What is a volcano?

• A. An opening in the Earth's crust from which lava, ash and gases erupt.
• B. A small earthquake that only affects a local area.
• C. A mountain formed by tectonic plates grinding past one another.
• D. A weather phenomenon associated with volcanic ash.

Answer: (A)

A volcano is an opening in the Earth's crust through which magma from beneath the surface erupts as lava, along with ash and gases. This vent or crater allows molten rock to reach the surface and release materials during an eruption, which can build volcanic mountains over time. The other statements describe earthquakes, tectonic-move–driven mountains, or weather-related events with ash—none define a volcano, which is specifically about that surface opening and its eruptive products.