Igneous intrusions are bodies of igneous rock which form due to the crystallization of magma underground . Igneous intrusions are also referred to as plutons . There are a number of different types of intrusions. These are classified based on their sizes and shapes.
Dikes are small, tabular intrusive rock bodies.
Dikes are considered to be discordant intrusive bodies because they cut across pre-existing rocks. Picture 1 shows a small, felsic dike approximately seven centimeters thick which is exposed in Joshua Tree National Park, California. Note that the country rock here is granite which had already crystallized before the intrusive activity which formed the dike. Picture 2 is of a handsample of diorite with a two centimeter-thick felsic dike cutting across from left to right. The rounded edges of this handsample indicate that it was transported some distance by a stream at Earth's surface.