The products of volcanic eruption consist of gases, rock fragments of different sizes And finally the lavas of different composition. Steam is the commonest of the volcanic Gases. It is discharged by active volcanoes in immense quantity as indicated by the great Height and enormous volume of the clouds that accompany many eruptions. These clouds Consist of the dust and ash by the up rushing column of steam and other gases. Due to Friction between ash particles and also to the atmospheric disturbance, the volcanic eruptions
And the rains that ensue are accompanies by electrical manifestations which culminate in Spectacular lightning discharges. Other gases of volcanic origin in order of abundance are C02 and smaller proportions of H, CO, S and Cl.
The pyroclastic materials that are blown into the air by the explosive activity of Volcanoes and shower down at various distances from the cone, vary very much in size from Fine volcanic dust and ash to big volcanic bombs. These materials are classified in the
1) Pieces larger than 32mm in diameter are called blocks if ejected as Solid and angular fragments.
2) Bombs if ejected as clots of molten lava and hence Somewhat roundish or ellipsoidal.
3) Lapilli or cinders which are between 32mm and 4mm.
4) Ash which are between 4 mm and 1/4mm and still finer material is called volcanic dust.
The volcanic ejectments in part are composed of solid rock and in part of a spongy, Cellular or vesicular character. This vesicular condition is due to the fact that while most of The gas is escaping into the air and carrying fragments with it, a miner part is expanding Within the particles of the liquid lava puffing them up into cellular forms.
The bombs, lapilli or cinders and most of the ash fall near the volcanic vent and thus Help build an edifice round it called the volcanic cone. The dust is carried for long distances And spread over an immensely large area. Microscopic volcanic dust from the great Catastrophic eruption of the Krakatao volcano in 1883 encircled the whole world and its Dispersion through the atmosphere was reasonable for the vividly colored and quite Spectacular sunsets that were seen during the following months. The projectile power of the Volcanoes is simply astounding. Thirty-ton blocks of rocks were blown out by the Stromboli
In 1930 to a distance of about 3 kms, even more astonishing are the huge blocks up to Several hundred tons carried forward for miles by the pelean clouds of Mont Pelie during the Great eruption in 1929.
Lavas: In volcanoes the liquid rock materials that comes out of the vent is called lava. As The magma issues out a lava it cools on the surf ace, darkens and crusts over. It is very Viscous and it’s under part may still be moving while the upper part crusts over and breaks Up into rough angular jagged blocks which are borne as a tumbling lava How, Its surface Consist of a chaotic blocks with sharp points. Such lavas are called block lavas. In Hawaiian Islands it is called AA. They are difficult to cross on foot.
In marked contrast to the block lava other lava flows harden with smooth surfaces Which have curious ropy or curved or corroded forms. They are called Ropy lavas in Hawaiian Island. Ropy lavas begin to crystallize at a higher temperature, minute bubbles of Gas escape quietly and the now congeals with a smooth skin which wrinkles into ropy and Corroded forms like those assumed by flowing pitch or coal tar.
When a lava of the ropy type flows over the sea floor or otherwise beneath the Chilling cover of water, it consolidates with a structure like that of a jumbled heap of pillows Appropriately called pillow lava by the time each emerging tongue of lava has swollen to The size of a pillow the rapidly congealed skin prevents its further growth. This type of Pillow lava is common in the submarine lavas associated with geosynclinals sediments of Former geological periods.
The mobility of a molten lava depends upon its composition. Silica rich lavas are Usually stiff and viscous and congeal as thick tongue before they have travelled far. The. Appearance and character of a lava, stream depend upon several factors. The chemical Composition of the magma determines the nature. Of The resulting rock whether it is Light Colored lava (rhyolite) or a black basalt or of intermediate. character as andesite.
Volcanic Eruption and Cones
The eruption of a volcano is often proceeded by earthquake and by loud rumblings Like thunder which may continue on gigantic scale during the course of eruption. The loud Rumblings are probably due to the movement of gases and molten rock that are held in under Great pressures. During one of the greatest volcanic eruptions like that of Tambora on Stunbawa Island of Indonesia in 1815, the land rumblings were heard over an areas of 1, 000 Kms radius.
The nature of a volcanic eruption is largely determined by the type of materials Ejected from the volcanic vent. These, products are gases, molten rock or solid fragments. A given volcano may emit all the three types of products but in some regions a certain Product predominates. Thus the Hawaiian volcanoes generally discharge very hot and highly fluvius lava from which the enclosed gases escape very rapidly. Such eruptions are non-Explosive. On the other hand, some volcanoes erupt with great explosive violence. Mt. Pelic on the island of Martinique in the West Indies is a typical example of explosive type. In May, 1902 it began to erTupt in a series of violent explosions but discharged almost no lavas.
Intermediate between these two extreme types is the one exemplified by Stromboli in the Mediterranean sea which erupts mildly every 15 minutes hurling small masses of partially solidified lava into the air. Steam also issues from the openings and forms a cloud, which Reflects the light form the incandescent lava.
Volcanic eruptions are broadly categorized into two types
(1) Central eruptions and
(2) Fissure eruptions.
Majority of the present day volcanoes belong to the central type but during certain Periods of geological time the fissure type eruptions have poured out lava and which covers The surroundings country as sheets of greater size than the limited flows of central types of Eruptions.
When eruptions takes place through a vertical chimney the orifice is widened into A crater with flaring sides by outward explosion and inward lumping. But with the Accumulation of volcanic products around the vent a conical or dome shaped mountain is Gradually built up. Volcanoes with the familiar cone and crater structure are said to be of the Central type, because the volcanic activity is centralized around a pipe-like conduit. The top Of a volcano generally is marked by a pit or crater, the crater may be widened to great size Due to repeated eruptions,