During volcanic eruption liquid, solid and gaseous materials are thrown to the air, which are collectively called as volcanic materials. This volcanic products can be classified into three categories; pyroclastic materials, lava and volatile materials.
During explosive volcanic activity, especially rhyolitic and andesitic eruption produces large volumes of shattered, fragmented rocks which are collectively called as Pyroclasts. Pyroclastic materials have different name according to their size.
Ash: pyroclastic materials with diameters smaller than 4mm is termed as Ash.
Cinders: They are small slag like solidified pieces lava.
Lapilli: Fragments of 4-32mm are termed Lapilli. They are pieces looks like a walnut.
Scoria: Fragments with vesicular texture and basaltic and andesitic in composition.
Pumice: Fragments with light coloured and extremely porous in character.
Volcanic Tuff: When solidified pyroclast (ash, cinder, lapilli, scoria) mixed with new lava flows produce a compacted volcanic materials called as volcanic tuff.
Volcanic Bomb: Fragments with diameter of 32mm-30cm and more than 100 tone in weight, circular or elliptical in shape, termed as volcanic bomb.
Blocks and Volcanic Breccia:
Fragments larger than 32mm called as blocks. Blocks which are originated as lava and have been thrown into the air while still molten become rounded or decorated with spiral patterns as they travel; such rounded and patterned forms are called volcanic bomb. Consolidated blocks are known as volcanic breccia.
Tephra: During heavy explosive eruption, pyroclastic materials and lava thrown to the air with greater height which some times reach to the lower portion of the troposphere and then it cools down and solidified rapidly and then it fall on the earth surface. This kind of air-fall deposition is collectively called Tephra. The size of the materials is about 2mm to more than 2m.
Nuees Ardentes: Eruption which produce very large quantities of gas may give rise to glowing clouds of ash, pumice and larger pyroclasts which move down slope at velocities in excess of 100km/hour. Glowing avalanches are known by their French name as Nuees Ardentes.
Though lava have many similar compositions, but two volcanic eruption is not exactly same composition. In fact composition of lava may vary from one eruption to another in the same volcano. Volcanoes, which are primarily erupting basaltic lava, common in ocean basins and rift zones of the continents. Andesitic lava erupt from volcanoes encircling the Pacific, where subduction zones occur.
Pahoehoe is a highly fluid lava that spreads in sheets. A thin, glassy skin forms on the surface of the hot flow and is dragged into ropy folds by the glowing lava beneath.
Aa is slower moving with a thick skin broken into jagged, clinker blocks forming on the surface and steep front of the flow. On some flows Iceland, Aa blocks are the size of a house, but elsewhere more like the size of a football. Local relief on a flow is usually several metres.
Under some circumstances, especially when erupted under water, lavas consolidated as a pile of tightly packed rounded masses, a foot or so in a diameter. These are known as pillow lava.