Question

which phrase describes a sill?
○ fills in valleys and rivers
○ is a hardened magma intrusion
○ cuts vertically through older rock layers
○ is built up by numerous eruptions over thousands of years

Explanation:

A sill is a type of igneous intrusion that forms when magma flows horizontally between pre-existing rock layers and solidifies. Let's analyze the options:

1. Filling valleys/rivers describes lava flows, not sills.
2. A hardened magma intrusion matches the definition of a sill, as it is magma that intruded rock and solidified.
3. Cutting vertically through rock describes a dike, not a sill.
4. Being built by multiple eruptions describes extrusive landforms like volcanoes, not intrusive sills.

Answer:

is a hardened magma intrusion