The cooling and solidification of magma within the earth's crust results in the formation of a

Analyze the question: It asks about magma that cools and solidifies within the earth's crust. That means the magma never reaches the surface. This is the key phrase.

Explain the concept: Igneous rocks form in two main ways:

• Extrusive (volcanic) features form when magma or lava reaches the surface and cools there. Examples are cinder cones, composite cones, and lava plateaus.
• Intrusive (plutonic) features form when magma cools and solidifies underground, inside the crust. Examples are batholiths, laccoliths, sills, dykes, and volcanic necks.

Step-by-step solution:

• A. Cinder cone: built from ash and cinders thrown out of a vent onto the surface, so it is extrusive.
• B. Composite cone: built from alternating layers of lava and ash erupted at the surface, so it is extrusive.
• C. Laccolith: a dome-shaped mass of magma that pushes up the overlying rock layers and then cools and solidifies underground, so it is intrusive.
• D. Lava plateau: formed when large volumes of fluid lava flood out over the surface through fissures, so it is extrusive.

Only the laccolith forms by cooling inside the crust.

Exam tip: Remember the prefixes. Extrusive means exiting onto the surface. Intrusive means inside the crust. When a question says "within the crust," think of intrusive features such as sills, dykes, laccoliths, and batholiths.