometers away from the vent.

GENETIC CLASSIFICATION OF TEPHRA DEPOSITS

• Ash tuff - rock dominated by ash; sometimes simply referred to as tuff.

• Lapilli tuff - rock dominated by lapilli.

• Tuff breccia - rock containing 25% to 75% blocks and/or bombs.

• Pyroclastic breccia - rock containing at least 75% blocks and bombs.

• Agglomerate - rock containing at least 75% bombs.

• Agglutinate - rock composed of fused, largely unrecognizable, basalt spatter fragments.

NON-GENETIC CLASSIFICATION OF TEPHRA

• Fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice.

• Heavier-than-air emulsions that move much like a snow avalanche.

• Most deadly of all volcanic phenomena.

PYROCLASTIC FLOWS

• Indonesian term for a volcanic mudflow.

• Lethal mixtures of water and tephra have the consistency of wet concrete.

• Maximum temperature of a lahar is 100 degrees Centigrade.

LAHARS

• Majority are produced by intense rainfall during or after an eruption.

• A tragic example of such an event was the 1991 eruption of Mt. Pinatubo in the Philippines. An estimated 700 people died from burial by the ensuing lahars, together with the collapse of structures beneath the wet ash.

LAHAR GENERATION

• Lahars can destroy anything by direct impact.

• Lahars can lead to increased deposition of sediment.

• Lahars can block tributary streams.

• Lahars can bury valleys and communities with debris.

EFFECTS OF LAHARS

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