Pyroclastic igneous rocks

The glassy or vitreous texture with no crystals usually indicates that the magma cooled extremely quickly and/or that it was so viscous that ions could not migrate to form crystals seed. Most glasses are related to pyroclastic igneous rocks. 

Tuff Tuff is a pyroclastic igneous rock with a fragmental texture and is often friable (loosely held... 

In the preceding paragraphs we have mentioned some of the researches that are necessary on the chemical substances of the earth s surface. We come next to aggregates, including the igneous rocks, the pyroclastic and sedimentary rocks, the oceans and other bodies of water, and the atmosphere. 

Igneous rocks may be divided into intrusive and extrusive types, according to their mode of occurrence. In the former type, the magma crystallizes within the Earth s cmst, whereas in the latter, it soiidifies at the surface, having erupted as lavas and/or pyroclasts from a volcano. The intrusions have been exposed at the surface by erosbn. They have been further subdivided on the basis of their size, that is, into major (plutonic) and minor (hypabyssai) categories. 

The finest pyroclastic material is called ash. Much more ash is produced on eruption of acidic than basic magmas. Acidic igneous rocks contain over 65% silica, whereas basic igneous rocks contain between 45 and 55%. Those rocks that have a silica content between acid and basic are referred to as intermediate, and those with less than 45% silica are termed ultrabasic. As mentioned, the reason for the difference in explosivity is because acidic material is more viscous than basic or basaltic lava. 

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