Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

What term describes a geological formation created when magma squeezes between two rock layers and hardens into a thin sheet?

1. Laccolith

2. Sill

3. Caldera

4. Batholith

The correct answer is: Laccolith

The geological formation described is best defined as a "sill." A sill is a type of igneous intrusion that forms when magma intrudes between existing layers of rock and solidifies, creating a thin, horizontal sheet of igneous rock. This process occurs when the magma does not reach the surface but instead is injected into the surrounding rock layers, causing them to be lifted and sometimes altered by the heat. Understanding the characteristics of a sill is crucial for differentiating it from other geological formations. For example, a laccolith also involves magma intruding between rock layers, but it is typically dome-shaped and causes the overlying rock to bulge upward significantly, rather than remaining relatively thin and flat like a sill. A caldera is a large depression formed after a volcanic eruption when a volcano collapses, and a batholith is a large, deep-seated intrusive igneous rock body that has solidified from molten material. Each of these formations plays a distinct role in geological processes and has unique features that help geologists identify them in the field.