A magma-filled fissure under pillow basalt is known as what?

The correct term for a magma-filled fissure under pillow basalt is a "dike." A dike refers to a type of igneous intrusion that forms when magma infiltrates and solidifies in fractures or fissures in existing rock layers. When this process occurs underwater, such as in mid-ocean ridges where pillow basalt is commonly found, the dike can lead to the formation of basalt columns as the magma cools and crystallizes.

In the context of pillow basalt—characterized by its rounded, bulbous shapes formed under water by the rapid cooling of basaltic lava—dikes often serve as conduits that transport magma from deeper sources to the surface. This is crucial for volcanic activity and the formation of new crust.

The other terms mentioned refer to different geological features. A batholith is a large mass of intrusive igneous rock that forms when magma cools and solidifies beneath the Earth's surface, typically over a larger area. A sill is a type of horizontal igneous rock formation that occurs when magma intrudes between existing layers of rock, rather than in a vertical fracture. Dyke is simply an alternative spelling of dike but does not change the definition or context.

Thus, choosing "dike" correctly