On terrestrial class worlds with an active geology, a tabular sheet of rock resulting from the intrusion of magma between layers of sedimentary rock, older beds of volcanic rock, or along the foliation planes of metamorphic rock. Unlike a dike, a sill is initially close to horizontal, though later events may turn the rock layers up at an angle. As on Old Earth, such intrusions may carry important ore deposits of rare elements.
- Dike, magmatic - Text by M. Alan Kazlev
On Terrestrial Class worlds with active geology, a vertical or near-vertical sheet-like intrusion of magma that forces its way through a layer of fractured rock. The magma flows through cracks in the rock and later cools and solidifies into a sheet of igneous rock. Magmatic intrusions that are closer to horizontal or run along sedimentary bedding planes are called sills.