Because they cool much quicker than intrusive igneous rock, extrusive
rocks have aphanitic
texture - they have very small mineral grains.
cool slower, maybe over
thousands of years, while plutonic intrusives cool as slowly as over
millions of years. Intrusive rocks have phaneritic texture -
medium-sized or very large mineral grains.
Igneous rocks often have an equigranular
texture - with a uniform size of mineral grains. Sometimes
minerals have formed significantly earlier than others.
If that's the
case, the rock has got a porphyritic
texture and the larger minerals are called phenocrysts.
large, the rock is pegmatite.
Sometimes air or gas bubbles have been present. This rock has a vesicular texture.
If an igneous rock has formed too quick for any minerals to form, it
has a glassy texture.
If it has formed from volcanic ash, it has pyroclastic
If it has formed under water it is called pillow lava.
When vesicles are filled by
low-temperature, secondary minerals, the texture is amygddaloidal.
When minerals are interlocked
like in granitoids, it is called interlocking
And when there is intergrowth of quartz and
feldspar in triangular or linear orientation like in pegmatities, the
texture is graphic.