ABSTRACT
The solidified lavas and intrusions constitute the igneous rocks. The molten material from which igneous rocks have solidified is called magma. Natural magmas are hot, viscous siliceous melts in which the chief elements present are silicon and oxygen, and the metals potassium, sodium, calcium, magnesium, aluminium, and iron. A volcano is essentially a conduit between the Earth’s surface and a body of magma within the crust beneath it. During an eruption lava is extruded from the volcanic vent and gases contained in the lava are separated from it; they may be discharged quietly if the lava is very fluid and the gas content small, but commonly they are discharged with explosive violence. Different styles of volcanic action are distinguished as fissure eruptions and central eruptions. The deposits formed by the consolidation of fragments ejected during an eruption are called pyroclastic.
