The geological formation that forms when magma flows in a horizontal direction along rock layers is called a sill.

Here's why:

* Sills are tabular (flat) intrusions of igneous rock that have solidified between existing rock layers.

* Dikes, on the other hand, are similar intrusions, but they cut *across* existing rock layers.

* Lava flows occur on the surface of the Earth, not within rock layers.

So, when magma flows horizontally between existing rock layers, it creates a sill.