A small volcanic mountain with a narrow base and steep sides is called a cinder cone.

Here's why:

* Small: Cinder cones are relatively small, usually less than 1,000 feet (300 meters) tall.

* Narrow base: They have a narrow base compared to their height, giving them a cone-shaped appearance.

* Steep sides: The sides are very steep, often sloping at angles of 30-40 degrees.

Cinder cones are formed by the accumulation of tephra, which is a mixture of volcanic rock fragments, ash, and cinders, ejected from a vent during an eruption. They are common features in volcanic areas around the world.