1. Batholiths: These are the largest intrusive bodies, often extending for hundreds of kilometers and reaching depths of tens of kilometers. They form deep within the crust, often as a result of the buildup of magma beneath volcanic arcs. Over time, the magma cools and solidifies, creating vast, underground granite masses.

2. Sills: These are tabular, sheet-like intrusions that solidify parallel to the existing rock layers. They form when magma forces its way between layers, often exploiting weaknesses or zones of low pressure. Sills can vary in size, from thin sheets to large, multi-layered intrusions.

3. Dikes: These are vertical or nearly vertical intrusions that cut across existing rock layers. They form when magma ascends through fractures or weaknesses in the crust, often feeding volcanic activity. Dikes can be relatively thin, but can extend for significant distances.

These are just a few examples of intrusive igneous rock formations. There are other types as well, such as laccoliths, stocks, and volcanic necks.