Basalt is one of the most widespread and best known igneous rock types. It is the main constituent of the enormous plateau basalts e.g. Deccan in India Typically basalt is a fine-grained, dark-coloured, dense rock which has a dull, granular appearance. It is also the volcanic equivalent of gabbro and is often porphyritic. Commonly basalt is vesicular, that is, it has holes (vesicles) which were once gas bubbles; later, these holes often fill with mineral matter (amygdales). Basalts which formed by underwater eruptions characteristically develop pillow-like shapes.
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