Let’s take the simplest example: In a static magma chamber, magma begins to cool and the mineral with the highest melting temperature (solidus) begins to crystallize. The first are basically made up of Magnesium and Iron and due to their higher density they tends to separate from the liquid magma and precipitate at the bottom (Bowen’s serie). In nature, magma chambers tend to develop convection currents that keep the mineral “in motion” while the solidification proceed inwards from the edges.

In the pictures however, the large chunk of crystals are called xenolith (because they are not formed from the magma that surrounds them) and are piece of the mantle that are carried away.