IGNEOUS ROCK - TEXTURES
Consisting of crystal grains that are large enough to be easily seen by the naked eye, the grains varying in size from 0.5 mm (1/32 in) in adesites to over 5 mm (1/4 in) in granites.
Made up of tiny crystals, which can only be identified using a microscope or powerful hand lends, they give the rock a flow texture, when aligned.
Composed of volcanic glass, sometimes the glass may be streaky, due to aphanitic bands, and may often contain micro crystals of feldspar (eg. obsidian).
These are volcanic rocks in which the magma has been shattered by an explosive eruption and so may consist of tiny silvers of volcanic glass, fragments of pumice, crystals or fractured rock; they may be unconsolidated or cemented together when fresh and altered to clays by weathering when not.
Larger crystals, phenocrysts, are embedded in a finer ground mass; some of the large crystals best being described as megacrysts that have grown in mearly solid rock by means of the replacement of other minerlas - a common feature in many granite.
Minerals are arranged in paralle bands, sometimes contorted as a result of the way the rock flowed while it was still hot and plastic.
Igneous rock Formation:
Igneous rocks are formed by Volcanic eruption. Molten rocks
called magma is found under high temperatures in the Earth's
interior. Some of this molten rock remains inside the earth and
some is ejected as lava onto the Earth's surface during volcanic
eruptions. When the molten rock cools and solidifies, it becomes
mineral crystals. The process of forming mineral crystals is called
crystallization. As the mineral crystals form, they join together or
interlock into masses of igneous rocks.
Igneous rocks are of two types.