What are dikes, sills, and laccoliths?

Dikes and sills are tabular or sheet-like igneous bodies. Dikes are discordant (they cut across rock
layers), and sills are concordant (they parallel the layering in rocks). Laccoliths are sills that inflate,
causing the overlying rock to bow upward. A laccolith has a mushroom-like geometry.

Pegmatites are composed of mostly felsic minerals, close in composition to granite. The minerals in
pegmatites are enormous, at least 1 cm across and up to many centimeters or even meters. Pegmatites
are found near large granite plutons and form from the water-rich magma that remains after most of
the granite crystallized. The water-rich magma travels through cracks and has time to cool slowly,
creating enormous minerals.

Basalt and gabbro are both felsic rocks, but basalt is aphanitic due to rapid cooling and gabbro is
phaneritic due to slower cooling. The same is true for composition but not cooling rate of andesite, the
fine-grained, and diorite, the coarse-grained, equivalents. Rhyolite is a fine-grained, rapid cooling
felsic rock, and granite is a coarse-grained, slow cooling felsic rock too. Basalt, andesite, and rhyolite
are increasingly felsic fine-grained rocks, and gabbro, diorite, and granite are increasingly felsic
coarse-grained rocks. The first set is extrusive, and the second set is intrusive

Aphanitic rocks are so fine-grained that the individual minerals are too small to be seen with the naked
eye. They form when the rate of nuclei formation exceeds the rate of crystal growth, and an aggregate
of small mineral grains forms. Phaneritic rocks are coarse-grained; the rate of growth exceeds the rate
of nuclei formation, and large mineral grains form.

If magmas of different composition come into contact in a magma chamber, they will mix. The result
will be a magma that is different from both of the original magmas in the chamber.