Transform boundaries are known for their shearing motion, where plates slide past each other horizontally. This kind of movement doesn't typically lead to the formation of new crust or significant melting, which are necessary for the creation of igneous or metamorphic rocks.

Therefore, the most likely type of rock to form at a transform boundary is metamorphic rock. Here's why:

* Fault Zones: Transform boundaries are characterized by large fault zones where rocks are broken and crushed.

* Heat and Pressure: The friction generated by the sliding plates creates intense heat and pressure along the fault zone.

* Metamorphism: This heat and pressure can cause existing rocks, such as sedimentary or igneous rocks, to recrystallize and transform into metamorphic rocks.

Examples of metamorphic rocks formed at transform boundaries include:

* Mylonite: A fine-grained metamorphic rock formed by intense shearing and crushing.

* Schist: A metamorphic rock with a foliated texture, indicating the presence of pressure and heat.

While igneous rocks are not directly formed at transform boundaries, they can be indirectly associated with them. For example, if a transform fault intersects a subduction zone, the resulting friction could trigger magma formation and lead to volcanic activity.

However, it's crucial to remember that the primary rock type associated with transform boundaries is metamorphic rock, formed through the process of metamorphism driven by the intense shearing forces.