Lexikon
extrusive igneous rocks are formed at the Earth crust's surface as a result of the partial melting of rocks within the mantle and crust. The melt, with or without suspended crystals and gas bubbles, is called magma. When it REACHes the surface, magma extruded onto the surface either beneath water or air, is called lava. The lava cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture. In terms of the composition the volcanic rocks are classified in three large groups: basalts (greyish black coloured), andesites (greyish, redbrown coloured), rhyolite (white coloured). In Hungary basalt rocks are to be found in the Balaton and Salgótarján area, while andesite rocks extend from the North Danube area to the Mátra. The Zemplén mountains are mostly made up of rhyolites and partly by andesites. See also volcanic rock, magmatic rock