Sill
Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"
Meaning
A sill is a flat, almost horizontal sheet of igneous rock formed inside the crust. It forms when magma (molten rock below the surface) pushes sideways along a weak plane between rock layers, then cools and hardens there. A thick body is called a sill, and a thin one is called a sheet. A sill is concordant, which means it lies parallel to the rock layers around it. It is an intrusive landform, so it shows at the surface only after erosion strips away the rock above it.
Example
The Great Whin Sill in northern England is a hard sheet of dark igneous rock lying between older rock layers. Where erosion has exposed it, it forms long, steep ridges.
Don't confuse with
- Dyke: a dyke stands almost upright and cuts across the rock layers (discordant). A sill lies flat between the layers (concordant).
- Laccolith: a laccolith is a dome with a flat base, fed by a pipe from below. A sill stays a fairly even, flat sheet.